Fault stabilities of the Fox Creek, Alberta, earthquakes: complications from naturally high formation pore pressures

A series of earthquakes in the vicinity of Fox Creek, Alberta have been linked temporally and spatially to hydraulic fracture stimulations within the unconventional Duvernay reservoir. We have recently developed a comprehensive regional model for the full stress tensor in this region by combining geophysical logs and pressure tests with dimensionless stress ratios obtained by inversion of earthquake focal mechanisms. Here, we apply this stress knowledge to examine the stability of fault slip on the planes associated with eleven intermediate magnitudes primarily strike-slip induced earthquakes in the area. One interesting characteristic of the reservoir is that its ambient pore pressure P_P, as determined using conventional pressure tests, is exceptionally high reaching ~90% of the measured minimum horizontal compression. The expected in situ stresses are resolved onto the fault planes and the stability assessed using a standard Mohr-Coulomb frictional criterion. Under this paradigm and assuming reasonable ranges of friction and fault cohesion, we find that none of the fault planes could remain stable at the ambient P_P within the rock mass. This holds true even for those planes of weakness that would not normally be considered to be optimally oriented with respect to the stress field. This suggests that the fluid pressure P_f existing naturally within the presumed pre-existing planes of weakness must be significantly lower given the historic aseismicity; conversely supporting the hypothesis that the events were triggered by briefly elevated values of P_f due to hydraulic communication between the borehole and the plane of weakness. Poroelastic effects need not be invoked; although without detailed knowledge of structure and fluid transmissivity neither effect can be ruled out. Diminished P_f within the planes of weakness may be indicative of fluid transport pathways of the hydrocarbons from the source rock Duvernay Formation to the overlying siliclastic Cretaceous sediments hosting numerous conventional reservoirs. This strategy is being used to study the more recent M_W ~3.8 earthquake of 04 Mar, 2019 and to understand the lack of induced seismicity in other adjacent portions of the Duvernay Formation.