FOX CREEK, A.B. – A University of Calgary study released yesterday says that fracking near a system of faults that researchers didn’t know existed triggered earthquakes west of Fox Creek in early 2015.
U of C geophysics professor Dr. David Eaton and former doctoral student Xuewei Bao conducted the study, which was published in Science magazine yesterday, after a series of earthquakes struck in an area where fracking operations were being conducted. In addition to mapping the previously undetected faults, the two researchers also say that earthquakes can continue to occur months after fracking has stopped. The research was prompted after the M3.9 earthquake rattled Fox Creek on Jan. 23, 2015.
The two scientists used data both from their own instruments, as well as from twelve existing area seismic stations to build a picture of what could have caused the earthquakes. They found that the previously undetected fault system ran parallel to two horizontally drilled wells. In one strand of the fault, hydraulic fracturing in both wells triggered small earthquakes by imposing mechanical stresses on the rock formations beneath the hydrocarbons-bearing zone — causing the fault to slip. Eaton noted that in that case, movement on the fault effectively stopped when fracking operations ceased. That behaviour is consistent with existing regulatory protocols to halt operations under certain conditions.
Further along the fault and more than two weeks after fracking injections stopped, the M3.9 earthquake occurred roughly four kilometres below the surface. This places the event close to the upper levels of basement rocks from the Precambrian period. Subsequent smaller seismic events persisted for a few months afterward, as the seismic activity migrated slowly from the basement back up toward the area where the fracking fluid had been injected.
“We didn’t expect to observe contrasting signatures for stress-induced and fluid-induced seismicity,” Eaton says. “This distinction may have important ramifications for hazard mitigation, depending on which process is inducing seismic activity.”
Eaton says that the new information from the study will help industry regulators learn to mitigate the risk of these induced earthquakes in the future.