This is the name given to the process that maintains hydrostatic pressure in a well bore greater than the formation fluid pressure but less than fracture pressure.
If hydrostatic pressure is less than formation pressure then unwanted formation fluids will enter the well bore. If the hydrostatic pressure of the fluid in the well bore exceeds the fracture pressure of the formation then the hydrostatic pressure of the fluid in the well bore will decrease possibly to a point where formation pressure will exceed hydrostatic pressure and formation fluids will enter the well bore. An overbalance of hydrostatic over formation pressure is maintained – this is normally referred to as a trip margin.
Secondary Well Control
If the hydrostatic pressure of the fluid in the well bore fails to prevent formation fluids entering the well bore the well will flow uncontrollably until all pressure is dissipated. This process is stopped using a blow-out preventer to prevent the well bore fluids escaping from the well – this is the initial stage of secondary well control – containment of unwanted formation fluids.
For the purpose of this guidance note, we are going to focus on primary well control. However, secondary well control will be discussed in a separate post.
As previously stated Primary Well Control is a process that maintains hydrostatic pressure in a well bore greater than the formation fluid pressure but less than fracture pressure to prevent unwanted fluid entering the wellbore.
What is Hydrostatic, Fracture Pressure and Formation Fluid Pressure?
The pressure exerted by a column of fluid depends on its density and vertical height or depth. Hydrostatic Pressure (psi) =Mud weight (ppg) x 0.052 (psi/ft) x True Vertical Depth (TVD)ft
psi = pounds per square inch.
ppg = pounds per gallon.
0.052 = pounds per square inch per vertical foot of a 1 pound per gallon fluid.
Since pressure is measured in Ibs per square inch (psi) and depth is measured in feet, it is convenient to convert mud weight from Ibs per gallon (ppg) to a pressure gradient in psi/ft.
The 0.052 conversion factor is derived as follows: 1 cubic ft. contains 7.48 US gallons. A fluid weighing 1ppg would weigh 7.48Ibs per cubic ft.
The pressure exerted by that one ft. height of fluid over the area of the base would be:
7.48 pounds/144 square inches = 0.0519 pounds per square inch (psi) Hence, a one ft. high column of 1ppg fluid would exert 0.052 psi on its base. This is the same as saying the pressure gradient of the fluid is 0.052 psi/ft.
ppg x 0.052 ( mud weight) = psi/ft. (pressure gradient)
Formation Fracture Pressure
Formation fracture pressure, or formation breakdown pressure is the pressure required to rupture a formation, so that whole mud can flow into it. The symbol PFB is usually used to denote this pressure.
Commonly this is expressed as a pressure gradient, GFB, with the units of psi/ft.
The formation breakdown pressure is usually determined for formations just below a casing shoe by means of a leak-off test (LOT). This test of the formation strength, also known as a formation integrity test or FIT, is effected after the casing has been run and cemented in place. This allows formations to be tested after the minimum of disturbance and damage due to drilling, and allows a clear indication of strength to be determined for one isolated zone.
Leak Off Test (LOT) Procedures and Calculations
Drill out new formation few feet, circulate bottom up and collect sample to confirm that new formation is drilled to and then pull string into the casing.
Close annular preventer or pipe rams, line up a pump, normally a cement pump, and circulate through an open choke line to ensure that surface line is fully filled with drilling fluid.
Stop the pump and close a choke valve.
Gradually pump small amount of drilling fluid into well with constant pump stroke. Record total pump strokes, drill pipe pressure and casing pressure. Drill pipe pressure and casing pressure will be increased continually while pumping mud in hole. When plot a graph between strokes pumped and pressure, if formation is not broken, a graph will demonstrate straight line relationship. When pressure exceeds formation strength, formation will be broken and let drilling fluid permeate into formation, therefore a trend of drill pipe/casing pressure will deviate from straight line that mean formation is broken and is injected by drilling fluid. We may call pressure when deviated from straight line as leak off test pressure.
Bleed off pressure and open up the well. Then proceed drilling operation.
Leak Off Test pressure in mud density
Leak off test in equivalent mud weight = (Leak Off Test pressure ÷ 0.052 ÷ Casing Shoe TVD) + (Current Mud Weight)
Leak off test in equivalent mud weight in ppg
Leak Off Test pressure in psi
Casing Shoe TVD in ft
Current Mud Weight in ppg
Note: Always round down for LOT calculation
Leak off test pressure = 1,600 psi
Casing shoe TVD = 4,000 ft
Mud weight = 9.2 ppg
Leak off test in equivalent mud weight (ppg) = (1,600 psi ÷ 0.052 ÷ 4,000 ft) + 9.2ppg = 16.8 ppg
Formation Integrity Test (FIT) Procedure and Calculation
Formation Integrity Test is a method to test strength of formation and casing shoe by increasing Bottom Hole Pressure (BHP) to designed pressure. FIT is normally conducted to ensure that formation below a casing shoe will not be broken while drilling the next section with higher BHP or circulating gas influx in a well control situation. Normally, drilling engineers will design how much formation integrity test pressure required for each hole section.
The formula below demonstrates how to calculate required FIT pressure.
Pressure required for FIT = (Required FIT – Current Mud Weight) × 0.052 × True Vertical Depth of shoe
Pressure required for FIT in psi
Required FIT in ppg
Current Mud Weight in ppg
True Vertical Depth of shoe in ft
Note: FIT pressure must be rounded down.
Required FIT (ppg) = 14.5
Current mud weight (ppg) = 9.2
Shoe depth TVD (ft) = 4000 TVD
Pressure required for FIT = (14.5-9.2) × 0.052 × 4000 = 1,102 psi
Maximum Allowable Annular Surface Pressure
The leak-off pressure, PLO, is determined as the maximum surface pressure, which the well could stand, with the hydrostatic load of mud in use at the time of the test. This can be described as the Maximum Allowable Annular Surface Pressure (MAASP) with that particular mud weight in use (meantime we shall leave aside safety factors).
Every time the mud weight is changed, the MAASP changes and must be recalculated.
MAASP = (GFB – CMUD) x Shoe Depth, True Vertical
If a Maximum Equivalent Mud WT is quoted for formation strength, then the same formula appears as:
MAASP = (Max Equiv. Mud Wt(ppg) – Current Mud Wt(ppg) x 0.052 x Shoe Depth, True Vertical
Frequently a safety factor is applied so that the ‘actual’ maximum is never applied.
This safety factor gives a margin for error. A leak-off test is not usually a precise or high accuracy test, so a margin, and a value somewhat lower than the formation fracture is used, particularly when the ECD is a major factor.
Formation Fluid Pressure (PF)
The formation fluid pressure, or pore pressure, is the pressure exerted by the fluids within the formations being drilled.
The sedimentary rocks, which are of primary importance in the search for, and development of oil fields, contain fluid due to their mode of formation.
Most sedimentary rocks are formed as accumulation of rock debris or organic material, underwater. Since over two thirds of the earth’s surface is covered with oceans, the vast majority of sedimentary rocks are laid down as marine sediments in the shallow seas around the land areas.
In general, areas of the earth’s surface, which are above sea level, are affected by the processes of erosion (breaking up and wearing down of the land masses). The debris is washed down into the shallow sea basins where it settles out onto the sea floor, the coarser material generally settling out closer to the shore than the fine silts and clays.
This process may continue for long periods as the earth’s surface slowly moves, some areas being pushed up to provide fresh surfaces for erosion, with adjacent sea basins slowly deepening to allow great lengths of sediment to build up. Thus sedimentary rocks contain water, usually seawater, as an integral part of their make-up. As the depth of sediment increases, the rocks are compacted, squeezing water out. The water contained within the rocks becomes progressively more salty as the relatively small molecules of water move through the pore spaces of the rock, while the larger salt molecule is retained.
The result of this is that the formation fluid pressure, or pore pressure, exerted by the water in a normal, open, sedimentary sequence is equivalent to that produced by a free-standing column of salt water, which is rather saltier and heavier than typical sea water.
An average figure for normal formation pressure gradient in marine basin sediment determined some years ago in the US Gulf Coast area is 0.465 psi/ft. This is the pressure gradient produced by a column of water of approximately 100,000 ppm chloride. In comparison, a typical value for seawater is 23,000 pprn chloride.
This gradient of 0.465 psi/ft. or expressed as an equivalent mud weight, 8.94 ppg is generally accepted as a representative figure for normal pore pressures in marine basins in the Gulf of Mexico. a common value for the North Sea is 0.452 psi/ft.
Abnormal formation fluid pressure, or ‘sur-pressures’ as they are sometimes known, are pressures greater than normal formation pressure and can rise for a number of reasons. They can be categorized as:
Gas Cap Effect
The above causes of Abnormal Pressure will be discussed in another post
As discussed previously, Primary Well Control is a process that maintains hydrostatic pressure in a well bore greater than the formation fluid pressure but less than fracture pressure to prevent influx of formation fluid to the wellbore.
Definition of Influx and Kick
An influx is an intrusion of unwanted formation fluids into the well bore which does not immediately cause formation pressure to exceed the hydrostatic pressure of the fluid in the well bore – but may do if not immediately recognized as an influx – particularly if the formation fluid is gas. A kick is an intrusion of unwanted fluids into the well bore such that the formation fluid pressure exceeds the effective hydrostatic pressure of the well bore fluid.
There are some warning signs to watch out for while drilling to ensure drilling mud weight (hydrostatic pressure of the well) is adjusted to mitigate formation fluid influx to the wellbore.
Kick or Influx Warning Signs
The indication of the presence of a kick or influx is:
Incorrect hold fill volume
If this sign is not noticed at an early stage, it should become progressively more obvious. In the more extreme case the hole would eventually stay full, or flow, while pulling out.
Hole keeps flowing between stands, while running in
The presence of some or all of these indications requires that a flow check is carried out to determine whether or not a kick is in progress.
DURING OR AFTER TRIPPING
Trip gas is a measure of swabbed gas over an entire trip. Often a short trip of 15-20 stands is made in order to circulate bottoms up and measure units of swabbed gas. Excessive units of trip gas may indicate the need for increasing the trip margin and/or reducing swab pressure.
The general definition of a warning sign – particularly while drilling is:
‘It is a sign to indicate a kick or some other reason’
However, vigilance and care are exercised to ensure that the reason for the sign is investigated.
The following warning signs are given in approximate order of priority. The drilling break is considered the most important and must always be investigated.
A sudden increase in rate of penetration is usually caused by a change in formation type. It may however signal an increase in permeability and an increase in formation pressure. Both these effects result in faster drilling.
The drilling break may be dramatic, though most commonly a gradual change is seen. A drilling break could indicate a kick is in progress though it is often a sign that conditions are changing and formation pressure rising, which may lead to a kick.
The appearance of gas cut mud at the surface usually causes concern, particularly bottoms up after a connections The reduction of bottom hole pressure owning to gas cutting can be critical on surface hole. However, due to the compressibility of gas, a fifty percent gas cut of mud at the surface changes the bottom hole pressure at 20,000 feet may be only 100 psi. The relative decrease in surface hole would be critical and the gas must be eradicated.
Gas cutting must not be ignored and the cause must be investigated.
Pump Pressure Decrease/Pump Stroke increase
Invading formation fluid generally reduces the total head of fluid in the annulus. The head of mud in the drill pipe is unaffected, so that there is a tendency for fluid to ‘U- tube’. This means that the pump does not have to provide so much energy and this may be seen as a pump pressured reduction. Depending on the rig installation, a small increase in pump rate may also be noted.
The effect is small, and may not be noticeable. The same effects are seen if a washout occurs, so it is necessary to confirm which is taken place, by doing a flow check. The presence of a continuous recording rnonitor of pump pressure and pump stroke rate on the drill floor means that quite small changes can be seen readily by the Driller.
Total Gas Levels
A gas detector, or hot wire device, provides valuable information. Such instruments measure changes in the relative amounts of gas in the mud and cuttings, but do not provide a quantitative value. Increase in the gas content can mean an increase in gas content of the formation being drilled, gas from cuttings and/ or an underbalanced pressure condition.
In conditions of normal pressure and normal overbalance, background gas should not vary significantly as the hole is drilled. Changes in background levels indicate possible conditions of concern. Increases in the normal background gas indicate the flow of formation gas into the mud or the presence of gas expanding from drilled cuttings. Background gas could mask connection gas, care must be exercised, especially top hole drilling.
Connection gas is a measure of gas which enters the hole whilst making a connections It is reported in units of gas over normal background gas. Estimating the time to pump mud from bottom and checking the gas detector recording can identify connection gas. After the swabbed gas passes the detector, the units should return to the background levels. If not, an underbalance condition could exist.
Connection gas can be eliminated if a sufficiently high overbalance exists, or if pulling speed is reduced and/or if mud properties are adjusted. However, connection gas can be used as an accurate indicator of formation pressure when drilling with close to a balanced pressure. Increasing levels of connection gas are a reliable warning of an underbalanced pressure condition. The relationship of normal gas content readings to the amount of increase can be used as an indicator of the need to increase mud density.
Change in Flow Properties
The presence of formation, such as hydrogen sulphide will effect the chemical and flow properties of mud. Gas and air will ‘froth’ or foam the mud at the surface, lowering the surface density and sometimes increasing the viscosity of the mud.
Formation fluids, particularly salt mater, which can enter the well bore, can also alter the chemical balance of the mud as well as reduce the density. The result can be a drastic change in chemical and flow properties of the mud. For example, salt water in the mud will cause a drop in pH as will hydrogen sulphide with a consequent increase in viscosity and fluid loss.
Sometimes the changing flow properties of a mud system can be a readable warning signal that the well is underbalanced and a kick is imminent.
Torque and Drag, Fill on Connections
Increases in torque and drag often occur when drilling underbalance through some shale intervals. As the result of this fluid in the shale expands, causing cracking, spalling and sloughing of the shales into the well bore. This condition can cause a build-up of cuttings in the annulus, excessive fill on connections and trips, a build-up in torque and drag and eventually stuck pipe. Increases in torque and drag can be a good indicator of abnormal pressure, especially if used with other indicators.
Plots of drilling rate versus depth are often difficult to interpret because of changes in drilling rate variables such as WOB, RPM and mud properties. In 1986, Jorden and Shirley developed a normalized rate of penetration equation from data gathered on the Gulf Coast. In their relationship, normalized drilling rate was defined as a function of measured drilling rate, bit weight and size and rotary speed in the equation shown below:
The authors provided correlation of field measured pressure data and ‘d’ exponent calculation. They showed that formation pressure could be estimated by first plotting ‘d’ values in shale versus depth, on semilog paper, and determining a normal trend line of decreasing value with depth in the normally pressured section. Then, by determining the differences between the extrapolated values of ‘d’ exponent and those calculated from actual data, the correlation was used to estimate the amount of overpressure at any depth.
The method developed with Gulf Coast data has been applied world-wide with moderate success. Since their original work, others have applied a correction for mud weight to obtain a modified drilling exponent. This is applied in much the same way as the ‘d’ exponent and sometimes it is plotted as 100/d versus depth, for direct comparison with plots of interval transit time.
The examination of shale cuttings and/or cores can provide information on formation pressures. Properties of shale such as bulk density, shale type, size, and shape can be related to abnormal pressures.
Several techniques, such as the graduated density column method or the mud balance method, are available to measure the density of shale cuttings recovered at the shaker. Care must be exercised to separate bottom cuttings from upper hole cavings. Also, cuttings must be properly washed and/or scraped to remove the outer layer of mud contaminated sample. Plots of shale bulk density versus depth are made and the normal trend of increasing density versus depth established. Changes from the normal trend can then be related to changes in formation fluid content, and hence formation fluid pressure.
Shale cuttings, which are drilled underbalanced, tend to produce larger than normal cuttings, larger volumes than normal, and shapes that are more angular, sharp and splintery in appearance. These effects are due in part to the fact that fluid trapped within pores of the shale at high pressure expands when exposed to the lower mud hydrostatic pressure. Therefore, drilling rates and hold size increases as shale continues to expand, crack, spall and slough into the well bore, thereby creating larger and different shaped cuttings or cavings. Volume of cuttings increases due to faster penetration rates and increased hole volume caused by sloughing and caving. Close observation of shale on the shaker along with other indicators can provide a basis for determining an underbalanced condition prior to taking a kick.
Excessive volumes of shale cuttings on the shaker may be an indication of an underbalanced condition. Shale is usually porous, but has little or no permeability. Fluids in the pores are subjected to formation pressure, but are not able to flow. However, if a differential pressure exists from the formation to the well bore, such as in the case of abnormal pressure, the fluid pressure causes weakening of the walls of the hole and spalling or heaving of shale into the hole. At the surface an increase in volume of shale cuttings is noted. These cuttings are splintery, angular, and generally larger than normal. If these conditions persist, the mud hydrostatic pressure is probably too low and a kick will occur white drilling the next permeable formation.
