The following points highlight the top three effective methods adopted for drilling tube-wells. The methods are: 1. Percussion Boring Method 2. Machine Percussion or Cable Tool Method 3. Rotary Boring Method.
1. Percussion Boring Method:
This method is suitable for soft and fissured rock formations. In purely soil formations this method gives very high working rate. This method consists in breaking and pulverizing the subsoil material by series of repeated blows with a cutter made of hard metal.
The pulverized material mixes with water and it is then removed. This method is also called cable tool method. Boring can be done manually or mechanically. Bores, as big as 30 cm in diameter, and over 200 to 300 m deep, can be very conveniently drilled by this method.
The blows are given by means of a plunger. The plunger consists of a hollow metal tube. A cutter is fixed to the bottom end of the tube by rivetting or welding. A ball or a flap valve made of steel is also provided at the bottom of the plunger. The valve is such that it allows slurry of pulverized soil material in water to enter the plunger. Once the slurry enters the plunger the valve closes and the slurry is prevented from coming out. Thus the valve has only one way action.
The plunger can be lowered and raised in two ways:
(i) By rope system, and
(ii) By rod system.
In rope system the upper end of the plunger is attached to a rope. The rope runs over a pulley. The plunger is lifted up and released suddenly to give a blow. Rod system is similar in principle to the rope system. Only difference is rod replaces the rope and consequently the rod operating machinery is also changed.
The disadvantage of rod system is, time is wasted in increasing or decreasing the rod length. The rod length can be increased or decreased by screwing or unscrewing small rod lengths. The actual manual boring procedure by percussion method.
A pit is dug at the site where a tube well is to be sunk. A casing pipe with a cutter shoe is inserted in the pit. A platform is clamped to the casing pipe. The platform is loaded by means of locally available material filled in jute bags. Over the casing pipe a tripod is erected and a pulley is fixed to it centrally. Rope runs over the pulley. One end of the rope is attached to the plunger. The diameter of the plunger is slightly less than the casing pipe (say by 6 cm). The arrangement is clear from Fig. 9.10.
Some water is poured in the hole before actual boring is started. The boring work is started by giving repeated blows by the plunger. When the plunger is filled with slurry it is taken out and the slurry is removed by turning the plunger upside down. The plunger is again lowered in position and the process is repeated. Thus the casing pipe is sunk slowly into the ground. When full length of the casing pipe is sunk additional pipe is attached at the top of the first pipe.
2. Machine Percussion or Cable Tool Method:
The machine used for tubewell drilling is called a drilling rig. It is used to increase the working rate. It is a truck mounted assembly and consists of a mast, a multiline hoist, a walking beam and an engine (Fig. 9.11). Also a string of tools including drill bit, drill stem, connecting links etc. are available to accomplish drilling.
Total weight of tool varies from 100 kg to 2000 kg to suit type of rock formation met with. The operation in principle is similar to manual method. The cable tool bit acts as a crusher for drilling in consolidated rocks. The drilling tool gives 40 to 60 strokes in one minute. The drill line is rotated to make a round hole.
As in the manual method water is added to the base to form the slurry. After the bore is drilled by 1.25 to 1.5 m the drilling tool is removed and slurry is taken out by means of sand pump or bailer. The bailer has one way valve which permits slurry to enter the bailer but does not allow escaping. After the bailer is filled it is lifted and emptied outside.
In unconsolidated formations well casing is inserted and simultaneously sunk to full depth to avoid caving in of the material. In soft formations drilling rate could be as high as 20 to 30 m per day. In hard rock formations the drilling rate could be as low as 2 to 3 m per day.
Wash-Boring or Water-Jet Boring:
This method is best suited for underground formations made of gravel, sand and clayey deposits. The casing pipe is sunk under the pressure of a water jet. The pipe with a nozzle at the end is lowered into the casing pipe. (Fig. 9.12) Water enters the pipe under pressure.
The pressure of water is still increased as it comes out of the nozzle. Due to the force of the jet of water the soil is loosened. The soil and water forms the slurry. It comes out of the bore hole through the space between the casing pipe and the jet pipe.
The casing pipe is slowly rotated. It then sinks in the loosened soil under the load. The load is placed at the top on the loading platform. If the soil is hard enough a chisel may be fixed to the jet pipe below the nozzle to accelerate the process. Then the jet pipe and the casing pipe both are rotated.
3. Rotary Boring Method:
It is also called hydraulic rotary boring method. This method can be successfully used for rock as well as unconsolidated formation. In this method drilling is done by means of rotating bits attached to lower end of hollow steel pipe. The pipe is attached to a mechanism at the surface which is rotated by power. The powdered rock and cuttings are removed by continuous circulation of a drilling fluid.
The hydraulic rotary drilling rig consists of a derrick or mast, a rotating table, a pump for injecting drilling mud, a hoist, and an engine. The drilling bit attached to the lower end of the drill pipe is provided with short nozzles to direct the jets of the drilling fluid down the faces of the blades of drilling bit. Fig. 9.13 shows the schematic diagram of the rotary boring method.
The rotating table turns the drill and in turn the drilling bit cuts through the formation. As hole deepens new lengths of drill pipe are attached. The drilling fluid or bentonite (clay) slurry is pumped down through the drill pipe. It comes out through the nozzles of the drilling bit at bottom. The mud then rises to the surface through the annular space between the drill pipe and the bore hole.
In the process the mud removes the rock fragments and cuttings with it. Once the drilling fluid comes to the surface it is taken in settling pit. In the pit the rock fragments settle down. Cleaned fluid can be used for recirculation. Sometimes there is a possibility that the drilling mud may seal low pressure water bearing formations. It is therefore necessary to control the quantity of mud and clay in the water.
The drilling fluid performs following functions:
(i) It supports the walls of the bore and prevents caving in of the sides.
(ii) It removes cuttings from the bore hole.
(iii) It checks the inflow of ground water into the hole when drilling is in progress.
(iv) It cools and lubricates the drill bit.
(v) It softens the underground formation and speeds up drilling.
The rate of drilling by this method for consolidated rock formations may vary from 10 to 15 m per day. In unconsolidated formations it may reach 100 to 150 m per day.
Main advantages of this method are the following:
(a) The drilling can go on continuously.
(b) The drilling rates are quite high.
(c) Casing pipe is not required. The mud forms a clay lining on the bore wall and it prevents caving.
(d) If the hole becomes unsuccessful it can be abandoned right away because removal of casing pipe etc. is not involved.
Reverse Rotary Boring Method:
This method is called reverse rotary method because the flow of drilling fluid is reversed as compared to the (hydraulic) rotary boring method. The drilling rig used for reverse rotary method is similar to that used for rotary boring method.
These are, however, two variations. The first is that the drill pipe of larger diameter (say 15 cm) is used. Secondly, large capacity special pumps are used to suck and discharge large size gravel through the drilling pipe. Schematic diagram of this system is shown in Fig. 9.14.
As a drilling fluid, only water is generally used because the water picks up fine particles from the sub-soil formation. The water moves into the bore hole through the annular space between the drill pipe and the walls of the bore. The water picks up the cuttings and the mixture is sucked upward by the pump through the drill pipe.
In the process big rock cuttings are also lifted up and removed. At surface the mixture is discharge into settling pit. By this method it is possible to drill wells of big diameter (upto 150 cm). This is the cheapest method of drilling large diameter wells in soft unconsolidated formations made of sand, silt or soft clay.