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- Title
States of In Situ Stress in the Duvernay East Shale Basin and Willesden Green of Alberta, Canada: Variable In Situ Stress States Effect Fault Stability.
- Authors
Shen, Luyi W.; Schmitt, Douglas R.; Wang, Ruijia; Hauck, Tyler E.
- Abstract
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 × 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 March 4, 2019, when local residents felt the MW 3.8/ML 4.2 event. We use geophysical logs and pressure tests within the targeted Duvernay Formation to construct maps of ambient pore pressure, vertical and minimum horizontal stresses. The maximum horizontal stress is constrained by the focal mechanism inversion and borehole‐based estimation method. We find that 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 quantitatively 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. Plain Language Summary: Knowing the complete stress tensor allows the assessment of a fault's slip tendency and thus the risk for induced seismicity. We study an area (200 × 200 km, near the city of Red Deer, Canada) where earthquakes, felt by local residents, are induced by hydraulic fracturing activities in the sedimentary Duvernay Formation. We measured the Duvernay's stress states using geophysical borehole logs, transient well test results, and information derived from the earthquake (i.e., focal mechanism). The stress tensors constrained using these direct measurements allow us to calculate the slip tendency of faults in our study area. We found that earthquakes can be triggered by varying fluid pressures in the faults. We then calculated the fluid pressures needed to activate faults in our study area and compared these values with the pressures of the nearby unconventional oil/gas reservoir; a susceptibility map is developed. We found that this map generally correlates well with the reported induced seismicity, except for one area. We reasoned that there might not be faults in ideal conditions to be activated near hydraulic fracturing wells there. The method presented here can be used for other studies that aim to understand the induced seismicity risk of areas that historically have few natural earthquakes. Key Points: Quantitative measurements for the stress tensor and pore pressure in an area with active hydraulic fracturing and induced seismicityDirect application of the stress tensor to understand the factors controlling a recent earthquake linked to hydraulic fracturingA stability map is built based on the difference between the formation pore pressure and critical fluid pressure that slips fault
- Subjects
SHEAR flow; HYDRAULIC fracturing; ROCK mechanics; INDUCED seismicity; BOREHOLES
- Publication
Journal of Geophysical Research. Solid Earth, 2021, Vol 126, Issue 6, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2020JB021221