Borehole drilling, Soil investigation, Geophysical, Environmental, Oil and Gas

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Rotary Drilling of Water borehole problems and Solutions

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

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.

Causes

  • 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
Surge Pressure

Solutions

Re-establishing circulation can involve several techniques:

  • Mud pressure (Pm) > formation pressure (Pf) – Decrease mud weight
  • 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.

Causes

  • 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.
Shale Swelling
Shale Sloughing

Shale Sloughing

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

Solutions

  • 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.

Causes

  • 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

Mechanical Sticking

Conditions:

  • 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

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.

Shale Cracks

Mechanical Sticking – Settled Cuttings

Causes: Low annular velocity and/or poor mud properties. Drill cuttings settle on the low side forming a cutting bed.

Settle Cuttings

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

Solutions

  • 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.

Basket Graple Overshot
Spear Fishing tool

Deviation

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.

Swelling Soil

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.

Under-gauged by Shale swelling

Talk to us for your upcoming Water Borehole Rotary Drilling Project

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How to Interpret Water Borehole Pumping Test

There are several ways of analyzing pumping test data, (The table below show the commonly used methods for pumping-test data analysis).

SNType of Aquifer Type of Pumping test data Names of methods
1Confine Aquifer 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
2Unconfined 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
3 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
4Leaky 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
5 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.
Borehole maximum 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

Where:

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 pumping height is 35m + 44m = 79m.

The energy consumed is:  

e = mgh/3.6 ∗ 10⁶ η  = 60000 ∗ 79m ∗ 9.8m/ s² / 3.6 ∗ 10⁶ ∗ 0.5 = 25.8 kwh

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?  
Transmissivity Interpretation

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.

Talk to us for your upcoming Water Borehole Pumping Test Project

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How to Perform Water Borehole Pumping Test

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 type of aquifer and confining beds.
  • 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:

Dipper

Dipper

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.

Pressure Transducer

Tape Method

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.

Steel Tape

How to Measure Pumping Rate

The most common method of measuring pumping rate is as follows:

Measured-Container Method

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:

where—

FR = flow rate, in GPM.

V = volume, in gallons.

T = time required to fill container, in seconds.

Flow-Meter Method

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.

Flow Meter

Weir tanks

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.

Weir Tank

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.

Step 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.

Gate Valve in Discharge Pipe

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).

Step-test procedure

Assuming that all the equipment is ready and people have been assigned their tasks, the procedure for conducting a step test is as follows:

  1. 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.
  2. 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).
  3. 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.
  4. 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.
  5. 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).
  6. Repeat the procedure for subsequent steps, progressively increasing the pumping rate for each step.
  7. 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/dayMinimum duration of constant discharge days (of constant 24 h discharge)
Up to 5001
500 to 1 0002
1 000 to 3 0004
3 000 to 5 0007
Over 5 00010
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:

Pumping  rate

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

Recovery 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:

  1. Switch the pump off and start the stopwatch at the same time.
  2. 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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How to Drill and Construct Deep Water Borehole

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.

Compressor and mud pump on a Rotary Rig
  • Install or dig a mud pit, whichever is applicable. Make the appropriate connections to the mud pump to ensure continuous cumulation 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.
Portable Mud Pit
Suction line connected to Mud Pit

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).

Tricone roller-rock bit
Drag bit

Starting the Drilling Operation

The first operation is spudding in (starting the borehole).

Rotary Table Kelly Drive

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.

Top Head Drive

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.

Rotary Rig component

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.

Connecting Elevator to Casing
Casing Slip

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).

Borehole Cement Grout

Borehole logging

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 Sampling

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.

Drill Cuttings

Geophysical Logging

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.

Geophysical Borehole logging Equipment

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.

Water Borehole Casing and Screen

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.

Water Borehole Gravel pack

Well Development

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.

Water Borehole Development with Compressor

Sanitary Seals.

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.

Sanitary seal and Well head Protection

Pumping Tests

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.

Pumping Test Setup

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Manual Water Borehole Drilling Design and Construction

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.

