Permeability Evolution and Water-Rock Reaction Mechanism of Dolomite Geothermal Reservoirs in Wumishan Formation During Reinjection

Geothermal reinjection is critical for sustainable exploitation of geothermal resources, yet the permeability evolution mechanism of carbonate geothermal reservoirs remains poorly understood. Taking dolomite geothermal reservoirs of the Wumishan Formation in the Xiong’an New Area as the research object, high-temperature and high-pressure dynamic flow experiments and TOUGHREACT simulations were performed to investigate the effects of injection pressure, temperature, and water chemistry on the permeability of matrix and fractured cores. Results show that the optimal injection pressure and temperature for both core types are 14 MPa and 70 °C. Under such conditions, the final average permeability of matrix and fractured cores is 0.0015 mD and 0.0287 mD, with CV values of 7.3% and 6.2%, respectively. Chemically treated water suppresses calcite dissolution by altering ionic composition and pH, thereby reducing permeability of both cores, while the reduction is less significant for fractured cores. Simulations indicate that reinjection water triggers slightly stronger calcite dissolution than dolomite precipitation and yields permeability enhancement. When treated water is injected, reaction intensity declines, Mg2+ concentration approaches zero, and dolomite precipitation becomes more thorough. Matrix cores follow an “activation-stabilization” pattern, whereas fractured cores exhibit a “decay-convergence” pattern and are more sensitive to pressure. This study reveals distinct responses of matrix and fractures to hydrochemical perturbations in dual-porosity carbonate geothermal reservoirs, demonstrating that permeability evolution during reinjection is jointly governed by reservoir pore type and hydrochemical conditions. The findings provide mechanistic support for targeted regulation of reinjection water chemistry.