The reverse circulation drilling system is quite unlike any of the other systems profiled in that it is a very large rig requiring a big footprint for both the rig itself, as well as the machine dug settling pit.
These rigs are primarily associated with drilling large diameter wells in unconsolidated material ranging from 24 inches to 50 inches in diameter and up to 1000 feet deep.
The cutting actions used in the reverse circulation system include the rotary cut and the rotary crush actions. Shown here is in this video rotary cut.
The unique direction of a water based flushing media is profiled. This fluid is sent down the annular space between the drill string and the bore hole, removing cuttings back up the drill string and into the settling pit. This pit must be very large to accommodate a sufficient amount of fluid, which is required to flush the large amount of cuttings generated in drilling a borehole of this diameter.
The excavated material needs to be replaced with fluid. As drilling continues, further increasing the quantity of fluid required for this drilling process.
In addition to the makeup fluid required, a significant amount of fluid is displaced and lost to the original formation as this system does not typically use bentonite and therefore does not entirely seal the hole during drilling. While increasing the demand for liquid use during borehole construction, this system has the advantage of reducing development time as compared to traditional mud rotary drilling.