Modelling the Reflectivity of a Carbon Dioxide Transition Zone

The seismic monitoring of carbon dioxide in geologic reservoirs is mostly focused on the characterization of accumulations of high saturation, due to their large seismic amplitudes. Nevertheless, low-saturation zones with dispersed CO2, or saturation transitions may have an important role in the propagation of waves within the reservoir, giving rise to amplitude and phase changes of the seismic signals. In this work, we consider a transition layer given by a linear CO2 saturation-depth profile, which in turns develops a non-linear velocity trend with depth. We model the theoretical reflectivity response of a simple reservoir model, based on the Sleipner field, with a given CO2 saturation transition zone. Our study entails a parametric analysis of the generalized P-wave reflection coefficient and its variations with ray angle (AVA) and frequency (AVF). Our results suggest that the characterization of CO2 transitions zones can be achieved with a combined AVA and AVF analysis. We have shown that discrimination between thin and thick CO2 zones seems feasible. In addition, the bulk CO2 saturation present in the reservoir may be estimated by considering its reflectivity in the frequency domain.