The rock breaking mechanism analysis of axial ultra-high frequency vibration assisted drilling by single PDC cutter

Axial ultra-high frequency vibration (UHFV)-assisted drilling technology that combines polycrystalline diamond compact bits and rotary ultrasonic machining technology is expected to improve rock-breaking efficiency and reduce the drilling costs of deep and ultra-deep wells. To better understand the rock-breaking mechanism under a UHFV load and provide a theoretical basis for practical engineering, this paper presents numerical modelling with the finite element method of dynamic cutting processes under different UHFV loads. The cutting force, mechanical specific energy (MSE), rock failure mode, and crack propagation process of a heterogeneous rock with natural cracks are analysed, and then compared with the steady load cutting condition. The simulation results indicate that when the excitation frequency is close to the natural frequency of the rock (approximately 25-30 kHz), the MSE and cutting force reach the minimum value. Compared with the steady load, the average cutting force and MSE under a 25 kHz UHFV load decreased by 49.4% and 28.5%, respectively. The number of cracks increased by 24.9%, while the crack volume and area reached the maximum. In addition, the shear crack ratio can be reduced under UHFV loads. The proportion of shear cracks in 25 kHz UHFV cutting was 5.3% lower than that in steady load cutting. Thus, axial UHFV-assisted drilling can effectively improve the rock-breaking efficiency by adjusting the magnitude of the excitation frequency. The results provide a theoretical basis for the new rock-breaking technology of PDC bit drilling.