Dual-Wall Reverse Circulation Methodology (PDF)
The dual-wall method uses flush-jointed double-wall pipe in which the compressed air moves by reverse circulation. Unlike conventional reverse circulation using drilling fluid which runs down the outside of the drill pipe down the annulus, the compressed air flow is contained between the two walls of the dual-wall pipe and only contacts the walls of the borehole near the bit. The most common drill pipe diameter for the dual-wall is 4 1/2-in. OD size. Male and female tool joints are used to connect the outer pipes; a connector sleeve with an “0” ring seals the joint between the inner pipes.
The circulation medium passes from the annular space between the two pipes, through a predrilled bit sub, and is discharged toward the cutting surface along the periphery of the bit sleeve; after entraining the cuttings, the circulation medium passes upward through the inner pipe.
At the surface, circulation medium enters the annular space between the inner and outer pipes by a special side inlet swivel. Circulation medium can consist of dry air, air and water, air and water with surfactants or water with clay or polymers. When air is used, uphole velocities in the dual-wall system average 4,500 to 6,000 ft/min. After passing down the annular space and up inside the inner pipe air passes with the formation sample into a cyclone that can be equipped with an automatic splitter.