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Underground Hydrogen Storage (UHS) has gained significant attention recently as an efficient means of storing green energy. The unique challenges of hydrogen, such as its low ambient density (Muhammed et al., 2022) and its high flammability with atmospheric oxygen across a wide concentration range (Dagdougui et al., 2018) necessitates the use of deep geological formations. Among these, depleted gas reservoirs stand out due to their proven containment security, reduced losses compared to aquifers and depleted oil reservoirs, and existing infrastructure that can be repurposed for hydrogen storage (Muhammed et al., 2022; Al-Shafi et al., 2023; Zivar et al., 2021). The residual native natural gas in depleted gas reservoirs, minimizes the need for cushion gas, (Carden and Paterson, 1979; Bragg and Shallenberger, 1976; Ahmed, 2010; Tarkowski et al., 2021) essential for pressurization and hydrogen withdrawal (Carden and Paterson, 1979).
This study investigates the influence of methane, as a proxy for natural gas, on UHS through microfluidic experiments simulating drainage and subsequent imbibition processes at conditions representative to shallow reservoirs. More specifically, the potential effect of the dynamic mixing between hydrogen and methane is investigated. Hydrogen and methane reflect a significant enthalpy of mixing, which increases with pressure, resulting in a reduced temperature of the mixture, thereby changing the thermodynamic properties of the resident fluids (Lewis et al., 1977; Xue et al., 2018). These temperature changes, significant at typical UHS pressures, may impact the overall UHS process, including storage capacity and withdrawal efficiency.
The experiments are conducted at conditions representative for shallow gas reservoirs i.e. 850 psi and 20-50 °C. Mixtures of 50 mol% H2 – 50 mol% CH4, 70 mol% H2 – 30 mol% CH4 are examined alongside pure H2 and pure CH4 injections. Moreover, the influence of dynamic mixing is investigated with the use of a dual port injection in which H2 and CH4 are injected simultaneously in a controlled way into the glass chip. The obtained images are initially processed with ImageJ followed by filtering and phase segmentation using Avizo Pro. This way, the gas phase storage capacity and withdrawal efficiency are calculated after each drainage and imbibition cycle, respectively. This is one of the first studies to evaluate the effect of dynamic mixing of hydrogen and methane with respect to UHS processes, to the best of the authors knowledge.
The analysis is ongoing, but preliminary results (Fig.1) reflect some differences in terms of the storage capacity and connectivity of gas ganglia in between the dynamic and static mixture injection cases, which are important for the understanding and optimization of UHS operations.
| Country | UK |
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