The type of clay mineral of which shales are largely composed varies slowly with increasing depth and the swelling clays, sometimes known as ‘gumbos’, progressively give way to the non-swelling type. Near the surface the principal clay minerals are calcium and sodium based montmorillonites and illites. With increasing depth of burial these alter slowly – towards the largely potassium based kaolinites.
This change can be determined roughly in a number of ways, as of which the Methylene Blue test for clay absorption lever determination, and Differential Thermal Analysis for structural water content determination are the best known and most widely used.
Flow line Temperature
The temperature gradient in the transition between normal and abnormal pressure zones often increases to about twice the rate of the normal temperature gradient. Increases of the mud temperature at the surface can also indicate the top of an overpressure section. Consideration must be given to circulation times, trip times, connection times, stabiliser temperature after tripping temperature of mud at suction pit, and other factors such is water depth. An increase in flow line temperature when used with other indicators can show the top of an overpressure section with accuracy.
It is important to note this indicator can be partially or totally masked in offshore drilling from floating vessels by the cooling effect of long lengths of riser and substantial air gaps.
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The aim of supervising borehole drilling is to ensure that boreholes are constructed according designed and all the data collected during the drilling are accurately recorded and reported to the relevant agencies. Good supervision is essential for a high-quality borehole, even if a competent Driller is employed. Without good supervision, the quality of the work may be compromised.
A poorly constructed borehole can fail after one year, resulting in wasted investment and disappointed users. Good supervision and quality control of water well drilling by trained professionals (geologist and engineers) to ensure that quality is not compromised are essential for the provision of long-lasting water wells.
A High quality and cost-effective constructed water borehole can continue to function through the lifespan of 30 to 50 years. In some African countries, as many as 60% of water borehole are not working because of poor and substandard construction. One of the best ways to tackle this problem is to improve the quality and professionalism and supervision of water well drilling.
Levels of Supervision
There are three levels of drilling supervision:
Full-time supervision: a supervisor stays with the drilling team throughout the drilling process, from the inspection to demobilization. On large drilling programs with multiple rigs, several Supervisors are deployed, and they stay in the Drillers’ camp and go out with them each morning. While this supervision level is ideal, the resources needed are not always available
Part-time milestone supervision: one Supervisor is in charge of several drilling rigs and may only witness crucial stages (milestones) of the drilling. The stages that must be carried out in the presence of the Supervisor need to be specified in the contract document and the consequences of not abiding by them stated. However, the Supervisor is expected to be promptly on site and should not cause undue delays. The milestones are:
check siting/site selection
termination of drilling
lining of the borehole
platform construction and pump installation (may be delegated, depending on contract).
The ‘Record Keeper ‘, one of the Driller team plays a very important role. He/she is designated to collating the measurements and preparing the forms at all stages of the process set out in the milestones above. This role should be specified in the contract documents.
3. End of contract supervision is not actually supervision but a site inspection when the Supervisor goes through the records and inspects the functionality of the borehole on completion. Where this is the planned level, the supervising role of the community members is particularly important. As in the case of part-time supervision, the role of the ‘Record Taker’ is also very important.
In all cases, the Supervisor requires a minimum level of equipment and needs to issue site instructions.
Vehicle: Ideally, the Supervisor should be independent. However, this may not be possible, in which case the Driller provides transport to and from the site.
Down-the-hole camera: useful for preventing arguments about casing lengths. In one example, a supervisor carried out a camera survey of several boreholes on a project. The Driller had hurriedly drilled the boreholes not allowing any supervision. Several of the holes were found to be open holes whilst it was specified that they be lined. He had to re-drill them. Cameras are getting cheaper. Every project should have one.
Others: Boots; hard-hat; clipboard; notebook; duplicate book; digital camera; global positioning system (GPS) device; mobile phone; caliper; spirit level (for checking verticality of drill mast and pedestal as well as slope of run-off drains); dip meter; measuring tape; simple calibrated V-plate for measuring borehole yield, magnifying glass; stop watch; pH stick meters and calibrants; iron-checker disc and reagents; bottle of hydrochloric acid if limestone is predicted and a first aid kit. Some other equipment are – depth meter, electronic dipper, tape, EC and PH meters, Global Positioning System (GPS)
The technical specification for the borehole should include the procedure for site instructions and the consequences for not abiding by them. Site instructions issued to the Driller by the Supervisor should be in writing in duplicate using carbon paper. The Driller should sign on the original and the duplicate instructions. The original is handed over to the Driller, and the Supervisor keeps the duplicate
Whichever level of supervision is adopted it is essential that community members are involved in the entire drilling process. This should foster the spirit of ownership and understanding of post-construction operation and maintenance. The need for this is even greater when either part-time supervision or end-of- project inspection is used.
Prior to the Driller’s mobilization or at the initial stages of the borehole construction, selected community members (school teachers, health workers, water users’ association members) are taken through the drilling process and are taught how to:
take the required measurements and record observations;
keep daily records such as start and end times of drilling and any breaks, and the reasons for them;
determine depth of drilling by counting the number of drill pipes lowered down;
record depth and time of the first water strike and other strikes when drilling with air;
count and record the length and number of casings and screens installed;
count the number of bags of cement used;
observe the installation of gravel and the sanitary seal, test pumping and whether borehole chlorination is undertaken.
From the information provided by the community supervision, the Supervisor can build an accurate account of the drilling progress which he/she can cross-check with the driller’s daily log.
What can realistically be expected of the community will depend on their level of literacy and numeracy, too. It should also be clear that community involvement can never replace an experienced supervisor.
Borehole Drilling Workflow – Responsibilities of the Driller and Supervisor
Check equipment; provide guidance on siting borehole; approve siting report.
3. Pre-Mobilisation Meeting
Raise specific questions regarding the contract requirements.
Together with the client, thoroughly discuss the design, materials and procedures for each step of the contract.
Submit program of work; submit samples of materials; move equipment to site.
Liaise with the community; approve drilling equipment & material; guide driller to site.
Position and operate the rig, collect sample and reports.
Monitor drilling; advise depth to stop drilling; log the borehole.
6. On-site Design Modifications
Install casing and screen; gravel pack; sanitary seal; report.
Instruct screening and casing depths; ensure gravel pack and sanitary seal properly placed.
7. Borehole development and site completion
Develop the hole; undertake test pumping; collect water sample; disinfect the hole.
Ensure water is clean; proper disinfection; supervise pumping test; ensure samples are taken and platform installed.
Remove all equipment and rubbish from site; report.
Ensure the site is restored to its former state.
Complete documentation and handover
Submit all records. Hand over.
Hand over borehole to community. Report.
Adapted from RWSN
Step 1: Inspection
Aim: To verify the capabilities of the Driller before a contract is signed.
Type of Equipment
1 drilling rig
1 drilling manager
1 mud pump
1 rig operator
1 water tanker
1 support truck
Adequate lengths of drill pipes to drill the deepest hole
3 rig assistants
Drill bits of the right diameter
Casing, gravel and filter pack, drilling mud
Example of basic equipment and personnel
Step 2: Borehole siting
Aim: To ensure that the borehole is drilled in the right place so that it has water that is accessible to users and protected from pollution.
Step 3: Pre-Mobilization Meeting
Aim: To ensure that the Driller and Supervisor are fully aware of their exact roles and responsibilities and contract details.
Step 4: Mobilization
Aim: To take the drilling project from contract signing to deployment of the drilling crew on site
Mobilization includes the following activities:
Contract: All borehole projects and supervision are based on a contract agreement. Once the contract has been signed, and pre-mobilization meeting held (Step 3), the mobilization phase starts. Procurement and contract management aspects are covered in Adekile (2012).
Program of works: The Supervisor should discuss the technical specifications and drilling procedure with the Driller, and discuss and agree the target depths. Then the Super- visor should ask the Driller to submit a program of works.
Community liaison: It is essential that, before the Driller arrives on site, the Supervisor or Project Manager has had several discussions with the Community about the project and details of the drilling process and their expected obligations and contributions with the main contact persons or Community representatives. The Driller’s representative should meet with the Community and agree a start date.
Equipment check: The equipment that is to be used by the Driller should be checked to make sure that it is all in working condition, and the same as, or equivalent to, what was examined in the inspection step.
Materials check: In some contracts, the suppliers, manufacturers, or sources of the material to be used, such as drilling fluid, casing and screens, are specified. The Driller should submit samples of the materials for the Supervisor’s approval. The slot size and wall thickness should be checked, for example.
Data collection forms: The format of drilling data collection to meet the contract requirements should be agreed on. The final version for copying will be agreed on site between the Driller and Supervisor, and signed by both parties once all the stages of the contract are completed.
Project filing system: Most of the data could be stored electronically, but hard copies are required for field use. A file (in duplicate) should be opened for every community and all records and data for the community stored in the file. Checklists for all stages of borehole construction (Annex B) are printed inside the flap of the folder and ticked as construction progresses. The original is kept in the office and the duplicate in the Drill Camp or site office.
Drill camp layout: On large projects where a Drill Camp is set up, the Driller should submit a drawing of the camp lay- out for approval. The main consideration in approving the plan is safety and sanitation: inflammable items should be kept away from likely sources of heat and fire; potential contaminants from water-supply sources and cooking areas; and PVC casing and screens are protected from direct sun- light, which makes them brittle. Where the project covers a large area, Satellite Fly Camps may be needed in the more remote parts to reduce the travelling time to a cluster of borehole sites. The same criteria as for the approval of the Drill Camp plan apply. Once all the above have been completed and approved, the Driller and the Supervisor are ready to move to site.
Step 5: Drilling
Aim: To ensure a high-quality borehole is drilled in a way that is safe and well-documented.
When the driller reach the project site. The following aspects are critical:
Safety: Drilling is a very hazardous activity. Safety of the workers on site is absolutely vital. Responsibilities for ensuring safety should be clearly set out in the contract. The Supervisor must be constantly vigilant to prevent accidents, and to minimize injuries should accidents occur.
Rig position: It is essential that the rig is horizontal and the mast vertical, otherwise a bent hole may result. Verticality of the drill pipe should be checked with a spirit level. The rig should be jacked on a robust wooden block so that verticality remains throughout. The rig should be positioned exactly over the pegged site. This is particularly important when the siting is undertaken by a consultant employed by the Client rather than the Driller. If the borehole is dry, there can be no argument that the borehole was not drilled on the specified location. The Driller should ensure that the weight on the drill string is adequate to maintain a straight hole. The use of a heavy drill collar is recommended on at least the first three meters of length behind the hammer. The first drill rod could have welded wings, adding weight as well as scraping to get a circular, straight bore. Also, the Driller should not drill with too much pull-down on the rods.
Monitoring drilling depth: The Supervisor needs to know the depth of the drill bit at all times to ensure that proper data logging is being done, to know the depth at which to tell the Driller to stop and to compare the drilled depth with the depth recommended in the contract. An unscrupulous Driller can try to rip off the Client either by pretending that the borehole has been drilled deeper than it actually has.
The drilling depth can be monitored by measuring the length of the drill pipe and multiplying the number of full pipes that have gone down into the hole.
Chalk can be used to mark the drill pipe: with the drilling rig set up with the first length of drill pipe and bit fitted, the drill bit is lowered to the ground. The drill pipe is marked “0” at the rotary table that centralizes the drill-pipe, and then chalk marks are made at 1m intervals up the drill-pipe, numbering the marks from 0 up- wards. Measured chalk marks are then made on subsequent drill pipes to be added. This procedure allows anyone on the drill team to know at a glance the exact depth of the drill bit from the ground surface. Note that if the hammer is changed to a longer one after drilling has commenced, the pipes will need to be remarked.
Penetration rate: This is the time taken to drill a particular interval. A fast penetration rate can indicate an aquifer, although this is not always the case. Less porous strata, such as fresh granites, are often slower to drill through.
Drilling fluids & air-lift yield: Drilling fluids are used to remove cuttings from the borehole and to stop the hole collapsing during drilling. The type of fluid should match the drilling method:
Down-the-hole-hammer: compressed air; water and air; or foam;
Rotary drilling: drilling mud (water and additive). Be aware that bentonite clay is commonly used but is outlawed in some countries because it can do permanent damage to the aquifer. Biodegradable polymers should be used;
Monitoring the drilling fluid colour and viscosity is the responsibility of the Driller. Viscosity is checked by measuring the flow rate of the drilling fluid through a Marsh Funnel. The Supervisor should ensure the Driller has a Marsh Funnel and it is properly used. In the case of air-percussion drilling, the air-lift yield should be measured using a V- plate or pipe/container. All observations and measurements are recorded every metre, using the marks on the drill pipe as a guide.
Drill cutting samples: To collect the samples, the Driller stops drilling, flushes all cuttings in the hole to the surface, resumes drilling, and then collects the cuttings. In air drilling, the samples are caught in a bucket placed in the stream of air jetting from the borehole. In mud drilling the samples are collected by inserting a spade into a small collection pit as the cuttings flow to the main pit. It is the Driller’s responsibility to ensure that the mud pump is of such rating and condition that it can lift the cuttings out of the hole. If the hole is not properly flushed, cuttings may become mixed up and not lifted out so that during lining, the casings do not get to the required depth. The drill samples should be bagged in strong transparent bags, labelled with indelible ink, and stored in a position that they will not be contaminated by site conditions or drilling operations. The label should contain the borehole number and location, sample number and depth. The sample could be collected and stored in a sample box. A photograph of the samples should be taken as a permanent record. In mud drilling, the samples would have mixed with the drilling fluid. The samples should be washed before bagging, but care should be taken in washing soft rock material, such as clays, as they could disintegrate in water.
The depth interval of collecting samples might have been stated in the Technical Specification, but drilling conditions may require that this is reviewed. It might have been specified that samples should be taken at every metre interval. However, in a deep borehole where the formation does not change rapidly, the interval could be increased to three metres. Equally, where there is rapid change in lithology, the Supervisor may change the interval to half a metre.
Description is based on identifying and describing:
the grain size and shape
the rock type
For example, samples from a sedimentary borehole could be described as:
0 – 2 m dark grey hard CLAY
2 – 4 m grey brown coarse angular grained loose SAND
4 – 6 m white medium to coarse partially compacted SANDSTONE
6 – 10 m white coarse partially compacted SANDSTONE
10 – 23 m white compacted SANDSTONE
Strata Log: Drill samples should be described and a strata log prepared by the Supervisor. Different methods are required for describing sedimentary rock samples and crystal- line rock samples. From the strata description, the Supervisor will prepare a graphic strata log which will form part of the final borehole report
Final borehole depth: It is the responsibility of the Supervisor to instruct the Driller to stop drilling when the right depth has been reached. The decision to end drilling will depend on the information gathered in the course of drilling. The factors will include:
what has been stipulated in the contract, which may be based on Client guidelines with respect to the average borehole depth in the area;
depth of the water strikes/aquifer;
static water levels;
estimated seasonal fluctuations in water levels i.e., changes in water levels as a result of recharge in the wet season(s) and groundwater discharge during the dry season(s);
the estimated yield from the borehole.
The typical signs for adequate yield and drilling depth vary with the type of formation and the drilling method. In the case of a yield which is obviously good, in a well that is to be installed with a handpump the final borehole depth should be at least 5 metres into the aquifer. It needs to allow for proper installation of the pump. It also should allow for 3 to 6 metres of sump (blank casing) below the screen as a sand trap.
However, if the yield is not clearly so good, continue to drill to the next strike horizon, until the yield is sufficient. The yield increments should be monitored with the V-plate. A 6m sump may be suitable where sand and silt are a problem. In cases where there is fine saprolite in the upper sections, these should be cased off to prevent silt from entering and filling the sump.
Unchanged from original state
Slight discolouration, slight weakening and dislocation
Considerably weakened, penetrative discolouration. Large pieces cannot be broken by hand
Large pieces can be broken by hand Does not readily disaggregate (slake) when dry sample immersed in water
Considerably weakened Slakes Original texture apparent
Soil derived in situ weathering but retaining none of the original texture or fabric
Description and Classification of Crystalline Rocks Based on Grades of Weathering and Dominant Minerals
For example, the log from a granitic terrain might read as follows:
0 – 6 m orange brown silty CLAY
6 – 16 m grey brown clayey fine SAND
16 – 23 m biotite granite GNEISS IV-III+
23 – 30 m biotite granite GNEISS III+
30 – 43 m biotite granite GNEISS I
Drill Report: The data from the drilling should be recorded both for the final design and as a reference for future borehole projects. The Driller needs to keep a daily drilling log which should be signed by the rig operator and the Supervisor at the end of each day. The Supervisor should insist that this is done – as Drillers often consider this an unnecessary intrusion into their work. The Supervisor should keep the record of the drilling activities and all measurements in a field note book. The most important data will go into the Casing and Well Completion Form, which will be collated, filed or bound together as part of the final project report and deposited with the appropriate office for future reference. Even data from dry or aborted holes needs to be recorded.
