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 fluids. Modeling studies have shown that the caprock boundary can significantly decrease updip migration speed of CO2 and increase structural trapping . The trapping phenomenon depends on the structure and topography, or roughness, of the caprock that 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, subscale 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. Less is known about the actual impact of subscale 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 regional scale caprock topography can be resolved using standard discretization techniques. The well-known vertical equilibrium (VE) model, which allows for partial integration of the multiphase flow equations, captures CO2 migration along variable caprock topography more reliably and efficiently compared to a standard full-dimensional simulator for systems in which the assumptions of vertical equilibrium and gravity segregation are valid. Therefore, caprock variability that exists below the scale of VE model resolution must be handled by upscaling. In this paper, we summarize the derivation of effective equations for fine-scale