Role of correlation structures of permeability field on residual trapping mechanisms and buoyancy-driven CO₂ migration

Previous research suggests that for many geological CO₂ sequestration reservoirs, up to 60% of the injected CO₂ will be trapped by capillary forces (irreducible saturation), a mechanism termed residual CO₂ trapping. More specifically, our recent models of ongoing field tests of geological sequestration suggest that residual CO₂ trapping can be maximized if the CO₂ plume rises a greater distance due to buoyancy (i.e., injection at the deepest part of a thick reservoir) and sweeps a larger area before coming in contact with low permeability caprock. Although this strategy maximizes the residual CO₂ trapping in theory because CO₂ plume contacts more pore spaces, it also increases the probability that upwelling- CO₂ may come into contact with faults or other leakage pathways. Geological heterogeneity seems to play greater role with respect to residual CO₂ trapping potential. To help clarify what processes and properties will maximize residual CO₂ trapping and minimize CO₂-buoyant flow, we conducted a systematic analysis of permeability (k) fields and its correlation structures. The k fields served as primary parameterization of a numerical model describing CO₂ migration during a 100-year simulation period. We compared various permutations of two-dimensional conceptual models, including homogeneous, random, homogenous with low-k lens, and anisotropically-correlated k fields. Using a Sequential Gaussian Simulation method, for most of models, we generated 10 realizations in each model permutation. In each simulation, the amount of mobile-, residual-, and aqueous-trapped CO₂ was calculated and the spatial distribution of the CO₂ plume was quantified using the first and second spatial moments. Both homogeneous and random simulation results suggest that the amount of residual trapped CO₂ increases as the effective k increases. These results imply that the overall velocity distribution, which governs the sweeping area of the CO₂ plume, is a critical factor for residual CO₂ trapping. However, as overall velocity (or k field) increases, we observed that the CO₂ plume reaches the caprock more quickly. In simulations of anisotropically correlated k fields with specific correlation length ratios of 25 m×10 m,50 m×10 m, and 100 m×10 m in x (horizontal) and z (vertical) directions, respectively, the CO₂ migration distance due to buoyancy force is shorter as the horizontal correlation length becomes greater. In addition, as the horizontal correlation length becomes greater, residual trapping increases because the CO₂ plume spreads farther laterally, sweeping a larger area. In sum, results of this analysis suggest that heterogeneous k fields with greater anisotropic correlation ratios tend to maximize residual trapping and minimize buoyancy-driven CO₂ migration. Our findings also suggest that k correlation structures, especially anisotropic media with specific ratios of correlation lengths, can strongly impact CO₂ trapping mechanisms by controlling velocity and tortuosity, which in turn determines the sweeping area of CO₂ plumes.