Subsurface hydrogen generation: Natural sources and in-situ hydrocarbon conversion technologies

Hydrogen, a low-carbon source of energy, is mostly produced from fossil fuels (e.g. coal or natural gas) in surface plants but accompanied by significant carbon dioxide (CO₂) emissions. As a low emission source, the potential of subsurface hydrogen, including naturally occurring (H₂) utilizing geological conditions without human interferences and in-situ produced H₂ from hydrocarbons based on thermochemical or biological technologies in reservoir, is underrated. To provide a better understanding of the generation process for supporting the exploitation of subsurface hydrogen, this work comprehensively reviews the chemical pathways for H₂ generation and the status of technology development. The natural H₂ source is discussed in the classification of rock-water reactions, radiolysis of water, biogenic hydrogen, deep-seated origin, and thermal maturation of organic matter while rock-water reaction appears (e.g. serpentinization) as the most common source. This classification of the generation mechanism could be a guide to discover more hot spots of hydrogen. For in-situ produced hydrogen from hydrocarbons, we introduced the chemical and biological pathways with underscoring the main reaction types and suitable conditions for various feedstocks and the corresponding up-to-date technologies, as well as summarizing the microbe species and influencing factors in the dark fermentation process. It shows the optimized H₂ production temperature is >700 °C for methane, 500−700 °C for light oil, heavy oil/bitumen and oil shale, and 500−1500 °C for coal. By further discussing the challenges and the environmental and economic impacts of subsurface H₂, this work reveals the potential and hinderance for subsurface hydrogen industrialization.