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

Abstract

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.