Microseismic expression of natural fracture activation in a tight sand reservoir
David Eaton and Aamir Rafiq, Department of Geoscience, University of Calgary
Mirko van der Baan, Department of Physics, University of Alberta
Activation of natural fracture networks can provide enhanced reservoir permeability
that is critical to the success of hydraulic fracture treatments in unconventional oil and gas
development. Microseismic monitoring is a widely used surveillance technology for
mapping the growth of hydraulic fractures as well as elucidating geomechanical
interactions with mechanical stratigraphy and pre-existing fracture systems. In an
unconventional oil and gas setting, interpretation of microseismic data typically focuses on
the spatial distribution of microseismic events to estimate stimulated reservoir volume and,
in some cases, to infer the character and geometry of discrete fracture networks.
This study makes use of downhole microseismic monitoring data from a two-well openhole hydraulic fracture treatment of an unconventional tight-sand unit deposited in a
Cretaceous marine barrier-bar complex in western Canada, together with coincident 3-D
seismic data. Combined interpretation of well log information with 3-D seismic attributes
provide a framework for interpreted partitioning of the reservoir into distinct depositional
units that may reflect competing sediment transport and tidal influences. These reservoir
zones are also expressed in statistical characteristics of microseismicity, which show
pronounced spatial variability in event magnitude and fracture azimuth. The microseismic
event clouds also reveal a relatively complex fracture pattern with evidence for partial
control on fracture azimuth by the regional stress field and upward growth of fractures (~
100m) through overlying coal beds into siliclastic rocks. Post-pumping microseismic
activity occurred along a series of localized linear trends that are oblique to the maximum
horizontal stress direction, suggesting that natural fractures were activated during and after
hydraulic fracture stimulation. Microseismic tremor activity, with waveform characteristics
similar to recently described long-period long-duration events, occurred episodically during
treatment, flowback and production. This phenomenon is interpreted to represent slowly
cascading slip along pre-existing fractures that are mis-oriented with respect to the current
stress field. Current research is focused on development of innovative techniques for
locating and characterizing tremor activity and joint interpretation of 3-D seismic and
microseismic data for reservoir characterization.