A method of modelling a fractured reservoir having application for petroleum reservoir development is disclosed utilizing a set of several families of equivalent blocks of regular shapes and sizes. The fractured reservoir is modelled by a complex porous medium made up of irregular blocks. A function defining the progress of an imbibition front within these blocks, whose derivative A′(X) is calculated, is determined. A function defining the progress of an imbibition front within regular equivalent blocks, whose derivative A′eq(X) is calculated, is then determined. This derivative, which constitutes at least two line segments with distinct slopes, depends on the dimensions of the equivalent blocks. Finally, the dimensions of the equivalent blocks are obtained by adjusting the two derivatives A′eq(X) and A′(X).