This article throws light upon the top seven methods used for construction of wells. The methods are: 1. Digging 2. Boring 3. Driving 4. Jetting 5. Cable Tool Drilling 6. Direct Rotary Drilling 7. Reverse Rotary Drilling.
Method # 1. Digging:
Wells in shallow and unconsolidated glacial and alluvial aquifers can be dug by hand using a pick and shovel. Loose material is brought to the surface in a container by means of rope and pulleys.
The depth of a dug well may vary from about 3 to 15 m depending upon the position of the water table. Dug wells usually have large diameter ranging from about 1 to 5 m. Dug wells penetrate about 4 to 6 m below the water table. The yield of the dug wells is generally small and is of the order of about 500 litres per minute.
Method # 2. Boring:
Hand-operated or power-driven earth augers are used for boring a well in shallow and unconsolidated aquifers. A simple auger has a cutting edge at the bottom of a cylindrical container (or bucket). The auger bores into the ground with rotary motion.
When the container is full of excavated material, it is raised and emptied. Hand-bored wells can be up to about 20 cm diameter and about 15 m deep. Power-driven augers can bore holes up to about one metre in diameter and 30 m in depth.
Method # 3. Driving:
In this method, a series of connected lengths of pipe is driven by repeated impacts into the ground to below the water table. Water enters the well through a screened cylindrical section which is protected during driving by a steel cone at the bottom. Driven wells can be installed only in unconsolidated formation which are relatively free of cobbles or boulders.
The diameters of driven wells are in the range of about 3 to 10 cm. Such wells can be constructed up to about 10 m, if hand driven, and up to about 15 m when heavy hammers of about 300 kg are used. The maximum yield of driven wells is usually around 200 litres per minute. The main advantage of a driven well is that it can be constructed in a short time, at minimum cost and by one man.
Method # 4. Jetting:
Jetting (or jet drilling) method uses a chisel-shaped bit attached to the lower end of a pipe string. Holes on each side of the bit serve as nozzles. Water jetting through these nozzles keeps the bit clean and helps loosen the material being drilled. The fluid circulation system is similar to that of direct rotary drilling method.
With water circulation maintained, the drill rods and the bit are lifted and dropped in a manner similar to cable tool drilling but with shorter strokes. Jet drilling is limited to drilling of about 10 cm diameter wells to depths of about 60 m although larger diameter wells have been drilled up to about 300 m by this method . Other drilling methods have displaced the jet drilling for deep and large diameter wells.
Method # 5. Cable Tool Drilling:
It is the earliest drilling method developed by the Chinese some 4000 years ago. A cable tool drilling equipment mainly consists of a drill bit, drill stem, drilling jars, swivel socket and cable (Fig. 4.6). The cable tool drill bit is very heavy (about 1500 kg) and crushes all types of earth materials.
The drill stem provides additional weight to the bit and its length helps in maintaining a straight vertical hole while drilling in hard rock. The length of the drill stem varies from about 2 to 10 m and its diameter varies from 5 to 15 cm. It weighs about 50 to 1500 kg. Drilling jars consist of a pair of linked steel bars and help in loosening the tools when these stick in the hole.
Under normal tension of the drilling line, the jars are fully extended. When tools get stuck, the drilling line is slackened and then lifted upward. This causes an upward blow to the tools which, as a result, are released. Swivel socket (or rope socket) connects the string of tools to the cable.
The wire cable (about 25 mm in diameter), which carries and rotates the drilling tool on each upstroke, is called the drill line. The cable tool drilling rig mainly consists of a mast, a multiline hoist, a walking beam, and an engine. Drill cuttings are removed from the well by means of bailers having capacities of about 10 to 350 litres.
A bailer is simply a pipe with a valve at the bottom and a ring at the top for attachment to the bailer line. The valve allows the cuttings to enter the bailer but prevents them from escaping. Another type of bailer is called the sand pump or suction bailer which is fitted with a plunger. An upward pull on the plunger produces a vacuum which opens the valve and sucks sand or slurried cuttings into the tubing.
While drilling through consolidated formations, most boreholes are drilled as “open hole”, i.e., no casing is used during drilling operation. In such conditions, the cable tool bit is essentially a crusher. On the other hand, there is a danger of caving in while drilling through unconsolidated formations. For this reason, casing pipe must follow the drill bit closely to keep the borehole open in unconsolidated formations.
For the driving operation of the casing pipe, a drive head is fitted to the top of the casing. The drive head serves as an anvil and protects the top of the casing. Similarly, a drive shoe made of hardened and tempered steel is attached to the lower end of the casing pipe.
The shoe prevents the damage to the bottom end of the casing pipe when it is being driven. Casing is driven down by means of drive clamps — constructed of heavy steel forgings made in halves — fastened to the top of the drill stem. Drive clamps act as the hammer face, and the up and down motion of tools provide the weight for striking the top of the casing pipe and, thus, driving it into the ground.
The procedure for drilling through unconsolidated formation consists of repeated driving, drilling and bailing operations. The casing pipe is initially driven for about 1 to 3 m in the ground. The material within the casing pipe is then mixed with water by the drill bit to form slurry.
The slurry is bailed out and the casing pipe is driven again. Sometimes, the hole is drilled 1 to 2 m below the casing pipe; the casing is then driven down to the undisturbed material and drilling is resumed.
The drilling tools make 40 to 60 strokes of about 40 to 100 cm length every minute. The drill line is rotated during drilling so that the borehole is round. The slurry formed by mixing of cuttings with added water (if not encountered in the ground) reduces the friction on the. cutting bit and helps in bailing operations.
