Abstract: The invention simulates flows in a geological reservoir having a heterogeneous pore size. From laboratory measurements on samples taken in the geological reservoir, pore size distribution classes are determined and a triple-porosity model representative of each class is determined. The flow simulator according to the invention implements the triple-porosity model, a thermodynamic equation of state accounting for an equivalent dimension of the pores of the small-size medium, fluid exchanges exclusively between the large-pore and small-pore media and between the small-pore and fracture media, and the capillary pressure as a function of the saturation in a small-pore medium.