In this article we will discuss about:- 1. Introduction to Tube Well 2. Types of Tube Wells 3. Types of Strainers 4. Methods of Drilling 5. Completion.
Contents:
- Introduction to Tube Well
- Types of Tube Wells
- Types of Strainers Used in Tube Well
- Methods of Drilling Tube Wells
- Completion of Tube Well
1. Introduction to Tube Well:
This well is a type of deep well, tapping ground water from one or several confined aquifers lying under the surface of the earth. As its name suggests, it is a tube inserted in the ground. The parts of the tube which come in level with confined aquifers are slotted and those which come in level with impervious layers are hollow pipes not having any slots.
The parts of the pipe which are hollow and do not have any slots at this surface are known as blind pies. The parts of pipe that have slots at their surface are known as strainer pipes or perforated pipes. Thus a tube well consists of strainer pipes as well as blind pipes. There are several types of strainers or perforated pipes. The discharge of tube well is several times more than open well.
The reasons of greater discharge are the following:
1. Tube wells can tap several water bearing stratum.
2. Water can seep into the tube well with several times more velocity than the critical velocity of flow in the case of open well. This becomes possible because strainer pipes allow only water to pass through and no sand particles.
3. There is no danger of wellbeing subsided.
4. Flow in the wellbeing radial and that too from several water bearing layers, the cross-sectional area contributing water to well becomes quite large and thus quite large discharge can be obtained from the tube well.
Tube wells may be constructed by the government or by the individual farmers. The average yield from a tube well is of the order of 40 to 50 lit/sec. Tube wells have been installed to give yield as high as 200 to 250 lit/ sec. The depth of the tube wells may vary from 20 m to as high as 500 m. The diameter of the bore hole is kept 60 cm for top 60 m depth. Below this depth of hole diameter is reduced to 56 cm. The diameter of strainer is 25 cm and draw down 10 m. Each deep tube well can irrigate about 15 hectares of land.
Besides deep tube wells, shallow tube wells are also constructed by small farmers. The depths of such tube wells vary from 15 m to 30 m or so. Such tube wells may give about 15 lit/ sec yield, if located at proper place. Each shallow tube well can irrigate 5 to 8 hectares of land.
Except in desert areas of Rajasthan, porous sand and gravel layers charged with ground water are extensively found in India. The entire area from Himalayas to Vindhya Chal, coastal area, Narmada Valley etc. consist of deep alluvial soils. Tube wells can be easily installed in such soils and are very useful for irrigation. It is in this context that the tube wells are assuming greater and greater importance in these regions in India.
It is very difficult to construct a tube well in rocky region. It is actually not required in such areas as land for irrigation is generally not available.
2. Types of Tube Wells:
The tube wells may be further classified into following three types:
1. Strainer wells
2. Cavity wells
3. Slotted wells.
All the three types of tube wells have been discussed one by one –
1. Strainer Type Tube Well:
It is the most commonly used type of tube well in our country. It is so common that whenever we talk of a tube well, it is automatically understood as referring to strainer type tube well. This tube well consists of blind pipes and strainer pipes or screen pipes. This tube well is generally unsuitable for very fine sandy strata, because in that case screen openings are likely to get choked easily.
The boring of the hole is generally carried out by using casing pipe of about 5 to 10 cm larger than the diameter of the well pipe. Thus a 15 cm diameter well, will require a bore hole of 20 to 25 cm diameter. After boring the hole, the well pipe assembly consisting of blind and strainer pipes is lowered into the bore hole. The lengths of blind and strainer pipes are so adjusted that the blind pipes about against impervious layers and strainer pipes against the water bearing aquifers.
A short blind pipe should be provided at the bottom so as to permit settlement of any sand particles that have passed through the strainer. The well is generally plugged at bottom by cement concrete.
2. Cavity Type Tube Well:
This is such a tube well which does not require any strainer. It draws its supplies from a cavity developed in the water bearing layer at the bottom end of the well. In strainer type tube wells flow of water through strainers is horizontal and radial. But in this well water enters well from bottom of the well without passing through any screen and flow of water is normal to the spherical cavity formed at the bottom.
