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
Induced earthquakes in some areas of the Duvernay Formations (i.e., of
areas near the city of Red Deer and Fox Creek) due to Hydraulic
fracturing (HF) operations nearby and seismic quiescence of the other
areas (of a similar level of HF activities) warrants geomechanical
investigations. Here, we start by quantitatively constraining the
magnitudes and orientations of minimum (Sh),
maximum (SH) horizontal stresses, vertical stress
(SV) utilizing both borehole measurements and
earthquake’s focal-mechanism (FM) solutions for a study area where a
newly emerging swarm of HF induced earthquakes are reported (near the
city of Red Deer, Canada). The apparent pore pressures
(PP) are also assessed through several transient
well testing results targeting the unconventional reservoirs. This
knowledge allows the fault stabilities for the high-profile HF induced
Red Deer (ML 4.2/MW 3.8)
earthquake to be assessed. The N-S (or E-W) aligned fault, revealed by
the FM solution, appears to be stable at a hydrostatic fluid pressure
but unstable when fluid pressure is increased to the level of ambient
unconventional reservoir pore pressures. The slip-tendency of the faults
in the region studied is assessed by calculating the required fluid
pressures to activate hypothetical faults; we find that the HF-induced
clusters geographically overlap with the zones of higher susceptibility.
High ambient pore pressure does not correlate with high susceptibility,
and large deviatoric stress is needed to cause HF-induced earthquakes.