Water Well Drilling and Development Scenario of Drilling Equipment in India

Though, irrigation wells. Are known to exit from ancient times, real impetus to water well drilling for irrigation came only after independence and particularly with the launching of the planned development programme.  Notable break through in water well drilling, however, came when two consecutive droughts, during 1966-67 and 1967-68 swept over the country and exposed the vulnerability and canal irrigation in times of crises. It was only thereafter concerted efforts were made to explore and develop the much dependable ground water resources of the Country. Keeping in view the knowledge and field experience acquired by the Author on the various types of drilling rigs and well constructions a narration of various drilling techniques, design and construction of tube wells has been discussed in this article so as to provide the basic ideas for the well construction in various Hydro geological formations including development scenario of drilling equipment in India.
Particular drilling method have become popular in certain parts of the Country because they are most effective in penetrating the local hydro geological formations thus reducing the cost of various operations. In water well drilling we may change the usual drilling procedure depending on the depth and diameter of the well, type of formation to be drilled etc. It is, therefore, obvious that no single method of drilling is good for all types of hydro geological formations. Successful drilling is, therefore, an art developed from the long experience and the application of good engineering practices by the drilling engineers and drillers engaged in water well construction.

METHODS OF DRILLING
The methods of drilling and water well construction are so numerous that only the basic principles and some of their genera1 applications can be described. The most commonly used methods for well drilling and construction are: * Digging
* Boring
* Driving
* Jetting
* Reverse Circulation Drilling
* Cable tool Drilling
* Conventional Mud
*
Rotary Drilling
* Down the hole drilling (DTH)

Detail drilling operation and procedures shall be followed in the subsequent articles with more information about well design and construction with economics.

DIRECT MUD   ROTARY DRILLING

Direct Rotary Mud Drilling method
is becoming the most effective and widely practical procedure of all available drilling systems and has been adopted by the water well industry as the conventional drilling system. In water well drilling, this method is mostly deployed for drilling in unconsolidated and semi-consolidated formations alluvial soil, sand stone etc. for small as well as large diameter bore holes of any required depth. In this method drill string is consist of Kelley drill pipe and bit. For better result to maintain verticality of hole stabilizer and drill colars are also used in drill string. Mud pump is used to inject drilling fluid through drill rod and bit to the bore hole. The drilling fluid carries the cuttings from the bore hole and also gives cooling effects to the bit as well as it makes a semi-prominent plastering on the wall of the bore hole. The hydrostatic pressure of the fluid prevents the collapse of the bore hole.  A rotary table arrangement with the rig gives a rotation to the bit through drill rod. Different 'types of bits are used to encounter different type of formation. This type of drilling is faster method then that of percussion drilling.

DOWN THE HOLE DRILLING (DTH) 
Down-the-hole. drilling
method involves a pneumatically operated bottom hole drill that efficiently combines percussion action of the cable tool drilling with the turning action of rotary drilling. In this method in hard rock formation consists of granite, gneisses basalt etc. The compressed air is used as a drilling fluid and removes the cuttings of the bore hole with the help of pressure and velocity head of the compressed air. The compressed air also provides the required percussion and rotary motion to the drill bit through a hammer attached with it. Hydraulic motor is used to obtain the required movement of the drill string. This is a very fast method of drilling in consolidated hard rocks. This method gives a very fast progress but when it encounters the fractures and water bearing zones the progress considerably drops down. Different type of size of hammers and bits are used considering the size of the bore hole and formation to be encountered. Depth and hole size of borehole depends on the capacity of the compressor. In hard rock area casing and well screens are not  required only while constructing a tube well.  Weathered and over burden portion can be cased to prevent the collapse.

CABLE TOOL DRILLING 
This method is best suited to drill boulder hard and semi consolidated formation. Drilling rate is much lower than that of any other drilling method.  This method employs string of tools consist of drill bit, a drill stem, a drilling jar and a rope socket. The tool string is alternatively raised and dropped thus allowing the bit to encounter the formation. A reciprocating motion of the drilling tool is mixes with using material and water, and slug is removed from the hole periodically by using bailor or sand pump as drilling progresses. 

Hard consolidated rocks are usually drilled without casing. But casing is always used in unconsolidated loose formation to prevent the hole from caving.