Step 6: On-site Design Modifications
Aim: To ensure that the finished borehole uses the aquifer efficiently, gives a long working life and low capital, maintenance and operation costs. The Code of Practice for Cost Effective Boreholes (Danert et al, 2010) provides illustrations of different borehole designs. The provisional design should precede the signing of the contract, because the design gives rise to the specification. The specification informs the Driller what to bring to site. Any design work on site involves modifications to, or finalization of, the design.
The Supervisor is responsible for on-site design modifications. Every borehole design is unique because it has to be adapted to the local geology, which cannot be predicted with absolute certainty. Borehole design involves selecting the appropriate dimensions and materials of the borehole, i.e. depth, casing and screen type and diameter, depth intervals of installation, and gravel pack zone. Borehole design factors are set out in Check- list 5 and described in further detail below. Most of the parameters listed above would have already been taken into consideration when writing the technical specifications of the borehole, but the information gathered in the course of drilling will steer the final design.
Depth: Taken from the Drill Report;
Formation: What type of aquifer is the borehole taking water from? Use local geological expertise and mapping where possible, but in general the three aquifer types are: Basement Complex; Consolidated sediments; and Unconsolidated sediments;
Yield: A borehole only needs to be drilled to a depth where the required yield can be sustained without contamination from surface water. Table 4 gives the ranges of yields from different formations. Selection of the other parameters, such as the borehole diameter and lining, should be geared towards meeting the required yield.
Drilled borehole diameter: The drilled diameter of the borehole needs to be large enough so that pump, casing, screens, gravel pack and sanitary seal can all fit without snagging. For handpump-fitted boreholes there are different schools of thought with respect to diameter, with some favoring smaller diameters, such as 6” to 6.5” and others arguing that this is inadequate to enable gravel packing to be properly installed without bridging. Anscombe (2012), a Driller with years of experience in southern Africa, argues that the reality is that most Drillers simply do not use a tremie pipe when installing gravel pack, with the result that the gravel pack is not properly installed. He thus argues that wells need to be drilled at an 8” diameter. This view has cost implications.
Casing and screens: Casings are blank pipes which prevent the borehole from collapsing. Screens are pipes which have slotted openings that allow water to flow into the borehole but prevent sediments from entering the borehole. The lining material can be galvanized steel, Polyvinyl Chloride plastic (PVC), Glass Reinforced Plastic (GRP) or bamboo. The two most often used are PVC and steel, and the choice between them depends on the depth of the borehole and the corrosiveness of the groundwater.
A 100mm diameter borehole casing will accommodate a handpump. Submersible pumps may require larger diameter casing depending on the required yield. Pump manufacturers usually prepare sets of curves showing the capacity of their pumps in terms of yield at particular depths or pumping heads and the diameter of the borehole that will accommodate the pump. The Supervisor should therefore check the diameter of borehole casing suitable for the intended pump. Generally, for motorised schemes in rural communities, a 150mm diameter is adequate. In the case of boreholes for small town supplies, or agriculture, a larger diameter will be required. In boreholes that are deep but have high static water levels (i.e. shallow depth of water level), a larger diameter casing, say 300mm, may be installed in the upper reaches of the borehole to house the pump (called the pump chamber), whilst a smaller diameter, 100-150mm, is used to line the lower parts and the aquifer.
In consolidated rock – such as the basement complex – the depth is rarely more than 60m. PVC casing and screens can withstand the pressure imposed by the formation. There is a school of thought that argues that the lower part of the drilled hole could be stable and can be left open and un- lined. In such cases, only the top weathered horizon is lined with a casing. In such holes, the annulus between the casing and the drilled hole should be grouted. However, it has been argued that these holes are not always sustainable, with some prone to siltation.
In unconsolidated formations, the entire borehole is lined to prevent the borehole from collapsing. Where the aquifer is more than 100m deep, the pressure exerted by the water and the rock formation is great, and steel casing and screen should be installed. In deep aquifers with slightly acidic water, Glass Reinforced Plastic may be considered as mild steel could corrode..
The Supervisor should ensure that the casings and screens supplied are new and conform to the specification. If in doubt, the diameter and the wall thickness should be checked with callipers. The Driller should provide a sample of the pipe cut in half, and the measurement taken in the middle. Measuring the thickness at the threaded end will not give the accurate figure. Table 5 gives the dimensions of casings, wall thickness and possible depths of installation from a pipe manufacturer.
It should be noted that drill pipes, casings, screens and other lengths are not always standard. Sometimes they are cut and re-threaded. Often, 3m “standard lengths” are actually 2.95m, or some other length.
Indication of installation depth m*
Outside x inside diameter in mm
Wall thickness in mm
50 – 75
110 x 103.4 (3½’’)
75 – 100
110 x 101.6 (3½”)
200 – 300
113 x 96.6 (4”)
50 – 75
125 x 117.6 (4½”)
75 – 100
125 x 115.4 (4½”)
PVC Casing and Screen Dimension (*Depth of installation mention may vary with ground condition)
Screens: are installed in the aquifer horizon. A borehole screen is a filtering device that serves as the intake portion of boreholes constructed in unconsolidated and semi- consolidated aquifers. The screen permits water to enter the borehole from the aquifer, prevents sediments from entering the borehole and serves to support the aquifer material. Increasing borehole diameter does not have much impact on water flow into the borehole, but increasing the screen length significantly increases the yield. Therefore, as much of the aquifer as cost permits should be screened. There is not much difference in the prices of PVC casing and screen, but stainless steel screens are very expensive and should be used sparingly.
Screen slot size: The total open area of the screen governs the amount of water that flows into the hole. Slot sizes are not a big issue with handpumps as the required amount of water is relatively small. It is enough to ensure that the aquifer material will be retained by the selected screen slot size. This can be checked by doing a sieve analysis of the aquifer material, but a quick method is to rub a sample of the aquifer material against the screen. An adequate slot size will allow the fines to pass through, whilst the coarse material remains outside. It has been noted that in some southern Africa countries, locally available casing tends to be rather coarse (1mm slot size). This will not always be adequate. If the aquifer is laden with silt, and the slots allow it to pass through, it can result in the wearing of pump seals, and ultimately in siltation of the well. In motorized schemes where a high yield is required, a large diameter screen may be installed as the total open area increases. Water flows freely through a screen with a large in- take area compared to one with limited open area. To prevent turbulent flow into the borehole which could cause encrustation and lower the lifespan of the screen, the velocity through the screen should not be more than 0.03 m/s. The minimum open area in the screen to permit non-turbulent flow can be calculated from the formula:
A = Q/30
where A is the open area in m2 and Q is the water flow in l/s (Macdonald et al, 2005)
Installing casing and screen requires great care and attention as it is easy to install blank casing in the aquifer horizon. Once the depth of the borehole and the depth interval for screening are known, a sketch of the proposed assemblage of casing and screen should be made. The casings and screens should be laid out according to the sketch and measured individually, totaled and checked that they conform to the sketch. They should be placed next to the drill collar ready to go into the well. The Supervisor should take a photograph of the layout for the record. Figure 8 shows a sketch of a strata log with casing and screen assemblage in a sedimentary terrain. Once all of the materials are inserted, the drilled depth needs to be reconciled against the casing and screen depth. If the discrepancy is more than 3m, there is need to reconsider whether the screen is actually sitting where it is supposed to, or if there has been some collapse of the well. If something is wrong, the contractor must remove the casing, clean the well, and re-insert it until the Supervisor is satisfied.
Joints should be strong enough to support the entire weight of the casing during installation. Threads should be intact. Both male and female threads should be properly cleaned with a brush and cloth before they are joined. Where non- threaded couplings are used, they should be cleaned and joined together by the solvent cement recommended by the manufacturer. Before the casing is lowered into the bore- hole, the Supervisor should ensure that the recommended time for the cement to set and form a water tight seal is observed.
This is critical, but sometimes the Driller may be in a hurry to leave the site and so shorten the time. Steel casing and screen should be joined by threaded joints that are water- tight. Where welding is used, the weld should be fully penetrating and continuous. If possible, welding of casings should be avoided as the weld can be a point for rusting and casing failure. It is also time-consuming and can put the casing out of true line. In addition, steel casings which are torch slotted on site corrode much easier than those which are bench slotted beforehand.
The casing and screen assembly should be lowered into the hole under the force of gravity. They should never be driven down. In fact, they should be lifted slightly (by 100mm) when they reach the bottom and held there while the gravel is inserted (see point 9). This ensures that they are straight in the hole and not spiraled. In some cases, centralizers are used to align the casing in the hole. A 3m length of sand trap should be part of the well design when boreholes are cased to the bottom and the bottom casing sealed with an end cap.
Gravel pack: is installed in the annular space between the borehole screen and the wall of the drilled hole. Often, the aquifer material is allowed to collapse against the screen, and the fines are washed out during development. This enables natural development to take place. Where the aquifer material is coarse and mobile, it is the preferred method. However, where this is not possible, artificial gravel packing is used. There are two types with different functions:
The formation stabiliser is coarse sand or river gravel installed in the hole to prevent the caving of formation material and damage to the screen. The material should be carefully chosen and sieved to make sure it is of uniform size and bigger than the slot size of the screen and will not flow into the borehole. It should not contain mica, clay or laterite. Large pieces should be sieved out as they can bridge in the annulus and prevent subsequent gravel from reaching the bottom. Granite chippings should not be used as gravel pack as they tend to be angular and may contain mica or harmful material that leach into the water. The material should be washed and carefully introduced into the hole through a tremie pipe to avoid bridging. It should extend several meters above the screened interval but stop at least 6m below ground surface.
A filter pack is installed around the screen in fine grained unconsolidated formations where an appropriate screen slot size cannot be found. The grain size of the filter pack material has to be selected in relation to that of the formation material. It should be coarser than the aq- uifer sand. The relationship, called the pack-aquifer ratio (P.A. ratio), is calculated from the formula:
Pack Aquifer ratio = 50% size of gravel pack/50% size of
The ratio should be between 4 and 6. For the procedure for sieve analysis and selection of appropriate filter material the reader should consult Driscoll, 1986: 406-409.
It is essential that the casing, screen and gravel pack are available on the site once drilling commences. Once the drilling pipes are withdrawn, the hole has a potential to collapse. Thus the casing and gravel pack need to be placed without delay. Under no circumstances should this wait until the following morning. In the words of Anscombe (2012): “Rods out– casing and gravel in – fast and efficient”.
Step 7:Borehole development and site completion
Aim: To prepare the borehole for use and install the pump and ancillary headworks and structures
Borehole Development Method: Borehole development is about cleaning the area of the aquifer immediately around the screens. The method of development should be stated in the technical specification. Figure 9 shows an example of air lifting. Air jetting can use a galvanised pipe, plugged at one end, with 8mm holes along the length so that the air-jet streams in the borehole are horizontal. This pipe, connected to the compressor, is raised and lowered repeatedly over the screen section, finishing in the sump.
Borehole Development Success: The Supervisor’s duty is to ensure that eventually, the water coming out from the borehole is clear of mud and is sand free. Samples of the water are collected in a clear container and checked to see that there are no sediments collecting at the bottom of the container. As part of this, the Supervisor needs to decide whether a borehole should be accepted or declared abortive. If the borehole is to be aborted, the Supervisor also needs to determine whether the Driller should re-drill the borehole at his own expense or not. This will depend on the terms and conditions of the contract. Although some contracts specify the duration of development (the minimum number of hours that must be spent on developing the hole), this actually depends on the time it takes for the water to be clean. Development should continue until the Supervisor is satisfied that the water coming out of the borehole is clean and sand free. Some boreholes clear within a couple of hours, some may take days to several weeks. Some only clear after several months of pumping. The latter is likely if air-percussion drilling has been used in very loose, clay-rich, silty, micaceous and saturated conditions – in other words not the right drilling technique.
Sanitary seal: It is essential to prevent contamination of the aquifer and to ensure that the users obtain safe, clean drinking water. When the Supervisor is satisfied with the yield, and development has settled the formation stabiliser or filter pack, then the annulus of the borehole is back-filled with the cuttings, or clayey soil, up to 6m below the ground surface. A sanitary seal is placed in the top 6m to prevent surface water which may be polluted from flowing down the bore- hole annulus into the aquifer. The sanitary seal should be cement slurry in the mixture of 25l of water to 50kg of neat cement, or bentonite.
Pumping test provides the means to determine the likely success of the borehole in terms of yield and drawdown. It provides information on the properties of the aquifer and on the borehole itself. Two types of pumping test can be conducted. A constant- discharge or aquifer test should always be carried out. This gives information about the drawdown resulting from a spe- cific pumping rate (usually a little greater than the design discharge). The test data can also be interpreted in terms of the aquifer properties. For a handpump, a 3- to 6-hour constant discharge test is adequate. If the borehole is going to serve a large population and a high yield is required, then a longer test of say 24 to 72 hours, or even longer (up to 14 days) may be undertaken.
A constant discharge test provides information about the aquifer in the vicinity of the well. The results of the constant discharge pumping test enable the short term performance of the well to be determined. However, it does not provide any information about recharge, seasonal fluctuations or long term performance. In other words, the pumping test does not give information about the long-term (multi-year) sustainable yield of the borehole. The long-term yield is the subject of groundwater resources evaluations, which focus on recharge and its variability from year to year.
The second type of test is a variable-discharge or well test, also known as a step-test. This type of test is used to determine the hydraulic performance of the well. The data from a step-test can be used to calculate the well efficiency. Step tests are rarely carried out on low-discharge (e.g., hand- pump) wells. However, in the case of a production well or for motorised schemes, the step test is very useful. Provided that the data are kept, undertaking another step test say five years later can enable a comparison to be made. Thus it is possible to find out whether the well has clogged up over time.
In a step-test the well is pumped at a succession of increasing discharges, each carried out for the same duration, typically one or two hours. There will usually be at least four steps, such as at 1/3, 2/3, 1 and 4/3 of the expected design pumping rate of the well.
National or international standards (e.g., BS ISO 1468:2003) should be used in the design of both constant-discharge and step pumping tests. These standards specify test pumping duration, discharge and other aspects of the conduct of the test, including measurement methods. During the pumping test, the Driller usually measures the water levels, discharge and time. The Supervisor is responsible for ensuring that the pumping test is carried out correctly. The Code of Practice for Cost Effective Boreholes (Danert et al, 2010) provides guidance on pumping tests, including a recording format. The pumping rate and the water level are measured at the same time and recorded along with the time of measurement. The pumping rate can be measured with a flow meter, but it can also be established by recording the time it takes to fill a container of known volume. This is measured several times during the test.
There are several ways of analysing pumping test data, and some are quite complicated. However, for the purpose of this guidance note, what is important to the Supervisor is whether the borehole will deliver the required amount of water for the required pumping duration or not. The specific capacity of the borehole, which expresses the relationship between the yield and the drawdown, is the most important quantity, i.e.
Specific Capacity=yield /drawdown (m3/h per m drawdown)
This enables the Supervisor to predict the likely drawdown at different pumping rates and whether the borehole can deliver sufficient water. By calculating the drawdown incurred by different pumping rates, and comparing that drawdown to the available vertical interval between the rest water level and the top of the well-screen (while allowing for likely seasonal water level variations, the effects of extended dry periods, and the interval occupied by the pump itself) it is relatively easy to determine a viable discharge for the well as drilled. A specific capacity of around 1 m3/h per meter drawn suggests that a borehole would be adequate for a handpump – a typical hand pump, drawing 1m3/h would incur only 1m of drawdown in this example.
Water quality testing: Groundwater from boreholes is often of good quality, but sometimes it may contain contaminants which render it unsuitable for domestic use without treatment. The contamination could be due to minerals dissolved into it from the rocks through which it flowed but results more often from biological contaminants owing to human activities. If the borehole has been properly sited and constructed, it should be possible to eliminate biological contaminants.
The technical specification would have given the parameters to be tested. It is the Supervisor’s duty to ensure that the samples are taken by the Driller in a clean bottle of at least1litre volume. Where the facilities are available, the sample should be analysed on site and then taken to an approved laboratory for further analysis. Note that some parameters change between sampling and reaching the lab and so need to be tested on, or close to the site (including pH, EC, dissolved oxygen, iron and micro-organisms).
In taking the sample, the bottle is rinsed several times with the water being sampled, filled and securely corked and labelled. Some countries have developed drinking water quality standards, and the Supervisor should analyse the results of the water quality testing on that basis. Where there are no country standards, the WHO standards should be used.
Particular attention should be paid to the values of the pH, conductivity, iron, manganese, nitrates, fluoride, arsenic and thermo-tolerant coliforms (TTCs). Groundwater containing iron is often reddish brown or black if manganese is present. It may also taste bitter. It poses no threat to health, but the taste and colour of such water may make it unacceptable to the consumer. Acidic water corrodes metallic plumbing material. High conductivity indicates a high level of dissolved solids, Consumption of groundwater high in fluoride, arsenic and nitrates is toxic.
