Relationship between pore structure and mechanical properties of shale on supercritical carbon dioxide saturation

Abstract

When fracturing a shale gas reservoir with supercritical CO2, the interaction between CO2 and shale will change the pore structure, thus affecting the mechanical properties of shale. To confirm the influencing mechanism of pore structure changes on mechanical properties, nuclear magnetic resonance, uniaxial compressive strength, and acoustic emission were performed on shale specimens from the Sichuan Basin before and after CO2 saturation. Results indicated that after CO2 saturation, the proportion of micropores and mesopores decreased, while the proportion of macropores increased, and both the uniaxial compressive strength and Young's modulus decreased. The effect of supercritical CO2 was more significant than that of gaseous CO2. The changes in the proportions of main aperture (10–50 nm) and macropores (>250 nm) are the main factors controlling shale's mechanical properties. The probability of a large-energy acoustic emission event is increased when shale specimen fails after CO2 saturation, indicating larger macropores volume proportion is more likely to produce large-energy acoustic emission events. Additionally, 12.84% of the proppant embedment and 29.34% of the fracture closure were caused by mechanical properties deterioration. The fracturing process should be optimized to reduce the negative effects of the mechanical properties deterioration in the development of shale gas with supercritical CO2.