Diamond Compact Cutter Studies for Geothermal Bit Design

Abstract

As a prerequisite to design of a geothermal drill bit using diamond compact cutters, the rock cutting characteristics of these compacts have been studied in a series of instrumented single-cutter continuous and interrupted turning tests on hard, high-compressive strength sandstone and other rocks of known properties. A range of compact rake angles was used and cutting forces were measured. Rake angles affected cutting efficiency (specific energy) at light cuts but were less significant at high penetration forces more characteristic of bit weights used in drilling. In continuous turning, cutter forces exhibit regular, repeating variations probably related to the alternate crushing and fracturing modes which the rock undergoes. In interrupted cutting, forces drop to zero, as expected, while passing through a void but on subsequent impact the forces are no greater than measured in continuous cutting. Thus, force fluctuations when cutting a composite of hard and soft rock should not be significantly greater than those generated in cutting each rock separately. Of considerable interest is the fact that cutting forces were found initially to be independent of speed over the tenfold speed range and low depths of cut covered. More sophisticated cutting experiments covering a wider range of conditions and rock types showed a significant effect of speed on force. However, the effects of changing speed were not as great as those of geometry and depth of cut. The results of these experiments have been used in generation of a preliminary bit design.

Experimental study of rock cutting under external hydrostatic pressure

“…As the depth of cut increases, the mean cutting force tends to be lower than predicted by the linear relationship [10]. Rock cutting tests conducted by several researchers on various types of rocks have shown that the optimum back rake angle of PDC bits varies from −5 to −25 • [11][12][13][14]. These and other experimental studies have contributed immensely to the understanding of rock cutting and have provided insights into the mechanisms of rock cutting.…”

Drag Bit Wear Model

“…3 The wear study suggested that increases in temperature result in exponential increases in wear rate. Laboratory wear studies in Jack Fork sandstone showed that PDC cutter wear was dependent on the cutter speed and wearflat temperature.…”

Mathematical modelling of performance and wear prediction of PDC drill bits: Impact of bit profile, bit hydraulic, and rock strength

“…A number of researches aimed to design the crown shape of the PDC bit by employing the principles of equal wear, equal volume of rock removed and power (Glowka, 1985, Hibbs andFlom, 1978). However, they ignored the effect of cutter-rock interaction; therefore, they did not efficiently reflect the force and the cutting wear.…”