The essential requirements for a cavity well to function efficiently is to have confined aquifer of good specific yield and the aquifer should have a stronger impervious material layer. The impervious layer is punctured and a cavity is developed by pumping, by a sand pump.
In the initial stage of pumping, fine sand comes out with water and consequently a cavity is formed. As the spherical surface area of the cavity increases outwards, the radial critical velocity decreases and sand particles stop entering the well.
The principle behind this well is same as that of a deep well. The only difference is that in case of open deep well only first aquifer lying below mota layer is punctured whereas in case of strainer tube well any aquifer lying at any position can be tapped.
3. Slotted Type Tube Well:
This well is made at such places where sufficient depth of water bearing stratum is not available even at a depth of 80 m to 100 m and also when suitable strong roofing layer is not available for the construction of a cavity well.
This well consists of a slotted wrought iron tube penetrating a highly pervious confined aquifer. The size of the slots may be 25 mm x 3 mm and spaced at 10 to 12 cm centre to centre. In order to prevent the entry of fine sand particles into the pipe, the pipe is surrounded by a mixture of gravel and bajri. This mixture is called shrouding.
First of all a casing pipe 40 cm diameter is lowered and aquifer is penetrated for a depth of about 5 m. The slotted pipe, sometimes also known as education pipe of 15 cm diameter is then lowered. Gravel is then poured from top in the annular space left between inside of the casing pipe and outside of education pipe.
The gravel is filled for 3 to 4 m higher than the slotted pipe. Now the casing pipe is withdrawn 5 cm at a time and well is developed with the help of compressed air pumped into the education pipe.
Finally when casing pipe is fully withdrawn, the annular space between the casing pipe and the education pipe is suitably plugged. By developing the well with the help of compressed air, the sand surrounding the gravel is freed from finer particles and the chances of
filter getting choked are very much reduced.
The difference between slotted tube well and strainer tube well are the following.
(i) Slotted well can tap only one water bearing strata whereas strainer well can tap many.
(ii) Slots of slotted tube well are protected by gravel shrouding so as to prevent entry of sand particles in the well. In case of strainer tube wells strainer is used to prevent sand particles entering the well. This well is not much in use and hence not very important.
3. Types of Strainers Used in Tube Well:
The strainer pipe is a perforated pipe which is provided with such an arrangement that only water can be admitted inside the pipe. Various patented types of strainers are available in the market.
1. Cook’s Strainer:
It is made from solid drawn brass tube and is very costly. It consists of wedge shaped slots at its surface. Slots are wider at the inside and finer at the outer surface. The size of the slots depends upon the coarseness of the sand in which it is to be used. It is not used much in India. The size of the slots may vary from 0.1 mm to 0.4 mm.
2. Tej Strainer:
It is just like Cook’s strainer. The strainer tube is made by folding and bending the brass sheet. Slots are cuts in the sheet in required gauge before sheet is bent to form a tube. It is very much manufactured in India. Its diameter is 7.5 cm and above and length 2.5 m. Vertical joint of the sheet is closed by brazing. If larger than 2.5 in length is required more than one lengths of strainer are joined together by screwed collar joint.
3. Ashford Strainer:
It is also a brass tube having round holes in it. A steel wire is wrapped over the brass tube and then a wire mesh is soldered to the wound wire. The wire is wound to maintain a clear space between strainer and wire mesh. The wire mesh is finally protected and strengthened by a wire net around it.
4. Leggett Strainer:
It is a very costly strainer. It is not used in India. It consists of a usual strainer which is also fitted with a special strainer cleaning device. The device can be operated from the ground. Whenever strainer is clogged this device is operated and strainer is cleaned without any difficulty.
5. Phoenix Strainer:
It consists of a cadmium plated mild steel tube. Cadmium plating keeps the strainer free from choking and corrosion. Its slots are also made from inside the tube with the help of a special machine.
6. Layne and Bowler Strainer:
It also consists of a slotted or perforated steel or wrought iron pipe. A wedge shaped steel wire is wound around the tube at suitable pitch.
