A document by Oumeng Zhang. Click on the document to view its contents.

Fault slip is controlled by the normal and shear tractions on a fault plane. A full understanding of the factors influencing induced seismicity requires quantitative knowledge of the in-situ stress tensor and fluid pressure. We analyze these variables for a 200 km × 200 km region with active hydraulic fracturing near the city of Red Deer, Canada. The levels of induced seismicity in the area were generally low before Mar 04, 2019, MW 3.8/ML 4.2 event that local residents felt. We use geophysical logs and pressure tests within the targeted Duvernay Formation to construct maps of ambient pore pressure, vertical and minimum horizontal stresses. Maximum horizontal stress is constrained from the focal mechanism inversion and borehole-based estimation method. We find a broad range of orientations are susceptible to slip and small perturbations of fluid pressure would promote displacement. This suggests that the differential variations in pore fluid pressure in the target formation may provide a metric of slip susceptibility; a map for the study area is developed. Areas of high susceptibility correlate with those experiencing higher levels of induced seismicity except for the Willesden Green oil field that has similarly elevated susceptibility and active hydraulic fracturing operations. The methods and results demonstrate how more quantitively constrained in-situ stresses developed from an ensemble of real field measurements can assist in assessing fault stability and in developing metrics for slip susceptibility.

Geophysical logs collected from a deep borehole drilled to the Canadian Shield in Northeastern Alberta shed valuable lights on the state of stress in stable cratons. Observed breakout azimuths rotate between three depth intervals, from N100°E at 1650-2000 m to N173°E at 2000-2210 m, and finally to N145°E at the bottom. No obvious fractures that might disturb stresses were found; and these rotating breakouts can be interpreted either as being due to a heterogenous stress field or formation elastic and strength anisotropy. The latter interpretation is favored because the breakout azimuths are strongly controlled by rock metamorphic textures as validated by their close correlations with both dip directions of foliation planes and polarization directions from dipole sonic logs. Monte Carlo realizations further demonstrate that anisotropic metamorphic rocks subjected to a uniform horizontal stress direction could result in the observed azimuth-rotating breakouts. The stress magnitudes inferred from this analysis, which incorporates both the rock anisotropy and weak foliation failure planes, suggest a normal faulting regime and a maximum horizontal compression direction consistent with that in the overlying Western Canadian Sedimentary Basin (NE-SW) and the motion of the North American plate. The inferred stress magnitudes are low and Mohr-Coulomb analyses demonstrate that the formation is not near the critical loading for slip on weak planes. However, more detailed investigations should be conducted since Monte Carlo calculations indicate that analyses from breakout widths, particularly when a conventional Kirsch-based formula is employed, are highly nonunique, allowing for large variations in potential stress states.