Rock physics analysis for time-lapse seismic at Schiehallion Field, North Sea

Rock physics analysis plays a vital role in time-lapse seismic interpretation because it provides the link between changes in rock and fluid properties and the resulting seismic data response. In this case study of the Schiehallion Field, we discuss a number of issues that commonly arise in rock physics analyses for time-lapse studies. We show that:1 Logarithmic fits of dry bulk (K-dry) and shear (G(dry)) moduli vs. effective pressure (P-eff) are superior to polynomial fits.2 2D surface fits of K-dry and G(dry) over porosity (phi) and effective pressure using all the core data simultaneously are more useful and accurate than separate 1D fits over and P-eff for each individual core.3 One average set (facies) of K-dry(phi, P-eff) and G(dry)(phi(,) P-eff) can be chosen to represent adequately the entire Schiehallion reservoir.4 Saturated velocities and densities modelled by fluid substitution of K-dry(phi, P-eff), G(dry)(phi, P-eff) and the dry bulk density rho(dry)(phi) compare favourably with well-log velocities and densities.5 P- and S- wave impedance values resulting from fluid substitution of K-dry (phi, P-eff), G(dry)(phi, P-eff) and rho(dry)(phi) show that the largest impedance changes occur for high porosities and low effective pressures.6 Uncertainties in K-dry(phi, P-eff) and G(dry)(phi, P-eff) derived for individual cores can be used to generate error surfaces for these moduli that represent bounds for quantifying uncertainties in seismic modelling or pressure-saturation inversion.