Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 8.
LINKING GEOMECHANICAL MODELLING AND MICROSEISMIC OBSERVATIONS AT WEYBURN
15
10
5
∆ f p
0
−5
−10
Injector
Producer
Above inj
Above prod
2 4 6 8 10 12 14 16
Timestep
Figure 8.11: Percentage change in fracture potential in the softer Weyburn reservoir (solid lines)
and overburden (dotted lines). f p near the injector is marked in red, near the producers in blue.
The shear wave splitting predictions are plotted in Figure 8.14. As with the stiffer case, little
splitting develops in the reservoir. However, in the overburden some significant and coherent splitting
patterns develop. Above the production wells the fast directions are orientated parallel to the well
trajectories (the y axis), while above the injection wells the fast directions are orientated perpendicular
to this (parallel to the x axis). This represents the development of significant stress anisotropy from
the initial isotropic state.
In Chapter 3 I observed an HTI fabric with a fast direction to the NW, perpendicular to the
NE well trajectories. This splitting was measured on waves travelling to geophones sited above the
injection well, from microseismic events located in or above the reservoir. Therefore, most of the
raypath is in the overburden. Therefore, the splitting they experience will be caused by anisotropy
in rocks above the injection well. As such, the predictions from the model, with fast directions
orientated perpendicular to the well trajectories above the injection well, do provide a good match
with observations made at Weyburn.
It appears, therefore, that the model with a softer reservoir in comparison to the overburden produces
event location and shear wave splitting predictions that match well with observations, while the
original model does not. This demonstrates the need to link geomechanical models with observations,
by doing so discrepancies can be identified and corrected for, leading to the development of improved
models.
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