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Impact of Stress-field Changes Induced by Hydraulic Fracturing on Subsurface Fault Stability Qian Gao Hydraulic fracturing is essential for economically producing hydrocarbons from shale reservoirs, and thus has been extensively used in completion of shale gas wells. However, fracturing formations can change in-situ stresses in the surrounding areas. There are concerns that hydraulic fracturing may potentially reactivate the subsurface faults nearby the treatment wells. The reactivated faults could cause many problems, such as early aborting/failure of fracturing treatment, fluid leakage along the fault and even seismicity events. 3D numerical models are developed based on finite element method. The comparison between analytical and numerical solutions indicates an excellent agreement has been achieved, which certificates the applicability of the numerical models to complex situations. Stress redistribution around hydraulic fractures can occur due to the opening of hydraulic fractures and poroelastic effects. Only the opening of hydraulic fractures is considered in numerical models. The results indicate that the initial in-situ stress contrast in horizontal plane has a strong influence on the extent of stress-reveral region and reoriented-stress region. When the generated stress contrast is larger than the initial in-situ stress contrast, there will be stress-reversal region in the vicinity of fractures. As the distance from fractures increases, the generated stress contrasts become smaller. When the generated stress contrast is less than the initial in-situ stress contrast, the stress trajectories do not reverse. Usually the stress-reversal region is in the vicinity of fractures, the reorientedstress region is beyond the stress-reversal region and around fracture tips. In this study, fault stability during hydraulic fracturing was investigated using 3D numerical models. Three typical faulting environments were considered. They are normal, strike-slip, and strike-slip/reverse faults. The orientation and relative magnitudes of insitu stress fields differ under different faulting environments, which in turn control the direction of fracture propagation. Three hydraulic fractures created simultaneously are considered in all case studies. It was found that the angle between fracture orientation and fault strike has a strong effect on the stability of a fault based on the change in the ratio of shear to effective normal stresses. Along the fault plane, the stability is strengthened in certain regions but weakened in other regions. The stress alteration patterns are different on the three types of fault. The normal faulting environment has the largest fluctuation in stresses and in the ratio of shear to effective normal stress. The reverse/strike-slip fault has the least perturbation on stresses and the ratio of shear to effective normal stress when hydraulic pressure is applied. This study provides insight on the stress redistribution during hydraulic treatment and the impact of hydraulic fracturing on fault stability. The results indicate that it is feasible to manage the stability of faults by adjusting fracture design.