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In this article we will discuss about:- 1. Methods of Drilling Wells 2. Drilling Programme 3. Well Revitalisation.
Methods of Drilling Wells:
Wells may be dug, bored, driven, jetted or drilled. Simple drilling methods like drive point, jetting, and hand boring can be adopted in favourable conditions for construction of shallow wells up to 25 cm diameter and 45 m deep. Selection of drilling equipment depends upon the hydrogeology of the formation, diameter and depth of the production well, availability of funds, maintenance and spares, production capacity, volume of work, operating crew and easy movement of the rig.
1. Boring:
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Boring of small diameter wells up to 15 m in clay, silt and sand may be done with hand-turned or power-operated augers, Fig. 11.1. Wells deeper than 5 m may require the use of a light tripod with a pulley at the top or a raised platform, so that the auger shaft can be lowered and removed from the hole without dissembling all shaft sections. The spiral auger is used to remove stones or boulders encountered. In caving formations boring is done by lowering the casing to the bottom of the hole.
When the casing pipe reaches the desired depth, well pipe with screen is lowered inside the casing pipe and the annular space between the well pipe and casing pipe is filled with gravel. The casing pipe is gradually pulled out with a winch and the pipe is rotated simultaneously. The well is developed by surging or by instant pumping or by using a compressor, till sand-free water is obtained.
Depending upon the diameter, depth of the well and water table, the well can either be pumped by suction lift or by installing a deep well pump. One or more wells can be coupled to get greater discharge. In the Cauvery delta in Tamil Nadu (south India) hand boring sets are extensively used to drill filter point wells up to 18 m and deep tube wells; coir rope and coir filters are used for filter points.
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Well points fitted to the lower end of tightly connected sections of pipe are driven either by hand method up to 10 m depth or operated by a drilling rig for depths of 15 m and above. Fig. 11.2 (a) shows the assembly for a purely hand driven method and Fig. 11.2 (b) shows heavier drive block assemblies commonly operated by a drilling rig or by hand with the aid of a tripod and tackle. Driven wells can be installed only in unconsolidated formations relatively free of cobbles and boulders.
Well points may be driven into the formation below the casing in drilled wells by the methods shown in Fig. 11.2 (a), (b). The hole is kept full of water while the screen is being set in heaving sand formations.
When the well point has been sunk to the required depth, the well has to be developed so as to yield sand-free water to its maximum capacity, farming a natural gravel pack around the well point. The development can be done by surging the well with a loose plunger or by pumping intermittently or by pouring water into the well time to time and backwashing the well. The yields are of the order of 90-250 lpm.
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Well points are usually driven in shallow coastal aquifers. Well point system with four or more drive points spaced 8 to 16 m apart (so that there is no interference) connected to a header pipe is used for irrigation purposes, Fig. 11.2.
Well point system can be used for dewatering the areas for construction works with a closer spacing of 1.5 to 6 m (so that their areas of influence overlap), Fig. 11.3.
Well point available in sizes of 3 to 8 cm diameter and 0.5 to 2 m lengths.
3. Cavity Wells:
If there is a relatively thin impervious formation or stiff clay layer available at a shallow depth underlain by a thick alluvial stratum, it is an excellent situation for a cavity well. A hole is drilled using the hand boring set and the casing pipe is lowered to rest firmly on the stiff clay layer, Fig. 11.4. A hole of small cross-sectional area is drilled into the sand formation and is developed into a big hollow cavity by pumping at a high rate or by operating a plunger giving a large yield.
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The force of a high velocity stream or jet of fluid loosens the subsurface materials and transports them upward and out of the hole. Drilling may be a combination of chopping and turning of the drill bits shown in Fig. 11.5. The spudding percussion action can be imported to the bit by means of a hoist or by quickly releasing the rope.
A combination of jetting and driving casing, washing out samples, can give information quickly. A tripod and pulley, winch and a small pump of approximately 680 lpm at a pressure of 3.5 to 5 kg/cm2 is used to force the drilling fluid (very often plain water) through a hose on to the drill pipe and bit. Small truck mounted jetting drills are manufactured. They can drill the hole, install the casing and screens, develop the well and install the pump.
The jetting method is suitable for unconsolidated formations for holes up to 15 cm diameter. Hard clays and boulders pose problems.
