Microbial side effects of underground hydrogen storage – Knowledge gaps, risks and opportunities for successful implementation

Abstract As the use of hydrogen gas (H2) as a renewable energy carrier experiences a major boost, one of the key challenges for a constant supply is safe and cost-efficient storage of surplus H2 to bridge periods with low energy demand. For this purpose, underground gas storage (UGS) in salt caverns, deep aquifers and oil-/gas reservoirs has been proposed, which are environments with potentially high microbial abundance and activity. Subsurface microorganisms can use H2 in their metabolism and thus may lead to a variety of undesired side effects such as H2 loss, H2S formation, methane formation, acid formation, clogging and corrosion. In this review, we summarize the current knowledge and methodologies on microbial related risks related to H2 UGS, including basic research on subsurface microbiology, results from field trials, and experience from other industries including oil & gas. For safe underground storage of H2 gas we recommend to analyze the relevant microbiological and geochemical characteristics of each site to be able to predict the most suitable storage strategy and establish a good monitoring and mitigation approach to follow and counter potential microbial side effects.