Investigations on the Stinger PDC cutter breaking granitoid under in-situ stress and hydrostatic pressure conditions

The Stinger PDC cutter has been widely used for deep hydrocarbon and geothermal drilling because it can significantly improve the rate of penetration (ROP) and extend PDC bit life in hard and abrasive formations. The hydrostatic pressure and in-situ stress in the borehole have a substantial influence on rock breaking for the Stinger PDC cutter. The purpose of this study is to investigate the impacts of confining pressure on the force response, rock breaking efficiency, and rock fragmentation process during cutting granitoid by Stinger PDC cutter cutting. A total of 51 group cutting experiments were conducted under various confining pressure. The influences of hydrostatic pressure, in-situ stress, cutting depth, and cutting angle on the cutting force and the rock breaking efficiency were compared and analyzed. In addition, a Particle Flow Code (PFC) 2D model was established to simulate cracks’ initiation and propagation and rock fragmentation process under confining pressure conditions. The results showed that the cutting force and mechanical specific energy (MSE) increase significantly with the growth of hydrostatic pressure and in-situ stress. The influence of hydrostatic pressure is larger than in-situ stress. The cutting force increases linearly with the increase of cutting depth and cutting angle. The MSE declines linearly with increasing the cutting depth and increases linearly with the rise of the cutting angle. Appropriately improving the cutting depth and decreasing the cutting angle can enhance rock breaking efficiency. The hydrostatic pressure promotes the macro-cracks propagating to the rock interior. But it will transform the rock failure mode from brittle to ductile. The in-situ stress can promote the cracks in front of the Stinger PDC cutter to propagate along the direction of in-situ stress and restrict the generation and propagation of bottom macro-cracks. The key findings of this work will help to understand the rock breaking mechanism of the Stinger PDC cutter under confining pressure and provide some quantitative data and guidance for the hybrid PDC bit design.