Stimulated Geologic Hydrogen: from Mechanistic Control to Engineered Rock Transformation

Abstract

Geologic hydrogen (GeoH₂) generated from subsurface iron-rich rock-water reaction (i.e., serpentinization), is emerging as a promising candidate for the next primary energy source. Yet, accelerating GeoH₂ production from geological to human time scales via enhanced serpentinization remains a formidable scientific and technical challenge. In this Review, we decipher the mechanistic control and explore strategies for accelerating in-situ, engineered iron-rich rock transformation into carbon-free GeoH₂ by orders of magnitude. Serpentinization rate is hindered by low porosity and permeability of source rocks, suboptimal temperatures, unfavorable water chemistry, inefficient Fe²⁺-to-Fe³⁺ conversion, thermodynamic constraints, and low reactive surface area. While closed-system experiments provide valuable mechanistic insights, open-system conditions with fluid circulation are more crucial towards economically viable GeoH₂ production. We assess stimulation techniques from enhanced hydrocarbon and geothermal recovery as tools to be adopted or adapted for increasing reactive surface areas for stimulated GeoH₂ production. We estimate that 7.40×10⁵ to 1.73×10⁶ million metric tons (Mt) hydrogen could be engineered over 20 to 50 years from about 10% iron-rich rocks within 10 km depth of continental crust. Enabling GeoH₂ as a viable energy source requires not only advancing scientific frontiers but also forming a global GeoH₂ research network and innovation ecosystem to address the critical scientific, technical, societal, economic, and policy challenges.