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| Titel |
Two-phase gravity currents in CO2 sequestration |
| VerfasserIn |
Jerome Neufeld, Madeleine Golding, Marc Hesse |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044151
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| Zusammenfassung |
| Geological carbon capture and storage (CCS), in which compressed CO2 is injected into deep
saline aquifers for permanent storage, forms an integral part of CO2 mitigation strategies. At
representative reservoir conditions CO2 is buoyant and may therefore leak into surface waters
or the atmosphere. The leakage of CO2 back into the atmosphere may be prevented by the
formation of disconnected immobile residual CO2 in the wake of the migrating plume. Here
we constrain the magnitude of residual trapping by considering a two-phase model
of the buoyancy driven propagation of a plume of injected CO2 within a saline
aquifer.
The buoyant rise of CO2 within saline aquifers is the principal mechanism through which
CO2 contacts the host reservoir. Most simplified models of CO2 migration have assumed that
the capillary transition zone is negligible relative to the current thickness and that the fluids
are separated by a sharp interface. The results anticipate that such currents quickly become
highly localized at the top boundary of reservoirs resulting in a concomitant reduction in
residual trapping. However, such single-phase models neglect both the interfacial
tension and large viscosity difference between the injected CO2 and the ambient pore
fluid.
The key challenge in two-phase gravity currents is the modeling of the variation in CO2
saturation with depth within the current. Here we use a standard model that considers the
functional dependence of the relative permeability and capillary pressure on saturation to
describe the two-phase flow. We anticipate that, after an initial transient, the extent of the
current is much greater than its depth and that the capillary pressures within the current are
balanced by gravity in this limit. This balance, called gravity-capillary equilibrium,
and the fact that flow is predominantly horizontal within the current determine
the saturation profile. Realizing that flow is driven primarily by gradients in the
hydrostatic pressure, as in single-phase flows, we then use the saturation profile in
combination with the relative permeability to determine the dynamics of the two-phase
current.
The two-phase model that results is attractive because the formalism captures many of the
features of the simpler single-phase models while providing an integrated physical and
mathematical framework for the modeling of geological CO2 sequestration. In
particular, by resolving the saturation profile, or capillary fringe, we are better able to
estimate the extent and depth of the two-phase current, thus providing more robust
estimates of residual CO2. Such two-phase models lay the groundwork to effective
and efficient characterization of storage reservoirs and promise to illuminate the
underlying physical processes governing the propagation of sequestered CO2 in the
subsurface. |
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