Abstract: Supercritical geothermal systems are at the forefront of enhanced geothermal system (EGS) development. Investigating fracture seepage evolution in supercritical environments is crucial for the sustainable exploitation of geothermal resources. To reveal the evolution of seepage characteristics in hot dry rock fractures under the action of supercritical water, we examined the permeability of fine- and coarse-grained fractured granite under varying temperatures (25−500 ℃) and triaxial stresses, using a self-developed high-temperature, high-pressure rock triaxial mechanical testing system capable of reaching 600 ℃. The results indicate the following:1) Temperature significantly influences crack permeability, which can be divided into two distinct stages as temperature increases, with peak permeability observed at 250 ℃. 2) Under combined effects of temperature and supercritical water-rock interaction, fracture permeability evolves through four distinct stages as temperature increases, with peak permeability occurring at 250 °C and 400 °C. Near the supercritical state, water-induced weakening of rock properties and dissolution of free surfaces enhance permeability, with 350 °C identified as the critical threshold for permeability evolution under supercritical aqueous conditions. Following exposure to supercritical water exposure, the fracture’s surface morphology is redistributed due to mineral dissolution, precipitation, and recrystallization. The fracture’s filling rate after water-rock interaction is 14%, which alters the seepage pathways and consequently affects permeability. 3) Coarse-grained granite exhibits reduced permeability at low and medium temperatures, attributed to its high thermal expansion and low elastic modulus. At high temperatures, the extensive water-rock reaction area in coarse-grained granite leads to a 32.67% higher permeability reduction compared to fine-grained granite. This study provides valuable insights for the efficient development of enhanced geothermal systems under supercritical conditions.

Key words: geothermal exploitation, fracture seepage, supercritical water, water-rock interaction, morphological characterization