A ring with black diamond bits or steel teeth is attached to a drill rod which is rotated. Water is circulated to remove cuttings. As the cutter advances, a core rises inside the ring, which is broken off time to time by feeding chilled steel shots with the circulating water. In diamond core drilling, the diamond bit is used instead of the shot bit and calyxite.
Diamond bits are used for cutting extremely hard and abrasive formations, loosely cemented or consolidated. These bits have the diamonds set in a hardened alloy steel matrix which will retain the diamonds under critical conditions. For diamond core drilling a fairly high rotating speed is required to get good, smooth and unbroken core samples, Fig. 11.6.
The equipment and working of the hydraulic or straight rotary method is very similar to that of a calyx drill, except that they are designed to drill large diameter holes to a greater depth at a much faster rate. This method consists of a rotating drill bit, Fig. 11.7, for cutting the borehole with a continuously circulated drilling fluid, forced through the hollow drill pipe on to the bit by a mud pump, for removal of the cuttings, which flow to a settling pit where the cuttings settle out and then overflow to a storage pit from where the mud fluid is again recirculated.
The properties of the mud fluid are such as to provide adequate support for the wall of the hole and are usually viscous mixtures of water and natural or commercial clays such as bentonite. The driller can control the fluid characteristics by checking the density of the mud by a balance and its viscosity by a Marsh funnel shown in Fig. 11.8.
The balance has a cup at one end and a sliding weight on the other portion of its beam. The cup is filled with the drilling fluid and the weight is moved until it balances the filled cup. The density of the fluid is then read on the balance arm; a fluid density of about 9 N per litre is usually satisfactory. The viscosity of the fluid is determined by closing the lower end of the Marsh funnel by a finger and filling the funnel to the proper level (about 1,500 cc) and then removing the finger to allow the fluid to discharge from the funnel. The time is seconds to drain 1,000 cc of the fluid is defined as the Marsh-funnel viscosity, expressed in seconds, which is usually in the range of 30-40 seconds. The driller has to keep the density and viscosity within the above limits by periodically adding water and/or clay to the drilling fluid.
The drilling mud forms cake on the wall of the borehole which seals the pores to prevent loss of fluid into permeable formations and prevents caving. The mud cake has to be effectively removed during development for an efficient well. Hydraulic rotary method is suitable for drilling 15-45 cm diameter holes 100-150 m deep in soft rock and unconsolidated formation. For large diameters, small holes are sunk and reamed to large size. This method is not suitable for boulder formation and requires more water (450-900 1pm), repairs and maintenance. The driller must not only be able to operate the drill but also should have knowledge of mud technology. The accurate sampling and logging of the formations penetrated may be difficult.
The volume of the settling pit should be at least three times the volume of the hole being drilled. A settling pit of 2 m long × 1 m wide × 1 m deep and a storage pit 1 m square × 1 m deep may be suitable for the drilling of 10 cm wells <15 cm holes), 35 m deep. A system of baffles may also be used to provide extra travel time in the pit to improve the settling.
The basic parts of a conventional rotary drill are—a mast and hoist, a power operated revolving table that rotates the drill stem and bit, a mud pump and a power unit or engine. Drag bits of either the fish tail or three way design, Fig. 11.9 (a), (b) are best suited in an unconsolidated clay and sand formations and roller type bits, Fig. 11.9 (c), in coarse gravel and boulder formations.
The speed of rotation of bit in the bore hole is 30 to 60 rpm. The drilling mud is essentially bentonite clay and the density of the mud fluid is 1.02 to 1.14 g/cc. The upward velocity of flow in the bore hole is 0.7 to 1 m/s. In unconsolidated materials, the drilling rate is around 100 m/ day.
7. Air Rotary:
The drilling fluid may be any fluid lighter than water like air, foam, etc.
Air drilling is used in fractured rocks. Air rotary is especially suitable for limestones. Air foam is used to remove cuttings. With dry air upward velocities are in the range of 10 to 30 m/s in the annular space between the drill pipe and hole. In foam type fluids the ratio of air to liquid is of the order of 200 :1. Stiff foam is preferred for drilling in unconsolidated materials.
