Selecting and designing the proper completion in naturally fractured reservoirs is always a challenge because of the mechanical and flow heterogeneities due to the fractures. Furthermore, when hydraulic fracturing is used to enhance the recovery, the interplay between the 3D stress field and the existing natural fracture systems becomes an important factor. Three mechanical scenarios might occur while fracturing the medium (Figure 1). First, the natural fractures may have no influence and the hydraulic fracture will propagate in a direction orthogonal to the minimum principal stress as expected in a classical model (Figure 1a). Second, only the system of natural fractures will be reactivated and eventually extended (Figure 1b). Third, both newly generated hydraulic fractures and natural fractures will intersect and propagate in a complex manner (Figure 1c). The tortuosity of hydraulic fractures will be greatly controlled by the anisotropy of the effective elastic medium due to 3D stress and natural fractures.