Analysis of Stress-Field Variations Expected on Subsurface Faults and Discontinuities in the Vicinity of Hydraulic Fracturing

Abstract

Summary In this study, a local stability evaluation method, slip-tendency analysis, is proposed on the basis of the Coulomb criterion to investigate the effects of hydraulic fracturing on stress-field variations and possibility of fault reactivation. The effects of net pressure and in-situ stress fields on the stability of faults are investigated in two typical faulting environments (i.e., normal and strike-slip faults). A 3D numerical model developed on the basis of the finite-element method (FEM) is also adopted to better understand the stability states around pressurized hydraulic fractures. The orientation and relative magnitudes of in-situ stress fields differ under different faulting environments, which, in turn, control the direction of fracture propagation and its geometry. It is found that the general patterns of slip-tendency distributions around pressurized hydraulic fractures are similar under different in-situ stress fields. Providing the normal and strike-slip faults with a same initial slip-tendency, the normal faulting environment demonstrates larger variations in slip-tendency than the strike-slip faulting environment. The comparison between analytical and numerical solutions indicates an excellent agreement was achieved, which certifies the validity of the proposed numerical models in 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 depends 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. For faults and discontinuities with an angle larger than 40° (i.e., with respect to horizontal direction) and distances less than 2.5 times the height of the fracture (i.e., from the center of pressurized fracture), the slip-tendency is greater than the initial value, indicating that the discontinuities within this zone are unstable and have the potential to slip. The developed model predicts that the unstable regions extend from fracture tips in both lateral and vertical directions. This generates relatively planar-distributed microseismic events, which are well-demonstrated in monitored field events. It was shown that the slippage of underground faults and other discontinuities could improve the fluid flow and transport by increasing the apparent permeability of the reservoir, such that the unstable regions could be recognized as permeability-improved zones.