Influences of subcritical and supercritical CO2 treatment on the pore structure characteristics of marine and terrestrial shales

To better understand and implement the CO2 sequestration project, it is of significance to investigate the interaction of shales with CO2 and its potential effects on the pore morphology. In this study, two marine shale samples and two terrestrial shale samples were prepared and treated with subcritical CO2 (30 ℃ and 5 MPa) and supercritical CO2 (80 ℃ and 20 MPa) in a geochemical reactor. Various methods, including low-pressure carbon dioxide adsorption (LP-CO2A), low-pressure nitrogen adsorption (LP-N2A), high-pressure mercury intrusion porosimetry (HP-MIP) and fractal theory, were used to gain insights into the changes in the shale pore structure after 14 days of CO2 saturation. According to the results, the phase states of CO2 obviously affected the variations of pore structure parameters during the physical and chemical reactions in shales. Interactions of supercritical CO2 with shales created a more obvious effect on the pore structure compared to those of subcritical CO2, which was attributed to the greater dissolution and expansion effect as well as the extraction mechanism associated with supercritical CO2. After exposing the shale samples to subcritical CO2, the pore size distributions (PSDs) of the treated shale samples were lower than those of the raw samples at all diameter scales, indicating that the number of pores decreased due to the reactions. Furthermore, it was found that after supercritical CO2 treatment, the micropore and mesopore structure parameters of the marine shale samples obviously decreased with an increase in macropore structure parameters, leading to the reduction in fractal dimensions in smaller pores, while the terrestrial shale samples appeared to represent a contrary trend. These findings will provide experimental evidence for further assessment of the mechanisms for CO2 geological sequestration with enhanced shale gas recovery.