Drilling log

Borehole diameter

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.

Borehole depth

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.

PVC Well screen and Casing

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.

Sump

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).

Sump

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.

Pipe joints

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.

PVC Pipe: Threaded and Socket type

Gravel pack

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).

Gravel pack

Sanitary seal

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.

Sanitary seal

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.

Casing Installation

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.

Cleaning Screens

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.

Gravel Pack Installation

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.

Top seal and grouting

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.

Air lifting with 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.

Well head or Cap

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Sources of Ground Water Pollution

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

Visible

• 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.

Tastes

• 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

Smell

• 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

Microorganisms

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.

source of underground water pollution
Microorganisms

Radionuclides

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.

source of underground water pollution
radionuclides iodine cesium

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).

source of underground water pollution
nitrate nitrite

Heavy Metals

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.

naturally occurring source of underground water pollution
heavy metals

Fluoride

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.

source of underground water pollution
Fluoride

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.

ground water pollution from human activities
bacteria and nitrates

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.

ground water pollution from human activities
animal feeding

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.

ground water pollution from human activities
fertilizers and pesticides

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.

ground water pollution from human activities
Industrial_Wastes

Household Wastes

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.

ground water pollution from human activities
household wastes

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.

ground water pollution from human activities
water treatment chemical

Talk to us for your upcoming water borehole construction and treatment requirement

Geodata Evaluation & Drilling LTD. offers borehole construction, maintenance and water treatment services. For your water services requirement. contact us at www.geodatadrilling.com Phone: +234 8037055441

Drilling mud in Water Well Drilling

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.

Bentonite drilling mud

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.

Polymer

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.

Marsh Funnel

Viscosity can be measured with a Marsh Funnel. The procedure is as follows:

  1. Hold funnel in upright position with index finger over the outlet
  2. 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.
  3. 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 DrilledMarsh Funnel Viscosity
Water (with no swelling clay)  0
Natural Swelling Clays32 to 37
Normal Conditions (including non-swelling clay and fine sand)40 to 45
Medium Sand45 to 55
Coarse Sand55 to 65
Gravel65 to 75
Coarse Gravel75 to 85

Application Guidelines

  • 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).

Pore-Pressure and Wellbore Stability analysis to mitigate High cost of unscheduled drilling events

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.

Relative cost of unscheduled events caused by wellbore stability problems

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.

PP=Pore pressure, SFG=Shear failure gradient, FG=Fracture gradient

Pore-pressure prediction

Pore pressure is the fluid pressure in the pore space of the formation. Pore-pressure analyses include three aspects:

  1. 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
  2. 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
  3. 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 gradient from seismic section

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.

Pore pressure prediction from LWD data

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:

  1. 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:
  • Drilling Rate
  • ‘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)

These are discussed in : “Pore-Pressure and Wellbore-Stability Prediction for Prevention of Drilling incident”.

2. Quantitative: Which ensure the quantification of the pressures acting in and around the well and the related risk levels. These include:

  • Overburden gradient calculation
  • Formation Pressure calculation by:
  • Equivalent Depth Method
  • Ratios Method
  • Eaton Method
  • Fracture Gradient Calculation by:
  • Eaton Method
  • Daines Method

These are discussed in “Quantitative Methods of Real-Time Pore Pressure and Wellbore Stability Detection”.

After the wellbore – Near Wellbore Stress-State

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).

insitu stress

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)

Near wellbore stress state and Shear stress

Hoop Stress σø

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.

Tensile failure due to negative hoop stress

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.

Bottom-hole Temperature

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.

Time-dependent effects.

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

  1. 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

Stress regime

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.

Rock strength determination

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 Stability

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.

Conclusion

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

Consultant Geologist in the Oil and Gas industry – Advantage of hiring one

Consultant wellsite geologists, in the oil and gas industry, provide contract services to clients by bringing skill and experience in different perspective of drilling and geology which allows the geology consultant to help companies identify and solve several different problems.

