Fracturing formations can alter in-situ stresses in the surrounding areas. There are concerns that hydraulic fracturing may potentially reactivate the subsurface faults nearby the treatment wells. In this study, a local stability evaluation method, slip-tendency analysis, is proposed on the basis of Coulomb criterion. The effects of net pressure and in-situ stress fields on the stability of faults are investigated. It is found that the patterns of slip-tendency distributions around a pressurized fracture are identical under different in-situ stress fields. Providing the normal and strike-slip faults have a same initial slip tendency, the normal faulting environment demonstrates larger variations in slip tendency than the strike-slip faulting environment due to the creation of a pressurized fracture.
Fault stability during hydraulic fracturing was investigated using a 3D numerical model developed based on the finite element method. Two typical faulting environments were considered. They are normal and strike-slip faults. The orientation and relative magnitudes of in-situ stress fields differ under different faulting environments, which in turn control the direction of fracture propagation. Scenarios with a single fracture were studied first, and then multiple hydraulic fractures created simultaneously were considered.
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. Numerical models and analytical solutions confirm the presence of both unstable and stable regions around the pressurized fractures. Fault stability during hydraulic operation dependents on the position of faults with respect to the hydraulic fractures. The critical angle and distance between fault and hydraulic fracture in analytical solutions are identified when a region transits from stable to unstable status. Provided that the slippage of underground faults and other discontinuities could improve the permeability of reservoirs, the unstable regions could be recognized as permeability improved zones. It is shown that shapes of slip-tendency contours around pressurized fractures depend on fault types.
URTeC 1579798
