Advancing underground hydrogen storage: Geological insights from natural hydrogen occurrences in porous media

Hydrogen storage in porous media is still in the research phase, especially as the literature has no proof of retrieving hydrogen stored in aquifers with high purity. A key challenge with hydrogen storage in porous media is the anticipated losses through processes like geochemical reactions, solubility, diffusion, adsorption, and microbial interactions. Thus, site selection is essential for efficient and secure hydrogen storage in the subsurface. The occurrence of natural hydrogen storage sites suggests that some geological conditions might enable hydrogen storage in the subsurface. Hence, this study investigates the properties of these sites to determine what lessons can be used for artificial geological hydrogen storage in porous media. Lessons learned from natural hydrogen sites address what kind of reservoir rocks are suitable for storage, what kind of rocks are good as seals, what reservoir and geological conditions of the reservoir minimize losses, and what additional data needs to be acquired before selecting a site for potential storage. Natural hydrogen was found occurring in both carbonate and sandstone formations. Findings from these sites show that carbonate reservoirs are associated with hydrogen concentrations exceeding 80 %, while sandstone reservoirs typically have concentrations of 70 % or lower. Shales and dolerites have acted as seal rocks. Most sites have average temperatures ranging from 20 to 50°. Larger H₂ concentrations are seen at depths shallower than 1300 m, and smaller hydrogen concentrations were seen at pressures greater than 400 bar. These findings provide some guidance for screening and selecting depleted reservoirs and saline aquifers for underground hydrogen storage.