Competition adsorption of CO2/CH4 in shale: Implications for CO2 sequestration with enhanced gas recovery

Abstract

The competitive adsorption of CO2/CH4 mixtures in shale is a prerequisite for optimizing the process of CO2 sequestration in shale with enhanced gas recovery (CO2-ESGR). In this study, pure and binary adsorption of CH4 and CO2 in shale was measured at pressures up to 13 MPa and the temperature of 323.15 K using volumetric method. Pure and binary adsorption showed that CO2 is preferentially adsorbed in shale, and the adsorption amount of binary CO2/CH4 increased with the increase of CO2 composition. The selectivity factor of CO2 over CH4 (SCO2/CH4) in all the tested samples is greater than 1 and closely related to the pressure and CO2/CH4 concentration, as well as the CO2 phase state. SCO2/CH4 generally decreased with the increase of pressure in the case of 50%CH4 + 50%CO2, while first decreased and then increased with the increase of pressure in the cases of 25%CH4 + 75%CO2 and 75%CH4 + 25%CO2 for most of the tested shale samples. The SCO2/CH4 for most of tested shale samples in the case of 50%CH4 + 50%CO2 is the largest, and the SCO2/CH4 in the case of 75%CH4 + 25%CO2 is greater than that in the case of 25%CH4 + 75%CO2, indicating that the SCO2/CH4 initially increased then decreased with the increase of CO2 fraction. Thus, the CO2 is suggested to be injected when the reservoir pressure reduced to a certain level, and the injection pressure, rate and amount of CO2 should be optimized, to obtain an optimal SCO2/CH4 to improve the efficiency of CO2-ESGR and maximize the CO2 storage capacity. Furthermore, the binary CO2 and CH4 adsorption data were predicted using various models. The Ideal Adsorption Solution Theory (IAST) model fitted the experimental adsorption data the best, followed by the Extended Langmuir (E-L) and Loading Ratio Correlation (LRC). The results provide a firm basis for optimizing the CO2 injection strategies and estimating CO2 storage potential for CO2-ESGR operation.