Summary

Downhole seismic monitoring of an unconventional tight-sand unit in central Alberta was undertaken during a multi-stage hydraulic fracture (HF) treatment in September 2012. Acquisition of continuous passive seismic data continued during the subsequent flowback and production periods spanning over 10 months, with the objective of developing a geomechanical model for microseismic activity associated with treatment, flowback and production of the unconventional gas reservoir. A total of 1660 microseismic events were located during the two-day HF treatment program. The distribution of microseismicity reveals a relatively complex fracture pattern, with evidence for partial control on fracture azimuth by the regional stress field; upward growth of fractures (∼100m) through overlying coal beds into siliclastic rocks; post-pumping microseismic activity; and evidence for a pressure shadow around the vertical monitor well (a recent gas producer). Several microseismic event swarms, with low S/P amplitude ratios characteristic of tensile failure, occurred within the first hour of flowback initiation and are interpreted as closure of unpropped fractures. Long-period, long-duration (LPLD) microseismic events occurred episodically during all stages of the acquisition and are interpreted as activation of slow slip on pre-existing fractures. A "tuning fork" configuration for the treatment wells provided decreasing separation of the treatment zones from toe to heel. This led to sampling of distinct reservoir facies and stress environments, as documented by variable microseismic response along the lateral treatment wells.

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