Influence of Super-Critical CO2 on the Strength and Fracture Behavior of Brine-Saturated Sandstone Specimens

The mechanical behavior of rock is one of the most important parameters to evaluate the potential for geological carbon dioxide (CO₂) sequestration (GCS); therefore, the study of rock strength evolution and fracture behavior after CO₂ injection is helpful in the long-term stability and safety of GCS engineering. In this study, uniaxial compression, Brazilian splitting and fracture tests were carried out on sandstone specimens with brine saturation or brine-super critical CO₂ (scCO₂) co-saturation. The influences of brine salinity and scCO₂ injection on the uniaxial compressive strength (UCS), Brazilian tensile strength (BTS) and fracture toughness of sandstone were investigated. The experimental results showed that the UCS, BTS and fracture toughness of brine-saturated sandstone increased with increasing NaCl concentration but decreased after scCO₂ injection. Furthermore, increments in the elastic modulus and average stiffness of brine-scCO₂ co-saturated sandstone were observed relative to those under brine saturation. To investigate the change of mineral composition and micro structure during brine immersion and scCO₂ injection, X-ray diffraction analysis, scanning electron microscopic observation and mercury intrusion porosimetry test were performed. Composition changes and dissolution of quartz were not observed, but many micro pores were created after scCO₂ injection, thus increasing the porosity and reducing strength and fracture toughness. Finally, the mechanism of brine-scCO₂ saturation in altering mechanical properties was discussed. These experimental results are expected to increase the understanding of the mechanical response of rock after scCO₂ injection in deep saline aquifers.