Experimental Study of the Rock Cutting Mechanism with PDC Cutter Under Confining Pressure Condition

The existence of confining pressure is an important factor that can't be ignored in rock cutting process. Rocks show high strength and strong plasticity under confining pressure condition (CPC), leading to the low rock-breaking efficiency of drilling bit, which is more prominent in deep formation drilling. However, the understanding of rock-breaking mechanism of Polycrystalline Diamond Compact (PDC) cutter under CPC is still unclear. Given that fact, in this paper, a series of rock cutting experiments on sandstone and granite under different confining pressures and atmospheric pressure conditions were carried out using a self-made true triaxial rock cutting device. The cutting force, mechanical specific energy (MSE) and cuttings mass fraction under confining pressure and atmospheric pressure conditions were studied, and the cutting grooves were analyzed. The results show that the existence of confining pressure greatly increases the difficulty of rock cutting, which is embodied by the increase of cutting force and MSE. In addition, the size of cuttings produced under CPC is smaller, and the proportion of powdery cuttings increases with increasing confining pressure. Meanwhile, the cutting grooves obtained under CPCs are smoother. Microscopically, the existence of confining pressure greatly enhances the contact stress between PDC cutter and rock, resulting in a larger area of dense core when PDC cutter contacts with rock. Confining pressure leads to the difficulty of cuttings discharge to a certain extent, which easily results in the secondary and repeated destruction of cuttings and increases the energy dissipation. In general, confining pressure leads to the lower damage degree of PDC cutter to the rock, which makes the rock difficult to be drilled. The research results can provide some special guidance for rock breaking in deep formation, and provide practical thoughts and enlightenment for rock breaking, bits arrangement and optimization under CPC.