Microstructure and mechanical properties alterations in shale treated via CO2/CO2-water exposure

Abstract CO2 injections into shale reservoirs can enhance the recovery of oil and gas, while simultaneously sequestrating CO2. However, with the injection of CO2, the microstructure and mechanical properties alteration of shales caused by CO2/CO2-water exposure is should be noticed, which is of great importance to predicting the efficiency of CO2-enhanced shale oil/gas recovery and safety of CO2 sequestration in shale reservoirs. Therefore, this study clarifies the influence of sub/super-critical CO2-water exposure on the microstructure and mechanical properties of shale. Results show that after Sub/ScCO2 and Sub/ScCO2 water exposure, the contents of carbonate and clay in shale were decreased. Additionally, the proportion of micropores and mesopores were decreased, while the proportion of macropores was increased, leading to the increase of porosity and the improvement of pore connectivity, and the decreases of the pore surface fractal dimensions D1 and pore volume fractal dimensions D2 in shale. The uniaxial compressive strength (UCS) and elastic modulus (E) of shale were also decreased. Acoustic emission (AE) analysis results indicated that the cracks closure stage and stable crack propagation stage was prolonged and shorten, respectively, and AE signals show obvious multifractal characteristics. ScCO2 induced greater alteration in pore structure and mechanical properties than SubCO2, and the alterations were enhanced for CO2-water exposure. UCS and E of shale samples show a good negative correlation with porosity, and a positive correlation with D1 and D2. UCS in shale is positively related to the proportion of micropores and mesopores, and negatively related to the proportion of macropores, which indicated that microstructure alterations inevitably affected the macroscopic mechanical properties of shale.