Effects of permeability on CO2 trapping mechanisms and buoyancy-driven CO2 migration in saline formations

We present the results from a series of numerical simulations to explore systematic k heterogeneity effects on both CO2 trapping mechanisms and buoyancy-driven CO2 migration. For this purpose, we generated various permutations of two-dimensional numerical models of subsurface porous media: homogeneous, random, homogenous with a low-permeability (k) lens, and isotropically/anisotropically correlated k fields. For heterogeneous cases, we used a sequential Gaussian simulation technique to generate ten realizations in each model permutation. In each simulation, the amounts of mobile, residually, and aqueously trapped CO2 were calculated, and the spatial distributions of the CO2 plumes were quantified using first and second spatial moments. Simulation results from both homogeneous and random k fields suggest that the amount of residually trapped CO2 increases as the mean effective k increases. These results imply that the overall velocity distribution, which governs the sweeping area of the supercritical-phase CO2 plume, is a critical factor for controlling residual CO2 trapping. However, as overall velocity (or effective k field) increases, we predict that the CO2 plume potentially reaches the caprock more quickly. In addition, results also show that the decrease of variance in ln k increases the amount of residually trapped CO2. In simulations of anisotropically correlated k fields, the vertical CO2 migration distance due to buoyancy shortens as the horizontal correlation length becomes greater. In addition, as the horizontal correlation length becomes greater, residual CO2 trapping increases and mobile CO2 decreases because the CO2 plume spreads farther laterally (i.e., it sweeps a larger area). In summary, results of these analyses suggest that heterogeneous k fields with greater anisotropic correlation ratios potentially maximize residual CO2 trapping and minimize buoyancy-driven CO2 migration. Our findings also suggest that when heterogeneous k fields have a certain structure such as a low-k lens or other hydraulic barriers (e. g., faults), the amount of residually trapped CO2 may increase and depend more on the geometry of geological structures than the magnitude of effective k.