Study on Composite Rock-Breaking Mechanism of Ultrahigh-Pressure Water Jet–PDC Cutter

The drilling industry is paying increasing attention to deep and ultradeep wells because of the gradual decline and depletion of recoverable resources on the shallow surface. However, the difficulty of conventional mechanical rock-breaking grows significantly with increasing drilling depth. It has been found that the effect of a high-pressure water jet combined with a polycrystalline diamond compact (PDC) cutter is significant and can greatly increase the efficacy of rock breaking. A composite rock-breaking experimental device with a high-pressure jet was designed to carry out composite rock-breaking experiments. Meanwhile, a composite rock-breaking numerical model of high-pressure water jet-PDC cutter was created by smoothed particle hydrodynamics/finite element method (SPH/FEM). After verifying the reliability of the numerical model through experiments, the key factors, including rock stress field, cutting force, and jet field, were extracted to analyze the composite rock-breaking mechanism. The results show that the enhancing effect of jet impact on rock breaking is mainly reflected in three aspects: (1) The high-pressure water jet can create a groove and crater on the rock surface, effectively unloading the rock stress at the bottom of the well and increasing the area of rock damage; (2) PDC cutter vibration can be efficiently reduced with high-pressure jet; and (3) the rock debris in front of the cutter is cleaned in time, avoiding the waste of energy caused by the secondary cutting and reducing the temperature rise of the PDC cutter. Besides, it has been investigated how parameters like jet pressure, nozzle diameter, impact distance, and cutting depth influence the effect of jet rock breaking. The findings indicate that the best rock-breaking efficiency and economy occur at jet pressures of 30–40 MPa. Correspondingly, in terms of nozzle angle, nozzle diameter, and impact distance, the ideal ranges are 60°, 1.0–1.5 mm, and 10 mm, respectively, wherein the ideal impact distance is approximately 10 times the nozzle diameter. This research is critical for the advancement of high-pressure jet drilling technology and the design of supporting drill bits.