How to Drill Tube Wells: Top 3 Methods | Geography

The important methods that are available for drilling tubewells are as follows:

1. Percussion Drilling (Also Known as Cable Tool Method), further classified as:

(i) Hand boring by rope or rod, and

(ii) Mechanical boring with power rigs.

2. Rotary Drilling, further classified as:

(i) Direct circulation hydraulic rotary,

(ii) Reverse circulation hydraulic rotary, and

(iii) Rotary-air percussion drilling.

3. Other Methods, consisting of:

(i) Auger method,

(ii) Core drilling, and

(iii) Water jet method.

1. Percussion Drilling:

(i) Hand Boring by Rope:

In the percussion method, the borehole is drilled by making vertical to and fro motion of suitable tools for dislodging the soil, which is subsequently removed. Simultaneously, a casing pipe is lowered in order to permit the drilling and to prevent the caving of the material into the bore.

The percussion method of boring can be done both by manual labour and by mechanical equipment. Hand boring with manual labour is the simplest and is extensively used. In this method a pit is dug at the site measuring about 2.0 to 2.5 m in diameter and of the same depth. The casing pipe (also known as boring tube) is lowered into the centre of the pit.

It is advantageous to fix a cutter shoe at the bottom of the casing pipe. The cutter shoe is of slightly larger diameter than the casing pipes itself so that a clear passage is provided for the casing pipes as they are sunk. After lowering the first casing pipe in the pit, it is clamped in position with wooden blocks. The casing pipe is then partly filled with water.

The actual boring is now started by operating the bailer. This is suspended by a wire rope passing over a pulley fixed to-a tripod stand. The wire rope passing over the pulley is tied to a winch so that wire rope can be wound or unwound as required for raising and lowering the bailer.

The tripod is so fixed such that the bailer comes centrally over the casing pipe. The bailer is a steel pipe of about 6 mm thickness, varying in length and diameter according to the size of the well, bored. It is about 5 cm less in diameter than the casing pipe and its length varies from 2 m depending upon the size of the bore.

A cutting edge is provided at the lower end and a hook on the upper end. A flap is fitted inside the bailer just above the cutting sleeve. When this bailer is operated vertically up and down in the casing pipe, a slight circular motion is also automatically imparted to it by the torsion in the wire rope.

During the downward stroke, the flap valve is forced to open and the loose material pounded at the bottom of the bore enters the bailer. As the upward stroke begins the flap valve in the bailer is closed due to the weight of the material inside and thus the loose material inside is retained.

At the end of each downward stroke, the cutting shoe cuts some more of the material at the bottom of the bore. This material is pulled up and enters the bailer in the subsequent strokes. After about 30 to 40 strokes the bailer is lifted out of the casing pipe by winding the rope on the winch and the loose material retained inside the bailer is emptied out.

The materials brought up by the bailer are the samples of the strata. The casing pipe is clamped with wooden sleepers at a convenient point. This platform is loaded with sufficient weight (sand bags can be conveniently used for the purpose) so that the casing pipe is forced down automatically due to heavy weight as the bailer excavates the material at the bottom.

The boring is continued and one casing pipe after another is screwed on and lowered till the required depth of the bore is reached. Water should be poured into the casing pipe from the top until water is struck in the borehole itself.

Sometimes, instead of wire rope, rods are used to connect the drilling tools. The advantage is that if hard strata is encountered the rod can be used for using a rotary drill instead of the percussion drilling. For alluvial strata the rope method is more suitable than the rod method as in the rod method much time is wasted for in screwing the rods each time the bailer is taken out for emptying its contents.

(ii) Mechanical Boring with Power Rigs:

The mechanical equipment for percussion drilling consists of a portable rig mounted on a truck chassis or trailer. A mast of at least 10 m high (foldable for easy transit), a two line hoist, one line for operating the drilling tools and other for extracting the bailer, a spudder for raising and dropping the tools and a diesel engine for powering these operations are the important components of this equipment.

