Gas adsorption characteristics changes in shale after supercritical CO2-water exposure at different pressures and temperatures

The supercritical CO2 (ScCO2)-water-shale interaction and its influence on adsorption characteristics of shale have significant impact on the estimation of the CO2 storage capacity. In this study, the influence of ScCO2-water exposure pressures and temperatures (P = 0, 10, 15, 20 MPa, T = 308, 323, 338, 353 K) on shale CH4 and CO2 adsorption characteristics were investigated. CH4 and CO2 adsorption tests, X-ray diffraction analysis, low-pressure N2 adsorption measurement were carried out on the shale samples before and after exposure. The results shown that after ScCO2-water exposure, the CH4 and CO2 adsorption capacity were decreased gradually with the increase of exposure pressure and the decrease of exposure temperature, due to the alterations of mineral composition and pore structure in shale. After ScCO2-water exposure, the contents of shale clay minerals, organic matter and carbonate were decreased, resulting in the decrease of specific surface area and micropore volume. With the increase of exposure pressure and the decrease of exposure temperature, the solubility and extraction ability of ScCO2-water were increased, then more significant mineral composition and pore structure alterations, as well as more significant changes on the gas adsorption capacities of shale were expected. The selectivity factor of CO2 to CH4 of shale shown a gradually decreased trend with the increase of exposure pressure and the decrease of exposure temperature, respectively, and were all greater than 1 for both the untreated and ScCO2-water treated shale samples at different exposure conditions, indicating that CO2 enhanced shale gas recovery and sequestration is feasible even after ScCO2-water exposure. To predict the CO2 storage capacity in shale gas reservoirs, the combined effects of exposure pressure and temperature on the adsorption characteristics of shale should be considered at the reservoir conditions.