Fluid composition controls on reaction pathways and pore structure evolution in CO2-enhanced granitic geothermal reservoirs

The chemistry of circulating fluid is critical for the long-term sustainability of CO 2 -Enhanced Geothermal Systems (EGS). This study investigates the effects of Na + - and K + -dominated fluid chemistries on fluid–rock reaction pathways and the associated pore-structure and permeability evolution of granite at 200 °C and 25 MPa over 5 days. The objective is to clarify the cation-specific controls of monovalent cations on mineral dissolution–precipitation processes under representative EGS conditions. Results show that plagioclase hydrolysis was the predominant geochemical reaction, accompanied by the precipitation of diaspore and minor kaolinite. High Na + concentrations inhibited plagioclase hydrolysis, while K + suppressed the dissolution of orthoclase and biotite, consistent with a common-ion effect. The dissolution of the more reactive andesine initiated an interface-coupled dissolution–precipitation process, generating a local interfacial environment with an elevated Al/Si molar ratio that favored diaspore precipitation and inhibited kaolinite nucleation. Mineral dissolution predominantly increased porosity and shifted pore sizes toward larger diameters. Estimated permeability showed an overall increase after reaction, although the magnitude of permeability change differed between the two theoretical models. These findings demonstrate that fluid composition plays a pivotal role in mineral reaction pathways and pore-structure evolution in granitic CO 2 -EGS reservoirs, providing a basis for assessing fluid-chemistry management in EGS operations.