Microearthquakes Associated With Long Period, Long Duration Seismic Events During Stimulation of a Shale Gas Reservoir
presented by Indrajit Das (from Zoback’s Stress and Geomechanics Group at Stanford)
Long period, long duration (LPLD) seismic events are relatively low amplitude signals that appear to be generated by slowly slipping faults during hydraulic stimulation of a gas shale reservoir. They are remarkably similar in appearance to tectonic tremor sequences observed in subduction zones and transform fault boundaries. In most cases, micro-earthquakes occur during the LPLD events, most likely generated on small fault segments associated with the slowly slipping faults responsible for the LPLD events. Interestingly, the hydraulic fracturing stages associated with the most LPLD events in the data set investigated lie exactly where there is a significant low amplitude anomaly in the 3D seismic data which we believe is due to a large density of pre-existing fractures and faults in this part of the reservoir. This region also shows the highest perturbation in pore pressure during hydraulic fracturing. From the spectrum of LPLD events we estimate that the moment carried by the larger LPLD events is ~10-20 times that of a Mw ~ -1 microearthquake. The relatively large size of these LPLD events suggests that slow slip on faults is an important process affecting the stimulation. Stimulating slip on these preexisting faults in response to elevated fluid pressures can help optimize field operations and improve recovery for shale gas reservoirs.
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