Reflection characteristics of linear carbon dioxide transition layers

Abstract

Seismic monitoring of underground CO2 accumulations is a subject of growing interest in applied geophysics. Due to their large impedance contrasts, attention is focused on accumulations of high CO2 saturation in most cases. However, low-saturation zones with dispersed carbon dioxide, or saturation transition layers, may have an important role inthe propagation of waves within the reservoir, giving rise to amplitude and phase changes of the seismic signals. With this motivation, we studied the reflectivity response of a simple reservoir model with a given CO2 saturation-depth profile, on a theoretical basis. We investigated the influence of the overall saturation, vertical extent, and spatial fluiddistribution of a carbon dioxide transition zone in the reflectivity of a reservoir. The parametric analysis entails the computation of the generalized P-wave reflection coefficient and its variations with ray angle (AVA) and frequency (AVF). The combined analysis of AVA and AVF can help to characterize and monitor CO2 transition layers within geological storage sites.