A Guide To Water Well Casing and Screen Selection   -   2

2.0  METHODS OF WELL CONSTRUCTION

2.1  Cable Tool

There are several methods of drilling water wells today. These relate to particular well design and casing and screen requirements. A brief discussion of these techniques is presented below.

Many wells today are being drilled by a method which is centuries old. Although cable tool (percussion) drilling rigs and tools have changed, the basic principles involved in this system have remained the same. The borehole is drilled by the pulverizing action of a reciprocating steel bit suspended from the drilling rig by a wire cable. As the bit strikes the bottom of the hole, the formation is crushed, creating cuttings which are removed by bailing. If the formation is loose and unconsolidated, the casing must be forced in to the hole periodically to prevent caving.

Several procedures are available for completing wells drilled by the cable tool method. If casing is installed as the hole is drilled, it may be perforated by down-the-hole tools, forming a screen opposite the water-producing formations. With most methods of down-the-hole perforating, a small aperture cannot be formed nor can the aperture size be precisely controlled. Consequently, finer-grained aquifers must be avoided. In general practice, the cable tool method lends itself more to drilling coarser, harder formations. In some regions casing is installed to the total depth and a screen telescoped through the casing to the bottom. The casing is then withdrawn partially to expose the screen to the aquifer or aquifers. A variation of this method, permitted under certain drilling conditions, is installation of casing to the top of the aquifer and drilling additional open borehole to accommodate the screen, which is installed by lowering into place. With the advent of rotary drilling methods, these techniques are rarely used.

Small diameter wells for domestic purposes, drilled in tight, consolidated formations, are constructed with cable tools or down-the-hole air hammer. These wells often only need a surface conductor casing installed through the unconsolidated overburden. Water is produced from the open hole. In some cases, a protective casing is installed to the depth of the pump.

Under favorable drilling conditions and where the aquifers are well known, screens in the form of preperforated casings can be installed as the borehole is drilled. Such preperforated casing are normally slotted vertically for greater compressive strength to withstand being driven into place. The driller must be sure that he can carry the screens to the planned depth opposite the aquifers.

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