Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

3.6. SUMMARY
3.6 Summary
• I have developed a novel approach for inverting SWS measurements for fracture properties. I
have also developed a method to generate synthetic SWS data in order to test the resolution of
inversion results.
• Synthetic inversions show the sensitivity of the various parameters in the inversion to the range
of shear wave arrival angles available. In particular, γ cannot be resolved for subvertical arrivals,
while δ is poorly constrained for both subvertical and subhorizontal (but not oblique) arrivals.
• I have made SWS measurements on the Weyburn microseismic events using a semi-automated
algorithm. The data quality is poor and few successful results are found.
• Using one fracture set to invert the SWS data, Phase IB images a fracture set striking at 138 ◦ .
Though this does match with one of the fracture sets observed in core samples, it is not believed
to be the principal set, which is NE-SW in core samples.
• A synthetic model shows that the range of S-wave arrivals make it easier to image the NW-SE
striking set, even if the NE-SW set is stronger. This may be why the NE-SW set is not picked
up in the real data.
• An alternative approach is to invert for two fracture sets. In this case, the Phase IB data image
two fracture sets striking at 150 ◦ and 42 ◦ , closely matching the fractures observed in core sample
work. However, the inversion finds the set at 150 ◦ is the strongest, which is not the principal
set in core and borehole work. We may have to seek a geomechanical explanation for why this
set has been opened.
• Inversions for the Phase II data failed to find a stable result. It is possible that the quality of
the SWS measurements were not good enough, or that, as the arriving waves have sampled very
different regions of the subsurface, that there is too much variation in anisotropic symmetry
system type, strength and orientation.
• Beyond this thesis, the inversion method I developed has now been successfully applied by several
authors to a range of oil field, hydraulic fracture stimulation and block mining scenarios.
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