Modeling dissolved Fe and H 2 inter-relationships under low pressure natural systems: Insights on long term hydrogen storage

Subsurface hydrogen storage is necessary to shift towards sustainable and zero-emission energy technologies, but geochemical data on the suitability of different reservoirs for hydrogen storage are scarce. Studies on complex chemical dynamics of aqueous Fe 2+ and H 2 have partially quantified the degree of loss for hydrogen gas in the subsurface at different operating pressures of hydrogen. However, a consensus regarding their thermodynamic relationships is lacking. In this study, we have investigated the magnitude of variation of hydrogen partial pressure in the subsurface in the presence of various concentrations of dissolved Fe 2+ through simulations. Observations imply that for considerably low partial pressures of hydrogen (~ 10 − 5 bars), a feature of many natural brines, decreasing activity of Fe 2+ by an order of magnitude can reduce the initial partial pressure of hydrogen by 3–4 orders of magnitude within a few years, due to enhanced reductive dissolution of the oxides. When pH2 of injected hydrogen exceeds 10 − 2 bars, magnetite becomes dominant as a secondary phase after the reduction of primary Fe 3+ oxides, leading to almost three orders of magnitude of H 2 (gaseous) loss that is almost independent of variation in Fe 2+ activity. Both processes are supplemented with a varying degree of Fe 2+ increase in the aqueous phase, supporting the release of Fe 2+ to the aqueous phase due to Fe 3+ oxide dissolution. These results point towards the degree of formation of magnetite as a potential controller of brine chemistry that depends upon nucleation kinetics and a threshold partial pressure for injected H 2 under low reservoir temperatures (50–100℃). These results directly apply to understanding the cycling of redox-controlled elements and injected hydrogen in subsurface aqueous systems.