Adsorption selectivity of CO2/CH4 in water-bearing organic nano-slit shale: a molecular dynamic simulation study

Abstract

To examine the mechanisms of CO2/CH4 competitive adsorption and behaviour of the gas-water interaction under nano-confined spaces, this study built heterogeneous shale matrix models with a variety of pore shapes by using the molecular dynamics simulation method. It has been shown that within the temperature range of 293.15K to 323.15K, as temperature increases, the adsorption capacity of montmorillonite and kerogen for gases decreases. The results indicate that temperature is not the main factor influencing the competition adsorption process of CH4 and CO2. On the contrary, when the pore size is increased from 15.5 Å to 18.5 Å, and the proportion of CO2 in the gas mixture is changed from CO2:CH4 = 1:1 to CO2:CH4 = 2.5:1, the improvement in the efficiency of CO2 displacing CH4 is more significant. Besides, compared with CH4, both the montmorillonite and the montmorillonite-kerogen fracture model exhibit stronger selective adsorption for CO2. Meanwhile, the adsorption capacity of kerogen for CO2 is lower than that of montmorillonite walls. Moreover, under the same temperature and pore size, montmorillonite and kerogen both have a high selectivity for H2O molecules. Therefore, the presence of H2O reduces the adsorption capacity of montmorillonite and kerogen for CO2 and CH4 molecules.