Microearthquakes Associated with Long Period, Long Duration Seismic Events During Stimulation of a Shale Gas Reservoir

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Long period, long duration (LPLD) seismic events are relatively low amplitude signals that appear to be generated by slowly slipping faults during stimulation of a gas shale reservoir. They are remarkably similar in appearance to tectonic tremor sequences observed in subduction zones and transform fault boundaries. The ratio of the amplitudes on the three components and apparent velocities indicate that these signals are predominantly shear waves. In most cases, a few 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. We believe this results from a large density of pre-existing fractures and faults in this part of the reservoir. An image log in a nearby horizontal well shows the highest density of fractures and faults in the same general area. This region also shows the highest perturbation in pore pressure during hydraulic fracturing, From the spectrum of LPLD events, it is apparent that a significant part of the low frequency energy of the LPLD signals is not being recorded due to the instrument response of the 15Hz geophones. Despite this, we estimate that the moment carried by the larger LPLD events is ∼10-20 times that of Mw ∼ -1 microearthquake. The relatively large size of these LPLD events suggests that slow slip on faults is an important process affecting the stimulation more than microearthquakes. Two processes appear to control whether a fault slips rapidly as a microearthquake or slowly and stably. Laboratory friction date indicate that shales with high clay and kerogen tend to slip stably (Kohli and Zoback, 2011). In addition, slip along poorly-oriented faults that occurs due to reduction of normal stress by high fluid pressure is expected to propagate slowly. Taken together, stimulating slip on preexisting, faults in response to elevated fluid pressures can help optimize field operations and improve recovery.