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As a result of washouts in the shale sections of the Lake Preston 1 well, the density log could not be usedand a global average was used instead. There are no shear wave measurements. The rock physics for theimpermeable member is given by the following trend curves,ρ = (280.586V P 0.265 ) ± 70V S = (−858.93 + 0.7700V P ) ± 121.92V P = (1748.7 + 0.3394d) ± 182.98(3.3)where ρ is bulk density in kg/m 3 and other quantities were defined in Equation 3.2.Trends for both endmembers include uncertainties which are given by the standard errors of the linearregressions. These uncertainties are an integral part of modelling undertaken in Section 4.1.3 of thisreport. A similar approach to incorporating uncertanties in rock physics relationships was taken by Chenand Dickens (2009).Given the trend curves for the two member and the net-to-gross, we can compute the impedance changeas function of CO 2 saturation. Figure 3.3 shows the impedance change for the reservoir layer in theconceptual model developed in this work for the SW Hub Project. There is a nonlinear relationshipbetween impedance change and the CO 2 saturation and, echoing Figure 1.2, it is easier to detect changesof CO 2 saturation at small saturations than it is for larger saturations. Even so, determining an exactsaturation value from seismic alone will be difficult.00.0impedance change (%)−2−4−6−0.5impedance change gradient−80 10 20 30 40 50CO 2 saturation (%)−1.0Figure 3.3: Impedance change as a function CO 2 saturation in the potential reservoir area for the SWHub CCS Project. A 5% change in saturation will cause a larger change in accousticimpedance if the initial saturation is 5% compared to the situation where the initial saturationis 30%.34 |