Borehole disinfection: The borehole should be disinfected after construction to kill bacteria that might have entered during construction. Chlorine is normally used as the disinfecting agent, leaving a residual in the disinfectant water. The amount of chlorine required depends on the volume of water in the borehole. WHO (2012) recommend that a liter of 0.2% chlorine solution is used for every 100 liters of water in the borehole. This corresponds to a concentration of 20 mg/l. After adding the disinfectant, the borehole should not be pumped for at least 4 hours, if not longer. Care must be taken when mixing and adding chlorine to the borehole as it is poisonous when not diluted.
Successful or abortive boreholes: The Supervisor will decide whether a borehole should be accepted or declared abortive; and depending on the terms of the contract whether the Driller should re-drill the borehole at his/her own expense or not. The success and suitability of a borehole for acceptance will depend on the following:
a) The depth to pumping water level is critical for hand- pumps as the maximum depth from which it is practical to lift water with a handpump is 80m. RWSN (2007) provides an overview of the operation depths of the various public domain handpumps. Since some allowance must be made for water level drawdown, for a hand pump, the static water level must be less than 80m below ground level. The static water level, together with national pump standards, will determine the handpump to be installed. Depth to static water level is beyond the control of the Driller and he/she should not be penalized for it. Such deep water levels may be encountered in sedimentary terrains. In the case of deeper water levels, motorized rather than hand pump will be required.
b) A dry borehole or one with a lower yield than the desired should be declared abortive. This may not be the fault of the Driller, but if the agreement is that the Driller is only paid for successful boreholes, then re-drilling is at his/her own cost. However, even after attempting drilling in 3 locations in a community, the yield from the borehole may fall short of the minimum allowed. At this stage, a major reassessment of the drilling strategy may be required, with appropriate contracts drafted.
In the meantime, where the shortfall is less than 30% of the minimum specified, and there are no other safe sources of water, then the Supervisor may decide to accept the borehole and complete it if there is no alternative to improve the water supply. However, this may not be a viable long-term solution for the community.
c). The sand content of the water should not be more than 10 parts per million by volume. The Supervisor should collect three 20L samples at the end of the pumping test. The volume of sand in the samples should not exceed 0.2 cubic centimetres. If a borehole should be abandoned because of excessive sand content, then the Driller shall be responsible for constructing another borehole at his/her own cost. The wrong drilling technique or poor gravel packs and well development cause this.
d). The turbidity of the water should not exceed the stipulated limit. In some circumstances, excessive turbidity is due to the characteristics of the aquifer and thus beyond the control of the Driller, who should not be held re- sponsible.
e). Every borehole should be cased straight and vertical. The Supervisor may ask the Driller to carry out a test for straightness and verticality. The Driller should provide the plumb and carry out the test. Should the plumb fail to move freely throughout the length of the casing to the required depth or should the borehole vary from the vertical in excess of two-thirds of the inside diameter of part of the borehole being tested per 30 meters of depth, the borehole should be re-drilled by the Driller at his/her own expense.
f). The Supervisor will determine whether the chemical and bacteriological quality of the water is adequate to serve as potable water supply. If the borehole becomes contaminated because of an action or inaction by the Driller, the Driller should be asked to disinfect the borehole and if necessary construct a new borehole at his/her own cost.
Platform casting: All boreholes need a concrete apron around the length of casing above the ground for protection against soil erosion and surface water flowing into the borehole. Handpumps also need a concrete platform to hold the pump stand, to drain away spilled water, and for water users to stand upon. There are several designs of pump platforms, some being circular and others rectangular. Some incorporate a drinking trough for animals or a wash pad for laundry. Platform casting is usually undertaken by a dedicated construction team and may take place after demobilization of the drilling equipment. However, the Supervisor will be responsible for ensuring that platform is built to the design specified by the Client in the contract and that the quality of the materials and the construction is good and durable.
Step 8: Demobilisation
Aim: To leave the site clean, safe and ready for use
On completion of the pump installation, the Supervisor must issue a Work Completion Certificate. For this, he has to ensure that the Driller has complied with all the stages, including the final ones, of the contract specification.
Before demobilization, the Supervisor should check that the borehole record has been completed and all information filled in.
Step 9: Complete documentation and handover
Aim: To provide a clearly documented record to help future operation, maintenance and repairs and hand over the completed facility to the Community
The finalisation and submission of drilling records (to the appropriate national authority and local government) is essential. The submission of borehole construction data is Principle 8 of the Code of Practice for Cost Effective Boreholes (Danert et al, 2010). This should be an integral part of the drilling contract, and thus require approval before payment of the invoice.
When the Supervisor is satisfied that the borehole is ready for use, a day is set aside for handing over the borehole to the Community or the Client. It is common practice for the handing over certificate to be signed by three people: the Supervisor, the Driller and the Community representative. During the defects liability period, the Supervisor will monitor and liaise with community members on the functionality of the boreholes during periodic visits. If there are any defects, they will instruct the Driller to make repairs at his own cost, depending on what is specified in the contract.
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Some problems in rotary drilling are minor and others are serious and can result in failure to complete a hole or even loss of equipment. Many serious problems start minor but can become serious if not recognized or handled properly.
For example, in a loose sand zone, the borehole walls can casts of particles and cause drilling fluid loss. By reducing or increasing fluid velocity, you can stabilize the wall and regain fluid circulation. However, if you do not recognize the condition and you continue drilling, the wall will casts of particles and create a cavity. The cuttings lose velocity, become suspended in the cavity, and tend to fall back into the hole when you add a drill pipe. This action can result in the drill pipe or the bit becoming stuck in the hole.
Lost circulation refers to a loss in volume of drilling fluid returning to the surface. The implication is that some fluid pumped down the drill pipe is entering the formations. The mud pit will lower, since some of the mud is used in forming a mud cake on the borehole wall; however, increased lowering can indicate circulation loss.
Losses can occur through open-graded sand or gravel or open joints in rock or when we drill in highly porous & permeable formation. .
A loss can occur when cuttings are not washed out and the borehole annulus becomes restricted, resulting in increased down-hole pressure.
Spudding also known as surge and swab (raising and lowering the drill string) the hole too violently can cause loss. Spudding helps wash cuttings, but down-hole pressures increase momentarily. Experienced drillers can estimate when spudding is safe.
Low formation pore pressure
Poor drilling fluid characteristic
Induced fracturing of formation from from excessive mud weight
Natural fractures or formation cracks
Re-establishing circulation can involve several techniques:
You can add commercial items such as chopped paper, straw, cottonseed, and nut hulls to the mud pit.
Squeeze or grout cementing (Sometimes, while the loss zone is grouted and re-drilled, the grout is lost into the formation. In this situation, use the following step
Set casing through the loss zone.
Fall-In or Cavings
Fall-in is material that accumulates in the bottom of the borehole after you stop circulation. This material is borehole-wall material that results from, sloughing caving or cuttings previously carried in suspension.
Fall-in or caving occurs when you encounter a loose, unstable formation and the drilling-fluid weight is insufficient to stabilize the formation.
If mud cake is not formed properly → water from mud goes in → water comes in contact with clay/shale → hydration → so size increases → but there is no space to accommodate this: Weight cannot be supported and this bulge breaks from inside → Slowly this caving keeps on increasing.
This is Shale erosion due to Annular Turbulence Caused when a heavy weighted drilling fluid flows at a high speed inside the borehole and creates turbulence leading to sloughing of shale in unconsolidated formations
If you anticipate or suspect fall-in, raise the drill bit off the bottom of the hole (20-foot minimum) each time drilling is interrupted. This will prevent the cuttings and fall-in from settling back around the bit until the problem is solved.
Good quality mud should be made so that an impermeable mud cake gets formed. i.e. add 1.1% – 4% CMC or mud extenders.
Stuck Drill String
The drill bit and any collars just above the bit are larger in diameter than the drill pipe. The string becomes stuck when cuttings collect on the bit and collar shoulder. This condition is called sanded in.
Drill String Stuck-up Indication
Cannot pickup the pipe
Pipe movement in the upward/downward is restricted but free circulation is easily be established.
Differential pressure sticking (High pressure of mud)
Formation of adhesiveness & thick Filter Cake → deposited after circulation stops (while pipe still on bottom)
Caving-in and crumbling of rocks
Substantial water loss
Drill cuttings, sloughing formations, shale deformation/cracks and salt flow pack off the annular space around the drill string.
A drill string is run too fast, such that it hits a bridge or a tight spot or the bottom of the hole
Mechanical Sticking – Shale Crack
Caused when filtrate from a water based mud enters the cracks present in the walls of the formation and cause swelling of the sensitive clays.
Mechanical Sticking – Settled Cuttings
Causes: Low annular velocity and/or poor mud properties. Drill cuttings settle on the low side forming a cutting bed.
Preventive measures for Stuck Drill String
Limit Differential Pressure → Use Minimum Mud Weight Required to Control Formation Pressures.
Reduce Contact Area (Use Low Solid Mud, Use Spiral Drill Collars, Use Centralizers/Stabilizers on pipes and casing,
Reduce Friction Factor (Control Water Loss, Use Filtrate loss Controller, Use Lubricants, Emulsions)
Reduce static time → Periodically Establish Circulation while Running Casing or Drill pipe in Deep Hole
Wash a small pipe down the annulus to the bit and jet the settled sand back into suspension. When the annulus is too small to pass a jet pipe, a part of the drill string may be lost.
Erode Mud Filter Cake – at High Fluid Velocity (speed up pumps) – When the annulus is small, excessive up-hole velocity can promote erosion of the filter cake in granular zones and allow caving against the drill pipe. If this occurs, try to maintain circulation and rotation, even if circulation is slight.
With smooth pipe (not upset), hammering up and down will sometimes dislodge the string. You can reestablish circulation and continue drilling – Be careful because hammering up and down can produce unfavorable compacting of the sand. In a hole of fine-grained soil or shale, where the alignment has significantly deviated and the drill pipe has wallowed into the wall, the pipe can become wall stuck. Pipe friction and relatively high borehole pressure can move the pipe tighter into the wallowed groove as you pull the string. An alert driller should recognize early stages of deviation and take measures to realign the hole.
Spot Special Fluid; Oil, Acid.
Reduce Mud Weight as Far as Possible.
Rotate Pipe – Keep Moving Pipe.
If circulation cannot be established: Cut pipe or Unscrew Joint – If possible fish out drill string.
Drill String Failure
When the drill string parts, leaving a portion in the borehole, the drill string is rung off. The portion in the borehole is a fish and attempts to retrieve the portion is fishing. Fishing tools include a tapered tap and an overshot die (see fig. below)
Ringing off is normally fatigue failure in the drill-rod joints caused by excessive torque or thrust (repeated flexing and vibration that crystallizes heat-treated tool joints) or by borehole deviation (with flexing of the string). Examine drill rods for signs of failure.
A deviated borehole is called going crooked. If you make the initial setup without plumbing the Kelly, you can expect the borehole to go crooked. A crooked borehole usually amplifies other problems and can make a borehole unsuitable for a well. You should always anticipate deviation, since the borehole naturally tends to spiral from bit rotation. Variations in the formation badness may start deviation. Excessive bit load magnifies minor initial deviation. Use all available guides and collars and a reduction in bit load to minimize deviation.
The in-hole effects of swelling soil (shale or clay) that absorbs water from the drilling fluid is squeezing. The result is a borehole that is under-gauged to the extent that you cannot pull the bit by normal hoisting methods. In such cases, you can cut back through the blockage with a roller rock-bit or a drag bit. Swelling can cause caving and failure of the wall. Keep water out of the formation to prevent swelling. Special polymer drilling fluid additives that limit water absorption are available. High quality bentonite forms a thin but highly impermeable filter cake.
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There are several ways of analyzing pumping test data, (The table below show the commonly used methods for pumping-test data analysis).
Type of Aquifer
Type of Pumping test data
Names of methods
I. Time Drawdown data
II. Unsteady distance drawdown data
III. Quasi-Steady/ Steady Distance drawdown data
IV. Recovery Data:
– Time Residual Drawdown data
– Time Recovery Data
– Theis Type curve method – Cooper-Jacob Straight line method
Cooper-Jacob Straight line method
– Thiem method – Graphical method
– Residual Drawdown-Time Ratio method – Cooper-Jacob Straight line method
Unconfined Aquifer without Delayed Yield
I. Time Drawdown data
II. Unsteady distance drawdown data
III. Quasi-Steady/ Steady Distance drawdown data
IV. Recovery Data:
– Time Residual Drawdown data
– Time Recovery Data
– Theis Type curve method – Cooper-Jacob Straight line method
Cooper-Jacob Straight line method
– Thiem method – Graphical method
– Residual Drawdown-Time Ratio method
– Cooper-Jacob Straight line method
Unconfined Aquifer with Delayed Yield
I. Time Drawdown data
II. Quasi-Steady/ Steady Distance drawdown data
– Type-Curve method – Newman Straight Line method
– Thiem method – Graphical method
Leaky Confined Aquifer without Storage in Aquitards
I. Time Drawdown data
II. Quasi-Steady/ Steady Distance drawdown data
– Walton Type curve method – Hantush Inflection Point method
– Type-Curve method
Leaky Confined Aquifer with Storage in Aquitards
I. Time Drawdown data
II. Quasi-Steady/ Steady Distance drawdown data
– Hantush Type Curve method
– Type-Curve method
Commonly used methods for pumping-test data analysis.
Some of these methods are quite sophisticated. However, for the purpose of this guidance note, what is important to the Supervisor is whether the borehole will deliver the required amount of water for the required pumping duration or not. The specific capacity of the borehole, which expresses the relationship between the yield and the drawdown, is the most important quantity of pumping test. (Step Test, Constant Rate Discharge Test and Recovery Test).
Specific Capacity=yield /drawdown (m3/hr. per meter drawdown)
By calculating the drawdown incurred by different pumping rates, and comparing that drawdown to the available vertical interval between the rest water level and the top of the well-screen (while allowing for likely seasonal water level variations, the effects of extended dry periods, and the interval occupied by the pump itself) it is relatively easy to determine a viable discharge for the well and whether the borehole can deliver sufficient water.
Calculation of Water Borehole Specific Capacity
A pumping test was run for 24hours with a yield of (2,500 m3/hr.) and the drawdown (21 meter) were measured. The Specific Capacity is calculated as 2,500 m3/hr. divided by 21 meter, or 119 m3/hr. per meter
Borehole Specific Capacity
The Specific Capacity obtained just after a well is drilled and properly developed is typically the highest value that will be produced and is the benchmark with which to compare all future values. As time goes by, the Specific Capacity will decline as plugging of the well’s perforations or filter pack occurs, as the pump starts to fail, or as static water levels change.
Specific Capacity tests should be performed at least semi-annually and water levels (static and pumping) should be collected monthly to provide early detection of potential well problems. Rehabilitation work should be initiated when a well’s Specific Capacity drops by 25% (Driscoll, 1986).
Specific capacity can also be used to provide the design pumping rate or maximum yield for the well as follows:
Calculation of Water Borehole Maximum Yield
For example, the specific capacity of a borehole is 27 m3/hr. Assuming that only 15 m of drawdown is available in the well, the maximum yield is calculated as the Specific Capacity times the maximum drawdown, or 27 m3/hr * 15 m, = 405 m3/hr. This should be verified with other pumping test method.
Maximum Yield or Design Pumping Rate of Water Borehole
Calculation to determine if the borehole maximum specific capacity is enough to meet the need of the population end user.
As previously mentioned, the most important result of pumping test is to determine that the borehole flow for the required pumping duration is sufficient to meet the daily need of the population (maximum exploitable flow):
Less than 14 hours in operation, the borehole is sufficient to meet the need of the population.
Between 14 hours and 18 hours operation, bear in mind that the borehole may become too small in the near future due to population growth or reduced water levels in the aquifer due to prolonged exploitation. Furthermore, using the borehole becomes more expensive.
Over 18 hours operation. The borehole does not meet the needs of the population. Depending on the circumstances, you can decide whether or not to equip the bore or drill another borehole. (Arnalich-water and habitat 2011).
Calculation example: A borehole has been drilled for semi-urban population of 8000 people. The pumping test reveals that the maximum capacity of the borehole is 11 l/s. Is this enough to meet the needs of the population assuming 100 liters per person per day?
With a daily demand of 100 liters per person, the quantity of water consumed is: 8,000 people * 100 l/person* day = 800,000 liters daily demand.
The hourly production is: 11 l/s * 3600 s/hour = 39,600 l/hr
The required number of operating hours is 800,000 l / 39,600 l/hr = 20.2 hours.
The borehole is too small to meet the needs of the population.
Boreholemaximum specific capacity and end users
Approximate Calculation of Operating cost
Given a certain maximum capacity of the borehole, you can calculate operating cost.
The main expense is derived from energy consumption. To determine whether the population is able to pay for the operating cost of the borehole, calculating the energy consumption cost is usually sufficient.
e = mgh/3.6 ∗ 10⁶ η ∗ cost rate per kWh for local electricity tariff
m = mass of water per day in kg (1 liter of water weighs 1kg)
h = the pumping height taken from the lowest drawdown point to the water surface at the point of delivery.
g = gravity of acceleration on the surface of the earth at sea level is 9.8 m/s2
η = total efficiency for the pump, around 60% (for frictional losses use value of 0.5 (50%).
