Fig. 4.12 Pull-back method of installing screen
In the case of rotary-drilled wells, setting the casing to the bottom of the hole and then pulling it back may appear to be extra and unnecessary work in view of the drilling fluid supporting the borehole wall. But this extra work prevents serious problems which may arise on account of premature caving in which may occur when the viscosity of the drilling fluid is reduced prior to development. This casing is also useful when there is likely to be a longer period between drilling and screen installation, and during which period a momentary loss of drilling fluid may cause partial collapse of the borehole.
A filter pack is generally placed in large-diameter wells by the reverse circulation of the fluid in the well as the filter pack material is fed into the annular space outside the screen by a continuous-feed hopper. When the filter pack material fills the space around the well screen, the transporting water is drawn upward through the screen openings (Fig. 4.13).
