Long Period, Long Duration (LPLD) Seismic Events Observed During Monitoring of Hydraulic Fracturing in Pennsylvania and West Virginia

Abstract

Hydraulic fracturing is a well-established completion technique to efficiently extract significant volumes of natural gas from unconventional formations such as shale, which would otherwise behave as impermeable formations. Diffusion of water outward from the newly created hydraulic fractures into the reservoir helps reactivate the preexisting faults and fractures and initiates shear failure on complex networks of preexisting planes of weaknesses. We carried out surface seismic monitoring of hydraulic fracturing in Greene County, Pennsylvania and in Monongalia County, West Virginia. We used a single broadband seismometer within the footprint of six lateral wells for surface monitoring in Greene County. In Monongalia County, we monitored the hydraulic fracturing operation at an active well pad with five seismometers. Common field observations related to microseismic fracture-mapping indicates preferential growth along the maximum horizontal stress. Recent findings on long period, long duration tremors suggest that “slow slip emission” along weaknesses that are unfavorably oriented in the ambient stress field is likely the dominant mechanism of deformation and plays a vital role in reservoir stimulation. We identified 117 high-amplitude, impulsive events and 473 long period, long duration (LPLD) events from the combined dataset of hydraulic fracturing in Marcellus shale in Greene County and Monongalia County. The timing and location of majority of the impulsive events does not favor any causal relationship with the hydraulic fracturing in the study area. We used a mine blast database and 3 component surface seismic recordings of blasts to accurately characterize these events, which could be misinterpreted for LPLD events. The LPLD events identified in this study show a low-frequency, low-amplitude precursor followed by a relatively high-frequency, high-amplitude primary S wave signal and similar to other long duration events identified in previous studies. Spectral analysis of LPLD events revealed an anomalous concentration of energy at low frequencies (1-30 Hz). During various stages of hydraulic fracturing, LPLD events were found to occur most frequently when the pumping pressure and rate were at maximum values. These observations suggest that long period, long duration events are generated in response to highly elevated fluid pressure and may play a significant role in the reservoir stimulation.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016