Numerical Simulation Experiments on the Long-Term Evolution of a CO2 Plume Under a Sloping Caprock

摘要

We have used the TOUGH2-MP/ECO2N code to perform numerical simulation studies of the long-term behavior of CO(sub 2) stored in an aquifer with a sloping caprock. This problem is of great practical interest, and is very challenging due to the importance of multi-scale processes. We find that the mechanism of plume advance is different from what is seen in a forced immiscible displacement, such as gas injection into a water-saturated medium. Instead of pushing the water forward, the plume advances because the vertical pressure gradients within the plume are smaller than hydrostatic, causing the water column to collapse at the plume tip. Gas saturations and updip CO(sub 2) fluxes are nearly constant, independent of time and position, in the upper, mobile portions of the plume. The CO(sub 2) plume becomes thinner as it advances, yet the speed of advancement remains constant over the entire simulation period of up to 400 years, with migration distances of more than 80 km. Our simulation includes dissolution of CO(sub 2) into the aqueous phase and associated density increase, and molecular diffusion. However, no convection develops in the aqueous phase because it is suppressed by the relatively coarse (sub-)horizontal gridding required in a regional-scale model. A first crude sub-grid-scale model was implemented to represent convective enhancement of CO(sub 2) dissolution. This process is found to greatly reduce the thickness of the CO(sub 2) plume, but does not affect the speed of plume advancement.