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Geological storage of CO 2 (GCS), also referred to as carbon sequestration, is a critical component for decreasing anthropogenic CO 2 atmospheric emissions. Stored CO 2 will exist as a supercritical phase, most likely in deep, saline, sedimentary reservoirs. Research at the Center for Frontiers of Subsurface Energy Security (CFSES), a Department of Energy, Energy Frontier Research Center, provides insights into the storage process. The integration of pore-scale experiments, molecular dynamics simulations, and study of natural analogue sites has enabled understanding of the efficacy of capillary, solubility, and dissolution trapping of CO 2 for GCS. Molecular dynamics simulations provide insight on relative wetting of supercritical CO 2 and brine hydrophilic and hydrophobic basal surfaces of kaolinite. Column experiments of successive supercritical CO 2 /brine flooding with highresolution X-ray computed tomography imaging show a greater than 10% difference of residual trapping of CO 2 in hydrophobic media compared to hydrophilic media that trapped only 2% of the CO 2 . Simulation results suggest that injecting a slug of nanoparticle dispersion into the storage reservoir before starting CO 2 injection could increase the overall efficiency of large-scale storage. We estimate that approximately 22% ± 17% of the initial CO 2 emplaced into the Bravo Dome field site of New Mexico has dissolved into the underlying brine. The rate of CO 2 dissolution may be considered limited over geological timescales. Field observations at the Little Grand Wash fault in Utah suggest that calcite precipitation results in shifts in preferential flow paths of the upward migrating CO 2 -saturated-brine. Results of hybrid pore-scale and pore network modeling based on Little Grand Wash fault observations demonstrate that inclusion of realistic pore configurations, flow and transport physics, and geochemistry are needed to enhance our fundamental mechanistic explanations of how calcite precipitation alters flow paths by pore plugging to match the Little Grand Wash fault observations.

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The chemical potential of water may play an important role in adsorption and capillary condensation of water under multiphase conditions at geologic CO 2 storage sites. Injection of large volumes of anhydrous CO 2 will result in changing values of the chemical potential of water in the supercritical CO 2 phase. We hypothesize that the chemical potential will at first reflect the low concentration of dissolved water in the dry CO 2 . As formation water dissolves into and is transported by the CO 2 phase, the chemical potential of water will increase. We present a pore-scale model of the CO 2 -water interface or menisci configuration based on the augmented Young-Laplace equation, which combines adsorption on flat surfaces and capillary condensation in wedge-shaped pores as a function of chemical potential of water. The results suggest that, at a given chemical potential for triangular and square pores, liquid water saturation will be less in the CO 2 -water system under potential CO 2 sequestration conditions relative to the air-water vadose zone system. The difference derives from lower surface tension of the CO 2 -water system and thinner liquid water films, important at pore sizes <1 3 10 26 m, relative to the air-water system. Water movement due to capillary effects will likely be minimal in reservoir rocks, but still may be important in finer grained, clayey caprocks, where very small pores may retain water and draw water back into the system via adsorption and capillary condensation, if dry-out and then rewetting were to occur.