The drilling tools consist of the drilling bit, drilling stem, drilling jar and a rope socket with mandrel. The drilling bit has a chisel shaped straight edge and is the principal part responsible for breaking the strata. As and when the straight edge becomes blunt it has to be dressed at the site.

The mandrel holds the cable rigidly in the rope socket. The rope socket holds the mandrel and is joined with the drilling jar. The drilling jar gives a jarring effect to the wire rope to avoid its getting stuck up in sticky formations. The drilling stem is used to connect the drilling jar on one end and the drilling bit on the other and it increases the weight of the drilling bit.

The process of drilling is the same as that in the hand boring method except that with the use of power, the speed of drilling is very much increased and large sized and deep boring in all types of strata becomes possible. The progress of percussion drilling depends upon the particular strata encountered as well as on the size and depth of the bore. In sandy or clayey strata progress can be as much as 25 to 30 m whereas in hard rock it may be just 1 or 2 m.

It is likely that during the drilling operation the wire rope may break dropping the drilling tools inside the bore or the tools may get jammed in the bore. In order to extract the tools in such cases, special tools known as fishing tools are to be used.

2. Rotary Drilling:

The rotary drilling is invariably by mechanical power and it is a powerful and quick method of drilling suiting almost all types of formations. Its rate of drilling in alluvial formations is very fast. It is suited for trial boring purposes.

The practical operation consists of rapidly rotating a column of pipes at the lower end of which a cutting bit (or also known as drill bit) is attached. The bit cuts a hole slightly larger than the diameter of the drill pipe so that the cut material can be removed through the annular space of the bore and the drill pipes.

As the drill bit rotates simultaneously a downward thrust is also applied on the bit through the drill pipe so that the cutting points penetrate through the strata. The drill bits have hollow shanks and have one or more centrally located openings for the flow of the drilling mud.

The rotary drill has a derrick just as in the case of percussion drill, but is usually of a stronger build as it has to withstand the turning effect of the rotating drill. The important parts of the machinery are a revolving table (also sometimes known as turn table), hydraulic pumps, line shaft and an engine or an electric motor.

The drill pipe (also known as drill rod), carries the drill bit at the bottom and is screwed at the top to a pipe of square section known as Kelly. The revolving table fits closely around the Kelly and rotates it along with the drill rod. Simultaneously, the Kelly can slide down along with the drill rod as the bore deepens. The turn table is revolved by a bevel gear arrangements connected with the line shaft and is controlled by a clutch.

In the rotary drilling method no casing pipes are used. Instead the material is cut, made into small particles and is removed by maintaining a constant circulation of drilling fluid under hydraulic pressure. Two principle methods are followed for accomplishing the removal of the cut material.

These are:

(i) Direct circulation hydraulic rotary method

(ii) Reverse circulation hydraulic rotary method and

(iii) Rotary Air Percussion Drilling.

(i) Direct Circulation Hydraulic Rotary:

In this method, mud laden water is forced down under pressure through the drilling pipe and the bore hole and while it comes up it brings along with it the drill cuttings.

The mud mixing is a special technique and the right consistency of the mud is to be maintained depending upon the local conditions. Local mud or special chemical like ‘Aquagel’ (a collodial clay) or bentonite are used. In addition to bringing up the drill cuttings this mud plasters the inside walls of the bore hole in order to prevent its collapse and also cools the drilling bit during operation.

For preparing the mud, a slush pit is made near the derrick and two pumps are usually installed for pumping the mud-one actually used for pumping and the other acting as a standby in case of any breakdown of the original pump. The standby pump is necessary as in case of rotary drilling the drilling has to be continuous till the completion of the bore.

Experience is necessary for operating this equipment as several precautions are involved. If a clay stratum is encountered the water used for drilling should be clear and no mud added. In certain formations, the casing pipes may have to be installed very quickly to prevent collapse of the walls. It is also essential that the pump supplying the mud and water should be constantly running and its stoppage during drilling may result in settlement of the cut materials at the bottom of the bore.

