Dual-wall reverse-circulation drilling uses dual wall RC Drill Rods with high pressure air as the circulating medium. Air is forced down the annular space between the inner and outer pipe and the cuttings are lifted up the inner pipe in a reverse-circulation manner. Continuous formation and water samples are returned, which have not been contaminated with drilling additives or mixed with other borehole material.
The outer pipe of dual-wall drill pipe is flush threaded. Inner pipe connections are made with an airtight double “0” ring seal (Figure 1). The inner pipe is held in place within each joint of drill pipe by a centering device.
An air compressor capable of providing an up-hole velocity on the order of 70 ft/ sec permits reliable logging of water, mineral or contaminant-bearing strata. A filter is required on the, compressor to prevent oil and foreign debris from entering the system. Water may be injected into the air stream as mist to aid in the recovery of samples, or circulated without air to control heaving san