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

To investigate the chemical damage mechanism following the injection of supercritical CO2 into a saline aquifer, a self-made thermal-hydrological-mechanical-chemical coupled experimental system of 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 rock 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 supercritical CO2 injected into a saline aquifer.The results show that the mass fraction of calcite and K-feldspar decreases gradually as the reaction time increases, and the mass fraction of calcium, potassium and their oxides also decreases, 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 K-feldspar, and finally illite. During the reaction time, the mass corrosion rate and Poisson 's ratio of reservoir rocks gradually increases, and the uniaxial compressive strength and elastic modulus gradually decreases.The uniaxial compressive strength of reservoir rocks shows an exponential function relationship with reaction time.The functional relationship between elastic modulus, Poisson's ratio and reaction time is established.The strength prediction model of reservoir rocks under chemical reaction is established.Taking elastic modulus as the damage variable, the relationship between the uniaxial compressive strength and the damage variable of reservoir rocks is obtained.