(ii) Reverse Circulation Hydraulic Rotary:

For drilling in heavy gravel or for drilling large sized bores the direct rotary method is not suitable as it is difficult to circulate the mud laden water. In such a case the reverse rotary can be adopted. In this method the water is pumped down through the space between the drill pipe and the bore hole.

The water is drawn out through the drill pipe by means of a centrifugal pump. The water brings along with it the cut material. The reverse rotary method requires more water than the direct rotary method.

(iii) Rotary Air Percussion Drilling:

In conventional rotary drilling energy is applied in two ways viz.:

(1) Rotation of the bit, and

(2) Static force exerted on the bit by means of weight on the bit applied by drill string.

Rotary-air percussion method of drilling adds a third source of supply of energy to the bit, in the form of percussion impact.

Addition of this third source considerably increases the rate of penetration. The common method of applying this force is by an air actuated single piston hammer (called the down-the-hole hammer) immediately above the bit at the end of the drill pipe. Percussion is accomplished by forcing compressed air through the hammer.

3. Auger, Core Drilling and Water Jet Methods:

In addition to the two principle methods for drilling tubewells there are some other methods which have limited applications and are to be used in certain specific conditions.

These are:

(i) Core Drilling:

The operation of the core drill is similar to that of the hydraulic rotary drill. The core drilling is suitable where hard rock strata are encountered. It is also used where uncontaminated samples of underground formations are required. The cutting tool is known as the crown which is a short piece of steel tube fitted at the end with black diamonds. Special steel shots are also used as cutting medium.

The crown is rotated with the help of the drilling pipes and it cuts through the strata causing a core to rise up in the hollow tube. These steel shots are also known as calyxite and when they are used the method is referred to as calyx core drilling, After a suitable length of the core is cut the tool is lifted and the core is removed. The operation is not continuous and hence time consuming.

During the drilling operation water is continuously pumped to the bore. This water serves the dual purpose of cooling the cutting tool and at the same time carrying the loose debris while returning to the surface through the annular space between the drift pipes and the bore hole.

(ii) Auger Method:

This method is suitable for drilling wells of small diameter generally for domestic use. The depth of boring is usually small (20 to 30 m) and the formations encountered should be soft. Augers are special devices used to loosen the soil.

The auger consists of a long steel shank. At the bottom a cutting edge is provided and at the top there will be a handle with which the auger can be turned. Drill rods of lengths 2 to 6 m are used to suspend the auger.

For wells of 7.5 cm to 10 cm dia hand operated augers are employed. In these wells, if the stratum is very coarse the end of the suction pipe is left open and a foot-valve is provided. If the stratum is fine, a strainer of about 2 to 3 m length is used at the bottom.

The strainer is usually a wire gauge put around a perforated pipe with a drive point. Hand pumps are fixed in these type of wells to lift water. Electrical motors with suitable gear drive are also used in order to ensure continuous supply of water.

(iii) Water Jetting Method:

In the water jetting method, drilling is done by means of a jet of water pumped into the jetting bit through the drill pipe. The jet due to its pressure loosens the material and the loosened material is brought up through the annular space between the drill pipe and bore.

This method requires a large amount of water and a suitable pump having capacity of not less than 65,000 litres per hour and capable of developing large heads. The water, however, can be re-circulated from a settling tank after the material brought up from the bore settles down.

This method of boring is very efficient and convenient for hard and tenacious soluble clays in which the rate of drilling by percussion method is slow. A combination of the percussion and water jetting method can give an efficient service as each can be used when the particular strata for which it is suited is encountered.

Choice of Boring Method:

The hand operated percussion rigs have the advantage of simplicity in construction and operation. The equipment is light and can easily be transported from place to place. But they are suitable for comparatively small size of bore (up to 15 cm) and their rate of drilling is also slow.

