| A pore-scale network model is developed to simulate formation and collapse of wetting and spreading oil layers in two- and three-phase flow systems. We focus mainly on oil layers and their connectivity at low oil saturations particularly in three-phase flow processes. A new configuration, i.e., cusp phase location, has been introduced to address connectivity and conductivity of the oil phase in non-spreading three-phase systems. Helmholtz free energy balance and Mayer-Stowe-Princen (MS-P) method are used to find threshold capillary pressures of two- and three-phase pore-scale displacements. Minimization of Helmholtz free energy for each relevant displacement enables the model to accurately determine the most favorable displacement, and hence, improve its predictive capabilities particularly for relative permeabilities, capillary pressures, and residual saturations for a given set of fluid and rock properties and saturation path trajectory. Multiple phases can reside in each pore (i.e., in corners, center, layers, and cusp phase locations). Different layer collapse displacement mechanisms, i.e., based on MS-P analysis or geometrical criteria, are implemented in the model. We have introduced four different layer collapse policies for oil or gas. The model is rigorously validated against three sets of experimental data to test its predictive capabilities with different porous mediums and fluid systems. All three-phase equilibrium interfacial tensions used for the prediction of three-phase experimental data were measured and used in model validation. The results indicate that the previously-used geometrical criterion for the collapse of oil layers yields lower residual oil saturations than the experimentally measured values in both two- and three-phase systems. Incorporation of the MS-P method and cusp analysis significantly enhances the agreement with experimental data. The validated model is then used to study the effects of wettability, saturation history, spreading coefficient, and initial oil saturation on three-phase oil, gas, and water relative permeabilities and residual oil saturation. |