The Wellsite Geologist effectively supervise geological operations at the wellsite during drilling and acts on behalf of the operating oil company, reporting to the Operations Geologist. The role is also analytical in nature, with geological interpretations used to check that the well is meeting geological targets and also to advise drilling personnel on the geological causes of problems experienced during drilling as drilling equipment and fluids interact with the rocks forming the borehole wall. Early recognition of unpredictable geological anomalies can lead to rapid and cost-effective solutions being applied, making the well safer as well as within budget.

Wellsite geologist

The wellsite geologist (WSG) and the company man (DSV), or drilling supervisor are usually the only oil producing company representatives at the rig. Both are oil company supervisors, but the geologist oversees a few teams while the company man supervises the entire drilling operation.

A Wellsite geologist is an oil company subsurface representative at well site or drilling location. They are involve in geological supervision at the well site

The basic function of wellsite geologist is to analyze drill cuttings obtain by mudloggers while drilling by identification and description of lithology with respect to depth which is an aspect of formation evaluation.

Wellsite geologist is often consultant who offer advice to the oil company and take some decisions in conjunction with the operation geologist at office in town. An example of this is when there is a need to stop drilling for casing or coring operation.

The geologist works under the supervision of operations geologists. They are located in the town offices and are the ones to whom wellsite geologist transmit all their report. Wellsite geologists are the main contact point between the oil rig and the geology and geophysics team in town. We communicate and discuss their intentions, plans and concerns to the teams at the wellsite.

What are the functions of wellsite geologist?

Wellsite geologist are responsible for well site geological supervision and all geological related activities at drilling site. The following are responsibilities of wellsite geologist:

Drill cuttings analysis and description

One of the basic duties is the identification and description of drill cuttings circulating out of the borehole with respect to depth. The description is often standardized and defined by each oil company. The wellsite geologist can classify the rock cuttings, check for evidence of borehole instability and confirm the presence of hydrocarbons.  Drilling cuttings are analyzed and described using a stereoscopic microscope under white reflected light. To help identify the presence of hydrocarbons, a UV Box (Ultraviolet Box) is also used. Hydrocarbons will have a variable, but identifiable, brightness when exposed to ultraviolet light.

Several other tests are performed to carry out the formation evaluation. Such tests involve chemicals such as hydrochloric acid, to detect calcium carbonate content, and phenolphthalein, to detect the presence of cement and differentiate it from the formation.

Drill cutting analysis

Data correlations and decisions

The wellsite geologist analyses and interpret  MWD/LWD data for confirmation of lithology, fluid type (oil, gas. water) and compare data gathered during drilling to prediction from seismic section and offset wells for determination of actual formation tops and reservoir sands. 

When offset well data MWD/LWD and wireline logs are available to the wellsite geologist, data correlation can be carried out which can help to foresee important events like significant gas changes, drilling breaks and potential hazards which occurred in offset wells. You may find the older offset logs usually printed in paper or in a pdf format. The raw data is usually in a LAS file format, which is the most common for the mudloggers and LWD/MWD services to distribute.

Wellsite geologist need to advise the base office and drilling team on the best course of action in several scenarios. As the field geologist you have the responsibility of advising the team to either carry on or stop drilling. Nowadays this is usually a decision made together with operation geologist at office base in town. One such example is selecting at which depth drilling operations must stop in order to set casing or take a core sample.

Data correlation

Formation evaluation services team supervision

There are a few teams and services which wellsite geologists supervise. These are the mudlogging, MWD and LWD, wireline logging, core handling, micro and nano paleontologists. The geologist performs quality control and assurance of these services and the data they provide. These requirements can change from Oil Company to oil company.

Formation Evaluation Services

Coring operations

The geologist is the key figure at the wellsite for taking decision with the office in town on when to stop normal drilling operations, as we approach coring point. Again, wellsite geologist need to use several correlations logs, drill cuttings , offset mudlogging, LWD data and other formation evaluation methods. When reaching coring point the drilling team starts to pull out of hole to proceed with the coring operation.

