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

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.