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Abstract

The properties of rocks, especially the thermal properties, porosity and permeability properties, are of great significance to accurately predict the operational performance of Enhanced Geothermal Systems (EGS). In order to reveal the evolution mechanism of rock properties of CO₂-based EGS reservoir during geothermal exploitation, experiments on the interaction between the mixed fluid of CO₂ and water (the proportion of water is 0, 51.205, 100%) and granite were carried out. Based on the change of microstructure characteristics of rock samples, the changing mechanism of rock properties is analyzed. The results show that the properties of rock samples exposed to pure CO₂ change most weakly, while those exposed to water change most dramatically. The porosity and permeability of rock samples exposed to water increase by 236.70% and 7497.17%, respectively, while the thermal conductivity and thermal diffusion coefficient decrease by 21.55% and 22.53%, respectively. The change of rock properties is closely related to the change of rock microstructure. The main mechanisms of microstructure change include thermal fracture caused by thermal stress, chemical reaction between fluid and rock, and the increase of micropores caused by CO₂ extraction of water inside rock. The microstructure change mechanism of rock samples is different in different reaction systems. Although the mixed fluid of CO₂ and water is weakly acidic and reacts with the reservoir rocks, the structure changes are the most drastic, but secondary minerals will fill the new micro-cracks and pores, delaying the influence of chemical reactions on the changes of rock properties.