Competitive Sorption of CO2 with Gas Mixtures in Nanoporous Shale for Enhanced Gas Recovery from Density Functional Theory

CO2 competitive sorption with shale gas under various conditions from simple to complex pore characteristics is studied using a molecular density functional theory (DFT) that reduces to perturbed chain-statistical associating fluid theory in the bulk fluid region. The DFT model is first verified by grand canonical Monte Carlo simulation in graphite slit pores for pure and binary component systems at different temperatures, pressures, pore sizes, and bulk gas compositions for methane/ethane with CO2 Then, the model is utilized in multicomponent systems that include CH4, C2H6, and C3+ components of different compositions. It is shown that the selectivity of CO2 decreases with increases in temperature, pressure, nanopore size, and average molecular weight of shale gas. Extending the model to more realistic situations, we consider the impact of water present in the pore and consider the effect of permeation of fluid molecules into the kerogen that forms the pore walls. The water-graphite interaction is calibrated with contact angle from molecular simulation data from the literature. The kerogen pore model prediction of gas absolute sorption is compared with experimental and molecular simulation values in the literature. It is shown that the presence of water reduces the CO2 adsorption but improves the CO2 selectivity. The dissolution of gases into the kerogen matrix also leads to the increase in CO2 selectivity. The effect of kerogen type and maturity on the gas sorption amount and CO2 selectivity is also studied. The associated mechanisms are discussed to provide fundamental understanding for gas recovery by CO2.