Investigating the mechanistic effects of supercritical CO2 on tight sandstone mechanical properties under varying water compositions

Supercritical CO2 fracturing technology has the potential to enhance the fracture network structure of tight sandstone, subsequently altering its mechanical properties. However, the mechanisms underlying these changes in mechanical properties vary due to differences in geological and water source environments. Therefore, this paper investigates the interaction between supercritical CO2 and various water-containing environments (dry-ScCO2, ScCO2–H2O, and ScCO2–brine), elucidating the mechanisms of change in mechanical parameters from the perspectives of mineral environment and variations in pore structure. These results indicate that the injection of ScCO2 induces mineral dissolution, accompanied by variations in the pore structure. Mineral dissolution primarily governs the changes in nanoscale and submicroscale pores, increasing them by 0.17%–0.54% and 0.2%–2.81%, respectively, whereas the formation of new minerals and salt crystallization mainly governs the changes in microscale pores (decreasing by 0.48%–2.87%). The mechanical damage resulting from the interaction between tight sandstone and ScCO2 is mainly controlled by changes in the microcomponents and structure of tight sandstone. Under dry ScCO2 and ScCO2–H2O conditions, mineral dissolution predominates, with a compressive strength reduction rate of 40.3% and an elastic modulus reduction rate of 18.5%, whereas under ScCO2–brine conditions, the formation of new minerals and the precipitation of salt crystals gradually become dominant, and the compressive strength and elastic modulus are essentially consistent with those of the original samples. Under deep in situ stress, the strength and elastic modulus of tight sandstone do not exhibit obvious weakening effects under different ScCO2 conditions. Under in situ stress conditions, the mechanical properties do not significantly weaken.