8. Reverse Circulation (Rotary) Drilling:
In this system, the direction of flow is reversed, i.e., from the annular space between the drill pipe and the wall of the hole through the bit into the hollow drill pipe upwards and discharged by the pump into the settling pit. The clear fluid returns to the borehole by gravity flow, Fig. 11.10. Relatively high velocity of the fluid in the drill pipe enables the cuttings to be carried to the surface without the deliberate use of clay or other additives to increase the viscosity. The boring is done without a casing and hydrostatic pressure is used to support the walls of the bore-hole during construction. Water level in the bore hole is about 2 m above natural level or at ground level.
The settling pit is about three times the volume of the material expected to be removed from the bore hole. Circulation of large amount of make-up water (2500-5000 lpm) by the use of a large capacity suction pump or by an air lift is required and minimum development is required to remove the light filter cake formed unlike the impervious mud cake in the direct rotary. Reverse rotary is best suited to drill large diameter wells 45—60 cm, 75—90 m deep in 8—12 hours, in soft unconsolidated formations. Drilling in coarse dry gravels poses the greatest difficulty.
The diameter of the hole is large in relation to the drill pipe in order that the velocity of the descending water in the annular space is low (30 cm/sec or less) and the bit and drill pipe are rotated at speeds varying from 10 to 40 rpm. The large diameters favour completion of the wells by artificial gravel packing. The suction end of the rig pump is connected through the swivel to the kelly and drill pipe and suction head of the pump limits the drill pipe lengths to 3 m. 15 cm drill pipes are commonly used so that cuttings of size up to 13 cm can be brought up through the pipe.
9. Cable-Tool Percussion Drilling:
The cable-tool percussion method consists of a tool string, comprising the drill bit, drill stem, drilling jars and rope socket, suspended by a cable from a walking beam (truck mounted) or operated from a diesel engine, which lifts and drops the tool string, Fig. 11.11. Water forms a slurry with the pulverised material which is bailed out at intervals. When drilling in dry formations, water must be added to the hole to form the slurry. Tools for drilling and bailing may be carried on separate lines or cables spooled on independent hoisting drums.
The casing closely follows the drilling bit, adding lengths of casing as the hole is deepened, to prevent caving. The top of the casing is fitted with a drive head which serves as an anvil. Periodic checks should be made with a plumb bob or carpenter’s level to ensure that a straight and vertical hole is being drilled. Usually drilling is started with a large diameter and the diameter is reduced telescopically after drilling certain depths.
This method is suitable for rock, medium hard, soft and boulder formation to drill 20— 45 cm and 75—180 m holes; drilling is relatively slow and casing has to be provided as the drilling progresses.
Some of the advantages of the percussion method are:
(i) Reasonably accurate sampling of the formation material can be readily achieved.
(ii) Rough checks on the water quality and yield from each water-bearing stratum can be made as drilling proceeds.
(iii) Minimum water is required for drilling, a point for consideration in arid regions.
(iv) Any encounter with a water-bearing formation is readily noticed as the water seeps into the hole.
(v) Water-bearing strata of poor quality may be grouted or sealed off.
(vi) Bits can be dressed in the field.
Fishing of lost or stuck parts may be difficult and utilises of many fishing tools.
10. Rotary-Cum-Hammer Drilling (Down-The-Hole Hammer):
In this method, a pneumatic hammer operated at the lower end of the drill pipe is used. It combines the percussion effect of cast tool drilling and the rotary action of rotary drilling. The bit is an alloy steel hammer with heavy tungsten carbide inserts which help in faster rate of penetration in hard rock areas. The diameter and depth of the hole is limited by the volume of air that can be exhausted through the hammer to remove the cuttings.
Compressed air must be supplied at pressure of 750 to 1350 kN/m2 (to remove the cuttings effectively) and free air supply of at least 9—10 m3/min for drilling 15 cm holes. The upward velocity in the space outside the drill pipe should be about 900 m/min. Proper rotation speed is from 15 to 50 rpm and 10 to 20 blows per second to bit. Reduced speed is best in harder and more abrasive rock. Down-the-hole hammer (DHD) drilling is the fastest drill suitable for hard rock areas to drill 15-20 cm holes, 120 m deep in 10-15 hours.
It uses compressed air and a foaming agent is injected into the air line under pressure by means of an injection pump to lubricate the bit and dust control. A flush pump is used for flushing the hole and bringing the cuttings to the surface. Air compressor, pump and prime mover are all mounted on one truck. Some of the DHD drilling rigs used in the hard rock areas are Ingersoll Rand, RMT, Halco 625 etc.
