Research on rock-breaking mechanism of cross-cutting PDC bit

Abstract

Cross-cutting PDC (Polycrystalline Diamond Compact) bit is an efficient drilling technology that increases the rock-breaking efficiency and prolongs the service life of the bit through forming mesh-like pattern in the bottom-hole. This paper studies the rock-breaking mechanism of the cross-cutting PDC bit with the combination of both experiment and numerical simulation. With the nonlinear dynamical model being established, the stress status within rock units, sliding fracture characteristic, plastic energy consumption and stress distribution along the cutter edge are analyzed in the cross-cutting process. Compared with unidirectional cutting, cross-cutting generates larger tensile stress within the rock unit and achieves lower plastic energy consumption. Moreover average stress on the cutter edge in cross-cutting is much smaller than that in unidirectional cutting. When cutters break the rock protrusions formed during cross-cutting process, cracks inside of the rock will rapidly run through the protrusion, generating brittle fractures within the rock and consequently producing sizable volumetric fractures. During cross-cutting, the rock is damaged not only because the shear failure, but also brittle fracture, which is conductive to improving the rock-breaking efficiency. Through unit experiment under cross-cutting condition, this paper studies the influencing regularity of experimental parameters (including the cutter diameter, back rake angle, cross-cutting angle, cutters spacing and rock hardness etc.) on the cutting load, the research achievements are conductive to the deeper understanding of rock-breaking mechanism of the new bit and provides theoretical basis for the application of the technology.