Well Loss in Ground Water

Well Loss:

  1. When water is pumped out of a well, the total drawdown caused includes not only that of the logarithmic drawdown curve at the well face, but also drawdown caused by flow through well screen and axial movement within the well.
  2. The latter drawdown is called well loss. Since turbulent flow generally occur near the well face, this loss may be taken to be proportional to Qn

Interference among wells

  1. When two well, situated near to each other, are discharging, their drawdown curves intersect within their radius of zero drawdown.
  2. Thus, though the total discharge is increased, the discharge in individual well is decreased due to interference.

Well Shrouding

  1. Well shrouding is a process of interposing coarse material such as gravel and coarse sand between the well-pipe and the aquifer soil to prevent finer particles of soil coming in contact with the strainer and chocking it. This is essential in sandy and unconsolidated formations of aquifer
  2. The shrouding increases the effective well diameter, acts as a strainer to keep fine material out of the well, and protect the well pipe from caving of surrounding formations.
  3. The amount of shrouding material per 30 cm length of the casing pipe can be calculated accurately beforehand. In the beginning, the shrouding material for 60 cm length is shovelled in from the top between the tube-pipe and casing
  4. The casing and the casing pipe is withdrawn 30 cm at a given time till the strainer is covered.

Well Development

  1. The well development is the process of removing fine material from the aquifer formation surrounding the strainer pipe and is aimed (i) increasing the specific capacity of the well, (ii) preventing sand flowing in, and (iii) obtaining maximum economic well life.
  2. The actual yield can be known only after well development. Thus, it also helps in determining the required characteristics of the pump and pump power unit to be installed.
  3. Depending upon the formation characteristics of the aquifer, a well may be developed by one of the following methods:

    • Development by pumping: Initially, the pump is started with a very low discharge. The fine particles start coming. Thus low speed is maintained till clear water comes. The discharge is then increased in steps until maximum discharge or well capacity is reached. The pump is then stopped and levels permitted to increase till it comes to normal.
    • Development by surging: In this method surging effect is created by up and down movement of a hollow surge block or a bailer. Calgon (sodium hexametaphosphate) is added to water so that it acts as dispersing agent for fine grained particles. When the surge block is moved up it sucks water in. When it is moved down, it forces water-calgon solution back in the formation. Further upward motion brings with it fine material. The surge block is connected to a string of hollow pipe from which the water charged with fine particles is pumped out continuously. The procedure is repeated by increasing the speed of surging till clear water comes out.
    • Development by Compressed air: In this method, the development is done with the help of an air compressor, a discharge pipe and air pipe. The air pipe is put into the discharge pipe and is lowered into the well tube, till the assembly reaches near the bottom of the strainer-pipe section. The lower end of the air pipe is kept emerging out of the discharge pipe is then started till a pressure of 5 to 10 kg/cm$^2$is built up. The air is suddenly made to enter the pipe, at this pressure with the help of suitable quick-opening valve. This sudden entry of air into well creates a powerful surge within the well causing loosening of fine material surrounding the perforations. When ice pressure decreases, water enters the well bringing the loosened particles with it. The continuous air injection creates an air lift pump and water carrying fine particles is pumped out. The process is repeated till clear water comes out.
    • Development by Back washing: In this method in addition to the compressor, a discharge pipe and an air pipe, and additional small pipe in used. The well is sealed at its top so that it becomes air tights. The discharge pipe and the air pipe assembly is lowered in the well, as in the previous method but the end of the air pipe is kept inside the discharge pipe. A small air pipe is fitted at the air tight cover, and is provided with a three-way cock. With the help of the three-way cock, air can be admitted to the well either through long air pipe or through small air pipe fitted at the top. Air is first made to enter the long air pipe. This forces air and water out of the well through the discharge pipe. When clear water comes, the valve is closed, and water level is allowed to increase in the well. The valve is then turned to the other side so that air enters through the small air pipe. This back washes the water from the well through the discharge pipe and at the same time agitates the fine particles surrounding the well.
    • Development by Dry Ice: In this method, well is developed with the help of two chemicals i.e., hydrochloric acid and dry ice. At first hydrochloric acid is poured into the well. The well is capped at the top and compressed air is forced into the well. The pressure of the compressed air forces the chemical into the formation. The cap is then removed and blocks of dry ice are dropped into the well. The sublimation releases gaseous carbon -dioxide, and a high pressure of this gas is built up in the well. On releasing the pressure, the muddy water is forced up in the form of a jet and is automatically thrown out of the well.
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