Reverse circulation requires that the drilling fluid flow from the compressor to the top of the annular space between the outside of the drill-string and the inside of the borehole, down the annulus, to bottom hole. After entraining the rock bit cuttings at bottom hole, drilling fluid flows with the cuttings, through bit, up the inside of drill collars and drillpipe, to the blooey line at the surface where the cuttings are disposed into pit. Many shallow and large diameter wells that are drilled with gas utilize reverse circulation technique. These wells are normally water wells, monitoring wells, and mining boreholes. The major advantage of using reverse circulation technique is to reduce the minimum gas volume requirement. This is achieved because of small cross-sectional areas inside the drillpipes where upward flow of drilling fluid with cuttings is carried out.

The theory and procedure for determining the minimum required volumetric gas flow rate in air, gas, mist, and unstable foam drilling in reverse circulation are the same as in direct circulation except that the cross-sectional area of the upward flow path is the cross-sectional area calculated based on the drillpipe ID. All equations derived for the direct circulation technique are applicable to the reverse circulation technique. The only two parameters that need to be noted are hydraulic diameter d _H and cross-sectional area of flow path A. They should be calculated based on drillpipe ID, not annulus diameters.

However, the engineering charts developed for the direct circulation...