The disc bit-a tool for hard-rock drilling

This paper traces the development of a radically different design of drill bit, describing its concept, laboratory work, and the latest field trials. Disc-bit technology is based on the single-rolling disc cutter. This cutter was first used successfully in rock on a tunnel-boring machine in 1956. By the early 1970s, virtually every rock-tunneling machine in the world had evolved from picks, tungsten carbide button cones, or multirow tapered cutters to the single-row disc. By then, practical field experience had demonstrated the superior performance of tunneling machines. Further, the Colorado School of Mines (CSM) began scientific studies and published data comparing the specific energy of excavation of various tools on the basis of full-scale laboratory testing. These studies clearly showed that the single-row disc cutter required less energy than conventional cutters used in roller cones to excavate a given volume of rock. With energy at the cutting face always a limited commodity, a lower energy requirement translated to a higher rate of penetration (ROP). Not until the mid-1990s was the concept of the single-rolling disc cutter applied to drill bits. Very small-3 1/4-in. Mini-Disc-cutters were evaluated at CSM and found to be extremely efficient and sufficiently robust to be used on drill bits as small as 7 7/8 in. An evaluation of an 8 1/2-in. disc bit was performed at the Gas Research Inst. (GRI) Catoosa Test Facility in Oklahoma. The U.S. GRI sponsored an effort to develop the concept through laboratory and field testing. Other field trials were sponsored by private and government grants, including the most recent test funded by a joint industry project (JIP) and managed by Petrobras.