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packed well with cable tool rig, gravel is slowly fed around the assembly into the annular space while the casing is withdrawn and there should always be about 3 Mts.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. |
