AVA seismic reflectivity analysis in carbon dioxide accumulations: Sensitivity to CO2 phase and saturation

Seismic monitoring of sequestered carbon dioxide (CO2) in underground deposits is a matter of growing importance. The subsurface monitoring of this greenhouse gas is possible due to the marked contrast between the physical properties of natural reservoir fluids and those of carbon dioxide after the injection. This technique makes necessary the investigation of appropriate seismic indicators to link seismic attributes to petrophysical properties, composition and state of the rock as well as pore-fluid type and in-situ physical conditions. With this motivation in mind, we use a Biot-Gassmann formulation to model the theoretical P-wave amplitude reflection coefficients vs. angle of incidence in the seismic range when a planar P-wave strikes the interface between a caprock and a porous sandstone which has its pore space saturated by a mixture of CO2 with brine or oil at different states (supercritical, liquid and gas). The effects of dissolution of CO2 in oil and the existence of a saturation threshold, above which a free CO2 phase develops, are included in the computations. Attention is particularly focused on the sensitivity of the classic best-fit amplitude variations with angle coefficients, to different degrees of CO2 saturation. We conclude from this analysis that the changes in seismic AVA attributes between 30 and 40 degrees can be useful to infer bounds on the CO2 saturation degree, to detect the presence of immiscible CO2 phase and, in some cases, to infer the physical state of the accumulations. (C) 2010 Elsevier B.V. All rights reserved.