Seismic Reflectivity of a Carbon Dioxide Flux

Abstract

In the context of the geological storage of carbon dioxide (CO2), the characterization of the injected CO2 in a reservoir is of prime importance for volume capacity evaluation and long-term site performance. In this article, we aim to characterize a CO2 accumulation in a deep layered aquifer by means of its seismic reflectivity. For modeling the vertical distribution of CO2 saturation in the reservoir, we solve the Buckley-Leverett equation with discontinuous flux function, which describes twophase flow in porous stratified media. To solve this equation numerically we employ a finite-difference relaxation scheme. The scheme entails an upwinding reconstruction for the spatial derivatives and an implicit-explicit Runge-Kutta scheme for time integrations. Once the vertical distribution of CO2 is obtained, we use a matrix propagator algorithm to compute in the frequency domain, the generalized reflectivity of the reservoir due to the injected gas. The behavior of this reflectivity controls the amplitude of seismic wave reflections and strongly conditions the detectability of the CO2 volume in the space-time domain. The numerical approach used in this article is easy to implement and allows to quantify the
reflectivity of the carbon dioxide distribution in a practical way. We show that the frequency behavior of the reservoir reflectivity may help to interpret the vertical accumulation of CO2, which can be useful as a basis for time-lapse geophysical monitoring.