Fracture extension and heat transfer mechanism of hot dry rocks with different grain sizes under hydraulic fracturing

The extension of hydraulic fractures in hot dry rocks (HDR) and their seepage–heat transfer characteristics are strongly influenced by mineral composition and grain size. In this study, three granites with varying grain sizes (fine-grained G1, G2, and medium-coarse-grained G3) were subjected to hydraulic fracturing under high-temperature and true triaxial stress conditions. A fractal-based seepage–heat transfer coupling model grounded in actual fracture geometry was developed. The results indicate that the BP of medium-coarse-grained G3 granite with lower mica content and higher quartz content is approximately 1.4 MPa lower than that of G1 granite, and the tortuosity of hydraulic fractures is reduced. Additionally, compared to the fine-grained G1 granite with high mica content and low quartz content, the fractal dimension of the fracture surface, exit temperature, and overall heat transfer coefficient (OHTC) of G2 and G3 granites all decreased, exhibiting a clear positive correlation among the three parameters. At an injection velocity of 0.15 m/s, the OHTC of G2 and G3 were 10.49 W/(m²·K) and 21.06 W/(m²·K) lower, respectively, than that of G1. Furthermore, both OHTC and the local heat transfer coefficient (LHTC) generally increase with flow velocity. The injection velocity exhibits a negative exponential relationship with the OHTC and a quadratic polynomial relationship with the outlet temperature.