Phase-Field Simulation of Counter-Current Imbibition and Factors Influencing Recovery Efficiency

Counter-current imbibition can improve the recovery efficiency of complex fractured reservoirs, but there are few studies on the pore-scale mechanism and the factors affecting the recovery efficiency. This paper attempts to track the microscopic oil–water imbibition process through phase field method simulation, revealing the distribution characteristics of oil and water phases at different stages, as well as the sudden change characteristics of pressure and velocity at the instant of oil film rupture. Then, the influence of fracture aperture, capillary number and viscosity ratio on oil recovery efficiency is discussed. Results indicate that the microscopic imbibition process can be divided into 4 stages: the oil film forms after oil–water contact, then the oil film ruptures to form oil droplets, then the oil–water line moves outward from the large pore, and finally the oil droplets gather to discharge from the fracture. It is also found that there will be sudden changes at the moment of oil film rupture, the pressure drops sharply and the velocity increases sharply. Moreover, there exists a critical fracture aperture which is approximately 10 times the average pore size, and if the fracture is smaller than the critical fracture aperture, a dead oil zone occurs, which affects recovery. Additionally, LogM-LogCa stability diagram is constructed which is mainly dominated by viscous forces, capillary forces. As the capillary number increases, the recovery efficiency shows an overall decreasing trend. When the viscosity ratio was greater than 10, there was no significant change in the recovery efficiency, influenced by the weakening of the dominant role of viscous forces. New findings are beneficial to enhancing the recovery efficiency of low permeability reservoirs.