Well: Shrouding and Development | Groundwater | Water Engineering

In this article we will discuss about well shrouding and well development.

Well Shrouding:

The shrouding is a layer of coarse material such as gravel and coarse sand interposed in the annular space between the well pipe and the aquifer soil. The shrouding prevents the finer particles of the soil coming in contact with the strainer and choking it, acts as a screen or filter introduced between the strainer and the aquifer soil to allow only water to pass through and not sand, and protects the well pipe from caving in of the surrounding formation. Further the shrouding also increases the effective well diameter. In general a shrouded well has a greater specific capacity than the one of the same diameter but not shrouded.

The shrouding is usually not required in rocky and consolidated formations, but it is essential in sandy and unconsolidated formations. It is also essential in a slotted type tube well where wire mesh (or screen) is not used. Such a tube well is usually provided with a thick shrouding and it is also sometimes known as gravel-packed well.

The shrouding material should have proper grain size distribution which depends upon the grain size distribution of the aquifer soil and upon the size of the slots or the well screen. When a casing pipe is used during drilling, the shrouding material is introduced before the casing pipe is taken out of the hole. In this case the quantity of the shrouding material per 0.3 m length is determined.

In the beginning the shrouding material for 0.6 m length is shovelled from the top in the annular space between the well pipe and the casing pipe. The casing pipe is then taken out by 0.3 m with the help of jacks or crane. Then the quantity of shrouding material for each 0.3 m is added and the casing pipe is taken out by 0.3 m until the strainer is covered.

Sometimes the shrouding material is placed through small pipes. A 50 mm diameter pipe is lowered in the annular space that is to be filled in by the shrouding material. The shrouding material is fed through a hopper provided at the top. The feeding pipe is gradually pulled up as the annular space is filled.

Water is fed into the pipe so as to aid in the downward movement of the shrouding material. For very deep wells the reverse circulation system of feeding the shrouding material is more suitable. In this case a pipe is lowered inside the well pipe and it is connected to a pump.

A circulation of water is established downwards in the annular space and upwards through the lowered pipe. The shrouding material is fed in the annular space and is carried downwards by the force of the water. However, in an improved method, the shrouding material is pumped down into position by using bladeless pumps developed for this purpose.

Well Development:

Well development is the process of removing fine material from the formations surrounding the strainers or the screened sections of the well pipe.

It is aimed at:

(i) Increasing the specific capacity of the well

(ii) Preventing the sand flowing into the well

(iii) Obtaining maximum economic well life.

The actual yield of a well can be known only after the well development.

The following methods of well development are commonly used:

(i) By pumping

(ii) By surging

(iii) By compressed air

(iv) By back washing

(v) By chemical (or By dry ice).

(i) By Pumping:

In this method the well is pumped in a series of steps from a low discharge to the one exceeding the design capacity. At each step the well is pumped until clear water is obtained, after which the power is shut off. The discharge rate is then increased and the procedure repeated until the maximum capacity of the pump or well is reached.

This irregular and non-continuous pumping agitates the fine material surrounding the well so that it can be carried into the well and pumped out. The coarser fraction entering the well is removed by a bailer or sand pump from the bottom.

(ii) By Surging:

In this method a surging action is imparted to the water in the well by up and down movement of surge block. A surge block is essentially a plunger which is made by mounting cylindrical wooden blocks with two leather or rubber sandwiched discs and two steel washers at the end of an iron pipe. The diameter of the surge block is such that it can pass easily through the well. The surge block is lowered into the well so that it is 3 to 5 m under water, but above the top of the screened section.

The surging action is transmitted to the screened section by the water column. Surging is started slowly and the speed is gradually increased upto the point the surge block can rise and fall smoothly without jerking. The downward stroke of the surge block causes back wash of water from the well which results in dislodging of the sand particles in the surrounding formation.

The upward stroke of the surge block causes sucking action and thus pulls the dislodged sand particles from the surrounding formation into the well. After surging for 10 to 15 minutes the surge block is taken out and bailer is lowered to the bottom of the well to estimate the quantity of sand pulled in and to bail out the sand. The process is repeated by gradually increasing the period of surging until the sand pulled in and accumulating in the bottom of the well becomes negligible.

