Investigation on real-time permeability evolution of granite under subcritical and supercritical water conditions

The in situ permeability evolution of granite is critical for efficiently developing subcritical and supercritical geothermal resources (200–400 °C). Herein, a high-temperature, high-pressure triaxial seepage system was employed to compare the real-time permeability evolution of K-feldspar-rich and plagioclase-rich granites with increasing temperature under only thermal effect (T), thermal–mechanical (T–M) coupling and subcritical/supercritical water thermal–mechanical–hydrological–chemical (T–M–H–C) coupling conditions. Combined with SEM and ICP–MS analyses, the results show that: Compared with T conditions, T–M coupling conditions strongly suppressed permeability enhancement, particularly within 300–500 °C. Under T–M–H–C coupling conditions, permeability increased slowly below 350 °C. With the subcritical-to-supercritical transition of the fluid medium from 350 °C to 400 °C, the permeability of both granite types increased sharply, reaching 9–13 times the values at 350 °C. At 500 °C, the real-time permeability reached the order of 10 −16 m 2 , 2–3 orders of magnitude higher than the initial values. Compared with the subcritical stage, hydrological–chemical coupling under supercritical water conditions more strongly enhanced granite permeability under high-temperature triaxial stress. During the subcritical stage, plagioclase-rich granite showed nearly stagnant permeability increase because of intense competition between mineral dissolution and precipitation. Meanwhile, K-feldspar-rich granite underwent limited water–rock reactions and showed permeability increases of more than one order of magnitude. During the supercritical stage, changes in fluid properties and enhanced convective–diffusive mass transfer promoted mineral dissolution and elemental migration, facilitating microfracture propagation and coalescence, thereby forming effective fluid-flow networks. Within 400–500 °C, the permeability difference between the two granite types progressively decreased, and the permeability enhancement relative to the initial values under T–M–H–C conditions was approximately 1–2 orders of magnitude greater than that under T–M conditions.