Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 9.
CONCLUSIONS
modelling the deformation. This is because the effects of fractures in the reservoir are not accounted
for in upscaling from core to reservoir scale. When this effect is accounted for, the model provides a
much better match with observation.
CCS regulators have not yet made clear what legal requirements will be for a CCS site to be
deemed acceptable throughout its period of operation. It has been suggested that for transfer of site
responsibility from operator to government at the end of operations, it must be demonstrated that
the actual behaviour of the site matches modelled behaviour, allowing accurate long-term predictions
to be made. This suggestion is intended for fluid-flow models, where the oil industry has had far
more experience in developing full-field simulations with history-matching to well activity and the
CO 2 plume as imaged by 4-D seismic surveys. The question remains whether geomechanical models
can be developed with the accuracy required to fulfil a legal obligation such as this Geomechanical
modelling is a less mature technique than fluid-flow modelling, and the parameter space available in
a geomechanical model is far broader than a fluid-flow simulation.
At present, it does not seem likely that geomechanical models will be a legal requirement for CCS,
where an inability to develop an accurate model would represent a failing of the site. Indeed, given
the current state of maturity of the technique, it seems that such a requirement would be difficult
to meet. However, there may be certain circumstances, for example a site that appears to have
a large amount of geomechanical activity, or where fracturing is a particular risk, where it may be
especially important to develop a good mechanical model to ensure safe storage. Perhaps microseismic
monitoring and/or surface deformation measurements can be used to indicate sites where accurate
geomechanical modelling is necessary.
Furthermore, even without a legal requirement for it, geomechanical modelling can still be a useful
tool, as demonstrated in this thesis. Geomechanical modelling in the forward sense can be used
to make predictions about observable effects should a reservoir behave in a certain manner (i.e., a
perfectly sealed case, a worst case, etc.). By linking forward geomechanical models with observables
such as microseismic events, it is possible to identify which models most accurately represent reality.
In essence, these models provide us with a tool with which to test hypotheses about how the reservoir is
responding geomechanically, allowing us to reject those that do not provide a close match with reality.
By doing so, we can improve our understanding of the risk to secure storage posed by geomechanical
deformation at a particular site.
It is still not clear whether microseismic monitoring should always, sometimes or never be used for
CCS. An important first step in such a monitoring project would be to establish the pre-injection level
of seismicity, and also to develop a good geomechanical model of the reservoir. This would aid in the
decision making process, providing information about the likelihood of generating observable seismic
events, for both the desired and worst case scenario. However, at present, because Weyburn is the only
storage site to deploy microseismic monitoring, it is difficult to draw more definitive conclusions. Given
the state of CCS with respect to political uncertainties and public acceptance, the most appropriate
approach must be to deploy monitoring overkill on early projects, thereby proving to the public
that CCS is safe, and providing the research community with the opportunities to determine which
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