Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

3.4. SWS MEASUREMENTS AT WEYBURN
dataset. The results are plotted in Figure 3.10. Although the NE-SW set has a higher density, it is
the NW-SE set, at ∼140 ◦ , that is imaged by the synthetic inversion, just as it is with the real data.
The explanation for this observation is that most of the events have arrived with azimuths close to
NW or SE. As a result, they are travelling subparallel to the NW-SE set, and close to the normal of
the NE-SW set. When shear waves travel parallel to the normals of a fracture set they are not split by
them. Hence, although in reality the NE-SW set has a higher density, it is at the wrong orientation
to be imaged by the arrivals available, and so the inversion images the NW-SE set.
This demonstrates how synthetic inversions can significantly enhance the interpretation of splitting
results. This example also demonstrates how important the range of arrival angles available can be in
determining what SWS can and cannot image. I conclude that while the splitting appears to image
the secondary fracture set, this does not necessarily imply that the NE-SW set is not also open, only
that I do not have the ray coverage to image it.
3.4.4 Modelling two fracture sets
An alternative approach to inverting the splitting results is to assume that two fracture sets are
present, and to attempt to find the strikes and densities of both. To simplify the inversion I neglect
the effects of any sedimentary fabric, as this was found to be small by the initial inversion (Figure
3.9). I list the inversion results in Table 3.3, and plot the results in Figure 3.11. The inversion finds
fractures striking at 42 ◦ and 150 ◦ , providing a good match with the fracture sets identified in core
samples and borehole image logs. When I examine the misfit surfaces, I note that the best fit fracture
densities trade off against each other (Figure 3.11b) - this is because the two fracture set orientations
are close to orthogonal. Bakulin et al. (2002) and Grechka and Tsvankin (2003) have shown that
the same stiffness tensor, C, and therefore the same SWS patterns, can be produced by a range of
fracture densities, so long as the fractures are close to orthogonal. This means that the absolute value
of fracture density for the two sets is not uniquely resolvable. However, I can determine the relative
strength of each set: in Figure 3.11b the 90% confidence interval shows that the best fit fracture
density for the set at 150 ◦ must be larger than the density of the set at 42 ◦ . This is in disagreement
with the core sample work, which finds that the set at 40 ◦ has a higher density. However, there
may well be geomechanical reasons for this disagreement, with injection activities altering the stress
conditions to preferentially open the set at 150 ◦ (which runs perpendicular to the horizontal well
trajectories at Weyburn). This will be discussed further in Chapter 8. I note now that because the
splitting occurs over the whole of the raypath, which includes the overburden, as the receivers are
placed above the reservoir, it is impossible to determine whether the fractures modelled are located
in the reservoir, overburden or both.
3.4.5 Phase II
I also perform the inversion technique on SWS measurements from Phase II. The results are plotted
in Figure 3.12. The 90% confidence intervals are very large, as the inversion does not appear to find a
reliable interpretation for subsurface structure. There are a number of possible reasons for this failure.
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