7. Brownlie Strainer:
Steel plates are bent in form of polygonal convolutes. Suitable perforation holes are made in convolutes. The convolute shaped perforated tube is enclosed in a wire mesh of copper wires. It is considered one of the best strainers as wire mesh remains slightly away from the perforated tube.
8. Esbee Strainer:
It consists of a steel frame made of iron rods and wound by coir. The coir acts as a screen which allows water to pass but prevents sand. Its ends are so rivetted to screwed pipe that they can be easily joined to the next pipe.
9. Mesh Strainer:
It consists of a metal plate in which slots are punched. The punched strip is not removed. It is mostly made from iron plate which is wound in form of a pipe after making slots in it. It is claimed that they do not choke.
4. Methods of Drilling Tube Wells:
Wells for domestic purposes which require small quantity of water are bored with auger turned by hand. Smaller wells in unconsolidated formations may be constructed by cutting action of a jet of water. None of these two methods is however applicable to deep tube wells.
Deep and high yield wells are constructed by drilling. There are various techniques of boring the well hole. All the techniques have their own merits and demerits, depending upon the formation in which drilling is to be done.
Most commonly used drilling methods have been described as follows:
1. Cable tools method or percussion drilling method.
2. Hydraulic rotary method.
3. Reverse rotary or jetting method.
1. Cable Tool Method or Percussion Drilling Method:
This method of drilling of the well hole is based on the use of striking force, (hammering and cutting) of the drilling bit, attached to the lower end of the cable. The drilling bit is alternately lifted and dropped in the hole to break the formation. In this method a standard well drilling rig consisting of a mast, a multiline hoist, a walking beam, and an engine are all mounted on a truck in assembled form so that equipment may be easily transported.
A string of cable tools, consists of a rope socket, a set of jars, a drill stem and a drill bit. A set of jars is used to aid in loosening tools struck in a hole and drill stem is used to provide length and weight to the cable tools. The whole assembly of cable tools may weight 100 kg to over 2000 kg.
A pit is dug at the site where well is to be bored. A casing pipe along with drive shoe is erected vertically in the pit and the string of drilling tools is inserted in the first length of the casing pipe. The string of tools is alternately lifted and dropped with the help of engine. After penetrating for 1 m to 1.5 m the string of tools along with bit is taken out of the hole and a bailer is inserted to remove the cuttings from the hole.
The bailer is a pipe fitted with a valve at the bottom. When it is dropped in the hole, the valve at the bottom opens and cut material enters the bailer. When bailer is lifted up, valve automatically closes and cut material remains held in the bailer. Bailer may be lifted and dropped several times so that it may be filled with the cut material. The bailer is then pulled out emptied at the surface. The length of the bailer varies from 3 m to 12 m, depending upon its diameter.
When cut material is fully bailed out of well hole, the string of tools is again inserted and penetration carried out further. The string of tools is slightly kept rotating so that a round hole is drilled. Water is also put in the drilled hole to form the paste with the cut material so that it may be easily bailed out.
Secondly water also softens the formation which is being cut by the drill bit. When one length of casing pipe is sunk, subsequent length is joined either by threading or welding. This method is used for drilling through consolidated rock materials.
In soft and fissured rock formations, drilling is carried out with the help of a tripod and manual labour. In that case truck mounted with mast, multiline, walking beams, engine, string of tools etc. are not required. Tripod erected over the proposed well hole acts as a mast and manual labour provides the force for lifting and dropping the cutting equipment.
In this case ‘sludger’ is used in place of string of tools. Sludger is more or less similar to bailer. It is made of steel pipe 2 to 4 m long fitted with a hard steel cutting shoe at the bottom. A flap valve is also fitted near its lower end which allows cut material to enter the sludger pipe and then does not allow it to fall down, when sludger is lifted out of drilled hole. Sludger is inserted in the hole to be drilled and is lifted and dropped by manual labour through a rope which passes over a pulley fixed to the tripod.
