Long-period, long-duration seismicity observed during hydraulic fracturing of the Marcellus Shale in Greene County, Pennsylvania

Hydraulic fracturing is a well-completion activity that enables the economic extraction of hydrocarbons from unconventional reservoirs such as shale, which are of a naturally low permeability. During hydraulic fracturing, the movement of water into the reservoir from newly created hydraulic fractures reactivates preexisting faults and fractures, initiating shear failure on a complex network of planar features. Although the energy released during hydraulic fracturing is small, the seismic emissions arising from shear failure can be detected using surface and borehole seismometers, and these emissions are an important diagnostic tool for evaluating the effectiveness of reservoir stimulation. To investigate seismic activity during hydraulic fracturing of six horizontal Marcellus Shale wells in Greene County, Pennsylvania, we deployed a surface seismometer within the footprint of the horizontal wells. We recorded 53 high-amplitude, impulsive events and 144 long-period, long-duration (LPLD) events. LPLD events identified in this study show a low-frequency, low-amplitude precursor followed by a high-frequency, high-amplitude primary S-wave signal and are similar to long-duration events (30-60 s) identified in previous studies. After a thorough investigation, we found no temporal correlation with seismic events reported in regional earthquake catalogs and data from USArray stations, suggesting that these observed LPLDs are not attenuated signals from regional earthquakes. Spectral analysis of LPLD events reveals concentrated energy between 1 and 30 Hz. LPLD events were found to occur most frequently when the pumping pressure and rate were at maximum values. Recent findings suggest that "slow-slip emission" along discontinuities that are unfavorably oriented in the ambient stress field is likely a dominant and vital mechanism of deformation during reservoir stimulation. We compared the radiated microseismic energy plus the theoretical fracture energy to the total hydraulic input energy and found an approximately 75% deficit in the energy budget. We propose that LPLD deformation accounts for some portion of this energy deficit.