As generator are frequently required, you need to know the unit fuel cost of the diesel. The average fuel consumption of a generator is 0.3 liters of diesel per kwh produced.
Calculation example: A borehole powered by a generator pumps 60,000 liters per day to a tank with a tank stand situated at elevation of 35m. The dynamic height of the borehole is 44m and the cost of diesel is #250 per liter. How much will it cost to pump a day?
The number of liters of diesel required is: 25.8 kwh * 0.3 liters diesel / kwh = 7.74 liters
The daily cost for the local population is: 7.74 liters * #250 /liter = #1,935
Operating cost and energy consumption
Water Borehole Transmissivity and Storage Coefficient
Data from constant-rate tests can be analyzed to derive the transmissivity of the aquifer. The storage coefficient of the aquifer can be calculated only if data from observation boreholes are available.
The goal of a recovery test, as in any aquifer test, is to estimate hydraulic properties of an aquifer system such as transmissivity, hydraulic conductivity and storativity (storage coefficient). One typically measures recovery after the termination of a constant-rate or step-drawdown pumping test to provide additional data for estimating aquifer properties.
If transmissivity value of 10 m2/day is calculated from pumping test analysis, the question is, what does that value mean? Is a value of 10 m2/day good or bad?
The answer mainly depends on what the intended yield of the borehole is. MacDonald et al (2005) carried out modelling using typical assumptions and parameters applicable to emerging countries, and came to the conclusion that for a borehole supplying 5,000 litres per day (20 litres per person for 250 people), the transmissivity value of the aquifer should be at least 1 m2/day.
An aquifer with a transmissivity value of 10 m2/day would be capable of yielding around 40,000 litres per day. By comparison, a public water-supply borehole in a typical sandstone aquifer capable of yielding about two million litres per day would have a transmissivity value of 300 to 400 m2/day when tested. Highly productive aquifers, capable of supporting major abstractions, can have transmissivity values of 1,000 to 2,000 m2/day.
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Pumping tests or aquifer performance tests is a field experiment in which a well is pumped at a controlled rate and water-level response (drawdown) is measured in one or more surrounding observation wells and optionally in the pumped well (control well) itself. The way in which the water levels respond to the pumping is then analyzed to derive information about the performance characteristics of the borehole and the hydraulic properties of the aquifer.
The Purpose of Pumping Test
Pumping tests can be undertaken for a wide variety of reasons, including the following:
To determine the reliable long-term yield (or ‘safe’ yield) of a borehole, and therefore whether or not the borehole can be regarded as a ‘success,’ and how many people it will be able to supply.
To assess the hydraulic performance of a borehole, usually in terms of its yield-drawdown characteristics. How much drawdown does it take to yield a certain amount of water?
To derive the hydraulic properties of the aquifer, such as transmissivity and the storage coefficient, or to reveal the presence of any hydraulic boundaries.
To test the operation of the pumping and monitoring equipment, to make sure that everything is working safely and efficiently, and if applicable, to confirm that the contractors have done their job properly.
To determine the effects of water abstraction on neighboring abstractions.
To determine the environmental impact of the abstraction. All groundwater abstraction eventually has an impact; it is only a matter of where, when and whether or not the impact is acceptable.
To provide information on water quality. Is the water quality suitable for the intended use? Is it stable in the long term? Are there likely to be any problems such as drawing in saline or polluted water after extended periods of pumping?
To optimize operational pumping regimes (especially from multiple-borehole sources), including selecting the most suitable pumping plant for long-term use, and estimating probable pumping and/or treatment costs.
Preparation for Pumping Test
Before commencing any pumping test, there are certain basic preparations that should be made. These include:
Information about the borehole or well that is about to be tested.
The geological characteristics of the subsurface i.e., all those lithological, stratigraphic, and structural features that may influence the flow of groundwater.
The thickness and lateral extent of the aquifer and confining beds:
The aquifer may be bounded laterally by barrier boundaries of impermeable material (e.g., the bedrock sides of a buried valley, a fault, or simply lateral changes in the lithology of the aquifer material);
Any lateral recharge boundaries (e.g., where the aquifer is in direct hydraulic contact with a deeply incised perennial river or canal, a lake, or the ocean) or any horizontal recharge boundaries (e.g., where percolating rain or irrigation water causes the water table of an unconfined aquifer to rise, or where an aquitard leaks and recharges the aquifer);
Data on the groundwater-flow system: horizontal or vertical flow of groundwater, water table gradients, and regional trends in groundwater levels;
Any existing wells in the area. From the logs of these wells, it may be possible to derive approximate values of the aquifer’s transmissivity and storativity and their spatial variation
Pumping test equipment
The basic equipment to measure the two parameters in pumping test (water level in the pumped borehole and the rate at which water is being pumped) are as follows:
Measuring Water Level
The most common methods of measuring water level are as follows:
The hand-held water level monitor, commonly known as a “dipper,” or M-scope is the most practical, robust and easily available method of monitoring water levels in boreholes and wells. The dipper probe is lowered down the borehole, and when it reaches the water surface, an electrical circuit is completed and a ‘bleep’ is heard. The water level is then read off a graduated tape in centimeters and should be converted to meters. See fig above.
Pressure Transducer for Water Level Measurement
The transducer is placed in a known position down the borehole (below the water level) and it measures the pressure at that point. This information can be used to deduce the height of the water above that point, and therefore the water level in the borehole.
Transducers (with built-in dataloggers) have an obvious advantage: they can be left unattended for long periods, while still taking frequent water-level readings.
Use this method to measure the depth to the static level in a shallow well. Conduct this test as follows:
Chalk one end of a weighted steel tape with carpenter’s chalk. Lower the tape into the well to a depth of 1 or 2 feet past the chalk. (You can use soluble felt-tip markers as an alternative to chalk.) Measure the wetted length of the tape and subtract the amount from the total length lowered below the reference point to obtain the water depth. This test is accurate to within 0.01 foot.
How to Measure Pumping Rate
The most common method of measuring pumping rate is as follows:
You can determine the flow rate from a well or pump by measuring the time required to fill a container with a known volume. With this method, use small containers for early measurements and large containers for later measurements. Also, use an instrument, such as a stop watch, for accurate time measurements. Use the following equation:
FR = flow rate, in GPM.
V = volume, in gallons.
T = time required to fill container, in seconds.
A turbine-type flow meter will give an acceptable flow-rate reading. This uses spring-loaded pistons that are deflected by the flow of water, and the flow rate is read off the graduated scales.
A weir tank is a thin-plate ‘V-notch’ gauging weir within a self-contained tank. Weir tanks must be installed exactly level, and there must be an accurate method of measuring the water level inside the tank, plus a conversion table supplied by the tank manufacturer (to convert the water levels into flow rates). See BS ISO 14686:2003 for more information about constructing and using weir tanks.
Alternatively, small weir tanks can be made using local materials, sometimes even a converted oil drum. These weir tanks should be calibrated by an independent flow-measurement method, in order to ensure reliable data. For accurate results, care should be taken to follow design advice (see BS ISO 14686:2003), especially about the thin-plate ‘V-notch’ itself.
Whichever method is used, it is important to measure the pumping rate frequently during the test since it will probably fall as the water level drops, and an average pumping rate needs to be calculated for use in the test analysis.
Types of pumping test
There are different types of pumping test. Three main types of pumping test will be discussed for the purpose of this guidance note, namely: Step test or variable discharge, Constant discharge test and Recovery test.
The step test (sometimes referred to as the step-drawdown test) is designed to establish the short-term relationship between yield and drawdown for the borehole being tested. It consists of pumping the borehole in a sequence of different pumping rates, for relatively short periods (the whole sequence can usually be completed in a day). There are many different ways to perform a step test, but the most common practice is as follows:
Start with a low pumping rate, and increase the rate with each successive step, without switching off the pump between steps.
Aim for four or five steps in total, with the pumping rates roughly spread equally between the minimum and maximum rates.
All steps should be of the same length in time, with somewhere between 60 and 120 minutes per step being common.
The pumping rate for the final step should be at or beyond the intended operational pumping rate when the borehole is fully commissioned. Of course, this depends on whether the pump being used for the step test is capable of that pumping rate.
Setting pumping rates
It is advisable to spend time, on the day before the step test itself, experimenting with the valve settings that are necessary to produce the required pumping rates for each step. Manually operated gate or globe valves are commonly used, and these are operated by a screw handle. Fully close the valve, then open it to the fully open position, counting the number of turns of the handle that are made between fully closed and fully open. Experiment with opening the valve different numbers of turns from the fully closed position, to achieve the different pumping rates for the steps, and make a note of the results.
Choosing step length
In practice, the length of each step depends on the number of steps and the total time available for the test (which is usually one day), but 60, 100 or 120 minutes are common step lengths. Ideally, the water level in the borehole will approach equilibrium at the end of each step, but this cannot always be achieved. Even if the water level has not reached equilibrium at the end of each step (in other words, if it is still falling slowly), the results from the test will still be useful. They provide a ‘snapshot’ of borehole performance under certain conditions, and can be compared with the results from the same test (same pumping rates and step length) repeated at another time, to see if the borehole performance has changed. If at the end of the planned time for the first step the water level is still falling quickly, the decision may be taken to extend the length of the step (and extend the length of subsequent steps to match). Further details on the theory and practice of step tests can be found in Clark (1977).
Assuming that all the equipment is ready and people have been assigned their tasks, the procedure for conducting a step test is as follows:
Choose a suitable local datum (such as the top of the casing) from which all water-level readings will be taken, and measure the rest-water level. The water level must be at rest before the start of the test, so the test should not be conducted on a day when the borehole is being drilled or developed, or when the equipment is being tested.
Open the valve to the setting for the first step (determined by prior experiment, as described above) and switch the pump on, starting the stopwatch at the same time. Do not keep changing the valve setting to achieve a particular pumping rate (a round number in litres per minute, for example). Rather, aim for an approximate rate and measure the actual rate (see 4) below).
Measure the water level in the borehole every 30 seconds for the first 10 minutes, then every minute until 30 minutes have elapsed, then every 5 minutes until the end of the step (the length of each step having been decided during the test preparations). If you miss the planned time for a water-level reading, write down the actual time the reading was taken. Record all the readings on the standard step-test form.
Measure the pumping rate soon after the start of the step, and then at intervals during the step (every 15 minutes would be reasonable). If there is a noticeable change in the rate of increase of drawdown or the pump sounds different, then measure the pumping rate at those times as well. If the pumping rate changes significantly (say by more than 10%), then adjust the valve setting to maintain as steady a pumping rate as possible throughout the step. Be careful not to over- adjust and make the problem worse.
At the end of Step 1, open the valve further, to the setting for Step 2, note the time (or restart the stopwatch) and repeat the procedures for measuring water levels and pumping rates (see 3) and 4) above).
Repeat the procedure for subsequent steps, progressively increasing the pumping rate for each step.
At the end of the final step (which will probably be Step 4 or 5), switch the pump off, note the time (or restart the stopwatch), and measure the water-level recovery at the same measurement intervals as for measuring the drawdown in each step. Continue for at least the length of a step, and ideally for much longer, until the water level approaches the pre-test level. See Chapter 6 for a full explanation of the recovery period.
Constant Rate discharge
The constant-rate test is the most common type of pumping test performed, and its concept is very simple:
The borehole is pumped at a constant rate for an extended period (from several hours to several days or even weeks) while the water levels and pumping rates are monitored.
If the most value is to be gained from constant-rate tests, water levels should be monitored in an observation borehole as well as in the pumping borehole (or better still, several observation boreholes at different distances from the pumping borehole).
The present guidelines concentrate on what to do with the data obtained from the pumping well alone. Data from constant-rate tests can be analyzed to derive the transmissivity of the aquifer. The storage coefficient of the aquifer can be calculated only if data from observation boreholes are available. (Which is assumed not to be the case here).
Observation wells are necessary in order to determine fully the aquifer properties. Table below gives guidance on the minimum durations that should be allowed for constant discharge tests.
Discharge rate m3/day
Minimum duration of constant discharge days (of constant 24 h discharge)
Up to 500
500 to 1 000
1 000 to 3 000
3 000 to 5 000
Over 5 000
Minimum Duration of Constant discharge Test
In certain situations, increases or decreases in these periods will be appropriate. Longer tests would be required for example to adequately assess the influence of boundaries. The effect of a recharge boundary is a slow-down (deceleration) in the rate of drawdown.
Where the recharge source is a specific feature, such as a watercourse or a lake, the time that elapses before the onset of this deceleration will increase in proportion to the square of the distance between the pumping well and the recharge source. Eventually, drawdown will stabilize for the remainder of the test.
If a delayed-yield effect occurs, the development of the time-drawdown relationship will be delayed. It is not possible to estimate accurately in advance the length of this delay unless it has occurred in nearby wells previously tested in the same aquifer. If a delayed yield is expected, an extension of the duration of the test should be considered.
Barrier boundaries have the effect of increasing the rate of drawdown and present a serious constraint on the yield of the well. The shorter periods given in Table below may therefore require extending by one or two days to observe the effects adequately, particularly if they appear towards the end of the period initially specified. (The pumping of another well in the same aquifer will have the same effect as a boundary, if the cones of influence of the two wells intersect).
The two main decisions to make with a constant-rate test are the pumping rate and the length of the test:
Typically, the chosen pumping rate is equal to the intended operational pumping rate when the borehole is fully commissioned, although some hydrogeologists prefer to set the test pumping rate 25-50% higher than the intended operational pumping rate. Information from a step test is very helpful in deciding this pumping rate.
The chosen rate also depends on how the borehole is going to be operated. Some boreholes are pumped at a high rate to fill up a storage tank or reservoir in a relatively short period, and then the water is used gradually (by gravity) from storage. The pumping rate for the test can either be the actual pumping rate when the pump is switched on, or the average long-term pumping rate (including the operational non-pumping periods). If the focus of the test is on long-term sustainability, then it would be better to use the average pumping rate.
Length of test
Ideally, a constant-rate test should be long enough for the water level to reach or at least approach equilibrium. How long it takes to do this depends on the hydraulic properties of the aquifer. Again, the step-test results will help in understanding how the aquifer responds to pumping. For a small or medium borehole, one or two days should be sufficient, but for a large borehole expected to supply a large population, one or two weeks are common.
Many aquifers behave differently in the wet season compared with the dry season; if possible, constant-rate tests should therefore take place at the relevant time of year. For example, if the borehole is intended as a source of water for critical drought periods, then it should be tested during the dry season, otherwise a false impression will be gained of the aquifer’s performance. Another good reason to conduct a test during a dry period is that the groundwater levels may be influenced by recharge from heavy rainfall, which makes it more difficult to interpret the test results.
Maintaining a steady pumping rate during a constant- rate test is sometimes a problem, especially if the chosen pumping rate results in a large drawdown. This is because for centrifugal pumps (the most commonly used type of pump) there is a relationship between pumping rate and pumping head. Incidentally, the pump must be set at a depth that is several meters below the deepest water level expected during the test.
Constant-rate Test Procedure
Assuming that all the equipment is ready and people have been assigned their tasks, the procedure for conducting a constant-rate test is as follows:
Choose a suitable local datum (such as the top of the casing) from which all water-level readings will be taken, and measure the rest-water level. The water level must be at rest before the start of the test, so the test should not be conducted on a day when the borehole is being drilled or developed.
Open the valve to the appropriate setting and switch the pump on, starting the stopwatch at the same time. Do not keep changing the valve setting to achieve a particular pumping rate (a round number in liters per minute, for example). Rather, aim for an approximate rate and measure the actual rate.
Measure the water level in the borehole every 30 seconds for the first 10 minutes, then every minute until 30 minutes have elapsed, then every 5 minutes until 2 hours have elapsed. After 2 hours, observe how quickly the water level is still falling, and decide an appropriate frequency for water-level readings until the end of the test. If the water level is falling very slowly, then a reading every 30 minutes or even every hour may be sufficient. If the test is to continue for several days, review the measurement frequency depending on the behavior of the water level. If you miss the planned time for a water-level reading, write down the actual time the reading was taken. Record all the readings on the standard form.
Measure the pumping rate soon after the start of the test, and then at intervals during the test (every 15 minutes would be reasonable for the first few hours, then decide a suitable frequency for the remainder of the test). If there is a noticeable change in the rate of increase of drawdown, or if the pump sounds different, then measure the pumping rate at those times as well. If the pumping rate changes significantly (say by more than 10%), then adjust the valve setting to maintain as steady a pumping rate as possible throughout the test, but be careful not to over-adjust and make the problem worse.
At the end of the test, switch the pump off, note the time (or restart the stopwatch), and measure the water- level recovery at the same measurement intervals as for measuring the drawdown. Continue until the water level has recovered to the pre-test level, or at least approaches that level. See the next chapter for a full explanation of the recovery period.
