Focused Review on Natural Hydrogen: Occurrence, Economic Viability, Perspectives, and Implications for Underground Hydrogen Storage

Natural hydrogen is a promising source of sustainable energy with an estimated in-place amount of ∼5.6 × 1 × 10 12 tonnes. If only a small fraction (e.g., 1%) of this reserve could be recovered, then it would suffice to decarbonize the world for 200 years. Naturally derived hydrogen is potentially cost-effective (about 1 US$/kg, commercially produced at a cost of 0.5 US$/kg in Mali, South Africa) compared to green hydrogen (3–5 US$/kg). Additionally, natural hydrogen provides an analogue for the development of underground hydrogen storage (UHS) of engineered hydrogen. Such significances of natural hydrogen underscore the rapid growth of research in this field. However, there are oppositions arguing that natural hydrogen is too scattered to be recovered economically. Here, we critically discuss the latest insights into natural hydrogen and the implications to UHS. We focus on the major gaps in knowledge about physiochemical and pore-scale behaviors of hydrogen in geologic environments as well as the uncertainties and controversies about the commercial viability of natural hydrogen and UHS. We highlight the uniqueness of the pore-scale behaviors of hydrogen compared to that of other gases such as methane and carbon dioxide (CO 2 ), calling for hydrogen-specific investigations and cautions needing to be taken when applying the established knowledge and know-how of natural gas or underground CO 2 storage to UHS. This paper offers objective distinct perspectives of the interface of natural hydrogen and UHS in the low-carbon future where geosciences and pore-scale behaviors of hydrogen serve as the underlying principles.