Hydraulic fracturing is a key technology for extracting geothermal energy from hot dry rock (HDR) reservoirs. However, conventional hydraulic fracturing (CHF) encounters significant challenges, including high fracture initiation pressure, simplistic fracture geometry, and the potential for induced seismicity. To address these issues, a novel soft hydraulic fracturing (SHF) method is proposed, incorporating a pulse injection strategy to enhance reservoir stimulation in Enhanced Geothermal Systems (EGS). To explore the fracture initiation and propagation mechanisms under SHF, and assess the effectiveness of pulse injection, true triaxial hydraulic fracturing experiments were conducted on high-temperature granite samples, with a comparative analysis to CHF. The fracture morphology was characterized using computed tomography (CT) scanning and 3D surface reconstruction techniques. The results indicate that SHF induces fatigue deterioration of rock mechanical parameters, substantially alters fracture geometry, and improves fracture conductivity. Compared to CHF, SHF reduces breakdown pressure by approximately 10–30%, increases the stimulated reservoir volume (SRV) by 129.56–311.59%, and enhances fracture conductivity by nearly an order of magnitude. Furthermore, temperature positively influences SHF performance, as elevated temperatures, combined with thermal stress and fatigue-induced weakening, contribute to the formation of a more complex fracture network comprising primary and branched fractures. Additionally, SHF significantly increases fracture tortuosity and roughness, expanding the fracture surface and fluid contact areas, which improves heat transfer and enhances heat extraction efficiency. This study demonstrates the feasibility and potential advantages of SHF for HDR reservoirs, offering theoretical insights for its application in EGS development.
Zhou et al. (Mon,) studied this question.