A Guide To Water Well Casing and Screen Selection   -   3

2.2  Rotary

During the last 30 years, the use of direct rotary and reverse circulation rotary drilling methods has come to dominate the construction of higher capacity water production wells. Both rotary methods are linked with the gravel envelope well design. With the direct rotary method, a rotating bit under controlled loading is applied to the formation. Water with additives to provide weight and viscosity is pumped down the drill pipe, through the bit, and circulates up the hole carrying the cuttings, which are separated and removed at the surface. Usually the finished borehole is accomplished in two or more stages. A smaller pilot bore is drilled first, then reamed to a diameter 6 to 12 inches greater than that of the casing and screen. The screen is selected and designed according to information gained through analysis of the cuttings* and electric logging. It is then installed with the casing in a continuous operation. Selected gravel is placed in the annular space between the casing and enlarged hole to stabilize the formation and provide a filter against fine sand or silt which might be present.

The reverse circulation rotary method varies from the direct rotary method in three major respects. The circulating fluid flows down the hole and up the drill pipe. While hydrostatic pressure against the formation maintains the wall of the borehole in both systems, usually no additives are mixed with the circulating water. Finally, under reverse circulation procedures, the hole is normally drilled without staging. Selected casings and screens are installed and gravel placed as in the direct rotary method.

Many factors are considered in selection of drilling method and well design. Among them are depth, diameter, hardness or formation, presence of fine-grained aquifers that need a gravel envelope filter, accessibility of site to availability of the quantity of water required for drilling. Rotary drilling construction, particularly reverse rotary, requires large amounts of water. Many factors are considered in selection of drilling method and well design. Among them are depth, diameter, hardness or formation, presence of fine-grained aquifers that need a gravel envelope filter, accessibility of site to availability of the quantity of water required for drilling. Rotary drilling construction, particularly reverse rotary, requires large amounts of water.

In some areas, gravel envelope wells permit the production of greater quantities of water than non-gravel envelope wells, but this is not always the case. Many high efficiency water wells are being constructed today by the cable tool method.

*See Appendix II.

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