DESIGN OF THE WELL ASSEMBLY
When the borehole has been completed to desired depth and sufficient water bearing zones are encountered, the well is completed for production. The well is then designed keeping in view the litho logical log or driller's log and electric log. In case where there is doubt about the performance of the aquifers, bailer test is conducted and the data obtained is also kept in view while designing the well assembly.

SELECTION OF SCREEN SIZE: 

Different type of strainer, slotted pipes, V-wire screen, brass, copper agriculture Strainer are used in wells where there is no coarser (sand) material available. The general practice in selecting the screen size is based after doing the mechanical analysis of the sand. The slot opening of the screen is so made that 40% of the formation is retained and 60% of which should pass through the slots. In such type of tubewells the gravel pack is generally eliminated.

DIAMETER OF SCREEN:
The problem of selecting an ideal well screen is finally one of the selecting. a screen of proper length, diameter, size, shape and number of slot opening. Screen dia is a factor that can be varied after the length of the screen and size of the screen opening has been determined. Screen length depends upon the grading of sand. To larger extent the natural characteristic of aquifer fixed this dimension leaving the dia as a factor that can be varied.

The entrance velocity equal to 0.10 feet to 0.25 feet second proved to be ideal if proper screen is selected.

lNSTALLATION OF WELL ASSEMBLY:
After deciding the size of well assembly and designing of suitable well screen the assembly is to be lowered in the bore hole. The process is carried out by lowering bottom most pipe first and continue the process by maintaining the same sequence. The upper part of the tube well is usually of higher diameter to accommodate the pump, known as housing. The length of the housing may vary on the basis of static water level and pumping water level. The pipe can either be joined by socket and screw or by welding.

In case of Rotary drilling, the assembly is lowered up to the designed depth and the annular portion between the pipe assembly and the bore hole is packed by pea-gravel.

For percussion drilling first well assembly is lowered but gravel is fed simultaneously with lifting of casing drive pipe. In case of consolidated hard rock drilled by DTH rig the usual practice is to keep the bore hole necked except the over burden portion where there is a possibility of collapse. In such wells gravel packing is not recommended.

GRAVEL PACKING:
For a gravel packed well, the size of the hole should be large enough to allow 3 to 6 inches of gravel pack around the screen and, the annular space between the hole or casing and the pump housing pipe should be sufficient for proper gravel feeding. In construction of gravel packed well with cable tool rig, gravel is slowly fed around the assembly into the annular space while the casing is withdrawn and there should always be about 3 Mts. of gravel above the casing shoe. Too little gravel above the casing shoe may result in rush of sand above the gravel pack and too much gravel may result in bridging the assembly sand will start coming up with the extraction of the casing.

When a gravel packed well is constructed with a rotary' rig, the hole should be thoroughly washed clean of cutting and finally reconditioned with fresh mud of good viscosity before lowering the assembly. The hole should be kept full of mud during lowering so that it does not cave in. After lowering the assembly should be suspended in tension and suitably guided.

WELL DEVELOPMENT:
The purpose of well development is to restore the natural conditions and also to increase the permeability of the water bearing formations around the screen, this is achieved by removing the deposits i.e. removing mud, silt and finer particles /of sand from the surrounding water bearing formation and thus building an envelope of coarser particles of the formation around the screen. Proper development of a tube well is essential to obtain optimum yield with minimum draw down, to eliminate sand in question and to lengthen the economic life of the well.

coarser particles of the formation around the screen. Proper development of a tube well is essential to obtain optimum yield with minimum draw down, to eliminate sand in question and to lengthen the economic life of the well.

The methods commonly employed for development are the operation with;

a. Surge block.
b. Air lift.
c. High velocity jetting.
d. Turbine pump .etc. etc.

All these methods are based on the principle of Surging or agitating the water in the formation for the purpose of cleaning the well. Over development or improper development of a well may result in formation of cavity in the aquifer due to the removal of too large quantity of fine grained material. In such a case overlying impervious material may collapse and pack around the screen thus causing a serious damage to the, productivity of the well.

Detail Well development procedures and economics shall be followed in subsequent articles to be published in drilling today future volume. Reserve your copy today to enjoy and learn more about the profession.

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