Determination of CH4, C2H6 and CO2 adsorption in shale kerogens coupling sorption-induced swelling

Gas adsorption can induce kerogen swelling with changes in pore volumes. In this study, hybrid Grand Canonical Monte Carlo/Molecular Dynamics simulations are conducted to investigate gas adsorption in kerogen and the corresponding sorption-induced swelling. A unified relationship between volumetric strain and an absolute adsorption amount is developed for different adsorbates. A theoretical model for calculating excess adsorption isotherms coupling swelling is proposed. Results show that (i) steep increases in C2H6 and CO2 adsorption isotherms at lower pressure indicate stronger affinities of C2H6 and CO2 than CH4; however, a larger size of C2H6 results in smaller accessible pore volumes and smaller maximum absolute adsorption amounts than CH4 and CO2. (ii) A linear relationship between volumetric strain and an adsorption amount is shown for CH4, C2H6 and CO2, separately. The volumetric strain caused by per unit of the absolute adsorption amount for C2H6 is the greatest because C2H6 with greater diameters has larger contact areas with pore walls for the same adsorbed amount. The maximum swelling upon CO2 adsorption is the largest (CO2 > CH4 > C2H6) due to its greatest absolute adsorption amount. (iii) Excess adsorption isotherms generated by our model are consistent with excess adsorption data calculated by variable pore volumes, which shows a large discrepancy compared to that determined using constant volumes, especially at high pressures. The difference between the fraction of free CH4 in our model and that calculated by using a constant volume is up to 23.2% at 323 K, 40 MPa. Therefore, this theoretical model can accurately determine free and adsorbed gas amounts in shale, further influencing prediction of hydrocarbon production and CO2 sequestration.