(iii) By Compressed Air:

In this method an air compressor is used which is connected to an air tank. The air tank has an outlet provided with a quick opening valve which is connected to an air pipe. Around the air pipe a discharge pipe is fitted. The air pipe and discharge pipe assembly is introduced into the well. The discharge pipe is of diameter smaller than that of the well pipe such that there is clearance of at least 20 mm all around between the discharge pipe and the well pipe.

The air pipe as well as the discharge pipe can be shifted vertically by clamps. Initially these pipes are held with their lower ends near the bottom of the screened section (or strainer) of the well pipe. The lower end of the air pipe is kept about 0.3 m lower than that of the discharge pipe. The valve connecting the air tank and the air pipe is kept closed and the air pressure is allowed to be built up from 690 to 980 kN/m² (7 to 10 kg (f)/cm²) in the air tank by the compressor.

The compressed air is then released suddenly into the well through the air pipe by opening the valve. The inrush of air into the well creates a powerful surge within the well due to which the pressure increases and the water flows outwards through the strainer into the surrounding formation where it loosens the fine material. The air valve is then closed so that the pressure decreases and the water from the surrounding formation enters the well bringing the loosened fine material with it.

The water is then pumped out from the well to remove the loosened fine material from the surrounding formation. The pipe assembly may be operated as a conventional air lift (or air jet) pump. For this the air pipe is raised so that its lower end is about 0.3 m higher than that of the discharge pipe and the compressed air is introduced into the well through the air pipe due to which water is pumped out through the discharge pipe.

The pumping is continued until water free from sand is obtained. The air valve is then closed and the air pipe is lowered again and the process is repeated until water obtained from the well is free from sand.

The same operation is repeated at different levels along the screened section of the well pipe by lifting the pipe assembly every time by a metre or more, until the entire length of the screened section has been fully developed. Finally the pipe assembly is lowered again to the bottom of the well and operated as a pump to clean out any sand that may have accumulated.

(iv) By Backwashing:

In this method the top of the well is fitted with an air tight cover. A compressor and discharge and air pipe are installed in the same manner as in the previous method. However, in this case the lower end of the air pipe is always kept about 0.3 m higher than that of the discharge pipe, because the pipe assembly is operated only as an air lift pump. In addition to these pipes a short air pipe is fitted at the top of the well. A three way valve is provided to connect the short and the long air pipes with the compressor.

In the beginning the compressed air is released into the well through the long air pipe which forces air and water out of the well through the discharge pipe. When clear water starts coming out of the well, the air supply is shut off by closing the valve and the water is allowed to return to its static level. The three-way valve is then turned to admit compressed air into the top of the well through the short air pipe.

This back washes the water from the well through the discharge pipe and at the same time agitates the fine material surrounding the well. Air is supplied through the short air pipe until air begins to escape from the discharge pipe.

The valve is then turned so that air is again supplied through the long air pipe so that the pipe assembly works as an air lift pump and the water is pumped out from the well. The operation is repeated till clear water comes and the well is fully developed.

(v) By Chemicals:

In this method use of chemicals is made for the development of a well. In one of the commonly used methods hydrochloric acid and solid carbon dioxide (dry ice) are used for the well development. The hydrochloric acid is first poured into the well. The well is capped at the top and compressed air is introduced into the well which causes the hydrochloric acid to enter the surrounding formation.

The cap is then removed from the top of the well and dry ice is poured into the well. The sublimation releases gaseous carbon dioxide and a high pressure of this gas is built up in the well. With the release of this pressure the muddy water is forced up through the well in the form of a jet. Thus the fine material is automatically forced out with the water.

The addition of any of the polyphosphates to water in the well helps the development process. These compounds act as deflocculants and dispersants of clays and other fine grained materials, thereby enabling the mud cake on the wall of a hole and the clay fractions in an aquifer to be more readily removed during development of well by any of the methods.

One such polyphosphate commonly used is Sodium-hexametaphosphate (Calgon) which when added to water acts as dispersing agent for clays and other fine grained materials. Calgon is often added to water in the well during development by surging or by backwashing.