Molecular simulation of adsorption behaviors of methane, carbon dioxide and their mixtures on kerogen: Effect of kerogen maturity and moisture content

The adsorption behaviors of methane (CH4), carbon dioxide (CO2) and their mixtures are vital to understand the process of CO2 sequestration and shale gas exploitation. In this work, four realistic kerogen models with different maturities (immature (IIA), beginning of oil window (IIB), middle of oil window (IIC), postmature (IID)) were built by the molecular dynamics (MD) method. The adsorption characteristics of CH4, CO2 and their mixtures on these kerogen models with various moisture contents (0, 0.7, 1.4, 2.1, 2.8 wt%) were investigated by the grand canonical Monte Carlo (GCMC) simulations. The influences of kerogen maturity and moisture content on the adsorption capacity, isosteric heat of adsorption and adsorption selectivity of gas molecules were discussed. Simulation results show that the maximum adsorption capacity of gas molecules increases with increasing kerogen maturity, but decreases with increasing moisture content, and the reduction decreases as the maturity increases at high moisture contents. The average isosteric heat of CO2 adsorption is relevant to the sulfur/oxygen content of kerogen models. The pre-adsorbed water (H2O) has a small effect on the gas isosteric adsorption heat when located in the middle of pores, but can reduce the CO2 isosteric adsorption heat by occupying the hydrophilic groups. Moreover, H2O molecules are observed to migrate and aggregate into growing clusters at higher moisture contents for kerogen IIC and IID models, increasing the gas isosteric adsorption heat. The CO2/CH4 adsorption selectivity gradually decreases to the equilibrium value with the rise of bulk pressure. Also, the selectivity decreases with increasing CO2 mole fraction for lower mature kerogen models (IIA and IIB), but increases with the CO2 mole fraction at low pressure for kerogen models of higher maturity (IIC and IID). Meanwhile, the selectivity increases for IIA, IIC and IID models, while decreases for IIB model as the moisture content increases. This study gains deep insights into the effect of kerogen maturity and moisture content on the interaction between CH4/CO2 and kerogen at microscopic scale.