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
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 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.