This method for evaluating fluid flows in a heterogeneous medium, uses a hybrid grid constructed from a CPG type structured grid and from a radial grid. The first stage includes locally deforming a CPG type grid into a non-uniform Cartesian grid. These local grid cell deformations correspond to the change from a “CPG” frame to a “Cartesian” frame defined by the deformation. These deformations are then quantified, by a deformation function φ, and applied to the radial grid so as to shift to the “Cartesian” frame. The second stage includes locally correcting the definition of this deformation function around the radial grid, so that the radial grid, in the “Cartesian” frame, keeps its geometrical characteristics. A hybrid grid is then generated in the “Cartesian” frame from the two grids thus deformed. Finally, this hybrid grid is deformed to return to the “CPG” frame, using the inverse of deformation function φ.