The influences of super-critical CO2 saturation on tensile characteristics and failure modes of shales

Abstract

To investigate the mechanism of super-critical CO2 saturation on the tensile properties of shale, shale specimens were saturated with Sc-CO2, and inert gas (Ar) was employed as control group. Brazilian splitting experiment with different layering inclinations (0°, 30°, 60°, 90°) showed that after Sc-CO2 saturation, the Brazilian splitting strength (BSS) and Brazilian splitting modulus (E) of shale were decreased by 11.9% and 5.7%, respectively. Regardless of any direction of stress loading, the bedding planes of shale had been damaged. The failure mode of shale transformed from single tensile to tensile-shear mixed failure after Sc-CO2 saturation and less energy was required for shale failure. The weakening of carbonate cementation made the bedding planes more prone to damage, and the number of shear cracks after Sc-CO2 saturation was twice as much. The X-ray diffractometer, scanning electronic microscope and nuclear magnetic resonance results showed that the increase of porosity caused by calcite dissolution and organic matter extraction was the fundamental reason for the deterioration of shale tensile properties. Furthermore, gas pressure made shale more compact, and also induced new cracks formation in the meanwhile. Therefore, after Ar saturation, E and BSS of shale were increased and decreased, respectively.