Analysis of nonlinear seepage characteristics and parameter evolution of fractured granite under thermo-hydro-mechanical coupling conditions

To elucidate the nonlinear seepage characteristics and evolution of key controlling parameters of fractured granite under coupled thermo-hydro-mechanical conditions, fluid flow through fractured granited were investigated using a self-developed high-temperature and high-pressure triaxial seepage testing apparatus. Single-fractured granite specimens with varying roughness were selected and subjected to experiments under a range of confining pressures (1-20 MPa), temperatures (25-90 ℃), and pressure gradients (1-12 MPa/m) to simulate the interactive effects of multiple factors. The geometric morphology of the fracture surfaces was accurately captured using three-dimensional laser scanning technology and digital reconstruction techniques. In combination with the Forchheimer equation and indices, such as the non-Darcy factor and the critical pressure gradient, the evolution mechanisms of volumetric flow rate, hydraulic aperture and hydraulic conductivity with respect to confining pressure and temperature were quantitatively analyzed. The results show that the seepage behavior of single-fractured granite markedly deviates from Darcy flow under high pressure gradients or flow rates, with the Forchheimer equation yielding an excellent fit (R²>0.98), thereby demonstrating pronounced nonlinear characteristics. Furthermore, an increase in confining pressure leads to the compaction of fracture asperities, significantly reducing hydraulic aperture and consequently suppressing permeability, while an increase in fracture roughness exacerbates the formation of closed zones, further hindering fluid flow. Additionally, elevated temperatures reduce fluid viscosity and enhance seepage velocity under low confining pressures. However, under high confining pressures, the expansion of microcracks and local surface wear partially offset this permeability enhancement. Overall, the study confirms that nonlinear seepage is pervasive in deep geological environments, as evidenced by the transition of hydraulic conductivity from nearly constant behavior to power-law decay with increasing pressure gradients. These findings provide a robust scientific basis for the safety assessment and design of fractured permeability in nuclear waste disposal repositories, geothermal developments, and other deep underground engineering applications, as well as critical experimental support for subsequent multi-field coupling numerical simulations and mechanistic refinements.