Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 3.
INVERTING SHEAR-WAVE SPLITTING MEASUREMENTS FOR FRACTURE PROPERTIES
If the subsurface structure being illuminated by the Phase II results is too complicated to be modelled
by one set of HTI fractures and a VTI fabric then the inversion may fail. Another possibility is that
the Phase II arrivals come from many different locations and travel through different layers, and so
are illuminating different zones of the subsurface that have differing properties. A final possibility is
that, in lumping together A and B class measurements in order to increase the number of results to
an amount worth interpreting I have included too many unreliable SWS measurements and there are
not enough A class measurements to conduct the inversion using these alone.
The inversion procedure has now been successfully applied to a number of settings, including:
the Yibal oil field, Oman (Al-Harrasi et al., 2010); the Valhall oil field, North Sea (Wüstefeld et al.,
2010a); Weyburn during Phase IB (this chapter and Verdon et al., 2010b,c); the hydraulic fracture
stimulation presented in Chapter 4 (Verdon et al., 2009, 2010a); a hydraulic fracture stimulation in
the Cotton Valley field, Texas (A. Wüstefeld et al., in prep., 2010); and during block-collapse mining
at Northparkes mine, Australia (Wüstefeld et al., 2010b). This method has also demonstrated the
potential to image multiple fracture sets (Verdon and Kendall, 2010). The wide ranging successful
application of this approach suggests that the approximations made in the rock physics model are
suitable for characterising fractures in many different settings. Therefore, I do not think that it is an
excessive complexity of the fracture system that has caused the inversion to fail. It is far more likely
to be a combination of the other two reasons given, that the splitting results are of poor quality and
that the arrivals may have imaged portions of the subsurface that differ excessively.
3.5 Discussion
When interpreting SWS caused by fractures, it is commonly assumed that the fast direction rotated
into geographical coordinates corresponds to the strike of the major fracture strike and/or the maximum
horizontal principle stress orientation, and that an increase in δV S corresponds to an increase
in fracture density. However, in reality, this may be an oversimplification. The presence of fractures,
sedimentary layering and other structures all combine to give the overall elasticity of a rock. The
respective contributions must all be understood before SWS can be interpreted with confidence. For
instance, with the Phase IB data the principal fracture set is not imaged, while the secondary set
is. This is because most of the waves have travelled normal to the principal fracture set, and so are
not split by them. This highlights the need to consider all the potential contributions to anisotropy
when interpreting SWS. It also demonstrates how detailed modelling can be used to infer fracture
properties despite an unfavourable source-receiver geometry.
As mentioned above, I invert for an orthorhombic symmetry assuming either a single set of vertical
fractures and a horizontal sedimentary fabric, or two sets of vertical fractures. Furthermore, the
model I use to estimate the fracture compliance is quite simple. These assumptions were made
in order to reduce the number of free parameters and therefore simplify the inversion, while being
appropriate for the reservoir analysed. They are not necessary conditions. I could certainly conceive of
situations where additional fracture sets, dipping fractures or dipping sedimentary structures, or more
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