Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

7.3. RESULTS FROM SIMPLE GEOMECHANICAL MODELS
This can be seen for the soft 1z100x5y and 1z5x5y models (Figure 7.1). The accumulated TWTT
change is at most 1ms, a small but detectable shift. At present no CCS project has looked for overburden
travel time-shifts, so we cannot know whether such shifts really do occur above CCS reservoirs,
but I noted in Chapter 5 that reservoirs where stress arching develops are at a greater risk of failure in
the overburden. TWTT shifts in the overburden might provide a tool to image whether or not this is
happening. However, the pore pressure changes simulated in the geomechanical model were large, as
were the contrasts between material properties in the reservoir and overburden, and yet the TWTT
shift is at the lower end of what is detectable. Therefore it is conceivable that stress arching can occur
without generating detectable overburden TWTT shifts. That said, more complicated and realistic
models may produce larger time-shifts. Further work is needed both in terms of better measurements
of overburden material properties for modelling, and in looking more diligently at changes in TWTT
from overburden reflections from real datasets.
7.3.2 Shear wave splitting
As we have seen in earlier chapters, shear wave splitting can be a useful tool for identifying nonhydrostatic
stress changes. Yet SWS has rarely been used to link seismic observations with geomechanical
deformation. This is in part because splitting is rarely measured in or above reservoirs as part
of industry standard practise, and partly because few rock physics models, such as Prioul et al. (2004)
and Verdon et al. (2008), exist that translate triaxial stress changes into variations in anisotropy. At
Valhall, a producing reservoir in the North Sea that is experiencing significant subsidence, Olofsson
et al. (2003) noted a characteristic ‘ring’ of SWS in the overburden marking the extent of the region
of depletion in the reservoir. Herwanger (2007) generated a geomechanical model of the Valhall reservoir,
and used the Prioul et al. (2004) model to predict the anisotropy, and thereby the SWS, in the
reservoir and overburden, finding that it is subsidence over the depleting reservoir that has generated
the SWS pattern. The good match between SWS observations and geomechanical modelling found by
Herwanger (2007) is promising, yet attempts to link seismic observation with geomechanical modelling
are still generally limited to overburden P-wave travel times. In this section I will generate predictions
about splitting generated by my simple models.
In Figure 7.2 I plot the fast directions and splitting magnitudes for a shear wave travelling vertically
through the 3 different shaped medium stiffness reservoirs (flat and extensive, long and thin, and
small). The resulting pattern is that at the centres of the reservoirs there is no anisotropy. SWS is
generated at the edges of the reservoir. Inside the reservoir, the fast direction is orientated parallel to
the edge of the reservoir, while outside the reservoir the fast direction is orientated perpendicular to
the edge. The explanation for this, in terms of stress redistribution around an expanding reservoir,
is illustrated in Figure 7.3. At its centre, the reservoir expands uniaxially, so no splitting develops.
However, moving towards the edges, the reservoir can expand preferentially into the sideburden, where
no pore pressure changes occur, meaning that the stresses perpendicular to the edge can be released in
comparison to the stresses parallel. Therefore, with larger horizontal stresses parallel to the reservoir
edge, the fast splitting direction becomes orientated in this direction. The sideburden is compressed
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