Lawrence Berkeley National Laboratory

The critical point of CO2 is at temperature and pressure conditions of Tcrit = 31.04oC, Pcrit = 73.82 bar. At lower (subcritical) temperatures and/or pressures, CO2 can exist in two different phases, a liquid and a gaseous state, as well as in two-phase mixtures of these states. Disposal of CO2 into brine formations would be made at supercritical pressures. However, CO2 escaping from the storage reservoir may migrate upwards towards regions with lower temperatures and pressures, where CO2 would be in subcritical conditions. An assessment of the fate of leaking CO2 requires a capability to model not only supercritical but also subcritical CO2, as well as phase changes between liquid and gaseous CO2 in sub-critical conditions. We have developed a methodology for numerically simulating the behavior of water-CO2 mixtures in permeable media under conditions that may include liquid, gaseous, and supercritical CO2. This has been applied to simulations of leakage from a deep storage reservoir in which a rising CO2 plume undergoes transitions from supercritical to subcritical conditions. We find strong cooling effects when liquid CO2 rises to elevations where it begins to boil and evolve a gaseous CO2 phase. A three-phase zone forms (aqueous - liquid - gas), which over time becomes several hundred meters thick as decreasing temperatures permit liquid CO2 to advance to shallower elevations. Fluid mobilities are reduced in the three-phase region from phase interference effects. This impedes CO2 upflow, causes the plume to spread out laterally, and gives rise to dispersed CO2 discharge at the land surface. Our simulations suggest that temperatures along a CO2 leakage path may decline to levels low enough so that solid water ice and CO2 hydrate phases may be formed.