If the friction on the outside of the casing pipe increases so much that the casing pipe cannot be driven any more or further driving might damage the pipe, a string of smaller casing is inserted inside the first one. Drilling is, thus, continued. Sometimes, two or three such reductions maybe required to reach the desired aquifer. The diameter of the well is reduced. If such situation is anticipated, casing in upper part should be of larger diameter.
The drilling process through consolidated formation, not requiring casing, would consist of repeated drilling and bailing operations only. The cable tool method has survived mainly because of its suitability of a wide variety of geological conditions.
It offers the following advantages:
(i) Cable tool drilling rigs are relatively cheaper.
(ii) The rigs are simpler and require little sophisticated maintenance.
(iii) The machines have low power requirements.
(iv) Borehole is stable during the entire drilling operate on.
(v) Recovery of reliable samples is possible at every depth.
(vi) Wells can be drilled in water-scarce areas.
(vii) Because of their size, the machines can be operated in more rugged, inaccessible terrains or in other areas where limited space is available.
(viii) Wells can be drilled in formations where water is likely to be lost.
Slow drilling rate, higher cost of casing pipe and difficulty in pulling back long strings of casing pipe are the disadvantages of cable tool drilling.
Method # 6. Direct Rotary Drilling:
The direct rotary drilling is the fastest method of drilling deep well of diameter up to 45 cm (or more with the use of reamers) through unconsolidated formations. The drilling bit is attached to a heavy drill pipe which is screwed to the end of the Kelly which is a drill pipe of square section (Fig. 4.7). The drill collar or stabiliser helps in maintaining straight hole in soft formations through its large wall contact.
The drill pipe is turned by a rotating table which fits closely around Kelly and allows the drill rod to slide downward as the hole deepens. The drilling rig consists of a mast, a rotating table, a pump, a hoist and an engine. The borehole is drilled by rotating a hollow bit attached to the lower end of a string of a drill pipe.
Cuttings are removed continuously by pumping drilling fluid (mixture of clay and water with some additives to make it viscous) down through the drill pipe and through the orifices in the bit. The drilling fluid, then, flows upward through the annular space between the drill pipe and the borehole, carrying the cuttings in suspension to the surface settling pits where the cuttings settle down in the pits.
The clear drilling fluid is pumped back into the borehole. The settling pits can either be portable or excavated for temporary use during drilling and then backfilled after completion of the well.
Usually, no casing is required during drilling because the drilling mud forms a clay lining on the borehole walls which prevents caving in of the formation materials. After drilling, the casing pipe with perforated sections opposite aquifers is lowered into the borehole.
Drilling rotary method has become the most common method due to its following advantages:
(i) Drilling rates are relatively high.
(ii) Minimum casing is required during drilling.
(iii) Rig mobilisation and demobilization are fast.
(iv) Well screens can be set easily as part of the casing installation.
Some major disadvantages of direct rotary method are as follows:
(i) Drilling rigs are expensive.
(ii) Maintenance of drilling rigs is costly.
(iii) Mobility of rigs is restricted depending on the slope and wetness of the land surface.
(iv) Collection of accurate samples requires special procedures.
(v) Drilling fluid may cause plugging of some aquifer formations.
Method # 7. Reverse Rotary Drilling:
Direct rotary drilling method is capable of drilling boreholes with maximum diameter of about 60 cm. High capacity wells, particularly those with filter pack, need to be much larger in size. Besides, the drilling rate becomes smaller with increase in borehole diameter in the case of direct rotary drilling.
To overcome these limitations of direct rotary drilling, the reverse rotary drilling technique has been developed. This technique is capable of drilling boreholes of about 1.2 m diameter in unconsolidated formation. Recently, reverse rotary method has been used in soft consolidated rocks, such as sandstone and even in hard rocks using both water and air as the drilling fluid.
In reverse rotary drilling, the flow of the drilling fluid is reversed in comparison to that of direct rotary drilling. The reverse rotary drilling rig is similar to that of direct rotary drilling rig except that it requires larger-capacity centrifugal pumps, larger-diameter drill pipe and other components also of relatively larger size.
The drilling fluid moves down the annular space between the borehole wall and the drill pipe, picks up the cuttings before entering the drill pipe through the ports of the drill bit. The drilling fluid along with its cuttings move upward inside the drill pipe which has been connected to the suction end of a centrifugal pump through kelly and swivel.
The mixture is brought to a settling pit where the cuttings settle at the bottom and the drilling fluid (i.e., muddy water) moves down the borehole again. The drilling fluid is usually water mixed with fine-grained soil only.
The hydrostatic pressure and the velocity head of the drilling fluid moving down the borehole supports the borehole wall. Therefore, to prevent caving of the hole, the fluid level must always be up to the ground surface even when drilling is suspended temporarily.
Advantages of the reverse rotary drilling method are as follows:
(i) The formation near the borehole is relatively undisturbed compared to other methods.
(ii) Large-diameter holes can be drilled rapidly and economically.
(iii) No casing is required during the drilling operation.
(iv) Well screens can be set easily while installing the casing.
(v) The boreholes can be drilled through most geologic formations excepting igneous and metamorphic rocks.
(vi) Because of low velocity of the drilling fluid, there is little possibility of its entering the formation.
Disadvantages of the reverse rotary drilling method are as follows:
(i) Large quantity of water is needed.
(ii) Reverse rotary drilling rig is costlier because of larger size of equipment.
(iii) Large mud pits are required.
(iv) Some drill sites may be inaccessible because of the larger size of the rig.