Recently, the growing demand for carbon-free energy has sparked an unprecedented interest in naturally occurring H₂, as it could represent a potential alternative resource to fossil fuels. Throughout the world, and since more than one century, numerous natural H₂-bearing geological fluids have been discovered, but to date, there is neither exploration strategy nor any resource assessment, as practical guidelines for hydrogen targeting are still missing. Here, we lay the foundation of a preliminary exploration guide based on a global ‘source-transport-accumulation’ understanding of H₂-concentrating process and a combination of techniques and data used for both conventional petroleum and mining exploration. Based on different case studies, belonging to contrasted geological settings, we will provide the first elementary bricks to evaluate the sources, migration and trapping of H₂ in the Earth’s crust.

Water radiolysis is a key process for hydrogen (H₂) and abiotic organic molecules generation in the Earth’s crust. The aim of this presentation is to provide some insight into this process from a radiochemist viewpoint. We will transpose the knowledge we gain from water radiolysis in the context of radioactive waste disposal to natural geological settings and draw important conclusions for deep microbial ecosystems development and abiotic organic synthesis. Some examples will be given about: (i) the relationship between H₂ production ant the nature of the emitted particle (α/β/γ) considered for water radiolysis, (ii) the boosted production of H₂ observed when aqueous solutions are in contact with some mineral surfaces such as rutile (TiO₂) and calcite (CaCO₃), (iii) the scavenging role of carbonate anions onto hydroxyl radical and the amplified yield of H₂, (iv) the switch from an inorganic world to an organic one through the carboxylate anions production from carbonate radiolysis.

Radiation chemistry is often overlooked by geologists who consider the process as anecdotic (apart for the thermal budget of Earth) in term of mass balance. However, water radiolysis is a large-scale ubiquist process in the crust and it does not need specific conditions to occur. We will show that at geological time scale, water radiolysis leads to a very diverse, reactive, and fun chemistry able to sustain life and even to create the condition for its emergence.

Temporary underground storage of molecular hydrogen (H₂) in depleted oil and gas reservoirs has recently attracted increasing research interest as it can support chemical industry demands and peak-shaving in the energy supply grid. Experimental parameters related to abiotic chemical reactions, microbial metabolism, and transport mechanisms of molecular hydrogen under elevated pressure conditions in such reservoirs are of potential relevance to these applications but have rarely been studied. As far as abiotic chemical reactions are concerned, since mineral coatings of hematite are a common feature in conventional reservoirs, a thorough understanding of the reactivity of the hematite-H₂ reaction system is of utmost importance. In addition, potential microbial growth in the pore space of reservoirs may affect the preset gas composition as well as petrophysical properties.

In this context, RWTH-Aachen University is coordinating the “H2_ReacT2” project, a follow up cooperation together with the Bundesanstalt für Geologie und Rohstoffe (BGR) and the Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum (GFZ). The overall objective is to gain a comprehensive understanding of relevant abiotic and biotic redox reactions, associated changes of petrophysical properties and molecular mass transfer within potential underground storage formations.

In this session, we will present analytical data of novel experimental approaches to study (1) the kinetics of the H₂-H₂O-hematite reaction system at low temperatures and elevated H₂ pressures, and (2) the effects of microbial metabolism of H₂ and overburden pressure on storage and transport properties of a typical reservoir rock, the Bentheimer sandstone.

Current renewable energies are unsteady resulting in temporary mismatches between demand and supply. The conversion of surplus energy to hydrogen and its storage in geological formations is one option to balance this energy gap. This study evaluates the feasibility of seasonal storage of hydrogen produced from excess wind power electricity in a saline aquifer in Castilla-León region (northern Spain). A 3D multiphase numerical model is performed to test different extraction well configurations during three annual injection-production cycles in a selected underground sandstone formation (Utrillas Fm) in the Duero basin. Results demonstrate that underground hydrogen storage in saline aquifers can be operated with reasonable recovery ratios. A maximum hydrogen recovery ratio of 78%, which represents a global energy efficiency of 30%, has been estimated. Hydrogen upconing emerges as the major risk on saline aquifer storage. However, shallow extraction wells can minimize its effects. Steeply dipping geological structures are key for an efficient hydrogen storage.

Recently, politics and industry has discussed green hydrogen as one of the carbon-zero energy sources of the future. Besides many other countries, Germany formulates clear goals for the energy transition from fossil to hydrogen energy in its "National Hydrogen Strategy". In order to ensure steady supply of hydrogen and to secure national reserves the underground storage of hydrogen (UHS) is increasingly coming into focus. Germany already has extensive experience in storing natural gas to cover supply shortages in the medium term, which follows same geologic principles as for hydrogen. In general, there are two main underground storage options for hydrogen. While in caverns such as salt-caverns operating UHS localities already exist, hydrogen storage facilities in porous media like aquifers or depleted gas reservoirs are still unrealized. In recent studies such as InSpEE and INSPEE-DS, the Federal Institute for Geoscience and Resources (BGR) provided a sound database of potential salt structures and salt horizons that could be used for UHS. This study gives an overview over the research made within all known types of underground hydrogen storage and discusses each their potential in Germany.