The impact of capillary heterogeneity on CO2 flow and trapping across scales

Capillary heterogeneity has been identified over the last decade as a key control on subsurface CO₂ flow behavior during geological CO₂ sequestration. These heterogeneities can be formed in all sedimentary rocks, ranging from slight variations in the sand grain sizes to extensive sequences of interbedded sands, shales, and limestones. Capillary heterogeneity has been largely, although not entirely, overlooked in subsurface flow modeling because it is assumed to only directly influence fluid redistribution over scales of centimeters to meters. However, even small-scale fluid movements can result in dramatic impacts on the mobility and trapping of CO₂ over kilometers. Therefore, neglecting capillary heterogeneity at multiple scales could potentially lead to errors in modeling and predicting field-scale plume migration. In this review paper, we aim to provide a consistent overview to (1) establish that capillary heterogeneity can have a major impact on CO₂ plume migration, (2) establish the respective length scales at which capillary heterogeneity matters, and (3) provide guidance for numerical modeling.

This review covers pertinent literature and extracts key observations from core to field scales. Experimental studies have shown that millimeter-decimeter scale capillary heterogeneity can cause the so-called capillary heterogeneity trapping in addition to pore-scale residual trapping. Even at such a small scale, capillary heterogeneity can already lead to complex upscaled constitutive relationships, such as flow-rate dependent and anisotropic relative permeability, which affects field-scale CO₂ migration even when field-scale heterogeneities are present. Under gravity-dominated flow regimes, centimeter-meter scale capillary heterogeneity can entrap a significant amount of CO₂ at the field scale, not only after imbibition but also during drainage. In certain cases, the presence of capillary heterogeneity can even completely stop the vertical movement of the CO₂ plume, hence greatly reducing leakage risks. At the meter-kilometer scale, the influence of capillary heterogeneity is more pronounced and can hinder or redirect CO₂ migration in both lateral and vertical directions.

The impact of capillary heterogeneity across multiple spatial scales poses a great challenge in modeling CO₂ migration at the field scale, because it is practically impossible to build a field-scale earth model with grid blocks at the millimeter scale. We recommend a hierarchical modeling approach to address this challenge. At the field scale, earth models are built to capture geological features and heterogeneities in high but still practical grid resolutions. For each facies or rock type in the field-scale model, high-resolution meter-scale “conceptual” models are built using millimeter-scale grid blocks to capture representative fine-scale bedding geometries and heterogeneities in various depositional environments, thereby bridging the gap between the subcore scale and the size of a field-scale simulation grid block. Upscaling is then used to preserve the smaller-scale flow dynamics of various rock types in field-scale simulations. Future work is needed to (1) refine, improve, and validate the hierarchical modeling approach; (2) build libraries of fine-scale bedding models for facies in various environments of deposition; (3) quantify multiscale capillary heterogeneity effects under subsurface uncertainties; (4) gain learning from different storage formations; and (5) establish best practices that balance accuracy and computational speed.