| Abstract |
Axial-torsional coupled percussive drilling (ATCPD) is a promising method showing merits in creating volumetric breaking patterns and improving drilling rate of hard rocks, and thereby has attracted more and more interest recently in hot dry rock (HDR) geothermal drilling. To determine the dynamic single-cutter-rock interaction behaviors of ATCPD in HDR, we adopted a 3D FEM based thermo-structural coupled modelling approach. In this approach, a damage-viscoplasticity model accounting for the high strain rates was employed to characterize the rock failure. Thermal boundary conditions were implemented by the concrete damage plasticity code. Based on this model, we simulated the cutting process of the single cutter at different percussive modes (axial percussion, torsional percussion, axial-torsional percussion) and analyzed the corresponding damage evolution processes. By conducting the sensitivity analysis, the effects of the temperatures, the dynamic impact loads, and the ratio of the axial impact to torsional impact frequency on the stress around the cutter were investigated. Rock scratch experiments were further conducted on granite specimens under different temperatures to validate the numerical model. The results indicate that the axial and torsional percussion contribute to the generation of rock cohesive and tensile damage, respectively. Unlike them, ATCPD shows more excellent damage performances of HDR, resulting in a 117.73% increase in average Mises stress. This promoting effect can increase the displacement of the cutter in the penetration direction and the cutting direction, so that the excellent ROP enhancement performance in the same rock breaking time is generated by the axial-torsional coupled percussion method. The penetration force and cutting force of the cutter within the same rock breaking depth, as well as the Mise stress inside the rock, decrease with increasing rock temperature, but the trend of forces changing with depth remains unchanged, and higher cutting and penetration displacement are generated in elevated temperature rock. This phenomenon can be validated by the fluctuation of the tensile stress and compressive stress. The larger the axial impact force and the torsional impact torque, the cohesive and tensile damage area will increase at high temperatures. The stress and displacement of coupled percussion rock breaking show significant differences with the change of impact frequency matching in different rock temperatures. That is, the efficiency of geothermal drilling will be influenced by the matching of the rock temperatures and the impact parameters. Our findings are expected to provide an in-depth understanding of percussive drilling mechanisms in HDR. |