In direct rotary drilling, the viscosity and uphole velocity of the drilling fluid are the controlling factors in removing cuttings effectively. Unless cuttings can be removed, drilling cannot continue. Because of limitations in pump capacity and therefore effective cuttings removal, most direct rotary machines used to drill water wells are limited to boreholes with maximum diameter of 22 to 24 inches (559 to 610 mm). This size may not be sufficient for high-capacity wells, especially those that are to be filter packed. To overcome the limitation on hole diameter and drilling rate, reverse circulation machines were designed; originally they were used only in unconsolidated formations. Recently, reverse circulation drilling has been used in soft consolidated rocks such as sandstone and even in hard rocks using both water and air as the drilling fluid. Also, as hole diameters increase past 24 in, the rate of penetration by direct rotary machines becomes less satisfactory.

In reverse circulation rotary drilling, flow of the drilling fluid is reversed when compared with the direct rotary method. The suction end of the centrifugal pump, rather than the discharge end, is connected through the swivel to the Kelly and drill pipe. The drilling fluid and its load of cuttings move upward inside the drill pipe and are discharged by the pump into the settling pit. Centrifugal pumps with large passageways are often used to pump the drilling fluid because they can handle cuttings without excessive wear on the pump. In operation, however, most of the cuttings do not actually enter the pump but bypass it by means of an ejector system. An uphole velocity of at least 150 ft/min (45.7 m/min.) is recommended. The fluid returns to the borehole wall to gravity flow. It moves down the annular space between the drill pipe and borehole wall to the bottom of the hole, picks up the cuttings, and reenters the drill pipe through ports in the drill bit.

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