Separating Pressure and Saturation changes using Seismic Attenuation

Abstract

Distinguishing between pressure and saturation is important when monitoring injection in hydrocarbon reservoirs and potential CO2 sequestration sites. Examples of other methods used for distinguishing between saturation and pressure include using amplitude differences between near and far offset stacks (Landro, 2001). Although very successful with isolating pressure changes, saturation changes were more difficult to isolate. Here, we investigate the possibility of using both velocity and attenuation changes derived from prestack timelapse datasets to distinguish between gas saturation and effective pressure changes. Seismic attenuation is very sensitive to changes in gas saturation and effective pressure so by introducing this measurement, separation of pressure and saturation changes can potentially be made more robust. In order to model and interpret attenuation changes, relationships between attenuation, velocity, gas saturation and effective pressure must be chosen. Choices for these relationships are limited and are broken into two categories; theoretical, and empirical. In this paper we do not describe in detail the choice of relationships available, however, with the complexity of theoretical relationships we have chosen to use an empirical relationship outlined by Koesomadinata and McMechan (2001). This empirical relationship is only valid for sandstones as it was generated from laboratory measurement of sandstones only. The relationship of attenuation vs. pressure and saturation and velocity vs. pressure and saturation can be seen in Figure .