During coring operation, Wellsite geologist evaluates the few cuttings coming out of the wellbore while cutting the core. When the core is at surface, geologist take core chips from each meter (3 feet) of the entire core to evaluate the presence of hydrocarbons and to decide if coring operations should continue or stop in order to resume regular drilling operations (usually reaching the Total Depth). We must handle, or supervise the handling, of the core on surface ensuring proper markings and saw cutting as per oil company standards.

Core handling

Casing point determination

The role of the geologist, for this operation, will be similar to the coring point approach as our main focus is analyzes of drill cuttings and correlating data with offset wells and ensure that there are no permeable / porous formations close to the bottom of the hole when we reach casing point, or in the rathole immediately below the casing shoe, as that increases substantially the risk of having losses during the cement job that will be performed after running the casing itself.

Geosteering and Horizontal drilling

Wellsite geologist (depending on the oil producing company) coordinate wellsite Geosteering operation in conjunction with Base Operation Geologist by analyzing and interpretation of real time data (Well inclination, Azimuth, correlation, Lithology, Biostratigraphy, reservoir porosity, formation dip and compare with the pre-drill geological model derived from seismic and offset Well data for decision-making as whether to increase inclination or to place the borehole trajectory higher in terms of TVD or to aim for a series of forward target points coordinates and to maintain direction/angle of inclination required at the bit when target is reached) while drilling horizontal Well.   

Geosteering and horizontal drilling

Reporting

Wellsite geologist have several reports to prepare daily, weekly, and at the end of the well. Some of the daily reports are the Daily Geological Report and the Lithology Log. These reports are updated with geological data, ongoing operations and important events.

There is also an End of Well Report or Final Well Report. This is produced and completed during the course of operations at the wellsite. They are delivered as updates to the operations and petroleum geologists during drilling operations. When drilling operations end these reports continue to be completed in the offices in town. They are updated until the end of all wellsite operations and only end when all the data from the entire well is obtained. Final completion of these reports will be carried out by the onshore geology team or the oilfield geologist, if asked to. There are several software packages available for us and these are usually provided by the oil company. Training on how to use them is one of the geological consultant’s responsibilities. For example, some of these software packages will allow you to produce the lithology logs, composite logs and other types of logs which may be required.

Safety and communication

HSE (Health, Safety and Environment) is a key aspect at the wellsite. The geologist is a leader and sets the example at all times encouraging others to work in safe conditions. Safety is one of the most important aspects of the entire operation.

Communication is also key to the success of the operations. Geologist communicate frequently with both the onshore office and the teams at the wellsite. This can reduce misunderstandings and mistakes. Make your teams feel comfortable enough to ask you anything in case of any doubts.

Talk to us for your upcoming wellsite geology consultancy requirement

We specialize in providing highly experience wellsite geologist to clients of all sizes in the oil and gas industry. Contact us at www.geodatadrilling.com Phone: +234 8037055441

                            

Quantitative Methods of Real-Time Pore Pressure and Wellbore Stability Detection

Our pore pressure computer program (Geopressure software)  for detection / analysis of abnormal pore pressures during mud logging (drilling) operation currently supports the following methods:

Overburden gradient calculation

Formation Pressure calculation by:

  • Equivalent Depth Method
  • Ratios Method
  • Eaton Method

Fracture Gradient Calculation by:

  • Eaton Method
  • Daines Method

Shear failure gradient (SFG) or borehole collapse pressure gradient calculation by:

  • Mohr-Coulomb failure criterion

Normal compaction trend line (NCTL) Methods

These are the most common methods in current use, and require a shale sensitive log, a porosity-sensitive log, and an overburden gradient curve corrected for water depth.

  • Compute overburden gradient and correct for water depth
  • Identify the shale points. Select porosity points at the shale points.
  • Determine the normally compacted interval in the porosity-sensitive log and fit it with a normal compaction trend line (NCTL)

Compare each point of the porosity log to the NCTL and compute abnormal formation pressure using an appropriate method.