If there is a problem during the test, such as an interruption to the power supply or a pump failure, then use your judgement, depending on when the problem occurs and how long it is likely to last. For example, if something goes wrong in the first few minutes, wait for the water level to recover and start again. If the failure occurs well into the test and can be solved quickly, just restart the pump and carry on. If it is going to take a long time to solve, it may be better to allow full recovery of the water level and start again. For long constant-rate tests, it is especially important to ensure that there is an adequate fuel supply to last the planned duration of the test.
The recovery test is not strictly a pumping test, because it involves monitoring the recovery of the water level after the pump has been switched off in step tests and constant-rate tests. Recovery tests are valuable for several reasons:
They provide a useful check on the aquifer characteristics derived from pumping tests, for very little extra effort – just extending the monitoring period after the pump has been switched off.
The start of the test is relatively ‘clean.’ In practice, the start of a constant-rate test, for example, rarely achieves a clean jump from no pumping to the chosen pumping rate. Switching a pump off is usually much easier than starting a pump, and the jump from a constant pumping rate to no pumping can be achieved fairly cleanly.
Similarly, recovery smooth out small changes in the pumping rate that occurred during the pumping phase, and there is no problem with well losses from turbulent flow. This results in more reliable estimates of aquifer properties when the recovery data are analyzed.
The water levels in the borehole are easier to measure accurately in the absence of turbulence caused by the pumping (especially in the early stages of the test, when water levels are changing quickly). Some people find that it is easier to take readings quickly with a dipper when the water level is rising than when it is falling.
Recovery tests represent a good option for testing operational boreholes that have already been pumping at a constant rate for extended periods. In these cases, the recovery test can be performed when the pumps are first switched off, followed by a constant discharge test when the pumps are switched back on again.
Ideally, the duration of the recovery test should be as long as is necessary for the water to return to its original level, which, theoretically, would be as long as the duration of the pumping phase of the test program. In practice, however, the recovery test is often shorter, partly for reasons of cost (keeping equipment and personnel on the site). It should not be too short however, because as described in relation to the constant-rate test, the data from the early part of the test are affected by well storage. If the data from the constant-rate test have been roughly plotted in the field on semi-log graph paper, this will give some idea of the length of time before the data become useful for calculating transmissivity (when they fall on a straight line).
The pump should not be removed from the borehole while the recovery test is taking place, because the sudden removal of the submerged volume of the pump and rising main will cause a sudden change in the water level in the borehole. For a similar reason, there must be a non-return valve (called a foot-valve in this context) at the base of the rising main. In the absence of a foot-valve, when the pump is switched off, the contents of the rising main will flow back down into the borehole and cause a sudden change in the water level in the borehole. Having said that, unless the foot-valve can be opened from the surface, the rising main will be full of water, and therefore heavy, when it is removed from the borehole. Thus, it may not always be practicable to carry out a recovery test.
Recovery Test Procedure
The procedure for undertaking a recovery test is as follows:
Switch the pump off and start the stopwatch at the same time.
Measure the water level in the borehole in the same way as for the start of the pumping test, that is, every 30 seconds for the first 10 minutes, then every minute until 30 minutes have elapsed, then every 5 minutes until 2 hours have elapsed. After 2 hours, observe how quickly the water level is still rising, and decide an appropriate frequency for water-level readings until the end of the test. If the water level is rising very slowly, then a reading every 30 minutes or even every hour may be sufficient. If you miss the planned time for a water-level reading, write down the actual time the reading was taken. Record all the readings on the standard form. Make sure the same datum is used for measuring water levels as for the pumping phase.
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After Drill-Rig Setup, connect the discharge piping. Depending on the type of drilling operation, connect either the air compressor or the mud pump to the standpipe on the mast.
Install or dig a mud pit, whichever is applicable. Make the appropriate connections to the mud pump to ensure continuous circulation of the drilling fluid.
If you use rotary drilling with mud, connect or place the suction line of the mud pump in the mud pit and fill the pit with water. Close the standpipe valve and prime the mud pump. Mix the drilling fluid in the mud pit by slowly circulating fluid through the mud pump.
Before you start drilling, you must select a drill bit. Consider the well diameter and the type of formations you will drill through.
The types of bits are Drag bits: Use for soil, unconsolidated materials usually found near the surface. Tricone roller-rock bits. Use for a variety of materials from soft formations to hard rock. (The bits are available in different degrees of hardness).
Starting the Drilling Operation
The first operation is spudding in (starting the borehole).
Drive Mechanism for Rotary Rigs
The drive mechanism for drilling operation is provided either at the rotary table Kelly drive (table drive) or at the swivel (top head drive).
On rotary rigs with top head drive mechanism, the bit with drill collar and subsequent drill pipes are connected directly to the drive shaft of top head drive. This process facilitate fast drilling speed and short auxiliary time for drill pipe connection – Step 1 to Step 5 below are not applicable in top head drive rotary rigs.
Step 1. Make up the drill bit on the Kelly and lower the bit to the ground.
Step 2. When the bit contacts the ground, engage the rotary table and clutch to start drilling. (Ensure to start drilling in true vertical direction. Straightness is particularly important for water boreholes in which long strings of casing and screens may have to be installed with a gravel pack filter).
Step 3. After the borehole advances 6 to 12 inches, engage the mud pump to start circulating the drilling fluid (which will be mud or air).
Step 4. After drilling down the kelly, stop the rotation and raise the kelly about 4 inches off the bottom of the borehole. Circulate the drilling fluid until all drill cuttings are removed.
Step 5. Disengage the mud pump, raise the Kelly, and remove the bit.
Finishing the Operation
After spudding in, use the following steps to finish the operation:
Step 1. Makeup the bit on a drill collar. Suspend the collar and bit in the borehole using the hoist or sand line. Support the drill collar in the hole using slips.
Step 2. Clean and lubricate the threads on the Kelly bar and makeup the Kelly to the threads. (On top head drive rigs, you makeup the drill string to drive shaft of drive head).
Step 3. Remove the slips, and lower the kelly so that the drive bushing engages the rotary table and that the tool string is lowered to the bottom of the borehole.
Step 4. Engage the mud pump again and continue drilling. After drilling down the kelly, circulate the drilling fluid until the cuttings are removed. Raise the tool string until the tool joint is 4 to 6 inches above the rotary table.
Step 5. Disengage the mud pump and set the slips to support the tool string. Disconnect the kelly from the string and set the kelly back to the cradle position.
Step 6. Run out the sand line, remove the end cap, and connect a hoisting plug to another drill collar or to a joint of drill pipe.
Step 7. Lift the drill steel with the hoist line or sand line and place the hoisting plug on the drill steel
Step 8. Remove the lower end cap. Clean and lubricate the threads. Make up the threads to the string in the hole with pipe wrenches.
Step 9. Lift the entire string slightly and remove the slips. Lower the string until the tool joint is 4 to 6 inches above the rotary table and the slips are reset.
Step 10. Make up the kelly to the tool string, remove the slips, engage the mud pump, lower the string to the bottom of the borehole, and continue drilling. Repeat the process until you reach the desired depth.
Step 11. Reverse the process to come out of the borehole.
Reaming – After drilling a drill stand down, it is advisable to back ream and enlarge the existing hole.
Installation of Surface Casing
If the borehole has a tendency to cave in during drilling (when drilling in loose, unconsolidated materials.) install surface casing when drill to the desired depth.
When installing the casing before the well screen, such as surface casing, the casing must be larger than the screen. Therefore, use a larger drill bit than the one used to complete the screen portion of the borehole. The decision to use surface casing should be made before mobilizing and should be based on the geologic information about the site.
Use the following steps to install the surface casing:
Step 1. Drill the borehole to a predetermined depth, and remove all the cuttings by circulating the drilling fluid. Withdraw the drill string and remove the bit.
Step 2. Set the kelly back in the cradle. Connect the elevators to the first section of surface casing that is lifted over the borehole by using a casing elevator and the hoist or by using a sand line.
Step 3. Lower the casing into the well and set the slips, which suspend the casing in the well, in the spider bowl.
Step 4. Disconnect the elevator. Hoist the next casing section with the elevator, and place it in the first section. Join the two sections. Slightly lift the string of casing, and remove the elevator from the lower section. Lower the casing, and repeat the process until you reach the surface casing depth in the well.
Step 5. Grout the casing in place with a cement grout. After the grout sets (about 24 hours), resume drilling operations using a drill bit that will fit inside the surface casing. Drill the well to the desired depth, case and screen the lower section of the well using the single-string method. (With this method, you install the casing and screen (that have been joined) in a single assembly).
For a borehole to be properly logged, the driller and supervisor need to know its exact depth at all times. This is necessary for the calculation of drilling charges, and while designing the borehole. First, make a note of the length of the drill bit and of any other tools that may be used to drill the hole. Put the bit on the ground and make a chalk mark, ‘0,’ on the first drill pipe against a suitable fixed point on the rig and at a known height above ground level, such as the drilling table (which centralizes the drill pipes in the hole). From then on, marks can be made on the drill pipe at regular intervals – say, every half meter – to record the depth of drilling and to assist in the logging of penetration rates.
Formation samples need to be obtained as drilling proceeds: the usual sampling interval is one meter or every stand drilled. There will be a slight delay as formation fragments are lifted to the surface by the circulating mud, but ensure to make a rough estimate or calculation of the up-hole lag time and actual depth at which cuttings were derived. Cuttings obtained from the shallow mud channel near the borehole should be washed in water to remove mud, and laid out in order (by the depth at which each was acquired) on the ground or in a sample box with separate compartments for each sample. They can then be logged (Identification of lithology and description with respect to depth) by the supervisor or site geologist and bagged if required. Samples should, of course, be labelled correctly with all information relevant to the job in hand.
The main attributes of a borehole log are accuracy and consistency; a good set of logs can be a useful resource when planning future drilling programs. Drillers must keep their own logs and notes and, as is often stipulated in contracts, these should be accurate; however, in practice, they cannot always be relied upon, especially if the supervisor is absent from the site for a period.
All geological samples and water strikes should be logged by the drillers and the supervisor, as this important information will be required for designing the borehole and the equipment to be installed.
Full borehole logging may also include geophysical logging, which is normally carried out only after a well has been completed.
Information about structural features and geological formations in a borehole can be remotely obtained by geophysical borehole logging techniques. The object of well logging is to measure the properties of the undisturbed rocks and fluids they contain. Geophysical logs can provide information on lithology, the amount of water in a formation, formation density, zones of water inflow, water quality, and other in situ parameters that cannot be derived from highly disturbed drilling samples. A suite of geophysical log data, including deep-penetration methods, will more or less complete the technical description of a borehole, but geophysical logging is a specialized field best left to geophysical contractors or hydrogeological consultants. A logging unit consists of a power supply, a receiver/data processing unit, and a cable on a powered winch that lowers special sensor probes (‘sondes’) into the borehole to measure various properties. The cable contains multi-conductors that transmit signals to the receiver console. Data, processed by computer, can be shown as a geophysical record on a graphic display, which should consist of a number of different structural, formation, and fluid logs. Specialized software packages enable manipulation, interpretation, and comparison of data. Multiple-sonde geophysical (‘suite’) logging can provide a substantial amount of information about the sub-surface conditions in and around a borehole.
Casing and Screen Installation
You can use the following method to install screens:
Step 1. Place a casing section in the well. Cap the casing or the screen on the lower end so materials from the bottom of the well will not enter the well. (Depending on if the casing or screen is designed to install at the business end of the borehole (Total depth – TD)
Step 2. Suspend a screen section over the well and attach the screen section to the casing section.
Step 3. Lower the screen and casing section. Suspend them in the well either by the elevator resting on the rotary table or by slips in the spider bowl.
Step 4. Add casing until the screen reaches the desired depth.
Gravel Pack Filtering and Backfilling
The last procedure when installing screen and casing is to place a gravel-pack filter around the screen and backfill material around the casing. If you place the screen in material such as gravel or very coarse sand, you may not need a gravel pack. Place the gravel filter material around the outside of the casing. Deposit the material to the bottom of the well. Add gravel to about 5 feet from the top of the screen. (Use the sounding method to determine the level of the gravel.) Add impervious backfill around the casing from the gravel pack to about 10 to 20 feet from the surface. If you use grout instead of impervious material, add a couple of feet of clay above the gravel to prevent the grout from entering the gravel filter. Bring the grout to the surface.
Frequently, when a well is first installed, the efficiency (production per foot of drawdown) is not satisfactory, and you must develop the well either by pumping or surging or both. Developing a well removes the remaining drilling fluid, breaks down any filter cake buildup on the borehole wall, and flushes the fines in the formation (adjacent to the grovel pack) into the well. Make sure that you pump the well of all fine sediments and sand with an airlift before installing the submersible pump. If you do not, the pump and components will wear out prematurely. To develop a well, pump or blow the drilling fluid out of the well. Agitate the water in the well to produce an alternating in and out flow through the well screen (or gravel pack). There are several methods available to develop a well.
All wells must have a sanitary seal to prevent contamination from surface runoff. Mix cement grout and place it in the annulus between the well casing and the borehole wall.
Extend the grout from the surface to the top of the backfill material (30-foot minimum). You should also pour a concrete platform (about 4 feet by 4 feet) around the casing at the surface with the casing extended at least 1 foot above the surface. The upper surface of the slab and the surrounding area should be gently sloping away from the well for better drainage. In addition to a surface grouting, you need to install a well seal (a type of bushing or packing gland) to prevent foreign materials from entering the inside of the well casing. You normally install the well seal when you install the pump, which is after you complete all development, testing, and disinfecting.
After installing a well, you should perform a pumping test. The test will show you if the well can produce the required amount of water. If the well is considered permanent, the pumping test should help you evaluate any future performance deterioration. Evaluation parameters are flow rate, time, and drawdown in the well.
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To achieve a good well design, a drilling log should be completed, a drilling log during the actual drilling process, from the drilling logs, the exact depth and length of the well screen, the depth and thickness of the gravel pack and sanitary seal can be determined.
The internal diameter of the PVC well casing is selected to fit the outer diameter of the pump that is going to be installed. The drilled diameter of the borehole in turn depends on the outer diameter of the PVC well casing. For the diameter of the borehole, it is important to realize the following:
The drilled diameter of the borehole should be at least 2-inch larger than the outer diameter of the PVC well casing to be able to place the gravel pack and sanitary seal. If this rule is not applied and the space between the PVC well casing and the borehole wall is too small, it is almost impossible to place the gravel pack and sanitary seal at the correct depth. Furthermore, the backfill may get ‘stuck’ on its way down (this is called ‘bridging’) and end up in the wrong position.
When the final depth for the bottom of the well-screen in the aquifer is reached, an additional two meter should be drilled. This is to allow for fine soil particles, suspended (mixed with the water) in the borehole, to settle prior to and during the installation of the well-screen and casing, by doing so, the determined well-screen depth can be maintained, and to accommodate a sump.
Drilling the borehole to depth
Finally, before the drilling pipes are lifted, the fluid used to drill borehole should be flushed with clean water to remove all fine particles that are suspended in the hole. If this is not done, the particles will settle at the bottom of the well influencing the final installation depth or enter the well-screen during the installation of the well casing.
Materials: PVC well-screen and casing
Several different PVC pipes exist, varying from cheap drain pipes to expensive, high class, slotted well screens and casing pipes.
For a potable water well use by a single household or a few households a deeper hole may be needed. In this case a self-slotted 4-inch PVC pipe can be used.
In large water projects (for Governments or NGOs) for communal potable water wells, equipped with a high horse power submersible pump, a deeper hole is needed 5 to 6-inch standard factory slotted PVC well casing pipes are often required (to maximize yield, to ensure high construction quality, and to accommodate the pump). These wells will be significantly more expensive, but of better quality.
Slots are the openings in the well-screen which allow groundwater to flow into the well. In theory the slot size (width) should be smaller than the mean size of the soil particles. However, in some countries only 1 factory-made slot size (1mm) is available. For low-cost wells, one can make the slots by hand using a hacksaw.
To increase the lifetime of the well-screen, it is advised to attach a 1-meter sump at the bottom of the well-screen, into which any particles entering the well screen from the aquifer can settle, without blocking the well-screen and pump.
The sump simply consists of a 1-meter plain PVC pipe, which is closed at the bottom end. To close the bottom of the sump a factory-made wooden or PVC cap can be inserted. Alternatively, the bottom end of the sump pipe can also easily be closed by some cutting and bending. Make 4 cuts in the bottom part of the sump pipe and heat the pipe end. Fold the four parts inside and allow the parts to cool (see photo).
Alternatively cut out 6-8 triangular parts. The remaining parts can be bent together to a point. Making a point will reduce the risk of ‘scraping’ the borehole wall when the well-screen is lowered into the borehole. To completely seal the bottom of the sump 10 cm of cement mortar should be poured in the sump.
