Mechanical Behavior of Sandstone Pressurized with Supercritical CO₂ and Water under Different Confining Pressure Conditions

Abstract

Understanding the mechanical behavior of a reservoir rock within the CO2-injection zone is essential for the safety of CO2 geological storage (CGS). The water saturation of the reservoir rock could be changed with the injection of huge amounts of supercritical CO2 (scCO2). Triaxial compression tests were performed on sandstone under dry, H2O-injected, scCO2-injected, and CO2–H2O biphasic conditions at effective confining pressures ranging from 10 to 50 MPa to investigate the mechanical behavior of the sandstone within the CO2-injection zone comprehensively. The results indicated that the strength of the sandstone was significantly reduced by H2O, scCO2, and CO2­–H2O biphasic fluids. The CO2–H2O biphasic fluid reduced the strength most significantly, followed by H2O, and finally scCO2. Water/scCO2 monophasic and CO2–H2O biphasic fluids significantly affected the cohesion of the sandstone, but they did not affect the internal friction angle. Compared with the strength of the H2O-injected sample, the strength of the CO2–H2O biphasic sample was reduced by −6.94%, 16.94%, and 3.28% at 10, 30, and 50 MPa, respectively. The CO2–H2O biphasic fluid significantly deteriorated the deformation modulus and stiffness and enhanced the ductility and compaction of the sandstone. A higher confining pressure could inhibit the enhanced cracking effect of scCO2. The CO2–H2O biphasic zone within the reservoir was the most dangerous because faulting, surface subsidence, and permeability reduction of formation might occur in this zone. The results provided new information on the evolution of mechanical changes in saline aquifers injected with scCO2 in CGS.