The key challenge of CO2 sequestration projects is to ensure that selected formations will provide for long-term storage of CO2 without significant leakage. This is also the case for sequestration projects in saline aquifers, where substantial evidence must be provided that injected CO2 will remain in the target formation. However, the time-scales that characterize the immobilization of CO2 in aquifers are not yet fully understood. In this paper, we investigate the time scale for post-injection immobilization of CO2 and proposed a new dimensionless time scale that includes the contributions of both capillary pressure and gravity forces. The Bond number (NB) is included in the approach to capture the impact of the relevant driving forces. We use numerical calculations to study the interplay between capillary and gravity forces on the predicted evolution of a well-defined CO2 plume in an aquifer. Scaling analysis for 1D and 2D systems is presented for characterizing the plume behavior in different flow regimes as dictated by the relative magnitude of gravity and capillary forces (Bond numbers). A majority of the previous efforts on scaling analysis has focused on processes that are driven by capillary forces alone. However, for sequestration applications the impact of gravity force may be significant. We demonstrate that plume migration at constant Bond numbers may display very different characteristics as a consequence of phase mobilities. The results of our simulations and scaling analysis apply directly to the planning of CO2 sequestration projects and provide new guidelines for estimating the immobilization time of supercritical CO2 as well as the amount of CO2 that can potentially be trapped in a given aquifer.

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