Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 4. A COMPARISON OF MICROSEISMIC MONITORING OF FRACTURE STIMULATION DUE TO WATER
VERSUS CO 2 INJECTION
P-wave arrives
first at the top
of the array
Injection depth is
between geophones
3 and 4
Moveout is
consistent down
the array
0.0 0.25 0.5
Time
Figure 4.9: P- and S-wave moveout for an event during CO 2 injection located well above the
injection depth and recording array. The energy arrives first near the top of the array, with
consistent moveout going down. This indicates that the energy is arriving from an event above
the array, and well above the injection depth.
4.4 Shear wave splitting
An alternative method to estimate the degree of fracturing is, as discussed in Chapter 3, to use shear
wave splitting. As with the Weyburn microseismic data, for both stages I use the semi-automated
approach of Teanby et al. (2004b) to compute the fast direction polarisations and time-lags. I invert
these measurements for fracture distributions using the inversion technique outlined in Chapter 3 and
Verdon et al. (2009).
4.4.1 Splitting measurements
Of the 780 potential SWS measurements (65 events × 12 receivers) during water injection, 45 provided
class A results as defined by Teanby et al. (2004b). This distribution of result quality is to be expected
for a typical microseismic dataset (e.g., Al-Anboori, 2006). Figure 4.12 shows a cylindrical projection
(Liu et al., 1989) of the splitting results as a function of arrival azimuth (θ) and inclination (ϕ). The
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