In practice, fracture gradient also is calculated and plotted with the pressure/overburden curves.

Estimating Overburden Gradient

  • For evaluation of formation pressure
  • For calculation of fracture gradient
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The Operator can hand-calculate an approximate overburden curve from formation bulk densities for several representative depths over the interval to be drilled. Representative bulk densities may derive from wireline log data, from ‘shale’ (cuttings) densities, or seismic (interval velocity) data.

If we have an electric log for the formation density, we can use it to calculate the overburden:

Divide the log into intervals of depth with similar density.

If the proposed well is offshore, the first two density intervals will be:

  • Air gap between elevation of flowline and mean sea level, with ρb= density of air
  • Water depth between mean sea level and sea bottom (mud line), with ρb = density of sea water.

Then fill the following form to calculate the overburden at the end of each depth interval:

Interval bottom (m)Thickness (m)Bulk density (kg/)Overburden pressure in the interval (kg/cm2)Total overburden pressure (kg/cm2)Overburden gradient (kg/cm2/10m)
1501501.06S=150*1.06/10 = 15.915.9GS= 15.9*10/150 = 1.06
4002501.70S=250*1.70/10 = 42.515.9 + 42.5 = 58.4GS= 58.4*10/400 = 1.46
7003001.8054.0112.41.61
10703701.8969.9182.31.70
      
      
Overburden calculation by depth

Plotting the overburden gradient versus depth gives a curve

overburden gradient vs depth

The equation of the curve is:

S= a(Ln(Depth))2+ bLn(depth) + c

The coefficients a, b and c are regional characteristics

If no density log is available,

a ”hard formation” or a “soft formation” set of coefficients a-b-c is used.

The default coefficients, known as ‘soft’ and ‘hard’ values, were constructed from data for a number of wells in two separate areas:

‘Soft’ coefficients (relatively pure shales)

a = 0.01304

b = -0.17314

c = 1.4335

‘Hard’ coefficients (siliceous shales)

a = 0.01447

b = -0.1835

c = 1.4856

Clients most often prefer to use sonic log densities, or seismic transit times, to calculate formation bulk densities. This set of calculations is known as the AGIP method (Belotti, et al., 1978).

The transit times of sound waves passing through a given formation can be used to define the porosity of the rock, as in the equation below:

Where:

⧍tlog = Transit time reading from the sonic log (μsec/ft)

⧍tm = Rock matrix transit time (μsec/ft)

⧍tf = Transit time of the formation fluid (μsec/ft)

∅ = Porosity (decimal value from 0 to 1)

In practice, the approximate value of ⧍tf is 200 μsec/ft. The values for ⧍tm can be approximated as below

LithologyDensity (kg/l)⧍tma (μsec/ft)  
Dolomite2.8743.5
Limestone2.7143.5 to 47.6
Sandstone2.6547.6 to 55.6
Anhydrite2.9650
Salt2.0267
Shale3*47*
Values of rock matrix and density

Three formulae describe the relationship between porosity and transit times for different types of sedimentary formations.

Consolidated formation
Unconsolidated Sand
Unconsolidated Clay

After approximating the porosity, the bulk density is a function of:

Where:

ρb = Bulk density for the interval, g/cc

ρm = Rock matrix density, g/cc

ρf = Formation fluid density (usually 1.03), g/cc

Requirements for formation pressure/fracture gradient FP/Frac Calculations

For estimated formation pressure calculations, you will need:

  • DCN (‘normal’ trend of compaction increase)
  • Overburden gradient
  • Normal formation pressure (mud density equivalent) or equilibrium density at a specific depth.

For fracture gradient calculations, you will need:

  • Effective vertical stress
  • Tectonic stress (if Daines Method is used)
  • Formation pressure gradient.

Estimating formation pressure Pf

Our Pore Pressure Engineers can calculate formation fluid pressures from any of the following data:

  • Seismic interval velocities
  • Normalized drilling parameters (‘d’ Exponent)
  • Shale density
  • Wireline or MWD logs, including resistivity/conductivity, sonic and direct measurement of downhole pressure
  • RFT/DST (providing direct measurement of pressures)
  • Kick.

