Phase-Field Modeling of Pore-Scale Oil Replacement by Spontaneous Imbibition in Fractured Porous Media

Horizontal wells with complicated fracture networks have become a key technical measure to improve the oil recovery of low-permeability and tight reservoirs in China. Spontaneous imbibition is regarded as the major oil recovery mechanism after fracturing. Due to the limitation of the observation scale, the traditional experimental methods cannot accurately describe multiphase fluid flow in the micro-/nanopore space of low-permeability and tight reservoirs, and the pore-scale oil recovery mechanism during spontaneous imbibition was not clearly understood. In this study, a novel mathematical model of oil replacement by spontaneous imbibition in fractured porous media is developed and then numerically solved using the phase-field method. By comparing the numerical results with the analytical solution of single-tube capillary-driven flow, which is widely described by the classical Lucas–Washburn equation, the accuracy of the proposed method is validated. The effects of rock wettability, oil–water viscosity ratio, interfacial tension, and fracture network on oil imbibition recovery are further explored. The results demonstrate that the pore-scale dynamic events of oil droplets including snap-off and coalescence can be well observed. The stronger the degree of water-wet and the lower the oil–water viscosity ratio, the higher the oil imbibition recovery. The oil–water interfacial tension exerts little impact on the oil imbibition recovery, while it can significantly affect the imbibition time. As the oil–water interfacial tension decreases, the imbibition time will become longer. The existence of a fracture network can enlarge the contact area of oil–water exchange, thus greatly improving the oil imbibition recovery during spontaneous imbibition. It is concluded that the pressure difference between fracture and matrix is of particular importance to achieve a high oil imbibition recovery in fractured porous media. The above understandings can provide a theoretical basis for the efficient development of similar reservoirs.