Simulations of long-column flow experiments related to geologic carbon sequestration: Effects of outer wall boundary condition on upward flow and formation of liquid CO ₂

Improving understanding of CO ₂ migration, phase change, and trapping processes motivates the development of large-scale laboratory experiments to bridge the gap between bench-scale experiments and field-scale studies. Critical to the design of such experiments are defensible configurations that mimic relevant subsurface flow scenarios. We use numerical simulation with TOUGH2/ECO2M and ECO2N to design flow and transport experiments aimed at understanding upward flows including the transition of CO ₂ from supercritical to liquid and gaseous forms. These experiments are designed for a large-scale facility such as the proposed laboratory for underground CO ₂ investigations (LUCI). LUCI would consist of one or more long-column pressure vessels (LCPVs) several hundred meters in length filled with porous materials. An LCPV with an insulated outer wall corresponds to the column being at the center of a large upwelling plume. If the outer wall of the LCPV is assigned fixed temperature boundary conditions corresponding to the geothermal gradient, the LCPV represents a narrow upwelling through a fault or well. Numerical simulations of upward flow in the columns reveal complex temporal variations of temperature and saturation, including the appearance of liquid CO ₂ due to expansion cooling. The results are sensitive to outer thermal boundary conditions. Understanding of the simulations is aided by time-series animations of saturation-depth profiles and trajectories through P-T (pressure-temperature) space with superimposed phase saturations. The strong dependence of flow on hydrologic properties and the lack of knowledge of three-phase relative permeability and hysteresis underlines the need for large-scale flow experiments to understand multiphase leakage behavior.