Ironic behavior of iron bearing phases during Underground Hydrogen Storage ?

Underground hydrogen storage (UHS) and utilization have gained considerable attention over the past few decades as major alternatives for fossil fuels. However, bulk of the challenges associated with its implementation lie within the foundation. One of such challenges is the (bio)geochemical interaction of stored hydrogen with reservoir materials. In particular, microbially induced redox reactions can pose threats to hydrogen storage due to fast consumption and subsequent generation of other gases (H₂S, CH₄ etc.). Recent experimental studies have suggested a variable degree of hydrogen consumption through redox interactions, when optimum conditions for such thermodynamic interactions are met inside reservoirs. One of the most abundant redox-sensitive phases is iron (oxy)hydroxide that can readily be reduced in the presence of hydrogen or other reducing gases. In this study, we experimentally evaluate the potential of iron (oxy)hydroxides in the presence of ultrapure hydrogen under reservoir P-T conditions. Using high P-T anoxic batch reactions, we obtain the reduction rate of iron (oxy)hydroxide phases under variable pH₂, water chemistry and water-mineral ratios. We find that higher valence reactive iron (oxy)hydroxide phases readily transform into more stable mixed valence phases such as magnetite, even at abiotic conditions and high pH₂ (>50 bars). We compare the results with ambient P-T batch experiments and find similar observations. Only the concentration of dissolved Fe(II) would determine the phase transformation of Fe(III) oxyhydroxides, not ultrapure H₂. This implies that stored hydrogen may not be consumed even if iron rich oxyhydroxide phases are abundantly present inside reservoirs. In an additional study, effect of pH and matrix carbonates were also evaluated. Dissolution of carbonates (e.g. calcite) release bicarbonate and increase the pH of the porewater, which in turn, may impede further reduction of Fe(III) (oxy)hydroxide phases. This is currently being investigated inside the high P-T batch reactor with externally controlled pH and variable temperatures. In summary, our results suggest that surface controlled non-redox reactions associated with reactive iron bearing phases may be more effective in determining the fate of injected hydrogen. These studies aim at expanding our fundamental knowledge pertaining to iron redox systematics in the subsurface and a successful implementation of UHS.