Comparison of the Absolute Adsorption of CH4, n-C4H10, and CO2 on Shale

Comprehensive knowledge of the absolute adsorption of light hydrocarbons and CO2 is significant for shale reservoir assessment and CO2 stimulation optimization. In this work, excess adsorption isotherms are first obtained for CH4, n-C4H10, and CO2 on typical shale. The simplified local density (SLD) theory is then used to obtain the adsorption-phase density for CH4, n-C4H10, and CO2 in organic pores, which is then employed for absolute adsorption calculation. The absolute adsorption of CH4, n-C4H10, and CO2 is compared to prove the potential of CO2 for shale hydrocarbon recovery as well as CO2 sequestration in shale reservoirs. The results show that CH4, n-C4H10, and CO2 can form adsorption layers and result in a much higher adsorbed phase density than that at the pore center. Based on the SLD theory, C4H10 shows the highest adsorbed density on the shale surface than CO2 and CH4 at all pressure ranges. In addition, absolute adsorption is higher than the excess values in line with the previous molecular simulation methods. Absolute adsorption is calculated in the order of n-C4H10 > CO2 > CH4, indicating the suitability of CO2 for CH4 recovery but also that it may not be feasible for recovering heavier hydrocarbons, i.e., n-C4H10. This study provides insights into the mechanism of shale resources recovery using CO2 method, which is theoretically crucial for shale resource assessment and production optimization.