Numerical investigation of the influence of discontinuity orientations on fault permeability evolution and slip displacement

Abstract

Abstract A pre-existing plane of weakness along the fault is comprised of a particular pattern of joints dipping at different orientations. The fault stress state, partially defined by the orientation of fault, determines the potential of slip failure and hence the evolution of fault permeability. Here the influence of fault orientation on permeability evolution was investigated by direct fluid injection inside fault with three different sets of fault orientations (45°, 60° and 110°), through the coupled hydromechanical (H-M) model TOUGHREACT-FLAC3D. The influence of joints pattern on slip tendency and magnitude of potential induced seismicity was also evaluated by comparing the resulted slip distance and timing. The simulation results revealed that decreasing the dip angle of the fault increases the corresponding slip tendency in the normal fault circumstance. Also, with changing joints dip angle associated with the fault, the tendency of the fault slip changes concurrently with the permeability evolution in a noticeable manner. Permeability enhancement after the onset of fault slip was observed with the three sets of fault angles, while the condition of 60° dipping angle resulted in highest enhancement. Joints pattern with a dip angle of 145° (very high dip) and 30° (very low dip) did not trigger a shear slip with seismic permeability enhancement. However, high dip and intermediate dip angles (135°, 50° and 70°) yielded high permeability in varying orders of magnitude. The large stress excitation and increasing permeability during shear deformation was noticeably high in intermediate joint dip angles but decreases as the angle increases. Article highlights The magnitude of injection-induced permeability enhancement is largely influenced by the fault and joint spatial orientations. With a slight change in the joint direction, there is an increasing possibility for fault to approach a different critical state of failure. Stress elevation at the point of failure is controlled by the orientations of fault/joint planes with respect to the direction of maximum principal stress.