Effect of H2-rich water percolation in porous limestone core: simulation of a leakage in a shallow carbonate freshwater aquifer

Underground Hydrogen Storage (UHS) builds on existing underground storage technologies used for town gas and CO 2 and involves storing hydrogen in various geological structures underground. However, some questions remain regarding this storage method, particularly concerning safety: an untight UHS site could lead to a hydrogen leak to the surface, where the gas could be intercepted by an overlying freshwater aquifer. H 2 would dissolve in the water, and this fluid could react with the surrounding rock. This study investigates the effects of hydrogen-saturated water flow through carbonate rock cores to simulate such an H 2 leakage in a near subsurface freshwater aquifer. Core flooding experiments were conducted on two carbonate facies, a grainstone and a boundstone, using a Hassler cell. Petrophysical and geochemical changes were monitored throughout the experiments. Results indicate that hydrogen remains largely non-reactive in its dissolved form when in contact with calcite. However, the water flow itself induces mechanical pore clogging, leading to significant permeability reduction without affecting the total porosity. A slight calcite precipitation within the pore network is also suggested. Additionally, a loss of dissolved H 2 is observed, likely due to fluid degassing rather than chemical consumption. These findings highlight the importance of considering mechanical processes in UHS scenarios and emphasize the need for further research on the long-term behavior of hydrogen saturated water in carbonate freshwater aquifers. • Water flow causes the detachment of calcite particles, leading to pore clogging and a decrease in permeability. • H 2 is not reactive under the tested experimental conditions. • Some calcite precipitation is likely to occur within the rock.