A numerical technique based on discrete element method is presented in this paper to model failure in reservoir rock. The technique is considered as a geomechanical method which is coupled with a fluid flow simulator based on finite volume method. The two coupled methods are viewed as a tool that can model hydraulic fracturing in intact and naturally fractured rocks. Plasticity model is also included to represent rock brittleness, where a simple numerical test is performed to show a direct effect of plasticity. Discrete Fracture Network (DFN) model is coupled with the tool to represent the natural fractures in reservoir rocks, where modeling of some injection scenarios with different DFN densities are given.