Abstract Countercurrent spontaneous imbibition is one of the basic processes in two-phase fluid flow in fractured porous media. Many researchers have studied modeling of this phenomenon and analytical and numerical solutions have been presented. Specifying the imbibition face (open face) wetting phase saturation, which acts as a boundary condition, has been one of the main challenges in this area. There is no method to specify the imbibition face boundary condition satisfactorily. In this study a new logical method is introduced to determine the open face boundary condition that is satisfactorily close to experimental results. This method considers different aspects of the process of countercurrent spontaneous imbibition and, based on formulation and effective forces in this process, three pieces of evidence are presented to prove that the wetting phase saturation at the imbibition face is a value that leads to maximum wetting phase and nonwetting phase flow with respect to saturation. Because flow rates ...