To meet the global commitments for net zero carbon emissions our energy mix must transition from fossil fuels. Hydrogen is gaining increasing recognition as a low carbon energy option to support this energy transition. Hydrogen is considered a low-carbon substitute for fossil fuels to decarbonise domestic and industrial heat, power generation and heavy-duty transport. It can also promote increased renewable energy uptake by acting as an energy store to balance supply and demand.

For hydrogen to be deployed at the scales required for net zero, we will need access to large-scale geological storage. This talk will present an overview of the most recent findings from research working to establish the feasibility of storing hydrogen in underground porous reservoirs. The talk will cover the results of research into the key biological and chemical reactions between the reservoir rocks, formation fluids and injected hydrogen that could compromise the storage complex and the key flow processes that influence hydrogen migration and trapping during injection and withdrawal. It will also consider the role of hydrogen storage within an integrated energy system.

The first Hydrogen storage caverns of Germany are planned to be constructed at the salt cavern field Epe in NRW, where 114 caverns, of which more than 50 are currently used for natural gas storage, are located. Since gas filled caverns experience convergence over time and thus cause subsidence at the surface, it is important to have a monitoring concept with high spatial and temporal resolution, to predict future subsidence and potential damage to infrastructure, but also to detect unexpected subsidence quickly and assist in identifying the cause.

Epe displays a complex surface deformation field, consisting of cavern convergence caused linear trends, as well as precipitation dependent seasonal and cavern pressure dependent contributions as shown in previous studies of the area.

As part of the SAMUH2-Project, funded by the German Federal Ministry of Economics and Climate Protection, we are working on a monitoring concept to incorporate the already established methods of yearly levelling and GNSS measurements into our approach of using Interferometric SAR (InSAR) time series, which provide not only high spatial, but also good temporal resolution. Here, we use Sentinel-1 SAR data from 2015 to 2022, and process time series by using a joint approach of persistent scatterer (PS) and distributed scatterer (DS) techniques.

Our results show good agreement of the InSAR time series with other geodetic measuring methods. We can distinguish the signals of the different source mechanisms well and can even model varying cavern convergence rates, depending on the extent of the yearly cavern depletion and filling.

In the current global race towards achieving self-sufficiency and sustainability in meeting energy demands with net-zero emissions, hydrogen has emerged as a promising solution. However, hydrogen's volumetric energy density is lower compared to conventional energy sources, and the storage conditions (pressure and temperature) for hydrogen on the surface are expensive and technically challenging.

To overcome the challenge of large-scale hydrogen storage, researchers around the world have proposed storing hydrogen in subsurface geological structures, such as salt caverns and porous reservoir rocks. While hydrogen storage in salt caverns is a more advanced concept, storing hydrogen in porous rocks such as depleted reservoirs and aquifers requires further research attention. This paper outlines a preliminary workflow for a feasibility study on the use of depleted gas reservoirs for hydrogen storage. Two different fields in Germany are used as examples to illustrate the process. One of the case study fields is still producing from the carbonates of the Zechstein Group, while the other is a decommissioned field that previously produced from unconsolidated Neogene sands. The workflow involves creating a static geological model and populating it with petrophysical parameters, followed by dynamic flow simulation for history matching. Hydrodynamical parameters for hydrogen are then introduced to simulate hypothetical storage cycles. A geomechanical model is then created incorporating the pore pressure data and material properties, to assess storage integrity, fault activity, and surface deflection in response to hydrogen filling. Overall, this workflow provides a comprehensive approach to evaluate the potential for hydrogen storage in depleted reservoirs.

In the frame of the SAMUH2 project water samples were taken at a geological pore gas storage. In the course of the abandonment of a natural gas storage, deep fluids could be recovered from a depth of 500 - 600 meters. A total of eight boreholes were sampled from injection and observation wells. The focus of the investigations was on the analysis of the chemical composition of the fluids as well as the characterization of the microbial biocoenosis and partly also their metabolic activity.

Organic acids were detected in varying concentrations and compositions in both the fluid samples taken at the injection and observation wells. Organic acid concentrations ranged from 0.1 to 730 mg/L. Gen copies of Bacteria, sulfate reducers (SRB) and methanogenic archaea were detected in all fluids by qPCR. A detailed characterization of the microbial community was carried out by microbiome analysis. A diverse microbial community was detected on fermenters and methanogenic archea. Sulfate reducers, on the other hand, were identified predominantly in the observation wells.

Several laboratory experiments demonstrated that the fluids of injection wells contained an active biocenosis capable of hydrogenotrophic methanogenesis. In contrast, only one observation well fluid demonstrated the activity of hydrogenotrophic methanogenic archaea. The capability of underground storage facilities for producing eco- ("green") methane is a further topic of this study.

Underground H₂ storage (UHS) in salt caverns, deep saline formations, and depleted oil/gas reservoirs has emerged as a reliable and safe technology for storing renewable energy and reducing carbon dioxide emissions. H₂ gas, however, is one of the most important electron donors for many subsurface microorganisms. During the multiple cycles of H₂ injection and withdrawal operations, a certain amount of H₂ is permanently lost due to various physical, chemical, and biological mechanisms. Although research in UHS in porous media is evolving, our understanding of the impacts of cyclic loading and microbial activity on H₂ recovery and loss mechanisms remains inadequate.

In this study, we present recent findings from a quantitative investigation of H₂ reconnection and recovery mechanisms in repeated injection-withdrawal cycles using a microfluidic pore network simulating shallow reservoir storage conditions (30 barg). Our results reveal that H₂ storage capacities increase with higher injection rates, ranging between approximately 10% and 60%. Additionally, we observed the growth of a typical halophilic sulfate-reducing bacterium in the hydrogen-saturated pore network for 9 days. Significant H₂ loss occurred due to microbial consumption within 2 days following injection into the microfluidic device. These results may have significant implications for hydrogen recovery and gas injectivity. Microvisual experiments provide critical observations of bubble-liquid interfacial area and reaction rate that are essential to the modeling that is needed to make long-term predictions. Our results contribute to improving the selection criteria for future storage sites, ensuring optimized and efficient H₂ storage and utilization.