Investigating the Nature of Hydraulic Fracturing-induced LPLD Events Using the USArray

Abstract:

Long-period long-duration (LPLD) seismic events are low-amplitude tremor-like signals that have been observed in microseismic datasets acquired during hydraulic fracturing operations. These events have been interpreted to be associated with slow-slip along pre-existing fractures that are presumed to have either high clay content or to be misaligned with respect to the current day principal stress directions. However a recent study suggested that regional earthquakes could be misinterpreted as LPLD events when recorded in a vertical downhole monitoring array, and that careful analysis must be undertaken when analyzing and interpreting such events. Using data from a hydraulic fracturing microseismic data set including well-documented LPLD events, together with data from the EarthScope Transportable USArray, we investigate the hypothesis that the documented LPLD events are regional earthquakes.

The NEIC earthquake catalog recorded two regional earthquakes (magnitudes >2.9) concurrent with the hydraulic fracturing operation. The observed LPLD events do not correlate in time with the catalogued regional earthquakes, however they do share many important seismic waveform characteristics. Interestingly, the EarthScope Transportable USArray recorded signals associated in time with the LPLD events at distances up to 350 km from the injection well.

The similarities in the signal characteristics suggest that LPLD events could be regional earthquakes that are too small in magnitude to be recorded and published in the NEIC earthquake catalog. The spatial coverage of the USArray enabled many of the LPLD events to be relocated outside the stimulated volume of the reservoir, thus verifying that they are regional earthquakes of relatively small magnitude rather than locally sourced events related to the hydraulic fracture stimulation process.