Natural hydrogen seeps as analogues to inform monitoring of engineered geological hydrogen storage

Abstract Engineered geological porous media hydrogen storage must be designed to ensure secure storage, and use appropriate monitoring, measurement and verification tools. Here, we identify and characterize 60 natural hydrogen seeps as analogues for potential leakage from engineered storage reservoirs to consider implications for monitoring. We report and compare the geological and environmental setting; seepage mode (dry gas/associated with water); co-released gases; seep rates and areal fluxes; temporal variation; seep structure; gas source; and composition. Seep characteristics are determined by local geological and hydrological conditions, specifically whether hydrogen gas is seeping through soils and unconsolidated sediments, fractured bedrock or into water. Hydrogen is typically co-emitted with other gases (CO 2 , CH 4 , N 2 ) with CH 4 , the most common co-emitted gas. The structural controls on seep location and characteristics are similar between hydrogen and CO 2 seeps. However, compared to CO 2 , hydrogen is more readily dispersed when mixing with air and hydrogen is more prone to being consumed or transformed via biotic or abiotic reactions, and so the quantity of leaked hydrogen can be greatly attenuated before seeping. Monitoring approaches should therefore be tailored to the local geology and hydrological conditions, and monitoring approaches to detect hydrogen and associated gases would be appropriate.