For drilling large size bores and for faster rates of drilling, power driven drilling rigs are to be adopted. Among the percussion and rotary rigs, rotary drilling is much faster. It is suitable for almost all type of formations and especially clay formations where percussion drilling may be slow.

However, rotary method may not be successful in alluvial formations with large boulders and cobbles as removal of big sized cobbles in circulating mud is difficult. In rotary drilling large quantities of water is needed. The rotary drilling is ideally suited for putting test bores for groundwater investigations.

In power operated equipment rotary cum percussion drilling rigs are available for drilling in all sorts of formations.

For tubewells installed in an open pit or well it will be difficult to use the rotary drilling rigs as in such cases mud circulation cannot be conveniently done. Percussion drilling rigs can be easily used in such cases.

In certain areas it may be required to prevent subsoil water from some strata to enter the tubewell, if the water is of unsuitable quality. In such areas if a rotary drill is used it will not be possible to ascertain the strata containing the unwanted water as the drill cuttings are mixed with water pumped from the surface. In such situations percussion drilling is preferable.

The auger method has a limited use for boring up to shallow depths and for wells of small diameter generally for domestic use. Core drilling is useful when the soil samples are to be extracted in the form of a core for investigation purposes or when hard rock strata are encountered.

Observations during Boring:

Certain observations during the drilling operation are important in order to decide upon the type of the tubewell and for proper location of the strainers and plain pipes with respect to the water bearing strata. A record of the strata samples (known as well log) indicating thickness of formations and their chief characteristics such as texture, colour and hardness is important. When water is struck, it should be examined in order to determine its suitability for irrigation.

In case of percussion drilling, the excavated material coming to the surface with the bailer indicates the type of stratum encountered. The level of water in the bore is an indication of the presence of the water bearing stratum. When the bore is passing through water bearing stratum, water rushes into the bore and depending upon its pressure the water level in the bore rises.

In a non-water bearing stratum, the water level inside the bore is very low as there is no water coming into it. A bailer test is normally done to determine the capacity of a water bearing stratum. Water is bailed out in quick succession for a couple of hours and if the water level inside the bore remains steady the stratum may be considered as satisfactory for location of strainer or for development of the cavity.

In case of rotary drilling, a noticeable increase in the volume of the returning circulation fluid indicates a water bearing formation. When drilling is done by the hydraulic rotary methods, there is a possibility of the samples being diluted.

Geophysical measurements are therefore adopted in bore holes to obtain a continuous log from surface to the bottom. These techniques are known as well logging methods and they provide information about the nature of formations occurring in the bore hole.

Sieve Analysis of Strata:

The particle size distribution of the aquifer material is useful in selecting the correct width of openings of the strainers in case of strainer type wells and to select the right gravel pack in case of gravel packed wells. The particle size distribution is obtained by what is known as sieve analysis of the material.

About 500 gm of aquifer material is allowed to pass through a standard set of sieves and the weight of the fractions retained on each sieve is recorded. These weights are then expressed as the percentages of the total weight of the sample.

A graph is plotted between the cumulative per cent of the material retained on a given sieve and all other sieves above it and the size of the given sieve. A smooth curve is drawn through the points plotted.

This is known as the ‘sieve analysis curve’ or ‘grain size distribution curve’ and it indicates at a glance how much of the material is smaller or larger than a given particle size. Table 8.1 and Fig. 8.6 indicate how such a curve is prepared.

From the curve, (Fig. 8.6) it can be inferred that 90 per cent of the material consists of sand grains larger than 0.4 mm or 10 per cent is smaller than this size.

The following terms are used for defining the size characteristics of aquifer materials:

1. Effective Diameter:

It is defined as the diameter of the grains (d₁₀) such that 10 per cent of the grains are finer and 90 per cent are coarser as determined by a test using standard sieves. It is a measure of the fineness of the aquifer material.

2. Uniformity Coefficient:

The uniformity coefficient gives the slope of the major portion of the grain size distribution curve and is defined as –