In the oil and gas industry, geological faults significantly influence the hydromechanical behavior of reservoirs through their sealing capacity, which enables reservoir compartmentalization. However, this sealing capacity can be compromised by production-induced deformations, leading to risks such as fault reactivation, seismicity and fluid leakage. In order to predict these potential problems, numerical models often represent faults as impermeable surfaces. This is a reasonable assumption given the width and properties of the fault core. However, fault zones also include a surrounding damage zone with properties distinct from the host rock, affecting the hydromechanical behaviour of the reservoir. For example, damage zones composed either by deformation bands or fracture networks can act as barriers or preferential paths for fluid flow, respectively. In addition, damage zones also can affect the geomechanical stability of faults. Therefore, a proper fault characterization is essential in order to improve forecasting of problems and adopt suitable production strategies. However, subsurface fault characterization is not an easy task due to uncertainties in interpretating seismic data to identify these zones. In this study, we present a numerical methodology for assessing the extent and properties of fault damage zones [1][2][3]. The methodology is based on the finite element method and representative constitutive models capable of simulating rock behaviour under different deformation mechanisms. The results show that both the geomechanical properties and the extent of the damage zone are strongly affected by the fault formation process. A sensitivity study regarding the model parameters is also performed to demonstrate their impact on the results.