The shearing of pre-existing fractures plays an important role in the permeability enhancement of shale reservoirs during hydraulic fracturing or refracturing treatments. The process reactivates pre-existing fractures around a hydraulic fracture causing them to slip and dilate and can also cause fracture propagation in the shear and tensile modes creating secondary cracks resulting in increased permeability. However, laboratory data on fluid flow and fracture slip in reservoir rocks particularly shale rocks are rare, and the mechanisms of permeability evaluation with shear slip and dilation are still not well understood. In this work, we present the results of laboratory scale shear stimulation tests and numerical simulations to illustrate fracture permeability changes with fracture shear slip and complex network formation. Eagle Ford shale samples containing a natural fracture have been used to run triaxial shear tests and injection-induced shear tests. The multistage triaxial shear test has been performed to measure fracture mechanical properties including shear strength, friction angle, normal stiffness, and shear stiffness; the injection-induced shear test has been used to investigate fracture dilatant shear slip and the coupled permeability evolution. The multistage triaxial shear test show that this type of Eagle Ford fracture has a 37° friction angle, and average 1.39*106 psi/in. normal stiffness and 1.11*106 psi/in. shear stiffness. In the injection-induced shear test, we achieved 6 times increase in flow rate even with only a small induced shear sliding (<0.1 mm or <0.004 inch). Furthermore, permeability evolution during injection-driven shearing tends to linearly evolve with the shear slip and dilation. The irreversible behavior of shear slip was found to explain the permeability hysteresis during shear sliding. The relevant laboratory data has been used in numerical simulations to quantify the impact of shear slip along natural fractures during stimulation. This has been achieved using a newly developed complex fracture network model which robustly simulates hydraulic fracture propagation in a naturally fractured reservoir. The numerical results indicate that shear slip induced permeability enhancement in ultra-low permeability reservoirs is a critical component of stimulation particularly when most of the natural fractures are mechanically closed and may not be favorable for proppant placement.