Driving Factors for Purity of Withdrawn Hydrogen: A Numerical Study of Underground Hydrogen Storage With Various Cushion Gases

The objective of this study is to improve the current understanding of the hydrodynamic processes arising when hydrogen is stored in subsurface porous media. This work compares the use of two cushion gases, namely carbon dioxide and methane, for hydrogen storage in a synthetic aquifer.

Abstract nano molecular structure. Water 3d spheres
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The use of hydrogen (H2) as an energy vector is expected to grow substantially in the future during the energy transition. This is because H2 has very high energy density by weight (approximately 33.6 kWh/kg) and can be produced via electrolysis reactions from renewable energies, ideally without producing any carbon emissions. However, several technical challenges remain unsolved in the value chain of H2 energy. In particular, H2 has very low mass density (approximately 0.08 kg/m3) and is very difficult to compress. Therefore, one of the central technical challenges is to provide sufficient, reliable, long-term storage capacity at scale for further development of the hydrogen economy.

As a concept inherited from natural-gas storage, underground hydrogen storage in geological formations is increasingly promoted as a possible solution to the problem of storage capacity. H2 is injected into the subsurface porous media for temporary storage and is withdrawn later when the demand increases. In this process, a certain amount of gas (usually about the same as the working gas) remains in the storage site to provide pressure support and prevent major water breakthrough during the withdrawal process. The preinjected gas is also known as the “cushion gas.” However, the cost of H2 generation, even with the most-developed and cost-effective method of steam reforming, is three times greater than that of natural gas. Therefore, it may be more economically preferable in practice to inject other cushion gases for H2 storage.

Two possible choices, namely methane (CH4) and carbon dioxide (CO2), are considered here. The former is currently used in the UK’s national grid, which would be tolerant to the impurity content of CH4 mixed in the back-produced hydrogen. The use of CO2 as cushion gas, on the other hand, may provide extra benefits of permanent carbon storage. However, the back-produced CO2 may have to be removed from the H2 and injected back into the subsurface.

The central objective of this numerical study is to compare the use of these two different choices of cushion gases for H2 storage in subsurface porous media. In particular, the authors focused on how the differences in the gas properties (density and viscosity) influence the flow behavior and thus the H2 recovery performance. Based on previous work, both viscous- and gravity-dominated scenarios were tested in a synthetic 2D aquifer model. The advantage of this method, which is based on the scaling theory, is to save computational efforts. The authors show how the choice of cushion gases drives the operational performance, including the H2 recovery factor and H2 purity, during the process of back production. The ultimate goal of this work is to achieve a detailed understanding of the interactions between hydrogen, cushion gases, native in-situ fluids, and the ambient rocks in the subsurface porous medium.

In the context of H2 storage, this paper focuses on both the amount and the purity of the H2 that is back produced. A series of very-fine-scale numerical simulations was performed in 2D vertical systems using a fully compositional simulator. A simple three-stage operation strategy (cushion-gas injection, H2 injection, and H2 production) was designed to trigger the flow behavior of interest. Based on scaling theory, the authors analyzed the effects of various mechanisms on the H2 recovery performance, from viscous-dominated to gravity-dominated flow regimes.

Viscous instability and permeability heterogeneity may strongly degrade the purity of the back produced H2. No matter which gas (CO2 or CH4) is selected as the cushion gas, the less viscous H2 infiltrates the cushion gas, meaning that the displacement does not proceed in a piston-like fashion. In the viscous-dominated scenario, H2 may even bypass the cushion gas of CO2, which subsequently leads to early breakthrough of the cushion gas and thus a dramatic reduction in H2 purity during back production. However, this effect does not arise in the case with CH4 as cushion gas. On the other hand, in the gravity-dominated case, the less-dense H2 accumulates above the cushion gas and there is no flow infiltration or bypassing occurring in cases studied here. Therefore, the overall H2 recovery performance is much better in the gravity-dominated regime than in the viscous-dominated regime.

Finally, the authors demonstrate that it is important to include the solubility of CO2 when used as cushion gas in aquifer systems. This is because CO2 dissolution in water may significantly reduce its gas volume and lead to early water breakthrough during back production.

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