Experimental study on CO2-brine-sandstone interaction and reservoir stability analysis under high temperature and high pressure

The deep brine aquifer serves as a natural reservoir for the CO2 Plume Geothermal (CPG) system, offering the potential for both CO2 geological sequestration and thermal resource exploitation. In this study, a seepage experiment was conducted to inject CO2-dissolved brine into reservoir sandstone under high temperature and high-pressure conditions. The experiment encompassed analysis of mineral composition, ion concentration of the seepage solution, permeability, porosity, and elastic modulus. The PHREEQC 3.0 software was employed to simulate the temporal variation of ion concentration and mineral composition in the seepage solution. Furthermore, our study discussed the interaction mechanism of CO2-brine-rock, while considering the influence of CO2 injection on the stability of the brine reservoir. The reaction between CO2, brine, and sandstone leads to partial dissolution of primary minerals such as K-feldspar, plagioclase, illite, and hematite. This dissolution is accompanied by an increase in quartz, calcite, and montmorillonite, as well as the formation of secondary minerals like dolomite, kaolinite, and chlorite. Consequently, the permeability and porosity of the sandstone exhibit an initial increase followed by a decrease. The concentration of K+, Mg2+, Ca2+, Fe2+, Al3+, and SiO2 in the solution initially increases rapidly and then gradually decreases due to the dissolution of primary minerals and precipitation of secondary minerals. Conversely, the initial concentration of Na+ in the solution experiences an initial decrease followed by a gradual increase back to its initial concentration. The concentration of Cl− remains relatively stable, consistent with the simulation results obtained from PHREEQC 3.0 software. The combined effect of CO2-brine-sandstone interaction and seepage results in a decreased elastic modulus upstream of the seepage point, while downstream of the seepage, the elastic modulus surpasses the initial value. Moreover, the elastic modulus gradually increases in the direction of seepage. These findings suggest that during the early stages of CO2 geological sequestration and thermal resource exploitation, certain sandstone reservoirs may experience a deterioration in elastic modulus due to the temporary dissolution of minerals caused by CO2-brine-sandstone interaction. However, over time, as carbon-fixing minerals (calcite, dolomite, etc.) and clay minerals (kaolinite, chlorite, etc.) precipitate, the elastic modulus may increase, thereby contributing to reservoir stability. These research findings provide valuable references for the implementation of the CPG system and similar projects.