Molecular Level Investigation of CH4 and CO2 Adsorption in Hydrated Calcium–Montmorillonite

Abstract

We have studied the mechanism of intercalation and methane adsorption from a H2O/CH4/CO2 mixture on a prototypical swelling shale component, Ca–montmorillonite. We employed ab initio molecular dynamics simulations at 323 K and 90 bar to obtain molecular level information on adsorption energetics, speciation, and structural and thermodynamic properties. Interaction of CH4 with 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 CH4 that level off once a CH4 layer is formed. Intercalated CH4, also exhibits large induced dipoles at lower hydration levels, when the interaction with Ca2+ cations are less hindered. CO2 displaces CH4 in the coordination sphere of the cations (in the interlayer) or on the surface, thereby driving CH4 extraction. Our simulations indicate that there is an optimal pressure range (∼70–90 bar) where scCO2-facilitated CH4 extraction will be maximized.