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During underground hydrogen storage in aquifers and depleted gas fields, hydrogen commonly coexists with methane used as a cushion gas. In this context, it is important to understand how the distribution of the gas phase composition evolves over time in the reservoir, as this affects the recovery efficiency of the stored hydrogen. In these systems, the methane and hydrogen trapped in the system may still redistribute over time, due to gradual dissolution and diffusion of the gas components in the aqueous phase. This process, known as Ostwald ripening, alters the connectivity of the trapped gas phase, and typically leads to the dissolution of smaller gas bubbles and the growth of larger ones. Previous studies have extensively examined the Ostwald ripening of single-component gases in porous media; however, the behavior of multicomponent gas systems remains poorly understood [1], [2].
In this study, we investigate multicomponent gas ripening at the pore scale, by imaging the long-term redistribution of a trapped gas mixture in sandstone samples using time-lapse X-ray micro-CT imaging. Since characterizing the behavior of a methane-hydrogen gas mixtures in opaque porous media is challenging due to their limited contrast in X-ray imaging, we employ krypton and helium as proxy gases for which the composition can be quantified with X-ray micro-CT. At the start of the experiments, a 50–50% mixture of krypton and helium is prepared based on partial pressures and equilibrated with 25% KI brine in a reactor at pressure-temperature conditions which represent hydrogen storage in shallow aquifers (4 MPa, 25-35°C). The gas mixture then is trapped within the porous medium through sequential drainage and imbibition cycles using the prepared gas mixture and brine, after which the sample is shut in and allowed to equilibrate. This experimental approach enables direct visualization of gas-phase composition evolution within the pore space under supercritical conditions and allows analysis of redistribution kinetics using helium as a proxy gas with diffusive properties similar to hydrogen.
Preliminary results confirm the suitability of helium as a representative for hydrogen and indicate a gradual, capillary-driven mass transfer process in which smaller gas bubbles dissolve and diffuse toward larger gas ganglia, ultimately leading to an equilibrium state. These findings provide new insights into long-term dynamics of gas-mixture following entrapment in porous media. The results are particularly relevant for natural gas reservoirs repurposed for hydrogen storage, and contribute to a better understanding of gas distribution, transport properties, and recovery efficiency in such systems.
| References | [1] S. Goodarzi, Y. Zhang, S. Foroughi, B. Bijeljic, and M. J. Blunt, ‘Trapping, hysteresis and Ostwald ripening in hydrogen storage: A pore-scale imaging study’, Int. J. Hydrog. Energy, vol. 56, pp. 1139–1151, Feb. 2024, doi: 10.1016/j.ijhydene.2023.12.029. [2] N. Bueno, L. Ayala, and Y. Mehmani, ‘Ostwald ripening of multi-component bubbles in porous media: A theory and a pore-scale model of how bubble populations equilibrate’, Adv. Water Resour., vol. 182, p. 104581, Dec. 2023, doi: 10.1016/j.advwatres.2023.104581. |
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| Country | Belgium |
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