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

摘要

A Sercel L22 seismometer was deployed at ground surface about 8200 ft. above six horizontal Marcellus Shale wells; three-component data were acquired for a six-month period that spanned the time when the wells were hydraulically fractured. The seismometer was buried beneath 20 in. of soil in an isolated location where it was well coupled to bedrock. Data analysis identified 53 high-amplitude, impulsive events and 160 long period, long duration (LPLD) events. The timing and location of high-amplitude, impulsive events suggest that they were not related to hydraulic fracturing in study area wells; rather, the events were likely associated with background seismicity or to pore pressure reduction due to gas and water production from offset Marcellus wells. LPLD events displayed a low-frequency, low-amplitude precursor followed by a relatively high-frequency, high-amplitude primary S wave signal. Spectral analysis of LPLD events revealed an anomalous concentration of energy at low frequencies (1-30 Hz). During hydraulic fracturing, LPLD events were found to occur most frequently when the pumping pressure and rate were at maximum values.