Abstract
We report here a series of long-period and long-duration (LPLD) seismic events observed during hydraulic fracturing in a shale gas reservoir. These unusual events, 10–100 s in duration, are observed most clearly in the frequency band of 10–80 Hz and are remarkably similar in appearance to tectonic tremor sequences first observed in subduction zones. These complex but coherent wave trains have finite moveouts obtained from cross-correlation. The moveout direction of the events confirms that they originate in the reservoir from the area where the fracturing is going on. Clear P- and S-wave arrivals cannot be resolved within the LPLD episodes but, in some cases, small micro-earthquakes occur in the sequences. Whether these micro-earthquakes are causal or coincidental is not known. It has also been observed that in three contiguous frac-stages, all LPLD events appear to come from two distinct places along one of two hypothetical fracture planes. Interestingly, the stages which have the largest number of LPLD events also have the highest observed pumping pressures during fracturing, the highest density of natural fractures, and the greatest number of micro-earthquakes. One possible explanation of these LPLD events is that the high pore fluid pressure during hydraulic fracturing stimulates slow slip on pre-existing fault planes. In the absence of elevated pressure, slip would not be expected on these planes as they are poorly oriented to the stress field. Slip on these fault planes may be occurring because the fluid pressure is close to the magnitude of the least principal stress. We observe a few events between pumping cycles perhaps indicating that, once triggered, these planes continue to slip due to the high transient pressure within the fault planes after pumping has stopped.
