Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 5.
GEOMECHANICAL SIMULATION OF CO 2 INJECTION
because, with a higher value of K 0 , the normal stresses decrease while the differential stress does not.
Therefore I infer that for these cases the injection of CO 2 is increasing the likelihood of shear failure,
and therefore are more likely to generate microseismic activity.
In reality, microseismic events will generally occur on pre-existing planes of weakness such as faults
or fractures. Such features are well below the element length scales used in these (and most other
finite element) models, and so cannot be explicitly included. This kind of approach therefore will not
be able to make exact predictions about microseismic event occurrence, either spatially or temporally.
Nevertheless, by examining where fracture potential is increasing, it should still be possible to identify
regions where more events are likely to occur, assuming that the planes of weakness on which events
could occur are distributed evenly through the rock.
5.4.3 Shallower reservoirs
The above results have considered the effects of reservoir geometry and materials on stress evolution.
They have been evaluated for a reservoir that at 3000m is quite deep. It is of interest to consider
the stress evolution of reservoirs that are shallower. With this in mind I have repeated the numerical
calculations, but with a reservoir at a depth of 1500m. All other modelling parameters have been
kept the same, and again I consider 3 geometries, each with three sets of material properties, giving 9
models in total. In Figure 5.14 I plot the stress path parameters K 0 and γ 3 for the shallow models. I
note that the stress path parameters for each reservoir are very similar to their deeper counterparts.
I conclude that the depth of the reservoir does not appear to affect the stress evolution as a result of
inflation.
5.5 Surface uplift
An alternative geophysical method that can and has been applied to monitoring of CCS is to use InSAR
to measure ground surface uplift above a reservoir. As the reservoir inflates with CO 2 injection, the
ground surface may be pushed up by an amount that is detectable using satellite-based methods. This
technique has been most notably demonstrated at the In Salah site, Algeria (Onuma and Ohkawa,
2009), where uplift of a few mm has been detected. However, without a geomechanical model that
simulates the reservoir and overburden it is difficult to relate surface deformation observations to
reservoir processes. Such models have been developed for In Salah by Vasco et al. (2008) and Rutqvist
et al. (2009). In Salah is located in the middle of a desert, a perfect environment for InSAR. As a result,
measurements are very accurate, with a sub-millimetre resolution. In more challenging environments
resolution may be as low as half a centimetre.
In this section I present the surface deformation above the simple reservoir models developed above.
The contours of surface uplift for the 3 different geometries are plotted in Figures 5.15 to 5.17. In
all cases the shape of the uplift mirrors the region of pore pressure increase above the reservoir. The
extensive reservoir (1z:100x:100y) shows the largest uplift, while for the small reservoir (1z:5x:5y) the
uplift is so small as to be obscured by numerical noise.
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