Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 9.
CONCLUSIONS
work would be required as well. This example proves that if CO 2 is injected at pressures that are too
large for a particular formation to contain, events with a detectable magnitude will occur, which can
be imaged using a downhole passive seismic array.
Shear wave splitting measurements made on microseismic data are useful as an indicator of fractureinduced
anisotropy. This technique was first developed using teleseismic waves with subvertical arrival
angles. The splitting from such waves is relatively easy to interpret, with fast direction corresponding
to fracture strike, and splitting magnitude giving the number density of fractures. However, microseismic
data has arrival angles which are often subhorizontal, making the splitting harder to interpret.
Nevertheless, I have developed a technique to invert for fracture properties using rock physics theory,
and shown that it is possible to identify fracture orientations using SWS despite a highly unfavourable
source-receiver geometry.
Splitting analysis on the Weyburn events reveals the presence of a principal fracture set striking
to the NW, and a weaker, poorly imaged set striking to the NE. Previous work on core samples do
confirm the presence of conjugate fractures with these orientations. However, there is a discrepancy in
that core analysis indicates that the set striking to the NE should be the dominant set. To understand
this discrepancy, and to improve the interpretation of what the microseismic event locations mean for
storage security, I have constructed geomechanical models to represent the Weyburn reservoir.
The state of the art in geomechanical modelling of reservoirs is to couple together fluid-flow and
finite element geomechanical models. Such models can be used to predict the stress evolution during
injection, and hence the likelihood of brittle failure and microseismic events. I have defined several
stress path parameters, and have used these to study the controls that reservoir geometry and material
properties have on stress evolution. I find that small reservoirs are more prone to stress arching
effects, so long as the overburden is sufficiently stiff. In contrast, flat, extensive reservoirs do not
tend to transfer stress into the overburden. I have found that the extent to which this can happen is
controlled by the smaller of a reservoir’s horizontal dimensions. The smaller reservoirs that transfer
more stress into the overburden are more likely to generate fracturing, both inside and above the
reservoir. This finding may be an important criterion when selecting potential sites for their carbon
storage potential.
I have also modelled the effects of reservoir geometry and material properties on the amount of
surface uplift. The results demonstrate that the amount of uplift can differ by orders of magnitude
depending on the reservoir geometry and material properties. The modelled uplift for the simple
examples ranges from several centimetres, which would be easily detectable, to sub millimetre-scale,
which would not be detectable even in very favourable conditions. This demonstrates the need to use
accurate geomechanical models both when considering the use of InSAR as a monitoring tool, and
also when inverting measured surface deformation for pore pressure change.
To be confident that geomechanical models are providing accurate predictions, it is necessary to
groundtruth and calibrate them with observations. There are a number of methods that can be used
to do this, one of which will be changes to seismic properties. It is known from empirical observation
that stress changes alter seismic properties, and that non-hydrostatic stress changes create anisotropy.
I have developed a model that can account for these effects, while being simple to use and easy to
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