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The Leading Edge / Volume 30 / Issue 7 / Seismic events
Long-period, long-duration seismic events during hydraulic fracture stimulation of a shale gas reservoir
The Leading Edge 30, 778 (2011); doi:10.1190/1.3609093
Indrajit Das and Mark D. Zoback
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. ©2011 Society of Exploration Geophysicists
Long Period Long Duration Seismic Events During Hydraulic Stimulation of a Shale Gas Reservoir*
and Mark D. Zoback
Search and Discovery Article #40761 (2011)
Posted June 30, 2011
*Adapted from e-poster presentation at AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 10-13, 2011
Geophysics, Stanford, Stanford, CA (firstname.lastname@example.org)
Geophysics, Stanford, Stanford, CA
We investigate two classes of unusual events of long duration and relatively low frequencies observed during hydraulic fracturing
operations in a gas shale reservoir. Multiple stages of hydraulic fracturing operations in five sub-parallel wells were monitored with an
array of seismometers deployed in the central horizontal well. When this well was fractured, the array was deployed in a vertical well.
Some of the unusual seismic events recorded are clearly tube waves that propagate along the monitoring well from the heel toward the
toe with a velocity along the borehole of ~1.5km/s. The tube waves propagate down the well from surface, but origin of the tube
waves is not known. The other unusual events are similar in appearance to non-volcanic seismic tremor sequences. They are of 10-50
seconds in duration and are observed in the frequency band of 10-40 Hz, which is much lower than the characteristic frequency band
of microearthquakes (100-300 Hz). Complex but coherent wave trains are observed in both the horizontal and vertical arrays. These
wave trains have very slight moveouts corresponding to apparent velocities ranging from 25 km/s to 9 km/s. The moveout recorded on
the vertical array indicates that they are not caused by a surface noise source; rather they result from a source in the reservoir.
Although it is difficult to resolve any clear P- and S-wave arrivals, one possible source of these low frequency, long duration events is
sub-seismic slow slip on pre-existing faults. The first of these unusual events were observed in the later part of the very first hydrofrac
stage of the first experiment and then in almost all the following stages in the five wells. One interesting observation is that they occur
before the pumping starts in some stages and even after the pumping stops in other stages. In our ongoing work we will be trying to
locate these events using waveform cross-correlation and double-difference tomography.
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