This work is devoted to numerical analysis of permeability in rocks with multiple fractures. We propose a discrete approach for porous media with dual porosity. The intact porous rock is first discretized by an assembly of impermeable blocks according to the Voronoi diagram. The pore space of the intact rock is replaced by an equivalent network of interfaces between blocks, which produces the same macroscopic hydraulic conductivity as the intact rock. An induced network of macroscopic fracture or cracks is then introduced into the discrete porous rock. A specific numerical algorithm is developed to solve the obtained dual-porosity discrete porous medium. A series of numerical studies are performed in order to verify the efficiency of the proposed method and to investigate influences of mesh sensitivity, effects of fracture geometry and distribution. The proposed model is then applied to the study of permeability evolution in rock samples submitted to biaxial compression tests with different confining pressures. It is found that the proposed model is able to correctly reproduce the progressive process of initiation and propagation of fractures and the related evolution of permeability.