Effect of Iron Mineral Transformation on Long-Term Subsurface Hydrogen Storage—Results from Geochemical Modeling

Large-scale subsurface hydrogen storage is critical for transitioning towards renewable, economically viable, and emission-free energy technologies. Although preliminary studies on geochemical interactions between different minerals, aqueous ions, and other dissolved gasses with H₂ have helped partially quantify the degree of hydrogen loss in the subsurface, the long-term changes in abiotic hydrogen–brine–rock interactions are still not well understood due to variable rates of mineral dissolution/precipitation and redox transformations under different conditions of reservoirs. One of the potentially understudied aspects of these complex geochemical interactions is the role of iron on the redox interactions and subsequent impact on long-term (100 years) hydrogen cycling. The theoretical modeling conducted in this study indicates that the evolution of secondary iron-bearing minerals, such as siderite and magnetite, produced after H₂-induced reductive dissolution of primary Fe³⁺-bearing phases can result in different degrees of hydrogen loss. Low dissolved Fe²⁺ activity (<10⁻⁴) in the formation water can govern the transformation of secondary siderite to magnetite within 100 years, eventually accelerating the H₂ consumption through reductive dissolution. Quantitative modeling demonstrates that such secondary iron mineral transformations need to be studied to understand the long-term behavior of hydrogen in storage sites.