Toward the Origin of Long-Period Long-Duration Seismic Events during Hydraulic Fracturing Treatment: A Case Study in the Shale Play of Sichuan Basin, China

Long-period long-duration (LPLD) seismic events have been recently observed over several unconventional reservoirs, and are postulated to play a significant role in accommodating deformation within the stimulation zone. However, the origin of LPLD events is still under debate. In this study, we present observations of tremor-like events during the multistage hydraulic fracturing treatment of a pilot horizontal well within theWeiyuan Shale play in southwestern Sichuan basin, China. These tremor-like signals recorded by the deep downhole monitoring array (15 Hz geophones) resemble the previously reported LPLD examples in terms of both duration and frequency content. In this particular case, with the concurrent records from the surface broadband array, we however confirm that these signals are located well outside the treatment area, and most probably originate from regional earthquakes that occurred during the treatment course within 250 km radius of the treatment well, instead of local sources directly related to the hydraulic stimulation. This study implies that the attenuated signals of regional seismicity may constitute a particularly deceptive pitfall for identifying and misinterpreting LPLD events during hydraulic fracturing, especially in seismically active regions. Our results also highlight the importance of surface broadband array in monitoring hydraulic fracturing activities.