The critical point of CO2 is at temperature and pressureconditions of Tcrit = 31.04oC, Pcrit = 73.82 bar. At lower (subcritical)temperatures and/or pressures, CO2 can exist in two different phases, aliquid and a gaseous state, as well as in two-phase mixtures of thesestates. Disposal of CO2 into brine formations would be made atsupercritical pressures. However, CO2 escaping from the storage reservoirmay migrate upwards towards regions with lower temperatures andpressures, where CO2 would be in subcritical conditions. An assessment ofthe fate of leaking CO2 requires a capability to model not onlysupercritical but also subcritical CO2, as well as phase changes betweenliquid and gaseous CO2 in sub-critical conditions. We have developed amethodology for numerically simulating the behavior of water-CO2 mixturesin permeable media under conditions that may include liquid, gaseous, andsupercritical CO2. This has been applied to simulations of leakage from adeep storage reservoir in which a rising CO2 plume undergoes transitionsfrom supercritical to subcritical conditions. We find strong coolingeffects when liquid CO2 rises to elevations where it begins to boil andevolve a gaseous CO2 phase. A three-phase zone forms (aqueous - liquid -gas), which over time becomes several hundred meters thick as decreasingtemperatures permit liquid CO2 to advance to shallower elevations. Fluidmobilities …