Experimental study on chemical damage and mechanical property degradation of reservoir rocks during process of CO₂ geological storage in a saline aquifer

To investigate the chemical damage mechanism of reservoir rocks following the injection of the supercritical CO₂ into a saline aquifer, a self-made thermal-hydrological-mechanical-chemical coupled experimental system of the supercritical CO₂-water-rock at constant temperature and pressure is established, which is used to test the changes of mineral components, microstructure, mineral elements and oxides of reservoir rocks before and after the reaction. And the changes of pH value and concentrations of Ca²⁺, K⁺, Mg²⁺ ions of the reaction solution are analyzed. Furthermore, this study focuses on elucidating the evolutionary patterns of physical and mechanical parameters, including mass, uniaxial compressive strength, elastic modulus, and Poisson's ratio. The experimental study reveals the chemical damage characteristics and mechanical property degradation of reservoir rocks after the supercritical CO₂ injected into a saline aquifer. The results show that the mass fractions of calcite and K-feldspar decrease gradually as the reaction time increases, and those of calcium, potassium and their oxides also decrease, resulting in the presence of corrosion cavities. The pH value of the reaction solution decreases from 7.19 to 5.68, and the corrosion rate of calcite is the fastest, followed by that of K-feldspar, and finally that of illite. During the reaction time, the mass corrosion rate and Poisson's ratio of reservoir rocks gradually increase, and the uniaxial compressive strength and elastic modulus gradually decrease. The uniaxial compressive strength of reservoir rocks shows an exponential function relationship with reaction time. The elastic modulus and Poisson's ratio show a cubic polynomial function relationship with reaction time. The strength prediction model for the reservoir rocks under chemical reaction is established. Taking the elastic modulus as the damage variable, the relationship between the uniaxial compressive strength and the damage variable of reservoir rocks is obtained, which reveals the chemical damage mechanism of reservoir rocks.