--> Abstract: Observations of Low Frequency, Long Duration Events in a Microseismic Dataset Recorded in a Horizontal Shale Gas Well, by Indrajit Das and Mark Zoback; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Observations of Low Frequency, Long Duration Events in a Microseismic Dataset Recorded in a Horizontal Shale Gas Well

Indrajit Das1; Mark Zoback1

(1) 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.