The pore structure of rocks and other materials can be determined through microscopy and subject to digital simulation to determine the properties of multiphase fluid flows through the material. To conserve computational resources, the simulations are preferably performed on a representative elementary volume (REV). The determination of a multiphase REV can be determined, in some method embodiments, by deriving a porosity-related parameter from a pore-matrix model of the material; determining a multiphase distribution within the material's pores; partitioning the pore-matrix model into multiple phase-matrix models; and deriving the porosity-related parameter from each phase-matrix model. The parameter's dependence on phase and saturation can then be determined and analyzed to select an appropriate REV size.