A method of multiphase flow upscaling of a fine scale geologic model. A local domain is defined from the fine scale geologic model. The local domain includes adjacent upstream and downstream coarse grid domains and a concatenated domain adjacent the upstream coarse grid domain. A coarse grid interface is defined between the upstream coarse grid domain and the downstream coarse grid domain. Boundary conditions are imposed on various faces of the components of the local domain. Fluid flow is simulated through the local domain for a plurality of time steps until the local domain is completely flooded. A fine scale solution of saturation and flow rate of individual fluid phases across the coarse grid interface is recorded at each time step. A pseudo-function is constructed by post-processing results of the simulation of fluid flow. The fine scale geologic model is upscaled to a coarse scale geologic model using the pseudo-function.