Development and application of a hybrid PDC bit for drilling the high temperature granite formation in the deep geothermal exploration well Fushenre-1

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.