Summary To address the low rate of penetration (ROP) in deep and ultradeep well drilling, we developed a coupled smoothed particle hydrodynamics (SPH)-finite element method (FEM) numerical model to investigate the synergistic rock-breaking mechanisms of high-pressure water jets and polycrystalline diamond compact (PDC) cutters. Four representative cutter geometries and key jet bit parameters were systematically examined. The model was validated against experimental measurements of cutting forces and crater morphology. Mechanistic analysis indicates that the combined rock-breaking process proceeds through three stages: compaction, small-scale fragmentation, and large-scale fragmentation. The initial jet impact promotes crater and slot formation. The jet-induced stress concentration around the crater periphery superimposes with cutter-induced stresses, expanding the fragmentation zone and facilitating crack initiation and propagation. In addition, jet sputtering generates reflected stress waves that, although attenuated, promote the progressive growth of existing craters. The results indicate that the double-arc cutter performs best under the following conditions: 40-MPa jet pressure, 2-mm nozzle diameter, 15° inclination angle, 40° azimuth angle, and 25-mm radial distance. For the tested sandstone with a uniaxial compressive strength of 32.5 MPa, this jet pressure corresponds to approximately 1.23 times the uniaxial compressive strength. Under these conditions, cutting forces are markedly reduced, and cuttings transport efficiency is improved because jet impact and bit rotation jointly shape the bottomhole flow field, promoting cuttings removal and mitigating secondary crushing. This numerical framework clarifies how jet parameters and cutter geometry jointly influence rock fragmentation and cuttings transport, providing a reliable basis for PDC bit optimization and ROP improvement in deep and ultradeep wells.
Cai et al. (Fri,) studied this question.