In practice, ‘d’ Exponent usually provides the primary pressure data, with the other processes used to verify or correlate the results.

Equivalent Depth Method

We can apply the Terzaghi equation for Overburden pressure, (S= σ+ Pf) . We know that we have the same compaction in A and B, so the stress S must be the same in both points! σA = σB

We can write:

σB =  SB – PfB and σA = SA – PfA

As σA = σB  we have:

SB –PfB = SA –PfA

Thus:

PfA = PfB + (SA -SB)

Overpressure applies the equivalent depth method via the following equation:

  DeqA=Sa –  Hb/Ha * (Sb – DeqB)

                           

Where:

DeqA = Equilibrium density at depth A

Sa= Overburden gradient at depth A

Ha= Depth A

DeqB= Equilibrium density at depth B

Sb= Overburden gradient at depth B

Hb= Depth B

Ratios Method

Application:’d’ Exponent, shale density, wireline/MWD logs(resistivty and sonic log)

Principle:

The difference between observed and ‘normal’ values of a parameter is proportional to the increase in pressure.

As an example, for ‘d’ Exponent, the calculation is:

where:

PF = Formation pressure gradient (mud density equivalent)

H= ‘Normal’ pressure gradient (mud density equivalent)

Eaton Method

Application: Interval velocities, ‘d’ Exponent, wireline/MWD logs(resistivty and sonic log)

The Eaton Method uses the principle that changes in the overburden gradient govern the ratio between the observed and ‘normal’ values of a given parameter.

Shale Resistivity:

Pf Gradient=OVBG – (OVBG – H)*(RshO / RshN)1.2

                                                                    

With H:  normal hydrostatic gradient

         RshN: Theorical shale resistivity on normal trend (B)

          RshO: Observed value of shale resistivity         (A)

         OVBG: Overburden gradient observed at observed depth

Sonic:

Pf Gradient=OVBG–  (OVBG – H)*( DtN /DtO)3.0

                                                                                 

With H:  normal hydrostatic gradient

         DtN : Theorical transit time on normal trend (B)

          DtO : Observed value of transit time     (A)

        OVBG: Overburden gradient observed at observed depth

Formation tests

DST or RFT tests give a direct evaluation of the Pf

Estimating Formation Pressure from Kick

In most kicks, the invading fluid does not enter the drill pipe. Thus, the Shut-in Drill Pipe Pressure (SIDPP) represents the amount by which formation pressure exceeds the hydrostatic pressure of the mud column.

Formation pressure equals the sum of mud hydrostatic pressure (inside the drill pipe) plus Shut-in Drill Pipe Pressure (SIDPP).

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Calculating Fracture Gradient

Liquid exerts pressure which is equal in all directions.

When solids are subjected to external force, it reacts by distributing internal load called stress -giving to stress ellipsoid.

If loading is perpendicular to eliminatory surface the stress is normal.

If loading is tangential to the eliminatory surface shear stress results.

σ = OVB – PF

Fracture occurs when the stress exceeds tensile strength of the rock.

The pressure in this case is fracture initiating pressure. – FP1

If the pressure is suddenly reduced the fracture closes.

To reopen the existing fracture less pressure required – FP2

A surge may open a fracture, afterwards the mud that was holding the hole may not hold any more. Stress is a pressure force per unit area and acts normal to the selected plane.

Stresses acting at any point can be resolved in to 3 mutually perpendicular stresses.

Stress concept

Maximum – σ1

Intermediate – σ2

Minimum – σ3

Relaxed area : Low topography σ1 is vertical and equal to the weight of the overlying rocks.

σ2 and σ3 are horizontal and normal to σ1 .    σ1 > σ2 = σ3

Extreme case:

Tectonically stressed area : Thrust faults etc. σ3 is vertical and equal to weight of overlying rocks.

σ1 and σ2 are horizontal.

