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To meet the forecast demand for underground hydrogen storage, additional storage capacity in salt caverns and porous rock-formations will be needed (IEA, 2023). The reactivity of molecular hydrogen can trigger different geochemical processes in porous storage formations, for example the reduction of Fe(III) in hematite (Fe2O3) to Fe(II) (Hydrogen-TCP, 2023). Due to the heterogenous nature of porous rock formations uncertainty regarding the impact of these processes remains. Here we present results of detailed petrophysical and petrographic characterization and geochemical laboratory experiments of Triassic sandstones from a former gas reservoir and underground gas storage site in Germany. The mainly red-brown colored sandstone is primarily composed of quartz grains and subordinate feldspar grains, both with hematite coatings, and partly pore-filling cements and clay cutans. But within the investigated 5 m reservoir section, some decimeter scale intervals are bleached to grey-beige as a result of different diagenetic influences. The aim of the study is to characterize the transport and storage properties as well as to quantify the extent of hydrogen-rock reactions for these two distinctly different appearances within the formation. Petrophysical results show different poro-perm characteristics between the red-brown and the bleached sandstone. Samples of the red-brown sandstone show higher porosity but lower permeability than samples from the bleached sections. Batch experiments with powdered sandstone samples from both intervals, synthetic saline formation water and hydrogen at a partial pressure of 10 MPa at 120°C and 20 MPa confining pressure show significant, but minor amounts of H2 being oxidized during the 14 days experiment for both the red-brown and bleached sandstone. This was counterintuitive as we expected to see more H2 oxidation by the hematite-rich red-brown sandstone. Petrographic investigations combined with Raman analyses revealed that iron-bearing grain coatings in the red-brown sandstone are mostly overgrown with quartz and plagioclase cements. These results indicate that, diagenetic bleaching, probably caused by migration of reducing fluids (Aehnelt et al., 2021), led to improved permeability while porosity was reduced, e.g. due to cement precipitation. The presence of reactive Fe(III) in the unbleached facies does not increase H2-mineral reactions, indicating that the quartz overgrowth of hematite-coatings protects Fe(III) from reacting with hydrogen. Thus in the studied formation, accessibility to reactive mineral surfaces (here hematite) is a controlling factor that can limit H2-rock reactions.
| References | • IEA (2023) Global Hydrogen Review 2023. Available at: https://www.iea.org/reports/global-hydrogen-review-2023 • Hydrogen-TCP-Task-42 (2023) Underground Hydrogen Storage: Technology Monitor Report. International Energy Agency - Hydrogen TCP, p. 153. Available at: https://www.ieahydrogen.org/wpfd_file/2023_report_task-42_technology-monitor-report-underground-hydrogen-storage_h2tcp/. • Aehnelt, M. et al. (2021) “On the origin of bleaching phenomena in red bed sediments of Triassic Buntsandstein deposits in Central Germany,” Geochemistry, 81(2), p. 125736. Available at: https://doi.org/10.1016/j.chemer.2020.125736. |
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| Country | Germany |
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