Molecular Level Investigation of CH4and CO2Adsorption in Hydrated Calcium–Montmorillonite

We have studied the mechanism of intercalation and methane adsorption from a H2O/CH4/CO2mixture on a prototypical swelling shale component, Ca–montmorillonite. We employed ab initiomolecular dynamics simulations at 323 K and 90 bar to obtain molecular level information on adsorption energetics, speciation, and structural and thermodynamic properties. Interaction of CH4with surface Lewis acidic sites (Ca2+, surface OH) results in large induced dipoles (∼1 D) that lead to relatively strong adsorption energies compared to interactions of the normally apolar CH4that level off once a CH4layer is formed. Intercalated CH4, also exhibits large induced dipoles at lower hydration levels, when the interaction with Ca2+cations are less hindered. CO2displaces CH4in the coordination sphere of the cations (in the interlayer) or on the surface, thereby driving CH4extraction. Our simulations indicate that there is an optimal pressure range (∼70–90 bar) where scCO2-facilitated CH4extraction will be maximized.