Mechanism Comparisons of Underground Hydrogen Storage in Heterogeneous Aquifers
Authors: Cunqi Jia , Abdulhamid Alsousy , Kamy Sepehrnoori 
Published: September 2023

ABSTRACT
Hydrogen provides a practical solution to reduce climate crisis causing by utilization of fossil fuels. Underground spaces provides a practical solution to large-scale and long-time of hydrogen storage. This work is motivated to study the influence of different physical mechanisms on the underground storage process, including geomechanical process, hydrogen adsorption on solid phase, hydrogen dissolution in water phase, hydrogen diffusion in water phase, hysteresis (relative permeability and capillary pressure), and capillary trapping. Results show that an obvious difference exists in hydrogen recovery factor with and without all physical mechanisms. Values of hydrogen recovery factor with all physical mechanisms are lower than values without different physical mechanisms. Hydrogen recovery factor without geomechanical process do not show an obvious difference with values with all physical mechanisms. Underground hydrogen storage process in aquifers is not dominated by the geomechanical process. Storage mechanisms, including hydrogen adsorption on solid phase, hydrogen dissolution in water phase, and hydrogen diffusion in water phase, have less influence on the hydrogen storage process. By contract, underground hydrogen storage process in aquifers is dominated by the hysteresis and capillary trapping mechanisms.

INTRODUCTION
Hydrogen, a clean energy carrier, has attracted lots of interest recently (Pan et al. 2021b, Tarkowski and Uliasz-Misiak 2022). Especially hydrogen utilization provides a practical solution to releasing CO2 emission duo the heavy use of fossil fuels (Li et al. 2021, Li et al. 2023, Tang et al. 2023). Underground geological structures, such as aquifers and depleted petroleum reservoirs, gradually obtain more attention from the scientific community and industry (Delshad et al. 2022). Because they can provide long-time, large-scale, and safe storage spaces for hydrogen, which further promotes the potential possibility of practical hydrogen utilization.
Some scientific studies have been conducted to test and answer concerns about underground hydrogen storage. Lysyy et al. (2021) examine different storage mediums for underground hydrogen storage. Different storage schemes are conducted within the gas, oil, and water domains. Results show that the final hydrogen recovery factor has a larger value in the gas domain, followed by the oil domain. Withdrawal hydrogen has the worst behavior in the water domain. As for a sensitive study, the effect of the working rate is relatively well studied. But there is still no unanimous conclusion. Kanaani et al. (2022) take three different working rates to compare the difference between hydrogen recovery factor. Results suggest that a larger value of hydrogen recovery factor can be obtained using a higher working rate. Wang et al. (2022) also use three different rates to study the effect of the working rate on hydrogen storage behavior. But the results are different with Kanaani et al. (2022). In this study, compared to using a high working rate, more hydrogen can be produced when a low working rate is adopted. Cushion gas is another common topic discussed in the literature on underground hydrogen storage. Zamehrian and Sedaee (2022) indicate that the utilization of cushion gas mainly affects the hydrogen behavior in the first injection and production cycle. The final hydrogen recovery factor is less affected by the use of cushion gas. Different cushion gases have different behaviors. Results show that when nitrogen is adopted as a cushion gas, more hydrogen can be produced during the first production process compared to CO2 as a cushion gas. Ershadnia et al. (2023) also study the effect of the cushion gas during the underground hydrogen storage process. Results show that the whole cycle of hydrogen production amount can be affected by the use of cushion gas. More hydrogen can be produced when using cushion gas. In this study, CH4 and CO2 are selected as cushion gases. By contrast, choosing CH4 as a cushion gas has better behavior than CO2. Heinemann et al. (2021) mention that the hydrogen recovery factor can be affected by cushion gas. If more cushion gas can be used, less water can be produced. Hagemann et al. (2016) mention that the injection of hydrogen can stimulate the chemical reactivity induced by different microorganisms and bacteria. Ebigbo et al. (2013) address the activity of methanogenic microorganisms that can consume some hydrogen and carbon dioxide due to metabolism, which can change the composition of the hydrogen storage system. In addition, there are also some studies that have also contributed to improving the understanding of the underground hydrogen storage process. Sainz-Garcia et al. (2017) compare the hydrogen storage behavior using different well types. Results reveal that when a horizontal well is used to perform a hydrogen storage cycle, a lower recovery factor can be obtained than a vertical well system. Meanwhile, a horizontal well system reduces hydrogen injection capacity. Alhotan et al. (2023) conduct a sensitive study on permeability heterogeneity. Iglauer (2022) optimum the geological storage depth for structural hydrogen storage. 1000 m depth is finally suggested. Luboń and Tarkowski (2021) analyze the dynamic carbon dioxide and hydrogen injection capacity using a reservoir model from Poland. Results show under the same injection time, the CO2 distribution area is much larger than H2. Also, some experimental measurements are conducted to build an underground hydrogen storage database (Li et al. 2015, Li et al. 2017, Pan et al. 2021a).