Simulation Research on Rock-Breaking Performance of PDC Cutter Under Composite Impact Based on Discrete Element Method

Abstract

ABSTRACT This paper introduces a PDC (Polycrystalline Diamond Compact) cutting rock model loaded by spring-mass-damping system by DEM (discrete element method), and this model allows force boundary conditions in the vertical and horizontal directions of the PDC cutter. Therefore, the model can simulate the rock-breaking of PDC cutter under vertical, horizontal, and composite impact load. The influence of the component and frequency of the impact on the rock-breaking performance and force of the PDC cutter was investigated. Lower amplitude composite impact load or only increasing vertical impact load may lead to increase of the fluctuation of PDC cutter contact force and peak force value; Increasing the amplitude of composite impact load both in horizontal and vertical directions, or only increasing the amplitude of horizontal impact load, has a significant effect on suppressing the vibration of cutting, and can improve the cutting depth, reduce the contact friction, and reduce the MSE (mechanical specific energy) of rock breaking. The simulated rock crushing state show that under the composite impact, increasing the amplitude of horizontal impact load is beneficial to increase number and length of transverse cracks, which promotes the large volume rock spalling, and the effect of high-frequency composite impact on suppressing PDC cutter vibration is more obvious. INTRODUCTION The polycrystalline diamond compact (PDC) bit has been widely used in oil and gas drilling industry since the 1980s (Feenstra, 1988). Many researchers have carried out a lot of research on indoor rock-breaking experiments and numerical simulation of PDC cutter and full-scale bit. The influence of rake angle, cutting depth, rock material and drilling parameters on rock cutting is investigated. With the development of bit processing technology, the impact load and temperature-pressure field are introduced into the research of cutting rock. In addition, a variety of non-planar structure cutters, such as conical cutter, axe cutter, stay cool multi-dimensional cutter, and Firestorm cutter are developed (Si et al., 2018). In terms of laboratory tests of PDC cutting rock, Black et al. (1986) compared the drilling and rock breaking characteristics of stepped PDC bit with positive displacement motor and turbodrill respectively through laboratory tests, and its economic benefits were evaluated. They established a functional relationship between the rate of penetration (ROP), bit torque and rotational speed, weight on bit (WOB) and drilling fluid flow rate. Glowka (1987) carried out a large number of tests of PDC single-cutter to investigate the effects of cutter size, wear degree, jet pressure and rock type on the rock-breaking performance and force of cutters. The cutting force model was established based on the assumption of the shape of the contact surface between the cutter and the rock. Then, the cutting contact model was extended to the full-scale bit, and a program was developed to predict the drilling performance and wear of the field test bit. Subsequently, Sinor and Warren (1987), Warren and Armagost (1988), Warren and Sinor (1989) and other scholars carried out a large number of experimental studies on topics such as drilling performance and wear prediction of different types of PDC bits. However, only some regular patterns were obtained because the experimental conclusions were based on the specific test bit. Detournay and Defourny (1992) performed the rock-cutting experiments of a PDC cutter with different wear degrees. The functional relationship between cutting force and vertical force was proposed by defining the geometric characteristics of cutters and rock-breaking specific energy. On this basis, the equation of the functional relationship between rock-breaking torque and bit weight of the full-size bit was established. The equation is often used as the boundary condition at the bit when modeling the drill string system now. The tests also studied the fracture pattern transition from plastic and brittle at different cutting depths, and the primary purpose of the investigations was to try to obtain the strength parameters of rock through scratch tests (Richard, 1999; Richard et al., 1998). T. Richard systematically studied the cutting and crushing process of rock, adopting a scratch test for the first time, which provided a reliable verification for the numerical model of subsequent studies.