A molecular dynamics study of spontaneous imbibition of water in silica nanoslits

Spontaneous imbibition plays an important role in enhancing oil recovery for shale oil reservoirs after hydraulic fracturing. In this work, the spontaneous imbibition of the water-oil system in the hydroxylated silica nanoslit is investigated by the molecular dynamics simulation (MD) method. The effects of slit width, temperature, and surfactant on the imbibition behavior are mainly considered from the molecular level. Our results indicate that among the simulated slits with widths of 2 nm, 4 nm, 6 nm, and 8 nm, the medium-width slits of 4 nm and 6 nm are relatively better suited for the spontaneous imbibition of fluids. Meanwhile, the simulation results are in good agreement with those obtained by a modified LW model in the nanoslit. For the imbibition system with the same slit width, the imbibition efficiency of water can be significantly improved by increasing the temperature. This is because the high temperature increases the kinetic energy of the water molecules, making them easier to break the hydrogen bonds between the water molecules and the silica surface. In the imbibition systems containing different concentrations of surfactant molecules, the imbibition velocity is usually faster with the increase in surfactant concentration. Compared with pure water, surfactant aqueous solutions can generally promote imbibition. However, excessive concentrations of surfactant may have the opposite effect.

The open-source software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) is used to perform the molecular dynamics calculations, and VMD (Visual Molecular Dynamics) software is used to visualize the simulation results. The intermolecular interactions are described by 12-6 Lennard-Jones and Coulombic potential functions. The Nosé-Hoover algorithm is used to control the temperature of the system. The periodic boundary condition is used for all simulations. The MD simulations are carried out over a time of 2 ns or 4 ns with a timestep of 1 fs under the NVT ensemble.