Coarse-Grained Molecular Dynamics Investigation on the Penetration Characteristics of Two-Phase Spontaneous Imbibition in Nanopores

Spontaneous imbibition constitutes a major means for the development of unconventional reservoirs. An accurate description of the flow characteristics and a clear understanding of the underlying mechanisms are crucial for efficient oil production from such reservoirs. However, the Lucas–Washburn equation that is often used to model the spontaneous imbibition behavior in conventional rocks is no longer valid for porous media with nanoscale channels. In this work, molecular dynamics simulations were used to investigate the influencing factors of two-phase spontaneous imbibition in nanopores, and the advancement of the oil/water interface was fitted and compared against a series of modified Lucas–Washburn equations. The results show that the non-negligible flow resistance of the oil phase is mainly responsible for the failure of the Lucas–Washburn equation in the nanoscale and that the effective viscosity that scales with the flow rate, rather than the slip length and dynamic contact angle, is a more significant parameter that should be included in the Lucas–Washburn equation to better describe the simulation data. The insights obtained in this work provide guidance for a deeper understanding of two-phase spontaneous imbibition in unconventional reservoirs and are the prerequisite for the selection and design of the optimal fracturing fluids for field applications.