Effective models for CO₂ migration in geological systems with varying topography

Geological CO₂ sequestration relies on a competent sealing layer, or caprock, that
bounds the formation top and prevents vertical migration of CO₂ and brine. Modeling
studies have shown that caprock topography, or roughness, can significantly decrease
updip migration speed of CO₂ and increase structural trapping. Caprock roughness can be
characterized at different spatial scales. For instance, regional-scale features such as domes,
traps, and spill points can be detected in seismic surveys and have been shown to affect
large-scale migration patterns. However, structural and topographical variability, known as
rugosity, exists below seismic detection limits but can be measured at the scale of
centimeters and meters using LiDAR scanning of formation outcrops. Little is known about
the actual impact of structural rugosity on CO₂ plume migration. Practically speaking, given
the large scales required to model commercial scale CO₂ storage projects and the limitations
on computational power, only seismic-scale caprock topography can be resolved using
standard discretization techniques. Therefore, caprock variability that exists below the model
resolution scale is defined as subscale and must be handled by upscaling. In this paper we
derive effective equations for CO₂ migration that include the impact of fine-scale variability
in caprock topography using static equilibrium upscaling, an approach that is adapted for the
vertical equilibrium modeling framework. The effective equations give estimates of the
impact of rugosity on CO₂ plume migration and trapping in large-scale systems.