Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

7.3. RESULTS FROM SIMPLE GEOMECHANICAL MODELS
1.) In the center of the
reservoir, the horizontal
stresses increase equally. No
anisotropy develops.
2.) The edges of the reservoir
can push into the sideburden,
opening cracks parallel to the
edge. Anisotropy develops with
fast direction parallel to the
edge.
3.) Outside the reservoir, the force from
the expanding reservoir closes cracks
parallel to the edge. This leave cracks
perpendicular to the edge. Anisotropy
develops with fast direction
perpendicular to the edge.
Figure 7.3: Cartoon depicting the horizontal stress redistribution around an expanding reservoir,
demonstrating the SWS patterns seen in Figure 7.2. The red dashes indicate the resulting SWS
pattern.
by the expansion of the reservoir, increasing the horizontal stress perpendicular to the reservoir edge.
As a result, the fast direction is orientated perpendicular to the reservoir.
The maximum splitting magnitude modelled here is below 1%. Even a splitting magnitude of
1% may be hard to detect given that the maximum raypath length for a vertical wave through the
anisotropic region is 75m. Therefore the splitting modelled here is at the limits of detectability.
Additionally, in Chapters 3 and 4 I found that when inverting for anisotropy using splitting from
microseismic events it is difficult to resolve more than one anisotropic fabric. Therefore detecting
different regions of anisotropy with orthogonally polarised fast direction will be very challenging.
Nevertheless, in Chapter 6 (Figure 6.9) I noted that the rock physics model does tend to underestimate
splitting. Furthermore, the presence of fractures could serve to amplify the amount of splitting
induced by stress changes. Additionally, while splitting of microseismic events might not be able
to resolve adjacent anisotropic fabrics, splitting of 9C controlled source seismic data, or Amplitude
Variation with Offset and Azimuth (AVOA), may be able to image such fabrics.
This SWS pattern has been generated because there is a discontinuous edge between where pressure
is increasing and where it is not. Therefore, these prediction have a limited applicability, because in
reality reservoirs are often not bounded in this manner. Nevertheless, a sealing vertical fault would
generate the kind of edge around which the modelled splitting pattern could develop. Such a feature
would be of great importance for storage security, as an undetected sealing fault would prevent the
dispersion of both the injected CO 2 and also the pressure wave through the target reservoir. We have
seen that small reservoirs (or the equivalent, such as a compartmentalised large reservoir) are prone
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