Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

4.7. DISCUSSION
4.7 Discussion
The data presented here come from a hydraulic fracture job, where fluids have been injected at
pressures greater than 40MPa with the intention of causing fracture, whereas at Weyburn injection
pressures are ∼20-25MPa, with the intention of minimising fracture. There are also differences in
geology between Weyburn and the case presented here. As such, direct comparisons cannot be made.
So far, little research has been conducted to compare the amount of fracturing induced by injection
of different fluids with different properties. Therefore, it is of interest to compare the patterns of
seismicity generated in this case. In both cases, microseismicity images vertical fractures propagating
away from the injection site. There are some differences between the patterns of microseismicity -
the seismogenic zone during water injection is limited to perforation depths, although it has a larger
lateral extent. The seismogenic zone during CO 2 injection does not extend as far laterally, but does
have microseismicity extending up to 100m above the injection point. The rates and magnitudes of
seismicity generated are also similar. There is a greater temporal spread of seismicity throughout the
water injection stage, while the majority of events during CO 2 injection occur at the beginning of the
stage.
However, the discussion of these minor differences risks missing the wood for the trees, because
overall, the patterns of seismicity induced by injection of the two fluids are remarkably similar. The
primary purpose of my analysis of this data was to investigate whether the increased compressibility
of CO 2 is the reason for the limited seismicity observed at Weyburn. Here, I have found that event
magnitudes are loosely correlated with injection pressure, and do not appear to show a dependence
on the fluid properties. The rates of seismicity are also very similar. Although at face value the
SWS measurements during water injection appear to suggest a higher fracture density, the geometry
of event locations and geophones has made it difficult for splitting measurements to provide good
constraints, so this conclusion is not robust. I conclude that, despite the differences in compressibility,
viscosity, density and relative permeability between the fluids, CO 2 and water have produced similar
patterns of microseismicity. Certainly, there is no evidence to suggest that CO 2 is a ‘softer hammer’
that will be less capable of inducing microseismic events.
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