On the nature of long-period long-duration seismic events detected during hydraulic fracturing

Abstract

Long-period long-duration (LPLD) seismic events are low-amplitude tremor-like seismic signals that have been observed in some microseismic monitoring data sets acquired during hydraulic fracturing operations. LPLD events have been interpreted to be associated with slow-slip along preexisting fractures which are presumed to have either high clay content or be misaligned with respect to the current day principal stress directions. However, a recent study shows that regional earthquakes, when recorded on vertical downhole monitoring arrays, have similar signal characteristics to LPLD events and that care must be taken when analyzing and interpreting such signals. Using data from a hydraulic fracturing microseismic data set where LPLD events have previously been identified and well documented, together with data from the EarthScope Transportable USArray, we investigated the hypothesis that the documented LPLD events are regional earthquakes. We determined that the LPLD events correspond with signals recorded on the USArray at distances of up to 350 km away from the injection well, although they were not listed in any regional earthquake catalogue. The spatial coverage of the USArray allows the sources of many of the LPLD events to be relocated outside of the treatment well area and thus suggests that they are regional earthquakes of magnitude smaller than M2.5 rather than locally sourced events related to the hydraulic fracturing stimulation process.

Long‐period long‐duration seismic events during hydraulic fracturing: Implications for tensile fracture development

“…However, Zecevic et al . [] analyzed the USArray data and suggested that small regional earthquakes could be the cause of some of the LPLD events observed by Das and Zoback [, ]. This suggestion aligns with the study by Caffagni et al .…”

“…Das and Zoback [2011, 2013a, 2013b associated the LPLD events in the Barnett Shale with superposition of slow shear-slip events on relatively large faults due to high fluid pressure or high clay content. However, Zecevic et al [2016] analyzed the USArray data and suggested that small regional earthquakes could be the cause of some of the LPLD events observed by Zoback [2013a, 2013b]. This suggestion aligns with the study by Caffagni et al [2015], who showed that regional earthquakes in central Alberta in Canada were recorded as LPLD events on downhole seismic arrays for monitoring hydraulic fracturing.…”

“…One explanation for the LPLD events observed during hydraulic fracturing is regional tectonic earthquakes [ Caffagni et al ., ; Zecevic et al ., ]. This explanation is unlikely for the LPLD events we identified in this study.…”

“…First, all the LPLD events occurred after the rock started to break according to the variation of treating pressure and slurry rate (Figure ), indicating that they are highly associated with the hydraulic treatment process. Second, signals observed on seismograms of the LPLD events are dominantly P waves, which are different from those of regional earthquakes that have large S wave signals [ Caffagni et al ., ; Zecevic et al ., ]. Third, the locations of the LPLD events are close to the treatment well at the corresponding treatment stage.…”

“…LPLD events are indeed reported from microseismic data in recent years from downhole and surface arrays [ Das and Zoback , , ; Yu and Zhang , ; Tary et al ., ; Caffagni et al ., ; Kumar et al ., ; Zecevic et al ., ]. These studies propose different mechanisms for the observed LPLD events.…”