11. Logging and Sampling:
A well log denotes the characteristics of the materials penetrated by a bore hole along a vertical line from the ground surface. It may be a geophysical log like the electrical resistivity, SP, gamma ray, sonic, temperature and caliper logging or photograph logging by a deep well camera which gives a 360° picture of the bore hole. The direct methods of logging are drill cutting log or sample log, core log, drilling time log, drilling bit behaviour log, mud characteristics log etc.
Samples of subsurface material obtained from different depths during the process of drilling operations are, in most cases, the best source of geologic information. The principle object of test drilling is to obtain samples that reveal the character, depth and thickness of various strata. In consolidated rocks, the cores obtained serve as representative samples. The drilling time log is an accurate record of the time required to drill each meter depth; and the penetration rate represents the character of the material being penetrated, Fig. 8.13.
The drilling bit behaviour during drilling is indicative of the type of subsurface material encountered. In the case of rotary drilling, the drilling action may be smooth and even, indicating a homogeneous formation, or crunchy if gravel and boulders are present. In the cable tool method, the bit may stick in clay formations and tend to retard the rebounce, whereas the reverse is true if hard rock is encountered.
Mud loss, mud thinning, mud thickening, water loss, etc., should be carefully observed and recorded against depth. This gives useful information regarding the nature of the formations encountered. Sudden and persistent mud loss may be due to cavernous limestones, major joints or faults, dry sand, etc., whereas the mud thickening is due to clay formation. When artesian aquifers are encountered mud thinning may take place. It is also a good practice to have a complete record of mud pressure, mud density and viscosity, pressure on drilling bit, etc.
Samples of subsurface materials obtained during the process of drilling operations are, in most cases, the best source of geologic information. Samples arc collected at regular intervals from different depths during drilling and analysed, by the well site geologist and a driller’s log prepared. Better sampling results are obtained in cable tool drilling than in other methods.
The resultant log a more dependent on the well site geologist and driller than on the drilling method. Representative samples of all the formations penetrated have to be collected for geological study, mechanical analysis and preservation for correlation and verification purposes. The samples are kept packed in well labelled sample sacks of cloth. Good logs and samples and a valuable asset.
All the various types of direct and indirect logs available should be in the same vertical scale and put side by side to form a composite log Fig. 8.13. Then the characteristics of the various logs should be carefully studied and analysed critically to interpret the lithology and stratigraphy to obtain the final interpreted log.
Drilling Programme:
If a commission is given for execution of a ground water development project, the three essential documents to be prepared are:
(i) The construction schedule,
(ii) The equipment and materials schedule, and
(iii) The costs schedule.
The construction schedule is quite the most important as this shows exactly the method of approach to the problem and provides the basis for both the other schedules and the smooth field operation of all phases of the project. For economy the work pattern must resemble as closely as possible the assembly line in a large factory. Machinery must be maintained in operation, if possible, 24 hours a day performing the job for which it is designed.
Well Revitalisation:
Existing open dug wells may be revitalised by boring, deepening and rock blasting. Vertical bores at the bottom of the existing open wells are recommended when it is found from hydrogeological studies that the ground water table in the vicinity is at a fairly higher level and water is under pressure at the bottom of the well and rises up if a trial bore is drilled at the bottom of the well. Horizontal and inclined bores may also be drilled to intercept the water-bearing fractures to augment the yield into the open well.
This method of drilling 36-48 mm diameter and 15-30 m long holes in rock by using extension drill steel equipment and compressed air, Fig. 11.12 and 11.13, is called extension hole drilling. The extension rods are connected as drilling proceeds by means of coupling sleeves. A pusher leg is used for horizontal hole drilling. A rock drill can also be utilised to drill short holes (1.5 m) to enable charging and blasting of the hard rock for deepening the well.
Compressed air is the motive power for working the rock drill which is a percussion-cum-rotary machine (about 2,000 blows per minute and 200 rpm). The rock drill imparts the blows on either the extension equipment or the drill steel which has a tungsten carbide insert brazed on it and shatters the rock; the cuttings are flushed out either with compressed air or water and the hole is drilled. A proper cycle of operation has to be established in which drilling, blasting and mucking are carried out one after the other.
The well should, for economic reasons, be sunk with minimum diameter and only the bottom portion scooped out to provide storage for water, which is percolating during night. Revitalisation holes should be drilled to puncture water-filled fissures located far away from the well.