Influence of small-scale heterogeneity on upward CO2 plume migration in storage aquifers
Abstract
Recent advancements in experimental techniques allow sub-core-scale heterogeneities to be quantified in a high resolution. Based on the observations of heterogeneity distributions in natural core samples, we perform simulations to study the influence of small-scale heterogeneities on large-scale CO2 migration during geological storage. We observe that even the heterogeneities at millimeter scale (the scale of a Representative Elementary Volume for sandstones) can affect large-scale buoyancy-driven upward CO2 migration. For the representative examples we study, ignoring small-scale heterogeneities can lead to an overestimation of the migration speed by a factor of two.
To analyze the cause of such overestimation, we introduce a dimensionless heterogeneity factor to characterize different levels of heterogeneity. The influence on CO2 migration is quantified with respect to a variety of heterogeneity factors, correlation lengths, and fluid viscosity ratios for isotropic and anisotropic media. Our findings suggest that small-scale heterogeneities should not be ignored in core analysis, even for cores that appear relatively homogeneous and do not have distinguishable heterogeneous patterns. In addition, relative-permeability curves measured from core-flood experiments under high-flow-velocity conditions (a common practice to eliminate capillary end-effects) should not be directly used in modeling low-velocity CO2 migration if small-scale heterogeneities are present.