Investigation on the rock failure characteristics and reliability of hybrid drill bits combing shaped PDC cutters and roller-cone elements

Hybrid bits, equipped with PDC cutters and roller-cone elements, have been widely utilized during drilling due to their exceptional performances in directional wells and interbedded formations. Meanwhile, the shaped PDC cutter offers a novel insight in enhancing the capability of bits. In this study, a numerical model is developed to investigate rock failure mechanisms under the combined action of roller cutters and shaped PDC cutters. Bottomhole pressurized conditions are simulated in the model by incorporating both in-situ stresses and hydrostatic pressures. Additionally, heat flux is applied to the top surface of the rock, thus studying the thermal conduction between drilling fluid and high-temperature rocks. Based on the variation of a series of indexes calculated in the present study, rock failure characteristics under different cutter groups and formation conditions are evaluated. The results indicate that roller cutters have a larger capacity to load the penetration force, with a smaller cutting force being delivered simultaneously, indicating lower torque during the drilling process. These characteristics can improve the impact resistance and reliability of bits in interbedded formations and directional wells. Under the same cutting depth, the largest penetration force is generated by the wedge cutter, which is almost twice that applied on the conical cutter. The loading of bottomhole pressures has negative influences on rock failure efficiency. In this research, the stinger cutter is poorly sensitive to pressures. Thereby, it is recommended for drilling into ultra-deep wells, thus weakening the effects of pressurized conditions. Thermal stress is induced by the temperature gradient in the rock. Correspondingly, a zone with tensile stress can be detected, which leads to a larger rock failure volume, cutting depth, and higher rock failure efficiency.