Effect of sub-/super-critical CO2 and brine exposure on the mechanical and acoustic emission characteristics of shale

Abstract

The effect of fluid-shale interaction on the mechanical behavior of shale is of great significance for the safety of long-term CO2 sequestration in shale formations. In this paper, uniaxial compression and Brazilian splitting tests were conducted to the examine the mechanical alterations of shale caused by various fluids (brine, subcritical CO2, supercritical CO2, subcritical CO2+brine and supercritical CO2+brine) soak. After soaking in various fluids at 45 °C for 10 days, the uniaxial compression strength (σc), tensile strength (σt), Young's modulus (Ec) and tensile modulus (Et) of shale were decreased. Supercritical CO2 induced more significant mechanical weakening in shale than subcritical CO2 as its much higher adsorption affinity in shale. Compared with the brine and corresponding single CO2 conditions, further mechanical weakening in shale was observed in the Sub/-or supercritical CO2+brine soak conditions. The weakening effect was further verified by acoustic emission (AE) analysis results, as the crack damage stress was decreased and the stable crack propagation was shortened, and the reduction and shortening were enhanced for the CO2+brine soaked samples. The fractal dimensions obtained by AE signals were increased, and show a positive correlation with the damage variables, which indicated that the fractal dimension of AE signals can be used to describe the damage in shale.