Geological Carbon Sequestration: Experimental Study of the Mechanical Response and Microscopic Mechanism of Shales Subjected to Gaseous CO₂ and ScCO₂

In Carbon Capture and Storage (CCS), a shale formation is regarded as a potential geological reservoir that sequesters CO2. The complex interaction between CO2 and shale alters the mechanical behavior and structural characteristics of shale formation, which is a critical issue for the capacity and safety of the CO2 sequestration. In this study, triaxial compression tests, acoustic emission monitoring, electron microscopy scanning, and X-ray diffraction analysis of shale subjected to various infiltration pressures (3, 6, 8, 10, 12, and 14 MPa) were conducted. And mechanical parameters and ringer count characteristics of shale subjected to gaseous CO2 and ScCO2 were obtained. Based on fractal theory, microstructural features and surface porosity of shale were quantitatively analyzed. The results demonstrate that (1) the compressive strength of shale infiltrated by gaseous CO2 was reduced by 5.66%; however, the reduction rate of shale compressive strength is as high as 29.75% under ScCO2. This phenomenon is mainly dependent on the strong adsorption and dissolution properties of ScCO2. (2) The timing of microfracture expansion in shales infiltrated by gaseous CO2 is advanced, and the acoustic emission signals have obvious stage characteristics during compression. Meanwhile, ScCO2 conditions have a clustered distribution of acoustic emission signals, with a significant increase in the frequency and band range of the ringer count rate. (3) The microstructure of shale infiltrated by CO2 has fractal self-similarity. The trend between the fractal dimension and infiltration pressure is the same as that of surface porosity. This study contributes to a better evaluation of CO2 storage capacity and the safety of geologic resource development.