Influence of Low-Temperature Fluid Thermal Shock on Hydraulic Fracture Propagation in Deep Shale

Deep shale subjected to thermal shock from low-temperature fracturing fluid in high-temperature and high-pressure (HTHP) environments will severely affect the propagating behavior of hydraulic fractures. This study conducted hydraulic fracturing physical simulation experiments under HTHP conditions, as well as numerical simulations of thermal shock-induced shale cracking based on the cohesive zone model (CZM), to systematically investigate the initiation mechanisms and propagation behavior of thermal cracks under thermal shock. Experimental results indicate that as the rock temperature increases from 25 to 200 °C, the thermal shock effect becomes significantly enhanced. The stimulated rock area (SRA) increases from 1.00 to 1.75, and the fracture fractal dimension (FD) rises from 1.92 to 2.01. Meanwhile, the breakdown pressure at 200 °C decreases by approximately 9.8% compared to that at 25 °C. Numerical simulation results show that higher thermal shock rates (with a shock duration of 25 s) lead to an increased number of thermal cracks (up to 31 fractures). Moreover, multiple cycles of thermal shock (10 cycles) can enhance the average crack length by 51% through cumulative damage accumulation. By leveraging thermal shock effects, more advanced hydraulic fracturing techniques may be developed for deep earth energy reservoirs, with the potential to reduce breakdown pressure and enhance fracture network complexity. These findings provide laboratory-scale evidence and mechanistic insights for future fracturing process optimization.