The Development of CO2 Plume in CO2 Sequestration in Aquifer

Geological carbon sequestration through injecting large-scale CO₂ into the deep saline aquifers represents long-term storage of CO₂. In the CO₂ sequestration process, the injected CO₂ is displacing water from the injection point and is expected to remain in the reservoir. Due to the nature of one phase displacing another phase in porous media, it is noted that different water saturation exists in the CO₂ plume during the displacement. The distribution of water saturations behind CO₂ front during the displacement is a function of fluid and rock properties, fluid-rock interaction, and injection operation. In this study, these factors are considered when developing new models to predict CO₂ plume evolution during injection. Mass conservation, multiphase flow, and equations of state are applied in the derivation of the models, which guarantees a rigorous approach in the investigation.

The modeling results indicate that CO₂ does not displace water entirely away from the plume. The shape of the CO₂ front is controlled by the relative permeability of two phases and capillary pressure. Water saturation profile from CO₂ injecting point to the displacement front shows that water saturation behind the CO₂ front increases outwardly, and the change in saturation is non-linear. The injection rate impacts the sharpness of the CO₂ front, thus leads to different gas plume sizes for the same injection volume. The outward movement of the CO₂ front decelerates as injection time goes on.

The research illustrates that injection experiences two stages: transient and steady-state, in which the displacement behavior and the development of gas plume vary. Although the duration of the transient stage is dictated by the size of the aquifer and is relatively short compared with steady-state stage, its influence on the development of CO₂ plume cannot be neglected when selecting gas compressor horsepower and determining injection rate.