Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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Conclusions
Geological storage will be safe, and more permanent than any human activity since Stonehenge.
Ron Masters, 2009
This thesis has investigated the use of microseismic techniques to monitor CO 2 injection and
storage, as well as using geomechanical models to guide the interpretation of these observations. A
summary of the findings has been provided at the end of each chapter, but in this section I will review
the key findings, highlighting novel contributions to the field and areas for future research.
The first part of this thesis has concerned itself with observations of microseismic events induced by
CO 2 injection. A concern is that the pressure changes caused by injection will generate fracturing in
the overburden, providing a leakage pathway for buoyant supercritical CO 2 . Microseismic monitoring
can image this process directly. Microseismic activity can also be used to image fluids moving through
the overburden. The aims of microseismic monitoring for CCS are, then, slightly paradoxical in that
geophones are placed in the ground with the hope that they will not record anything. They are there
to provide a warning if things go wrong. This is not the only CCS monitoring technique that aims
to detect nothing: for example if no change is detected during shallow aquifer fluid sampling and
soil gas flux measurement, then the storage site will be deemed successful. Microseismic monitoring
should provide a much earlier warning of leakage than soil gas fluxes and shallow aquifer sampling. In
contrast, some techniques such as 4-D controlled source seismic monitoring aim to find a detectable
change after injection, so long as it is limited to the target reservoir zone.
I have presented the results of over 5 years of microseismic monitoring at Weyburn. There have
been less than 100 events over this period, indicating that the reservoir is undergoing little deformation,
and that the CO 2 is generally moving through the reservoir aseismically. This is encouraging with
regard to security of storage. The few events that are observed are generally located near to the
production wells to the NW and SE, and many could be located in the overburden, though event
depths are not well constrained. Although it may at first glance be worrying if microseismicity is
identified in the overburden, this does not mean that the events are caused by CO 2 moving into the
caprock. Indeed, inasmuch as that the events are located near the producers, while the CO 2 plumes
centre on the injectors, it is very unlikely that the events recorded represent leakage. This matches
4-D seismic observations that do not show any fluid migration above the injection wells.
This of course begs the question: what would microseismic monitoring detect if CO 2 injection was
causing fracturing and creating leakage pathways in the rock. I have attempted to answer this question
by comparing hydraulic fracture data from CO 2 and water injection. This example is not directly
equivalent to CCS scenarios, as the injection pressures used are much higher. Nevertheless, this can
be considered to be a worst case scenario, exactly what microseismic monitoring will be deployed to
detect for CCS projects, and so lessons can be learnt. During injection, the event locations image
the growth of fractures from the injection site, both laterally and above the injection point. If such
observations were made at a storage site, and in particular if events are detected moving well above
the injection depth, then the injection regime would have to be reconsidered, and possibly remediation
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