Molecular dynamics simulations of wettabilities on shale in three- and four-phase systems

The interfacial properties between three-phase fluid and shale are crucial in carbon dioxide flooding and sequestration. Here, we present an investigation on fluid-solid interfacial properties in the H₂O/CO₂/decane/shale four-phase system under geological conditions using molecular simulations. The four-phase system exhibits distinct three-phase contacts due to the varying occurrence state of fluids. The method (J. Colloid Interface Sci. 585 (2021) 320-327) for calculating the wettabilities of three-phase systems is not directly applicable to four-phase systems due to the lack of interfaces. The method is extended to four-phase systems by conducting simultaneous simulations of two such systems with different fluid phase positions, ensuring all interfaces are accessible. We found that the relative interfacial free energies at the fluid-solid interfaces and the wettabilities of the H₂O/CO₂/shale and CO₂/decane/shale contacts in the four-phase system are similar to those observed in the corresponding three-phase systems. However, the wettabilities of the H₂O/decane/shale contact in the four-phase system significantly differ from those of the H₂O/decane/shale three-phase system. The introduction of a CO₂ phase not only enhances the wettability but also amplifies this effect, especially under conditions of lower temperatures and higher pressures. For instance, the wettability increases by 35° at 298 K and 6 MPa. This variation in wettabilities is primarily attributed to the substantial influence of CO₂ on interfacial energies in the interfaces of decane-solid and H₂O-decane. The elevated impact of CO₂ at lower temperatures and higher pressures is explained by the enhanced CO₂ adsorption in both fluid-solid and fluid-fluid interfaces. © 2024 Elsevier B.V.