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.