Rock cutting mechanism of special-shaped PDC cutter in heterogeneous granite formation

Special-shaped Polycrystalline Diamond Compact (PDC) cutters show the superiority for improving rate of penetration (ROP) in hard abrasive formation drilling. However, few attentions have been paid on its geometric design and rock cutting mechanism. To deepen the understanding of special-shaped PDC (SPDC) cutter's rock-breaking mechanisms and provide a certain reference for the selection and application of SPDC cutters in the specific target formation, this paper firstly proposes two new principles (loading performance (LP) and cutting performance (CP)) as well as their evaluation indexes for evaluating the cutting process of cutters from the relationship between geometry, aggressiveness, stress state, working life, and rock cutting efficiency of SPDC cutters. Then, using gray-white granite as the research object, the finite element heterogeneous granite model, whose relative percentage error of all calibration quantities with experiment are less than 7%, based on Voronoi tessellation and the granite cutting model of 10 types cutters under confining pressure are established. Finally, the cutting results are discussed, and the CP of all cutters are evaluated. It's found that the ductile-brittle failure mode in the removal area and the influence region of cutting process determine the rock failure mode of SPDC cutters. The confining pressure has an inhibitory effect on rock cutting efficiency, it's because the high confining pressure leads to ductile dominated failure mode in removal area. The confining pressure weakens the LP and CP of cutters, but has no obvious effect on their stress distribution state. With the increasing of confining pressure, “ductile-plastic stick-slip cutting” effect is produced in the removal area. The conical-shaped cutter (whose CP coefficient is 4.3 × 10−3 - 5.4 × 10−3 N−1) and saddle-shaped cutter (whose CP coefficient is 3.4 × 10−3 - 4.8 × 10−3 N−1) are the optimal selection for heterogeneous granite formation. On the contrary, the three-blade cutter and cusp-shaped cutter are unsuitable for heterogeneous granite formation. This research leads to an enhanced understanding of rock-breaking mechanisms of SPDC cutters, and provides the basis to select of SPDC cutters for the specific target formation.