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.