Researchers had been capable of observe stress adjustments earlier than and through an earthquake induced by hydraulic fracturing in Alberta, Canada, with the assistance of a phenomenon referred to as seismic anisotropy.
Seismic anisotropy refers to adjustments within the velocity of seismic waves depending on the “cloth” of the rocks that the waves cross by. The sample of anisotropy adjustments 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 adjustments in anisotropy, the researchers hope to watch the processes that happen when hydraulic fracturing causes induced earthquakes, mentioned Nadine Igonin of the College of Texas at Austin and colleagues.
The worldwide analysis staff analyzed anisotropy adjustments noticed in 300 seismic occasions that had been recorded on a dense seismic array of 98 geophone stations put in place to watch 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 occurred throughout the examine.
“This case examine was notably attention-grabbing as a result of, whereas the hydraulic fracturing occurred 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 thus far above the reservoir unit.”
The geophone arrays had been buried on the backside of shallow, near-surface wells, as an alternative of the deeper, downhole arrays usually used for seismic monitoring. “Our findings required detailed monitoring of anisotropy adjustments throughout your entire space of hydraulic fracturing, which might solely be achieved with a floor or close to floor array, with monitoring stations unfold over a large space,” mentioned 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 attributable to totally different processes, together with tensile hydraulic fracturing opening, microseismic occasions on current faults or fractures and aseismic slip.
The anisotropy adjustments had been most like people who could be noticed within the case of a community of hydraulic fractures shaped after fluid injection, the researchers discovered, particularly a case the place the expansion of these fractures was restricted by intersecting with an current fault within the area.
Utilizing this info to evaluate stress adjustments 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 adjustments over time, Igonin mentioned, might be helpful info for managing and mitigating induced seismicity not simply in hydraulic fracturing, but additionally in wastewater disposal, carbon storage, and enhanced geothermal power manufacturing.
“Now we have noticed a spread of various mechanisms by which faults have been reactivated by injection, together with direct stream of injected fluids into faults, in addition to extra advanced results involving the geomechanical switch of stress by the subsurface, as described on this paper,” she mentioned.
“If we’re to provide more practical methods to mitigate induced seismicity, then we have to totally perceive the relative contributions of those totally different results in reactivating subsurface faults.”