Researchers had been capable of observe stress modifications earlier than and through an earthquake induced by hydraulic fracturing in Alberta, Canada, with the assistance of a phenomenon known as seismic anisotropy.
Seismic anisotropy refers to modifications within the velocity of seismic waves depending on the “cloth” of the rocks that the waves cross via. The sample of anisotropy modifications on the Canadian website was almost certainly created by a community of tensile hydraulic fractures created throughout fluid injection, the scientists conclude in Seismological Analysis Letters.
By mapping modifications in anisotropy, the researchers hope to observe the processes that happen when hydraulic fracturing causes induced earthquakes, stated Nadine Igonin of the College of Texas at Austin and colleagues.
The worldwide analysis group analyzed anisotropy modifications noticed in 300 seismic occasions that had been recorded on a dense seismic array of 98 geophone stations put in place to observe hydraulic fracturing of the Duvernay Shale Formation in Alberta’s Fox Creek space. Hydraulic fracturing within the area has produced a number of induced earthquakes, together with a magnitude 4.1 earthquake that befell throughout the examine.
“This case examine was notably attention-grabbing as a result of, whereas the hydraulic fracturing befell within the Duvernay Shale formation, the noticed seismicity occurred within the overlying Wabamun formation, 300 meters above the hydraulic fracturing zone,” Igonin defined.
“Therefore, there have been necessary unanswered questions as to how the hydraulic fracturing had managed to reactivate a fault to this point above the reservoir unit.”
The geophone arrays had been buried on the backside of shallow, near-surface wells, as a substitute of the deeper, downhole arrays typically used for seismic monitoring. “Our findings required detailed monitoring of anisotropy modifications throughout your entire space of hydraulic fracturing, which may solely be achieved with a floor or close to floor array, with monitoring stations unfold over a large space,” stated Igonin.
The researchers discovered that the power of seismic anisotropy elevated in some areas and decreased in others systematically, surrounding the hydraulic fracturing injection properly. They then in contrast the noticed anisotropy to fashions of stress change brought on by totally different processes, together with tensile hydraulic fracturing opening, microseismic occasions on present faults or fractures and aseismic slip.
The anisotropy modifications had been most like people who can be noticed within the case of a community of hydraulic fractures fashioned after fluid injection, the researchers discovered, particularly a case the place the expansion of these fractures was restricted by intersecting with an present fault within the area.
Utilizing this data to evaluate stress modifications on the hypocenter of the magnitude 4.1 fault, Igonin and colleagues’ mannequin confirmed a big stress change related to fault failure 24 hours earlier than the earthquake.
With the ability to resolve these sorts of stress modifications over time, Igonin stated, might be helpful data for managing and mitigating induced seismicity not simply in hydraulic fracturing, but in addition in wastewater disposal, carbon storage, and enhanced geothermal vitality manufacturing.
“We’ve got noticed a spread of various mechanisms by which faults have been reactivated by injection, together with direct movement of injected fluids into faults, in addition to extra advanced results involving the geomechanical switch of stress via the subsurface, as described on this paper,” she stated.
“If we’re to provide more practical methods to mitigate induced seismicity, then we have to absolutely perceive the relative contributions of those totally different results in reactivating subsurface faults.”