Casing and well-screen pipes are usually joined by glued sockets. The more expensive purpose-made casings and well-screens with a wall thickness of at least 5 mm are threaded. When the pipes are glued together, it is very important to clean and roughen both ends, the inside of the socket and the outside of the pipe to be glued. Then, put sufficient glue all around on both ends and put the pipes together in one move.
The gravel pack fills the space between the aquifer (sand particles) and the well screen preventing the wall of the hole from collapsing on to the well-screen and may serve to filter some of the fine sand particles from entering the well.
The gravel should consist of a grain size (generally 1.5 – 3mm) which is just larger than, and no more than twice to three times, the slot size of the well-screen. Good size gravel looks more like coarse sand, rather than gravel. The grains are best when round in shape. Such material can often be found on river beds or lake shores. The best way to prepare suitable gravel is using maximum and minimum sized sieves (grains which are too small or too big are sieved out).
It is essential to install a sanitary seal if the well needs to yield good quality water. The sanitary seal can consist of cement-water mixture (cement grout).
The water and cement are mixed until a thick slurry is created (26 liters of water to one 50 kg bag of cement will make about 33 liters of cement grout). If cement grout is used as a sanitary seal, first a half meter of clay should be back filled on top of the gravel pack to prevent the grout from penetrating the gravel pack.
Water Borehole Construction
Step 1, Preparations
Prepare all materials needed for the installation and backfilling. Measure out the effective length of the PVC pipes and cut the last pipe to a length, allowing 1 meter to be left above ground level, after installation. Number the pipes in order of installation.
Centralization of the well screen
To prevent the slots from becoming blocked with clay due to scraping of the well screen against the borehole wall during installation, the well-screen should be centralized. Centralizing the well-screen in the borehole also allows the gravel pack to settle equally around the screen, leaving at least 1-inch of gravel all around the well-screen. Centralization can be achieved by attaching spacer rings or centralizers with an interval of every 3 meter around the well-screen. The spacer rings can be made of PVC rings, which can be attached on 4 sides around the well
Step 2, Installation of the PVC pipes
A practical method of lowering the PVC pipes into the borehole is to use a rope (see picture). One end of the rope is attached to the drill rig and the other end is wrapped three times around the pipe to form a self-closing loop. The rope is used to prevent the casing and screen slipping into the borehole while adding a new length of pipe. Install all the prepared pipes, and leave 1 meter (see step 1) of pipe above ground level so that the well-screen is placed at the correct depth.
Step 3, Cleaning and flushing the well-screen
When the well-screen has been installed at the correct depth, the pipes and screen should be flushed in the case of fluid drilled boreholes. Pour water into the PVC pipes and allow the dirty water to overflow out of the borehole. If the added water only enters the well slowly (or not at all), this could indicate blockage of the well-screen slots by clay or fine material from the borehole wall. Extra water pressure in the casing and well-screen should then be created by adding a plunger or surge block (or simply a plug of cloth), which is then moved up and down in the casing. Repeat this process until the water directly flows away when added. Continue flushing with clean water until the water which is coming out of the borehole is clean. Only then the gravel pack should be installed.
Step 4, Installation of the gravel pack
The gravel pack is now poured in the annular space around the pipe. At the same time the PVC pipe is moved from side to side to guarantee an easy passage for the gravel down to the screen. Pour in the gravel slowly, to prevent bridging (gravel getting stuck at the wrong level). Use the measurement tape or tool to measure the depth to the top of the gravel and fill to 1-2 meter above the top of the well-screen.
In fluid drilled holes, water will overflow from the PVC casing pipe, as the gravel is dropped around the well-screen. Water will stop overflowing the PVC casing pipe when the entire length of the well-screen has been backfilled.
Step 5, Installation of the sanitary seal
When the gravel pack has settled to the right depth (always measure!), the sanitary seal can be installed Prepare the cement grout and pour it into the borehole in the same way (if cement grout is used for the sanitary seal remember to use clay for the first half meter on top of the gravel pack!) Measure to ensure the sanitary seal was installed at the right depth.
Step 6, Filling the annular space
The rest of the annular space is filled up by cuttings and cement grout (see below). Always pour in the material slowly, while moving the casing to prevent bridging of the material.
Step 7, Installation of the top seal
A sanitary top seal of 3-5m thickness should be placed from 3-5m below ground to the surface. The top seal is usually made of cement grout.
Step 8, Well development
‘Well development’ is necessary to maximize the yield of the well and optimize the filter capacity of the gravel pack. This is achieved by removing the fines and drilling fluid additives, and settlement of the gravel pack.
After drilling some of the fines and drilling fluid additives remain behind in the borehole and are blocking the pores of the surrounding aquifer and the new installed gravel pack. After they have been removed by well development the water will be able to move freely from the aquifer to the well screen. During well development also the gravel pack will settle and become more compacted, ensuring that there are no large voids into which aquifer material (sand) could later collapse. The settled gravel pack will filter out some of the fines from the aquifer. During flushing of the well-screen and the borehole, already some of the fine particles and drilling fluid additives were removed. However, normally this first well development is not enough and more extensive development needs to be carried out after completing the installation process. The remainder of well development takes place after the backfill has been placed and the cement grout of the sanitary seal has hardened (this hardening process takes at least 24 hours).
Several techniques are available for well development, The most effective technique is airlifting: Airlifting is a very suitable development tool, by which high flow rates and shock waves can be established with a compressor.
Step 9, Construction of the head works and Well cap.
Finally the head works and well cap should be installed. This apron will prevent surface water and contamination to flow into the borehole directly.
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Ground water is a resource found under the earth’s surface. Most ground water comes from rain and melting snow soaking into the ground. Water fills the spaces between rocks and soils, making an “aquifer”. Many families rely on private, household water borehole and use ground water as their source of fresh water.
Generally, the deeper the borehole, the better the ground water. The amount of new water flowing into the area also affects ground water quality. Ground water may contain some natural impurities or contaminants, even with no human activity or pollution.
The first step to safeguard drinking water is to understand and spot possible pollution sources. Pollution sources can be divided into two groups:
Naturally occurring contaminants, such as naturally occurring minerals.
Past or present human activity. Things we do, make, and use — such as mining, farming and using of various chemicals.
Several sources of pollution are easy to spot by sight, taste, or smell. The following are (Quick Reference List.)
Quick Reference List of Noticeable Problems
• Scale or scum from calcium or magnesium salts in water
• Unclear/turbid water from dirt, clay salts, silt or rust in water
• Green stains on sinks or faucets caused by high acidity
• Brown-red stains on sinks, dishwasher, or clothes in wash points to dissolved iron in water
• Cloudy water that clears upon standing may have air bubbles from poorly working pump or problem with filters.
• Salty or brackish taste from high sodium content in water
• Alkali/soapy taste from dissolved alkaline minerals in water
• Metallic taste from acidity or high iron content in water
• Chemical taste from industrial chemicals or pesticides
• A rotten egg odor can be from dissolved hydrogen sulfide gas or certain bacteria in your water. If the smell only comes with hot water, it is likely from a part in your hot water heater.
• A detergent odor and water that foams when drawn could be seepage from septic tanks into your ground water well.
• A gasoline or oil smell indicates fuel oil or gasoline likely seeping from a tank into the water supply
• Methane gas or musty/earthy smell from decaying organic matter in water
• Chlorine smell from excessive chlorination.
Note: Many serious problems (bacteria, heavy metals, nitrates, radon, and many chemicals) can only be found by laboratory testing of water.
Naturally Occurring Sources of Pollution
Bacteria, viruses, parasites and other microorganisms are sometimes found in water. Shallow wells — those with water close to ground level — are at most risk. Runoff, or water flowing over the land surface, may pick up these pollutants from wildlife and soils.
This is often the case after flooding. Some of these organisms can cause a variety of illnesses. Symptoms include nausea and diarrhea. These can occur shortly after drinking contaminated water. The effects could be short-term yet severe (similar to food poisoning) or might recur frequently or develop slowly over a long time.
Radionuclides are radioactive elements such as uranium and radium. They may be present in underlying rock and ground water. Radon — a gas that is a natural product of the breakdown of uranium in the soil — can also pose a threat. Radon is most dangerous when inhaled and contributes to lung cancer. Although soil is the primary source, using household water containing Radon contributes to elevated indoor Radon levels. Radon is less dangerous when consumed in water, but remains a risk to health.
Nitrates and Nitrites
Although high nitrate levels are usually due to human activities, they may be found naturally in ground water. They come from the breakdown of nitrogen compounds in the soil. Flowing ground water picks them up from the soil. Drinking large amounts of nitrates and nitrites is particularly threatening to infants (for example, when mixed in formula).
Underground rocks and soils may contain arsenic, cadmium, chromium, lead, and selenium. However, these contaminants are not often found in household wells at dangerous levels from natural sources.
Fluoride is helpful in dental health, so many water systems add small amounts to drinking water. However, excessive consumption of naturally occurring fluoride can damage bone tissue. High levels of fluoride occur naturally in some areas. It may discolor teeth, but this is not a health risk.
Ground water pollution from Human Activities
Bacteria and Nitrates
These pollutants are found in human and animal wastes. Septic tanks can cause bacterial and nitrate pollution. So can large numbers of farm animals. Both septic systems and animal manures must be carefully managed to prevent pollution. Sanitary landfills and garbage dumps are also sources. Children and some adults are at extra risk when exposed to water-born bacteria. These include the elderly and people whose immune systems are weak due to AIDS or treatments for cancer. Fertilizers can add to nitrate problems. Nitrates cause a health threat in very young infants called “blue baby” syndrome. This condition disrupts oxygen flow in the blood.
Concentrated Animal Feeding Operations
On these farms thousands of animals are raised in a small space. The large amounts of animal wastes/manures from these farms can threaten water supplies. Strict and careful manure management is needed to prevent pathogen and nutrient problems. Salts from high levels of manures can also pollute groundwater.
Fertilizers and Pesticides
Farmers use fertilizers and pesticides to promote growth and reduce insect damage. These products are also used on golf courses and suburban lawns and gardens. The chemicals in these products may end up in ground water. Such pollution depends on the types and amounts of chemicals used and how they are applied. Local environmental conditions (soil types, seasonal snow and rainfall) also affect this pollution. Many fertilizers contain forms of nitrogen that can break down into harmful nitrates. This could add to other sources of nitrates mentioned above. Some underground agricultural drainage systems collect fertilizers and pesticides. This polluted water can pose problems to ground water and local streams and rivers. In addition, chemicals used to treat buildings and homes for termites or other pests may also pose a threat. Again, the possibility of problems depends on the amount and kind of chemicals. The types of soil and the amount of water moving through the soil also play a role.
Industrial Products and Wastes
Many harmful chemicals are used widely in local business and industry. These can become drinking water pollutants if not well managed. The most common sources of such problems are:
• Local Businesses: These include nearby factories, industrial plants, and even small businesses such as gas stations and dry cleaners. All handle a variety of hazardous chemicals that need careful management. Spills and improper disposal of these chemicals or of industrial wastes can threaten ground water supplies.
• Leaking Underground Tanks & Piping: Petroleum products, chemicals, and wastes stored in underground
storage tanks and pipes may end up in the ground water. Tanks and piping leak if they are constructed or installed improperly. Steel tanks and piping corrode with age. Tanks are often found on farms. The possibility of leaking tanks is great on old, abandoned farm sites.
• Landfills and Waste Dumps: Modern landfills are designed to contain any leaking liquids. But floods can carry them over the barriers. Older dumpsites may have a wide variety of pollutants that can seep into ground water.
Improper disposal of many common products can pollute ground water. These include cleaning solvents, used motor oil, paints, and paint thinners. Even soaps and detergents can harm drinking water. These are often a problem from faulty septic tanks and septic leaching fields.
Lead & Copper
Household plumbing materials are the most common source of lead and copper in home drinking water. Corrosive water may cause metals in pipes or soldered joints to leach into your tap water. Your water’s acidity or alkalinity (often measured as pH) greatly affects corrosion. Temperature and mineral content also affect how corrosive it is. They are often used in pipes, solder, or plumbing fixtures. Lead can cause serious damage to the brain, kidneys, nervous system, and red blood cells.
Water Treatment Chemicals
Improper handling or storage of water well treatment chemicals (disinfectants, corrosion inhibitors, etc.) close to your well can cause problems.
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Drilling mud, also called drilling fluid is use for water borehole drilling. Drilling mud is pumped down the hollow drill pipe through the by-bass hose to the drill bit, where it exits the pipe and then is flushed back up the borehole to the settling pits at surface. Water remains the primary constituent of water well drilling fluids. Drilling fluid is used to:
Lift soil/rock cuttings from the bottom of the borehole and carry them to a settling pit.
Allow cuttings to drop out in the mud pit so that they are not re-circulated (influenced by mud thickness, flow rate in the settling pits and shape/size of the pits);
Prevent cuttings from rapidly settling while another length of drill pipe is being added (if cuttings drop too fast, they can build-up on top of the bit and seize it in the hole).
Create a film of small particles on the borehole wall to prevent caving and to ensure that the upward-flowing stream of drilling fluid does not erode the adjacent formation.
Seal the borehole wall to reduce fluid loss (minimizing volumes of drilling fluid is especially important in dry areas where water must be carried from far away);
Cool and clean the drill bit; and lubricate the bit, bearings, mud pump and drill pipe.
Drilling mud is created by thoroughly mixing water with clay usually bentonite to a desired consistency. Drilling fluids must be mixed thick (viscous) enough to bring soil cuttings up from the bottom of the hole to the surface, yet not so viscous as to prevent their settling out in the mud pits. It is, therefore, very important to understand the properties of drilling muds and their proper use:
The ability of a fluid to lift cuttings increases rapidly as viscosity (the degree to which a fluid resists flow under an applied force) and up-hole velocity are increased. After cuttings are brought to the surface, however, it is essential that they drop out as the fluid flows through the settling pit. The desired results are obtained by properly designing the mud pits, controlling the viscosity and weight of the drilling fluid and adjusting the pump speed.
During the drilling process, solids accumulate in the drilling fluid – especially when drilling silt, clay or weakly consolidated shale. The thickness of the drilling fluid often needs to be adjusted during drilling by adding more water and/or removing some of the accumulated cuttings from the settling pit.
Fluid which is too thick will be difficult to pump and will cause unnecessary wear of the mud pump since cuttings will not have settled out of the mud before the mud is pumped back down the borehole. It will also make it difficult to remove the mud from the borehole walls and adjacent aquifer during well development and rate of penetration is also potentially reduced.
If the mud is too thin, cuttings will not be brought to the surface and the drill bit and drill pipe may get stuck in the borehole by settling cuttings. In addition, thin mud can result in excessive migration of mud into the formation, thus decreasing the potential yield of the well.
Always start drilling with clean water as the drilling fluid; keep it as clean as possible during drilling to minimize subsequent well development problems. In clay-rich formations, the water will quickly mix with natural clays in the borehole to form a thin clay slurry, it should be replaced with clean water or a drilling mud prior to the water bearing zones. If this is not done, the natural clays will be pushed into the aquifer and will not break-down with development, thus seriously restricting well yield.
In sandy soils, bentonite clay (sodium montmorillonite) must be mixed with the drilling water to increase its viscosity and keep the borehole from collapsing (just a small amount of bentonite is required). While better than natural clays, bentonite does not readily break down its cohesive structure and it can be difficult to remove from the borehole and aquifer and has to be removed by heavy pumping equipment during the well development. Therefore, it is NOT recommended to use bentonite for manual (low cost) drilled boreholes.
It is always advisable to thin out the drilling mud before setting the casing. Many drillers switch to a polymer drilling fluid. (To switch from drilling mud to polymer, pump the drilling mud out of the mud pits and replace the fluid with a properly stabilized drilling polymer).
Drilling polymers are organic additives which take the place of natural clay. After several days, organic additives breakdown to a fluid as thin as water, and it can be thoroughly flushed from the well. Adding chlorine to the well during development will accelerate this breakdown and allow for faster development of the well.
Polymers may be added to drilling mud to improve the overall performance. Drilling mud makes a better wall cake. Polymer is better at increasing the viscosity of the drilling fluid. Polymers can be slowly sprinkled into the mud pit. If fed at too fast a rate, lumps will form.
How to Use a Marsh Funnel
Viscosity is a measurement of a fluid’s resistance to flow: the greater the resistance, the higher the viscosity. The viscosity of drilling mud is influenced by the gelatin-related density and the solids content. The viscosity can be controlled by adding drilling mud and adjusting the pH. The viscosity should be adjusted depending upon the type of material being drilled, the drilling rate, and the hole size.
Different types of clay have a wide range of hydration potential. The more the clay hydrates, the more it expands and has more lifting ability. Selling clays like bentonite and montmorillonite are preferred because the clay particles are much thinner and come apart more easily than those of other clays. When properly hydrated in water, these clays can swell to approximately 10 times their original volume. Bentonite and montmorillonite hydrate only in fresh water.
