Wettability behavior of montmorillonite and pyrophyllite in CO2 geological sequestration: Insights from molecular dynamics simulation

Abstract Understanding the interactions among shale clay minerals, water, and carbon dioxide (CO2) is essential for advancing CO2 geological sequestration in shale reservoirs. In this study, molecular dynamics simulations were conducted to explore how surface properties and CO2 affect wettability at the molecular scale. Four surface models were constructed: three montmorillonite surfaces (Mnt 1, Mnt 2, and Mnt 3) and one pyrophyllite surface (Prl) as a control. In Mnt 1 and Mnt 2, Si-Al substitutions occur in the upper Si-O tetrahedral layer, and Mg-Al substitutions occur in the Al-O octahedral layer, with sodium ions placed on the upper and lower surfaces, respectively, to neutralize negative charges. In Mnt 3, substitutions occur in the lower Si-O tetrahedral layer, with sodium ions located on the lower surface. The pyrophyllite model (Prl) contains no isomorphic substitutions. On the Mnt 1 surface, a continuous water film persists even under high CO2 pressure. Mnt 2 shows little sensitivity to pressure changes, exhibiting only slight variations in contact angle. By comparison, Mnt 3 and Prl display a marked increase in contact angle with rising CO2 pressure, with pyrophyllite reaching 180° at about 25 MPa, indicating strong CO2 affinity. Mnt 3 and Prl exhibit a decrease in contact angle with increasing temperature, while the contact angles of Mnt 1 and Mnt 2 remain insensitive to temperature. Hydrogen bonding plays an important role in regulating surface wettability. Furthermore, sodium ions on Mnt 1 significantly enhance wettability by promoting water adsorption. The interaction energy between Na ions and water is notably higher than that between Na ions and CO2, underscoring their key role in maintaining hydrophilicity. Our results provide molecular-level insights into CO2 behavior in water-bearing shale formations and offer theoretical guidance for optimizing CO2 sequestration strategies in shale reservoirs.