Underground hydrogen storage in caverns: Challenges of impure salt structures

Hydrogen is expected to play a key role in the future as a clean energy source that can mitigate global warming. It can also contribute significantly to reducing the imbalance between energy supply and demand posed by deploying renewable energy. However, the infrastructure is not ready for the direct use of hydrogen, and large-scale storage facilities are needed to store the excess hydrogen production. Geological formations, particularly salt caverns, seem to be a practical option for this large-scale storage as there is already good experience storing hydrocarbons in caverns worldwide. Salt is known to be ductile, impermeable, and inert to natural gas. Some cases of hydrogen storage in salt caverns in the United States, the United Kingdom, and Germany reinforce the idea that salt caverns could be a viable option for underground hydrogen storage especially when the challenges and uncertainties associated with hydrogen storage in porous media are considered. However, cavern construction and management can be challenging when salt deposits are not completely pure and mixed with non-soluble strata. This review summarises the challenges associated with hydrogen storage in salt caverns and suggests some potential mitigation strategies linked to geomechanical and geochemical interactions. The Zechstein salt group in Northern Europe seems to be a feasible geological site for hydrogen storage but the effect of salt impurity particularly at deep offshore sites such as in the Norwegian North Sea should be carefully analysed. It appears that mechanical integrity, geochemical reactions, hydrogen loss by halophilic bacteria, leaching issues, and potential hydrogen diffusion are among the major issues when the internal structure of the salt is not pure.