Influence of Supercritical CO2 Exposure on CH4 and CO2 Adsorption Behaviors of Shale: Implications for CO2 Sequestration

The interaction of shale CO2 during the CO2 sequestration and enhanced shale gas recovery process has significant influence on the adsorption properties of shale. In this study, the influence of supercritical CO2 (ScCO2) exposure on CH4 and CO2 adsorption behavior of shale was studied. The pore structure and functional groups of different shale samples before and after ScCO2 (P = 8 MPa, 12 MPa, 16 MPa; T = 35 degrees C) exposure were measured by low-pressure nitrogen adsorption and the Fourier transform infrared spectroscopy (FTIR) method, respectively. Moreover, CH4 and CO2 adsorption isotherms of shale samples before and after ScCO2 exposure were also obtained to reveal the mechanism of the influence of ScCO2 exposure on CH4 and CO2, adsorption behaviors of shale. The results indicated that after treatment by ScCO2 the adsorption capacity of CO2 and CH4 in all tested samples was decreased. The main reason for the decrease is the change of pore structures, mineral compositions, and surface chemistry in shale caused by ScCO2 exposure, as confirmed by the low-pressure nitrogen adsorption and FTIR analysis results. According to the results of low-pressure nitrogen adsorption desorption analysis, the total specific surface area, fractal dimensions of the pore structure, specific surface area, and pore volume of the micropores in shale were decreased, while the total pore volume and average pore size in shale were increased after exposure to ScCO2. FTIR analysis indicated that the contents of silicate, carbonate, and aromatic hydrocarbon in shale were decreased. The changes of pore structure and surface chemistry in shale are also related to the treatment pressure of ScCO2 and types of shale. After treatment by ScCO2, the selectivity ratios of CO2 over CH4 (alpha(CO2/CH4)) were also changed with a variety of trends.