DENHyMS

Scenarios for achieving carbon neutrality by 2050 predict a sharp increase in the use of hydrogen (H2). As with natural gas, large volumes of H2 will need to be stored underground, raising the question of storage integrity and the risk of leakage of this highly diffusive gas, which has significant flammability and explosion limits in air. At the same time, numerous projects are emerging around the world, focusing on the potential of natural H2, produced naturally by various processes in the earth's crust or deeper down, to become a resource. Natural H2 is often detected by its leakage at the surface, when it reaches the critical zone, the Earth's most superficial layer and the site of air-water-rock interactions. Studying how natural H2 migrates or is trapped is therefore a source of information for the monitoring strategies we need to implement to ensure the integrity of future underground H2 storages. Through the DENHyMS project, we propose to set up an in-situ laboratory to study the migration of natural H2 in the critical zone (0-100 m depth), using a documented case of natural leakage to provide information both on the integrity of storage sites and for natural H2 prospecting. The special feature of this project will be to study migration, reactivity and trapping in two different compartments of the critical zone: its deep part, or saturated zone, and its superficial part, or unsaturated zone, enabling us to conduct our research in a multiphase environment. This multidisciplinary project, involving seven partners, proposes to drill at least two dedicated wells to monitor the behavior of multiple gaseous species (H2, CO2, O2, CH4...) in saturated and unsaturated zones. Geological, geophysical and geochemical field studies will initially be used to select the best site for drilling. These studies will be conducted in line with best practice in terms of constructive dialogue with local stakeholders. During drilling, rock samples will be taken and characterized mineralogically and microbiologically. These samples will be monitored for degassing in the laboratory over a two-year period, in order to study degassing and productivity kinetics. Once the boreholes are equipped, geochemical and microbiological monitoring of the saturated and unsaturated zones will be carried out over two years, to describe gas migration, emission variability, the influence of redox reactions, and the role of the aquifer and bacterial consortia on H2 production or consumption. Complementary geological and geophysical work will be carried out to understand how certain seasonal variables (e.g. degree of water saturation of soil/rock formations) influence gas migration, and to help develop a conceptual fluid migration model. These data will be integrated into a reactive transport model describing gas/water/rock/biota reactivity in the critical zone. This model will simulate water and gas flows in a medium with variable water saturation, taking into account the reactive processes that can affect gas concentration. As the geochemical calculations can be coupled to the advection and diffusion equations, the model will ultimately enable us to assess the H2 flux existing in the study area. A PhD student and a post-doc will contribute to the field studies, interpretation and modeling of geochemical and microbiological data. By questioning our understanding of the role of aquifers, bacteria, geology, diffusion and reactivity, we will provide key elements for understanding how the subsurface promotes or limits H2 migration. This information will be useful both for improving underground storage integrity control strategies and for the search for natural H2-producing zones.