Report on the Geological And Reservoir Conditions of the Underground Storage of Hydrogen and the Potential Synergies Of Underground Gas Storage Sites and Lessons Learnt

Executive Summary

Underground hydrogen storage (UHS) is emerging as a critical component of Europe’s clean energy transition, providing a means to buffer large quantities of energy to balance intermittent renewables. This report presents a comprehensive overview of UHS in Europe, blending general background, engineering considerations, and academic research insights. Key findings include:

• Role in Energy Transition: UHS enables power-to-gas-to-power schemes, where surplus renewable electricity is converted to hydrogen and stored underground, then re-electrified when needed. This flexibility helps stabilize energy supply and demand, supporting decarbonization targets. European strategies like the EU Hydrogen Strategy recognize green hydrogen as a clean fuel, spurring investments in its production and storage.
• Suitable Geological Formations: Hydrogen can be stored in porous rock formations (depleted oil/gas reservoirs and deep saline aquifers) or in engineered caverns in rock (especially salt caverns). Each option has distinct characteristics.
  • Salt Caverns: Offer high injection and withdrawal rates and an essentially impermeable container for hydrogen. Salt formations in northern Europe are abundant (e.g. North Germany, UK, Netherlands), giving significant storage potential.
  • Deep Saline Aquifers: Widespread and potentially large-scale, but require more characterization. Historical use of aquifers to store town gas (50–60% hydrogen) in Europe has demonstrated feasibility.
• Technical Considerations: UHS faces unique technical challenges. Hydrogen’s low density and high diffusivity mean larger volumes are needed to store energy equivalent to natural gas, and careful sealing is required to prevent leakage. Cushion gas requirements differ by storage type (salt caverns need ~20–30% of volume as base gas, versus ~50% in depleted reservoirs). Material compatibility is crucial: hydrogen can embrittle steel pipelines and well casings, so suitable alloys must be used (recent tests have identified steel grades that perform well in hydrogen environments). Additionally, microbial reactions underground can consume a small fraction of hydrogen or generate contaminants (e.g. methane or H₂S), necessitating gas treatment on withdrawal.
• Economic and Environmental Factors: Salt cavern storage is expected to have the lowest levelized cost for large-scale, fast-cycling hydrogen storage, thanks to high deliverability and minimal cushion gas needs. Depleted fields and aquifers offer lower upfront cost by leveraging existing geology, but may incur costs for cushion gas and purification of output gas. Environmentally, UHS is considered safe when sites are properly selected and managed. Salt caverns have an excellent sealing record (used for natural gas for decades without leakage), and porous reservoirs used historically for town gas did not experience major issues, though continuous monitoring is required. Preventing hydrogen leakage protects both energy yield and mitigates any indirect greenhouse effects of hydrogen in the atmosphere. Overall, UHS allows long-duration energy storage with minimal land footprint, unlike above-ground storage which would require massive tank farms or expensive liquefaction and high cost safety measures.