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To assess the risks of Geologic Carbon Sequestration (GCS), it is crucial to understand the fundamental physicochemical processes that may occur if and when stored CO2 leaks upward from a deep storage reservoir into the shallow subsurface. Intermediate-scale experiments allow for improved understanding of the multiphase evolution processes that control CO2 migration behavior in the subsurface, because the boundary conditions, initial conditions, and porous media parameters can be better controlled and monitored in the laboratory than in field settings. For this study, a large experimental test bed was designed to mimic a cross section of a shallow aquifer with layered geologic heterogeneity. As water with aqueous CO2 was injected into the system to mimic a CO2-charged water leakage scenario, the spatiotemporal evolution of the multiphase CO2 plume was monitored. Similar experiments were performed with two different sand combinations to assess the relative effects of different types of geologic facies transitions on the CO2 evolution processes. Significant CO2 attenuation was observed in both scenarios, but by fundamentally different mechanisms. When the porous media layers had very different permeabilities, attenuation was caused by local accumulation (structural trapping) and slow redissolution of gas phase CO2. When the permeability difference between the layers was relatively small, on the other hand, gas phase continually evolved over widespread areas near the leading edge of the aqueous plume, which also attenuated CO2 migration. This improved process understanding will aid in the development of models that could be used for effective risk assessment and monitoring programs for GCS projects.

Plain Language Summary Geologic carbon sequestration is a strategy that is currently being considered to help reduce emissions of greenhouse gases to the atmosphere. This strategy involves injecting carbon dioxide deep underground in the hopes that it will safely stay there for long periods of time. However, the gas may eventually leak out of the storage location and travel back up toward the ground surface. If this happens, it could have negative effects on water resources, ecosystems, and human health. In order to better understand those potential risks, operators of carbon sequestration projects need to understand where the gas will go when it leaks. This study helped to improve that understanding and build predictive capability about carbon dioxide movement underground. Laboratory experiments were performed in a system that was designed to mimic an underground environment. Carbon dioxide was injected and sensors were used to measure where it went. Results showed that layers of different material slow down the movement of the gas.