Viscosity can be measured with a Marsh Funnel. The procedure is as follows:
Hold funnel in upright position with index finger over the outlet
Pour the drilling fluid through the screen in the top of the funnel until the drilling fluid reached the marked line just beneath the screen.
Remove the finger from the outlet and measure the number of seconds it takes to fill the accompanying container up to the marked 1 quart line
The following guidelines can be used to assess whether drill mud is thick enough:
Material Being Drilled
Marsh Funnel Viscosity
Water (with no swelling clay)
Natural Swelling Clays
32 to 37
Normal Conditions (including non-swelling clay and fine sand)
40 to 45
45 to 55
55 to 65
65 to 75
75 to 85
Adjust pH: The pH of the drilling fluid can affect performance of the drilling mud. Drilling mud will have maximum hydration where the pH is between 8.0 and 9.0 Use 1/4 pound of soda ash per 300 gallons of drilling fluid to bring water from a pH of 7 up to a pH of 8.5.
Build and Maintain Viscosity: Drilling fluid must have enough time to hydrate. Pump the drilling fluid through the 3-way valve and recirculate the drilling fluid back through the pits. Check the viscosity before drilling. Proper viscosity enables the drilling fluid to effectively bring up drill cuttings and to build a good wall cake. The wall cake helps support the borehole and keep it from collapsing when drilling in unconsolidated material.
Control the Borehole: Loosing fluid to the formation typically causes borehole problems. The higher the fluid loss, the greater the potential for weakening the formation to the point of collapse or thickening the wall cake – either of which can get you stuck. Have a barrel of thick drilling mud available. Add the thicker mud to the mud pit for a quick thickening of the drilling fluid.
The viscosity of the drilling fluid is also a function of the rate of flow for the pump and the size of the borehole. The bigger the borehole, the lower the upper velocity of the fluid. At lower velocities, the viscosity is higher because electric charge on the clay particles will hold in a tighter bond. This is why the clay in the drilling fluid tends to gel when the fluid is at rest. If drilling stops, even for a few minutes, raise the drill bit off the floor of the hole to avoid drill cuttings from trapping the bit. Pull the pipe out of the hole if drilling stops for an extended time (overnight).
A lack of accurate pore-pressure prediction and wellbore stability analysis can result in unscheduled drilling events, such as blowouts, kicks, hole washouts, wellbore break- out, and stuck pipe. Undetected abnormal pore pressure and wellbore instability also adds to drilling nonproductive time and increase drilling costs in millions of dollars and sometimes leads to abandoning the well before reaching its objective.
Integration of predrill pore-pressure and geomechanics analyses with real-time monitoring consistently provides an effective way to prevent drilling failure and improve well-construction efficiency.
Abnormal pore pressures can cause serious drilling incidents, such as unwanted fluid influx to wellbore (Kick) and well blowouts, if the pressures are not predicted accurately. This can lead to erroneous mud-weight design plan that can contribute to wellbore instability.
A lack of an accurate wellbore-stability prediction can also cause borehole breakouts and in hole closure, pack off, and collapse in cases of tensile and compressive shear failures leading to mud loss and lost circulation through hydraulic fractures and in severe cases can lead to a total lost borehole. Estimated cost to the drilling industry for hole stability problems range from 600 million to 1 billion dollars annually.
When the mud weight, or equivalent circulating density (ECD), is less than the pore pressure, the wellbore experiences splintering failure in shale formation. In this case, wellbore washouts or fluid kicks resulting from underbalanced drilling may occur.
A well may not have fluid kicks in an underbalanced-drilling scenario if impermeable formations that is not over-pressured are penetrated. When the mud weight or ECD is less than the shear-failure gradient or borehole collapse pressure gradient, the wellbore experiences shear failure (or wellbore elliptical enlargement, breakout, or collapse). Wellbore fracturing occurs when mud pressure exceeds the capacity of near-wellbore rock to bear tensile stress and the drilling fluid creates hydraulic fractures.
The drilling-induced fractures may cause drilling-fluid losses and even a total loss of drilling fluid returns (lost circulation). Maintaining wellbore stability and preventing these costly problems require an accurate prediction of the conditions that cause wellbore failures, including pore pressure and safe-mud-weight operating window.
Pore pressure is the fluid pressure in the pore space of the formation. Pore-pressure analyses include three aspects:
Pore pressure analysis before drilling a well include: Seismic data analysis and interpretation in the plan well location; Well-logging, and drilling data in offset wells if available
Pore pressure analysis while drilling a well (qualitative and quantitative): Drilling parameters and mud-logging data; Logging-while-drilling (LWD) or measurement while-drilling (MWD) data
Pore pressure analysis after drilling the well: Wireline log analysis (Sonic log, Resistivity log etc.)
Pore pressure prediction from seismic data analysis before drilling
Bowers (1995) proposed that the seismic interval velocity and effective stress have a power relationship. On the basis of this relationship, pore pressure can be obtained from seismic interval-velocity data (Reflection times to transit times) in the planned-well location. The seismic data transform in terms of depths interval velocities and interval travel times are used for pressure gradient calculation for determination of (Overburden pressure and gradient, Pore pressure and gradient, Fracture pressure and gradient).
Pore pressure prediction from wireline logs and logging while drillings logs
The analysis of wireline logs before drilling for offset wells and logging while drilling (LWD) allows the calculation of:
Overburden pressure and gradient
Pore pressure and gradient
Fracture pressure and gradient
The wireline logs generally used for pressure prediction and evaluation are: Sonic Logs, Induction Logs (Resistivity Logs) and Density Logs.
Pressure prediction and evaluation from drilling and mud-logging data
The acquisition and interpretation of drilling and mudlogging data represent a very important group of techniques which have the advantage to be available more or less in real time while drilling. These methods can be:
Qualitative: Which, if analyzed in their completeness, can provide significant information about the actual status of the well and alert the drilling team of dangerous and abnormal conditions while drilling. Among the qualitative techniques base on drilling and mudlogging data include:
‘d’ exponent, Sigma log
LWD (Resistivity, Density, Sonic)
Drag and torque
Mud pit level, Return flow, Pump Pressure (kick)
After Lag Time: Gas, (BG, CG, Pump off Gas) MW (out), Cuttings Shape/Size, Lithology (anhydrite, known marker, etc.), Shale density, Shale factor, Temp(out)
Before drilling, rock stress is described by the in-situ stresses; effective overburden stress, effective minimum horizontal stress, and the effective maximum horizontal stress. These stresses are designated by (σ1, σ2, σ3).
As the hole is drilled, the support provided by the rock is removed and replaced by hydrostatic pressure. This change alters the in-situ stresses. The stress at any point on or near the wellbore can now be described in terms of: Radial stress acting along the radius of the wellbore; Hoop stress acting around the circumference of the wellbore (tangential); Axial stress acting parallel to the well path. These stresses are designated by (σr, σø, σz)
Hoop stress is dependent upon wellbore pressure, in situ stress magnitude, orientation, pore pressure, hole inclination and direction. Wellbore pressure is directly related to mud weight/ECD.
For a vertical wellbore with equal horizontal stresses, hoop stress is dependent upon the mud weight and the magnitude of the horizontal stresses and is equally distributed around the wellbore
A deviated well creates unequal distribution of hoop stress around the wellbore due to the redistribution of the horizontal and vertical stresses. Hoop stress acting on a cross-section of the wellbore is maximum at the sides of the wellbore perpendicular to the maximum stress. The same is true when drilling a vertical well in an in-situ environment of unequal horizontal stress. Hoop stress is maximum at the side of the wellbore perpendicular to the maximum horizontal stress.
Axial Stress σz
Axial stress is oriented along the wellbore path and can be unequally distributed around the wellbore. Axial stress is dependent upon; in situ stress magnitude and orientation, pore pressure, and hole inclination and direction. Axial stress is not directly affected by mud weight.
For a vertical well with equal horizontal stress, axial and vertical stress are the same. Axial stress in a deviated well is the resolution of the overburden and horizontal stresses.
Radial Stress σr
Radial stress is the difference in wellbore pressure and pore pressure and acts along the radius of the wellbore. Since wellbore and pore pressures both stem from fluid pressure acting equally in all directions, this pressure difference is acting perpendicular to the wellbore wall, along the hole radius.
Hoop (σø), radial(σr), and axial (σz) stress describe the near wellbore stress-state of the rock. Mechanical stability is the management of these stresses in an effort to prevent shear or tensile rock failure. Normally the stresses are compressive and create shear stress within the rock. The more equal these stresses, the more stable the rock.
Whenever hoop or radial stress become tensile (negative), the rock is prone to fail in tension. Many unscheduled rig events are due to loss of circulation caused by tensile failure.
Mechanical stability is achieved by controlling the parameters that affect hoop, axial, and radial stress.
Wellbore stability controllable parameters:
Mud weight (MW)/Equivalent circulating density (ECD),
Mud filter cake,
Well path – Inclination and azimuth,
Drilling / tripping practice.
Time dependent effect
Mechanical stability of the well is also impacted by drilling fluid/formation interaction. Chemical instability eventually results in mechanical failure of the rock in shear or tension.
Effect of Mud Weight/ECD
Mud weight, ECD, and pressure surges on the wellbore directly affect hoop and radial stress. An increase in MW decreases hoop stress and increases radial stress. Similarly, a decrease in MW increases hoop stress and decreases radial stress. The result on wellbore stability is dependent upon the magnitude of the mud weight increase/decrease.
Mud Filter Cake and Permeable Formations
The filter cake plays an important role in stabilizing permeable formations. An ideal filter cake isolates the wellbore fluids from the pore fluids next to the wellbore. This is important for hole stability and helps prevent differential sticking as well.
If there is no filter cake, the pore pressure near the wellbore increases to the hydrostatic pressure; the effective radial stress is zero. The simultaneous decrease in effective hoop stress causes the stress-state to move left in the stability envelope; decreasing the stability of the formation. An ideal filter cake helps provide for a stable wellbore. The chemical composition of the mud and permeability of the formation control the filter cake quality and the time it takes to form.
Hole Inclination and Direction
The inclination and direction of the wellbore greatly impacts the stability of the well. Unequal distribution of hoop and axial stress around the circumference of the well tends to make the wellbore less stable.
For Equal Horizontal Stress: Drilling a horizontal well causes the hoop and axial stress distribution around the wellbore to change. Before drilling from vertical, the hoop stress is equally distributed. As angle increases to horizontal, the hoop stress on the high and low side of the wellbore decreases, but the hoop increases greatly on the perpendicular sides.
Temperature changes associated with mud circulation during drilling may alter the rock properties. The change in rock properties may reduce or enhance borehole failure depending on the thermal effect. Temperature fluctuations may also influence the stress distribution around the borehole. As the temperature increases, the tangential and vertical stresses will increase. However, temperature fluctuations will not influence the stress anisotropy around the borehole as the thermal effect should alter the tangential and vertical stresses by an equal amount.
Reactive shale instability is also time-dependent, and is governed by two intrinsic mechanisms: (a) consolidation and (b) creep. Consolidation is due to pore pressure gradients induced by fluid communication between the mud and pore fluid. Creep is described by a change of strain at a constant effective stress level. Both of these mechanisms will result in hole size reduction. In practice, it is difficult to distinguish between creep and consolidation effects. In general, consolidation will occur shortly after loading, while creep will govern later deformation. The mud pressure and properties, and the temperature in the rock may vary during drilling operations, which in turn enhance borehole instability. All these parameters make it more difficult to directly pursue the time-dependent effects. The best approach is to quickly isolate the rock with a casing to minimize the potential borehole instability.
Providing a stable wellbore
Potential Stability Indicators
If the answer to any of the questions below is “yes”, preventive measures should be taken:
Indications of tectonic activity in the area?
Sudden pressure transition zones expected?
Adverse formations expected (reactive shale, unconsolidated or fractured
formations, abnormal or sub normally pressured zones, plastic formations?
Is wellbore inclination greater than 30?
2. Identify Stress Regime
σ1= Greatest effective stress
σ2= Intermediate effective stress
σ3 = Least effective stress
3. Determine Magnitude of In Situ Condition (sv , sh , sH)
Overburden – sv (Obtained from density logs of offset wells).
Formation Pore Pressure -pp (Estimated by seismic and logs).
Minimum Horizontal Stress – sh (Determined by LOT and/or logs).
4. Use Core Tests or Logs to Determine Formation Rock Strength or Use Logs to determine: Effective Compressive Stress. Rock strength is estimated through correlations with sonic density logs since slow sonic velocity and high porosity generally relate to lower rock strength.
5. Select Mud System and Determine Mud Weight Window: Stability spreadsheets and analysis tools are used to determine the mud weight window for each hole section.
6. Avoiding Stability Problems
Select an inhibitive mud for reactive formations.
Casing points should allow for mud weight windows determined from stability analysis
Maintain mud weight/ECD in stability window. Use down hole. ECD monitoring tools in critical wells.
Optimize well trajectory based on drilling days vs. stability.
Plan for effective hole cleaning and stuck pipe prevention.
Follow safe drilling practices. Control ROP, surge pressures.
Mechanical instability has stated earlier is related to incorrect mud weight /ECD and/or well trajectory. Too low mud weight can cause hole cavings or collapse resulting in stuck pipe. Too high mud weight /ECD can cause excessive fluid losses to the formation or total loss of returns
Warning Signs of Mechanical Stability Problems
Large size and volume of cavings over shakers.
Erratic increase in torque/drag.
Hole fill on connections or trips
Stuck pipe by hole pack-off /bridging.
Restricted circulation /increases in pump pressure.
Loss of circulation.
Loss/gain due to ballooning shales.
Preventing Mechanical Stability Problems
The constraints on wellbore pressure are dictated by formation pressure on the low end and fracture strength on the high end. Hydraulics planning must also consider minimizing the shock load imposed to the wellbore.
Measures to prevent/correct mechanical stability problems include:
Increase the mud weight (if possible). The mud weight values should be determined using a stability analysis model and past experience if drilling in a known field.
If drilling fractured formations, it is not recommended to increase MW. Increase the low-end rheology (< 3 RPM Fann reading).
Improve hole cleaning measures. Maintain 3-rpm Fann reading greater than 10. GPM for high-angle wells equal to 60 times the hole diameter in inches and half this value for hole angle of less than 350.
Circulate on each connection. Use back reaming and wiper trips only if hole conditions dictate.
Minimize surge/swab pressures.
Monitor torque/drag and the size and amount of cuttings on shakers.
Wellbore stability analysis
Borehole collapse could be predicted by adopting compressive failure analysis in conjunction with a constitutive model for the stresses around the borehole.
The most commonly used failure criterion in wellbore stability analysis is Mohr-Coulomb criterion This criterion involves only the maximum and minimum principal stresses, σ1 and σ3, and therefore assumes that the intermediate stress σ2 has no influence on rock strength. This failure criterion has been verified experimentally to be good in modelling rock failure, based on conventional triaxial tests (σ1 > σ2 = σ3). On the other hand, in practice, the Mohr-Coulomb criterion has been reported to be very conservative in predicting wellbore instability.
When drilling near massive structures such as salt domes or in tectonic areas, the horizontal stresses will differ and are described as polyaxial stress state (σ1 >σ2 > σ3). A new true-triaxial failure criterion called the Mogi-Coulomb criterion has been developed to calculate the resultant shear stress in polyaxial state. This failure criterion is a linear failure envelope in the Mogi domain (τoct-σm,2 space) which can be directly related to the Coulomb strength parameters, cohesion and friction angle. This linear failure criterion has been justified by experimental evidence from triaxial tests as well as polyaxial tests. It is a natural extension of the classical Coulomb criterion into three dimensions.
As the Mohr-Coulomb criterion only represents rock failure under triaxial stress states, it is expected to be too conservative in predicting wellbore instability. To overcome this problem, Geodata Evaluation & Drilling Engineers utilized a new 3D analytical model to estimate the mud pressure required to avoid shear failure at the wall of vertical, horizontal and deviated boreholes. This has been achieved by using linear elasticity theory to calculate the stresses, and the fully-polyaxial Mogi-Coulomb criterion to predict failure.
Determining the safe-mud-weight range is critical to improve well planning, prevent wellbore-stability problems, and reduce borehole drilling-trouble time in the oil and gas industry. Accurate pre-drill pore-pressure prediction and well-bore-stability analysis are key to improving drilling efficiency and reducing risks and costs.
Seismic data, regional geology data, formation-pressure measurement, and well-log data from offset wells can be used for predrill pore-pressure prediction.
Pore-pressure profile, in-situ stress, rock strength, image log, caliper log, and drilling events in offset wells can be used to obtain a valid wellbore stability solution for predrill wells. Real-time analysis can be performed while drilling, either on site or remotely, to update the predrill model, reduce uncertainty, avoid drilling incidents, and increase drilling efficiency.
Talk to us for your upcoming wellbore stability analysis solution
We have specialized software and highly experienced Drilling engineers to provide training to your drilling department workforce in wellbore stability analysis solution. Contact us at www.geodatadrilling.com Phone: +234 8037055441