ABSTRACT

Microseismic source mechanisms (obtained through moment-tensor inversion) provide an understanding of the hydraulic fracturing behavior of a stimulated reservoir, knowledge of which can help to improve production and minimize seismic risk. Seismic source inversion, Gutenberg-Richter b-value analysis, and geomechanical considerations are carried out to investigate the fracturing behavior for a microseismic data set recorded in the West Pembina field of central Alberta, Canada. A spatial pattern in the fracturing behavior seems apparent in the source mechanism results, showing strong tensile components in between the two observation wells and parallel to the treatment well, and shear-dominated failure mechanisms outside of this zone. This gives the impression that the reservoir experiences two spatially different fracturing behaviors. However, reliability tests using an identical monitoring geometry demonstrate that the change in behavior coincides with a region of unreliable moment-tensor solutions. This region occurs between the two observation wells, casting doubt on the recovered tensile failure mechanisms, thus indicating that only shear-dominated failure is likely to occur. This demonstrates the importance of taking into account the reliability of moment-tensor solutions when interpreting the fracturing behavior of a reservoir. Conversely, analysis of b-values indicates changes in the behavior between the reservoir (high b-values) and the sandwiching formations (b-values closer to one), which is likely due to a lower differential stress within the reservoir, caused by surrounding load-bearing formations. Geomechanical considerations also explain the observed polarity changes in shearing mechanisms as caused by either forward and then reversed slip on preexisting weaknesses or opposing shear motion on either side of the main hydraulic fractures.

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