i. Operations in Tube Well Construction
Tube well construction can be divided into four major operations. These are drilling operation, casing installation, well screen installation and well development for getting clear water without sediments.
ii. Drilling Methods
The different methods of drilling include cable tool percussion drilling, rotary drilling,hammer drilling and core drilling. Depending on the geological formation, suitable drilling method is selected and drilling rigs of various capacities are available in each category. Cable tool drilling and rotary drilling are the commonly used methods.
Sometimes, particularly for large production wells and where water quality is particularly important, drill a small-diameter pilot hole before drilling the well bore.From information obtained from the pilot hole, determine aquifer formations and groundwater quality at various depths and then optimize the final well design for the specific hydrogeological conditions at the site. Appropriate materials (screen,casing, gravel) can then be ordered in a timely fashion prior to the final drilling.
Cable Tool Percussion Drilling: In cable tool drilling, bore hole is made by lifting and dropping a string of heavy tool suspended on a cable. A bit at the bottom of the tool string strikes the bottom of the hole, crushing and breaking the formation material. Water is added to form slurry of the crushed material. If there is water in the bore hole from formation material, then no external water is added. Due to reciprocating motion of the tool, the crushed material is mixed with water and forms slurry. The slurry formed is removed from the bore hole at some intervals.Otherwise, the accumulated slurry decreases the impact of falling bit which results in retarded penetration of tool. This method is suitable for all types of formations but takes more time. It is the most preferred methodn for drilling in boulders and hard rocks.
Rotary Drilling: Rotary drilling is very fast and is suitable for making large bore holes. In this method a rotating drill bit is forced downwards. Drilling fluid is circulated through this drill bit. This method is more economical for making deep holes in unconsolidated formations. However, it is not recommended for drilling inboulders and hard rocks since progress of drilling will be very slow in these formations.This method needs lot of water for circulation and could pose difficulties in water scarce areas.
The major items of rotary drilling method are the drill bit and the drilling fluid.Different kinds of bits are tailor made to suit drilling in different types of formations.The drilling fluid could be either simple mud water or carefully prepared special solutions of desired viscosity. This drill fluid is pumped down through the drill pipe and come out through nozzles in drill bit. The drill fluid then moves upward in the annular space around the drill pipe to the ground surface. On its way to the surface,the drill fluid picks up the cut material of formation and carry it in suspension. The fluid is then diverted to a sedimentation pit where bulk of the cut material settlesdown. The drill fluid is then pumped back to the drill hole. Thus the main function of drill fluid is to remove the cut material from bore hole. The drill fluid also prevents the caving in of the bore hole due to its fluid pressure.
iii. Well Log
As drilling proceeds, various kinds of formation materials are encountered. The record of chief characteristics and depth of each formation layer from surface is called a well log. This well log gives information on where water bearing formations are located. Well log information will help in finalizing the well design, which includes a determination of the depth of the well screens, the size of the screen openings, and the size of the gravel pack material. Well casings and well screens are arranged in sequence on the basis of well log to tap good quality water from different aquifers. If the water bearing layer contains fine sand and silt, gravel packing around the screens can be employed to get the water free from particles.If saline or brackish water is there in some layers, these layers need sealing so that the brackish or saline water does not enter the well. The next step after installation of casings and screens is the well development.
iv. Well Development
Tube wells need to be developed to prevent entry of sand, increase the yield of a well and to extend the life of a well. It involves reversing the flow of water from the tube well in to water bearing formations with force and then allowing back flow in normal direction. This results in loosening of small particles between large particles during reverse flow and flow back in to well during normal flow. These small particles from well are removed. After well development the formation material in water bearing layer becomes more porous which allows easy flow of sand free water in to well.
Tube well construction can be divided into four major operations. These are drilling operation, casing installation, well screen installation and well development for getting clear water without sediments.
ii. Drilling Methods
The different methods of drilling include cable tool percussion drilling, rotary drilling,hammer drilling and core drilling. Depending on the geological formation, suitable drilling method is selected and drilling rigs of various capacities are available in each category. Cable tool drilling and rotary drilling are the commonly used methods.
Sometimes, particularly for large production wells and where water quality is particularly important, drill a small-diameter pilot hole before drilling the well bore.From information obtained from the pilot hole, determine aquifer formations and groundwater quality at various depths and then optimize the final well design for the specific hydrogeological conditions at the site. Appropriate materials (screen,casing, gravel) can then be ordered in a timely fashion prior to the final drilling.
Cable Tool Percussion Drilling: In cable tool drilling, bore hole is made by lifting and dropping a string of heavy tool suspended on a cable. A bit at the bottom of the tool string strikes the bottom of the hole, crushing and breaking the formation material. Water is added to form slurry of the crushed material. If there is water in the bore hole from formation material, then no external water is added. Due to reciprocating motion of the tool, the crushed material is mixed with water and forms slurry. The slurry formed is removed from the bore hole at some intervals.Otherwise, the accumulated slurry decreases the impact of falling bit which results in retarded penetration of tool. This method is suitable for all types of formations but takes more time. It is the most preferred methodn for drilling in boulders and hard rocks.
Rotary Drilling: Rotary drilling is very fast and is suitable for making large bore holes. In this method a rotating drill bit is forced downwards. Drilling fluid is circulated through this drill bit. This method is more economical for making deep holes in unconsolidated formations. However, it is not recommended for drilling inboulders and hard rocks since progress of drilling will be very slow in these formations.This method needs lot of water for circulation and could pose difficulties in water scarce areas.
The major items of rotary drilling method are the drill bit and the drilling fluid.Different kinds of bits are tailor made to suit drilling in different types of formations.The drilling fluid could be either simple mud water or carefully prepared special solutions of desired viscosity. This drill fluid is pumped down through the drill pipe and come out through nozzles in drill bit. The drill fluid then moves upward in the annular space around the drill pipe to the ground surface. On its way to the surface,the drill fluid picks up the cut material of formation and carry it in suspension. The fluid is then diverted to a sedimentation pit where bulk of the cut material settlesdown. The drill fluid is then pumped back to the drill hole. Thus the main function of drill fluid is to remove the cut material from bore hole. The drill fluid also prevents the caving in of the bore hole due to its fluid pressure.
iii. Well Log
As drilling proceeds, various kinds of formation materials are encountered. The record of chief characteristics and depth of each formation layer from surface is called a well log. This well log gives information on where water bearing formations are located. Well log information will help in finalizing the well design, which includes a determination of the depth of the well screens, the size of the screen openings, and the size of the gravel pack material. Well casings and well screens are arranged in sequence on the basis of well log to tap good quality water from different aquifers. If the water bearing layer contains fine sand and silt, gravel packing around the screens can be employed to get the water free from particles.If saline or brackish water is there in some layers, these layers need sealing so that the brackish or saline water does not enter the well. The next step after installation of casings and screens is the well development.
iv. Well Development
Tube wells need to be developed to prevent entry of sand, increase the yield of a well and to extend the life of a well. It involves reversing the flow of water from the tube well in to water bearing formations with force and then allowing back flow in normal direction. This results in loosening of small particles between large particles during reverse flow and flow back in to well during normal flow. These small particles from well are removed. After well development the formation material in water bearing layer becomes more porous which allows easy flow of sand free water in to well.
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