A platform is attached at the upper end of the casing pipe and weight is placed on it to help drive the casing pipe slowly. When sludger is full with the cut material, it is taken out and emptied. This process is continued till boring upto the desired level is achieved. If some rock formation is encountered embedded in soft ground, it can be cut using a string of drilling tool instead of sludger.
In both the cases mentioned above, the record of material found at different depth is maintained. The positions of strainer pipes and blind pipes are decided based on this record. This record reveals what type of formation exists at what depth. Strainer pipes are positioned against aquifers and blind pipes against aquicludes.
2. Hydraulic Rotary Method:
This method is used for drilling large bores in unconsolidated formations. This is a very fast method of drilling well holes. This method consists of a drilling bit attached at the bottom end of a string of hollow pipes. Drilling mud or Bentonite slurry, is continuously pumped down through the string of hollow pipe and released through a nozzle in the drill bit.
This released drill mud is then carried up through the annular space between the drill pipe and the drilled hole, along with cut and loosened material. In this case no casing pipe is required since the drilling mud forms a clay lining and supports the walls of the drilled hole.
The rising drilling mud carrying cut material is taken to the settling basin where cut material settles. The mud is recirculated to the hole. In order to maintain required consistency, clay and water are added to the circulating mud from time to time. Complete record of settled materials is maintained and type of formations at different depths predicted on its basis.
After reaching the desired level, the drilling equipment is taken out and well pipes comprising strainer as well as blind pipes are lowered at appropriate levels. After fitting the well pipes the well hole walls which are coated by clay or Bentonite are to be washed. Washing of the walls is done by lowering drill pipe and drill bit in the well again and forcing water containing calgon (sodium- hexa-meta-phosphate) in the bore.
In this case of drilling, drill bit is the most important equipment. It has hollow shanks and one or more centrally located holes for jetting the mud into the bottom of the hole. There is a drill rod made of heavy pipe. At one end of drill rod drill bit is attached and at other end it remains attached to a square rod known as kelly. A rotating table rotates the drill rod and slides downwards as the hole deepens. Rotating table remains closely fitted to the kelly.
3. Reverse Rotary Method:
This method is also known as jetting method. It is a modified form of hydraulic rotary method. It is also useful for making large wells in unconsolidated formations. The tools required for this method are hollow drill, a drill pipe, and water swivel. The cuttings are removed by a suction pipe of a large capacity centrifugal pump.
The walls of the hole during drilling remain supported by hydrostatic pressure acting against the film of fine-grained material deposited on the walls by drilling water. Clay or bentonite are not necessary to be added to the drilling mud.
The mixture of water and cuttings is passed through settling tank and after settlement the effluent is again recirculated. Casing and cleaning of the walls is done in the same way as in hydraulic rotary method.
5. Completion of Tube Well:
During boring of the well hole by any method it should be ensured that bored hole remains straight and vertical.
If well hole is located in consolidated formation, water enters the well hole directly and no casing is required as all the surroundings are quite stable. But if formations are unconsolidated a casing is necessary to support the outside material and also to help in freely admitting the water into the well. The casing should either contain perforations or its lower part may be replaced by a screen or strainer so that water may enter the well.
Sometimes in order to increase the effective diameter of the well, the screen or strainer is surrounded by a layer of gravel. Gravel layer keeps fine material out of the well. Such gravel packed well will have a greater specific capacity than one of the same diameter not surrounded by gravel. The thickness of the gravel may vary with type of formation and method of drilling. However a minimum thickness of 15 cm is generally used.
While drilling a new tube well it should be ensured that it would not affect the discharge of other nearby existing tube wells. The distance of the proposed well from existing wells should not normally be less than 1.5 km. The diameter of the well pipe may be reduced theoretically, as we go deep down the well; but it is not recommended.
A single sized or at the most two sized tube may be used in actual practice. The velocity of flow through the tube is generally limited between the range of 1.5 m/sec to 4.5 m/sec. Once the design discharge of the tube well and the velocity of flow are known, the diameters of the pie can be easily found out.
The nearest available size of the pipe in the market may then be adopted. Normal life of a tube well in Northern India is about 20 years.