A sensitivity study based on 2D synthetic data from the Gullfaks Field, using PP and PS time-lapse stacks for fluid-pressure discrimination

Abstract

We investigate how combined use of time-lapse PP and PS seismic stacks can be utilized for discrimination between pressure and saturation changes. The theory is based on a combination of the well-known Gassmann and Hertz-Mindlin models. To simulate the amplitudes on PP and PS sections we compute the stacked reflection coefficients within a given angular span. Cross-plotting the PP and PS amplitude changes against the pressure and saturation changes, enables simultaneous estimation of pressure and saturation changes. The method is tested on a realistic 2D finite-difference time-lapse seismic dataset from the Gullfaks Field. The uncertainties in the estimated saturation and pressure changes are expressed by the uncertainties in PP and PS stack amplitudes. We find that this method is strongly dependent on the in situ stress conditions, and especially that low values for the initial effective pressure mean low discrimination sensitivity.