Geothermal energy is a clean and renewable energy with significant potential. During the drilling process of geothermal development, conventional drill bits often suffer from significant wear and severe cutter breakage. The triple ridged PDC cutter has been widely incorporated into drill bit designs to effectively enhance rock breaking efficiency by fully utilizing its ridged structure. To investigate the influence of key cutting parameters on rock breaking performance in mixed cutter PDC drill bits and to improve the rock breaking efficiency of triple ridged cutters in granite formations, this study conducted indoor multi cutter mixed cutting experiments on granite. A comprehensive rock breaking performance index was defined based on the Mechanical Specific Energy (MSE) of cutting units, overall axial impact force, relative variation in tangential force across stages, reduction amplitude, and the mass fraction of large sized rock debris. Further analysis and optimization were performed on the tangential force in different stages, MSE, and the comprehensive rock breaking index to determine the optimal cutting parameters. Subsequently, a customized triple ridged PDC cutter bit was developed and tested in the Fushenre-1 well. The results indicate that during multi-cutter mixed cutting in granite, the integrated rock breaking effect between cutters results in a 21.22%–24.27% reduction in the average tangential force during simultaneous cutting, compared to the sum of the tangential forces from multiple cutters performing overlapping cuts. Additionally, the overall MSE of the cutting unit decreases with increasing front cutter depth and cutter height difference. Considering the comprehensive rock breaking index and the overall trends of various objective functions, the optimal cutting parameters for the “triple ridged cutter + planar cutter” combination were determined as follows: front cutter depth of 2 mm, cutter height difference of 0–0.5 mm, and front cutter spacing of 4–6 mm. Field tests demonstrated that the customized triple-ridged PDC cutter bit increased the rate of penetration (ROP) by 24.51%–27.00% compared to conventional drill bits. The findings provide a valuable reference for the optimization and selection of cutting structure parameters in hybrid PDC drill bits.

The oil and gas stored in deep and ultra-deep carbonate reservoirs is the focus of future exploration and development. Conical PDC (Polycrystalline Diamond Compact) cutter, which is a new kind of PDC cutter, can significantly improve the rate of penetration (ROP) and extend PDC bit life in hard and abrasive formations. However, the breakage characteristics and failure mode of the conical PDC cutter cutting carbonate rock is still masked. In this paper, a series of single-cutter cutting tests were carried out with the conical and conventional PDC cutters. The cutting force, rock-breaking process, surface morphology of cutting grooves and cuttings characteristic were analyzed. Based on the derived formula of the brittle fracture index, the failure model of carbonate rock was quantitatively analyzed under the action of conical and conventional cutter. The results show that the average cutting force of the conical cutter is less than that of the conventional cutter, which means greater stability of the cutting process using the conical cutter. Carbonate rock with calcite as the main component tends to generate blocky rock debris by conical cutter. The height of the cuttings generated by the conical cutter is 0.5 mm higher than that generated by the conventional cutter. The conical cutter exhibits enhanced penetration capabilities within carbonate rock. The accumulation of rock debris in front of the conventional cutter is obvious. Whereas, the conical cutter facilitates the cuttings transport, thereby alleviating drilling stickiness slip. At different cutting depths, the conical cutter consistently causes asymmetric jagged brittle tensile fracture zones on both sides of the cutting groove. Calculations based on the brittle fracture index demonstrate that the brittle fracture index of the conical cutter generally doubles that of the conventional cutter. For carbonate rock, the conical cutter displays superior utilization of brittle fracture abilities. The research findings of this work offer insights into the breakage process and failure mode of carbonate rock by the conical cutter.