When fracture occurs S3 is overcome.

Fracture Pressure  F = S3= σ3 + PF                 σ3 = K X σ1

F – PF = K X σ1

          K  X (OVB – PF)

( F – PF) / (OVB -PF) = K (Stress Ratio).

Thus K can be calculated after LOT. Values of K differ with depth.

Hence a plot of Depth X K is necessary to get proper value of fracture pressure.

Poisson’s Ratio

Eaton introduced Poisson’s ratio to account for variable overburden gradient.

The ratio of lateral unit strain to the longitudinal strain in a body that has been stressed longitudinally within its elastic limits.

Measure an ability of the rock to deform within its limits.

Fracture Pressure F = [ μ/ ( 1- μ)] σ1+ PF

Consider flat lying stratum of semi-infinite extent and weight of overlying strata is the only source of stress.

σ H = [ μ / ( 1- μ)] σ1         μ = 0.25

= [ 0.25  / ( 1 – 0.25) ] σ1

= 1 / 3 (σ1 )

Calculating Frac: Eaton Method

As described previously, Eaton uses Leak-off Test results to compute Poisson’s ratio; this ratio is then use to determine the corresponding fracture gradient:

The fracture gradient at a specific depth is then calculated as a function of:

Calculating Frac: Daines Method

The Daines Method refines the Eaton calculation by allowing for a variable Poisson coefficient based on rock type drilled, and by introducing a correction factor for tectonic stress.

The basic Daines calculation is:

Frac = σt + σ (μ)/(1 – μ) +    PF ; where σ is vertical effective stress

Data from the first Leak-Off test (in a compacted formation) allows back-calculation of the ratio of superimposed tectonic stress:

σt = Frac –  σ ( (μ  )/(1 – μ)+    PF )

To determine the tectonic stress at the Leak-off Test depth:

σt = Frac- σ’1  X  μ/1 – μ+    P

Where:

σt = tectonic stress

Frac = Formation fracture gradient (mud weight equivalent); determined from leak-off test pressure

σ’1 = maximum effective compressive stress

μ = Poisson’s Ratio, as determined from table on next slide

P = estimated formation pressure gradient (mud weight equivalent).

Daines suggested the following Poisson’s ratios for different lithologies (use the one that most closely corresponds to the rock type at the Leak-off Test depth:

Rock typeμRock Typeμ
Clay0.17 to 0.50Sandstone cont. 
Conglomerate0.20poorly sorted, shaly0.24
Dolomite0.21fossiliferous0.01
Limestone: Shale: 
micritic0.28calcareous0.14
sparitic0.31dolomitic0.28
porous0.20siliceous0.12
fossiliferous0.09silty0.17
argillaceous 0.170.17sandy0.12
Sandstone: kerogenaceous0.25
coarse0.05 to 0.10Siltstone0.08
medium0.06Slate0.13
fine0.03Tuff, glassy0.34

After Daines, Journal of Petroleum Technology, 1982

The maximum effective compressive stress is determined as follows:

σ’1 = S – P

Where:

S = overburden gradient

P = estimated formation pressure gradient (mud weight equivalent).

Indicators from Wellbore Instability

When the mud weight is inappropriate, wellbore instability events occur while drilling, which can help to diagnose the overpressure and to adjust mud weight in real-time drilling operations. Wellbore instability can be classified into two categories:

  • Shear failures
  • Tensile failures.

Shear failures

When the downhole mud weight is less than the shear failure gradient (SFG, or borehole collapse pressure gradient), the wellbore experiences shear failure.  Shear failure is mainly caused by the condition in which the applied mud weight is lower than the SFG. The indicators of shear failures while drilling include hole enlargement (borehole breakout), hole closure, tight hole (overpull), high toque, hole fill after trip, hole bridging, hole pack-off, and hole collapse.

 Some of these indicators may be caused by swelling shale when the water-based mud is used because of the chemical reaction between the mud and the shale formation. Therefore, it needs to identify the causes of the failures.

