This work presents a methodology for determining the interface between wetting and non-wetting phases inside a three-dimensional (3D) porous space at a given equilibrium state. The work is limited to the study of mechanical equilibrium. Mass transfer between different phases is not considered. The method is based on a 3D extension of the opening: method from image analysis, applied on 3D stochastically reconstructed porous microstructures. The advantage of the presently proposed methodology with respect to percolation networks conception is that simplifying assumptions regarding the geometry of the porous space are not required. In fact, invasion of wetting fluid into a real porous structure in imbibition and wetting fluid retention at the later stages of drainage occur spatially through a complex structure of corners and intrinsic irregularities of pore surfaces that are very difficult to model by using percolation networks. Simulation results were compared with experimental data related to mercury intrusion and water-oil capillary curves for a Berea sandstone.