IRON ACTIVITY UNDER REDUCING CONDITIONS - EFFECTS ON SUBSURFACE HYDROGEN STORAGE

Geochemical information on the viability and feasibility of hydrogen storage in subsurface formations of different lithologies is scarce. Iron (Fe), an abundant element in earth’s crust, plays a crucial role in regulating the geochemical interactions in subsurface reservoirs due to its potential to alter redox conditions. In this study, using equilibrium modeling and in the absence of microbial activity (abiotic), we evaluated the role of various dissolved iron species in controlling the behavior of injected hydrogen in the subsurface.

Under reducing conditions, Fe can either reside in sulfides (pyrite), carbonates (siderite), silicates (greenalite) as Fe²⁺, or as a dissolved ionic state in aqueous fluids. With increasing temperature and at higher activities of HCO₃^-(>10^-3) and Fe²⁺(~10^-4), siderite is supersaturated within the circumneutral pH range. Whereas, for lower activities of Fe²⁺(<10^-6), most of the Fe stays in the aqueous phase at very low temperatures. Although the actual stability of siderite depends upon kinetic constraints pertaining to its formation, calculations suggest that high temperature (~200^oC) transformations of such Fe²⁺ bearing to Fe³⁺ bearing phases (magnetite) yield different results in terms of dissolved CO₂ concentrations also. Each mole of siderite to magnetite transition yields one mole of CO₂ in addition to H₂, while direct formation of magnetite (or oxyhydroxide) from aqueous Fe²⁺ generates solely H₂. Thus, reducing systems with low Fe activities are suitable for long term storage considerations due to the reduced potential for CO₂ generation with temperature. These post depositional alterations also serve as sites for methanogenesis.

Thermochemical Reduction of Sulfate (TSR), suggested to take place abiotically at low temperatures (Mougin et al., 2007), can potentially decrease the solubility of Fe by precipitating pyrite for the entire pH range of brines. Pyrite dissolves into aqueous Fe²⁺ under less reducing conditions (fH₂ <10^-6) and with increasing temperature; results in the formation of H₂. Systems with very low Fe activities are thus conducive for hydrogen storage as all Fe is present as dissolved ions. Considering the overall complexity for net consumption and production of hydrogen gas in the subsurface, we need to carefully evaluate the activities of redox-sensitive ions like Fe for appropriate site selection.