Here we use a vertical well as an example to illustrate the relationship of wellbore instability and pore pressure. Based on Mohr-Coulomb failure criterion, the minimum mud weight to avoid borehole shear failure can be obtained from the following equation:

Pm  = 1 – sin φ/2 (3σH – σ h – UCS) + pp sinφ   

Where 

Pm  = minimum mud pressure or collapse (shear failure) pressure

 φ = angle of friction of the rock

 UCS = rock uniaxial compressive strength

pp = Pore pressure

σH, σ h = the maximum and minimum horizontal stresses, respectively.

The horizontal stresses are most important parameters for analyzing wellbore stability, which can be obtained from either field measurements or calculations

The equation above shows that the shear failure is directly related to the pore pressure; a higher pore pressure needs a heavier mud weight to keep the wellbore from the shear failure. Therefore, wellbore instability can be used as an indicator of an overpressured formation.

Tensile failure

Tensile failure occurs when the mud pressure exceeds the capacity of the near-wellbore rock to bear tensile stress. If the downhole mud weight is higher than the fracture gradient, the formation will be fractured to create hydraulic fractures (or drilling-induced tensile fractures). Real-time indicators of drilling-induced tensile failures include hole ballooning, drilling mud losses, and lost circulation. Reducing mud weight, adding lost circulation materials (LCM), or applying wellbore strengthening technique are possible cures for the drilling-induced tensile failures.

Procedures of Real-Time Pore Pressure Detection

For real-time pore pressure detection and monitoring, the following steps can be performed:

  • Construct predrill petrophysical and pore pressure model and calibrate the predrill model to offset wells if they are available. The model includes methods of resistivity, sonic, Dxc, and so on. The model should include uncertainties and address drilling challenges and potential issues.
  • Apply the model to the real-time well. It particularly needs to have a calibrated NCT for each method.
  • Connect the model to real-time data (e.g., use Connect WITSML to connect LWD and MWD tools), so that the real-time data can be automatically loaded to the model. The model can then automatically calculate pore pressures based on the NCT using the real-time LWD and MWD data.
  • Compare the real-time calculated pore pressure to downhole mud weight (ESD, ECD); to determine if the mud weight is sufficient, particularly it needs to identify whether or not the mud weight is less than the pore pressure gradient. Only comparing the real-time calculated pore pressure gradient to the mud weight is not enough to conclude an underbalanced drilling status. Therefore, it also needs to combine to other real-time indicators of pore pressures.
  • Adjust the models (mainly NCTs) based on the following data if they are available: real-time pore pressure measurement, well influx, mud pit gains, kicks, mud gas data, mud losses, drilling parameters, and borehole instability events (e.g., cavings, torque, fills, and pack-offs).
  • Alert and inform the rig for action when the pore pressure is lower (underbalanced) or close to the downhole mud weight.
  • Liaise with technical expert group on all issues related to unplanned drilling operations, ECD, and pore pressures.
  • Make postwell knowledge capture and transfer within the appropriate organizations and systems.

                                               

The real-time monitoring should ensure that:

  • Pore pressure is continuously monitored and indicators of the abnormal pressures are identified;
  • Real-time pore pressure methods, estimates, and updates are discussed routinely with all involved monitoring parties to provide a consistent interpretation to the rig operations;
  • Abnormal pore pressure events are identified as soon as possible;
  • The abnormal events, including significant observations, changes, or updates in pore pressure estimates, if they are occurring or imminent, need to be communicated to the operations (e.g., operation geologist and drilling engineer) quickly;
  • The appropriate actions of operations (e.g. raising mud weight when the pore pressure gradient is lower than the downhole mud weight) are taken quickly.

Talk to us for your upcoming Pore-Pressure and Wellbore-Stability Prediction requirement

Geodata Evaluation & Drilling LTD. Pore pressure consultant use specialist software to provide pore pressure profiles for your wells which are calibrated to offset well behavior for determination of optimum mud weight window for successful drilling operation. Contact us at www.geodatadrilling.com Phone: +234 8037055441

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