Abiotic Redox Reactions of H2 with Iron-containing Minerals under Geologic Storage Conditions

Abstract

Summary The occurrence of H2 in natural geologic systems rarely exceeds small concentrations despite significant natural H2 formation processes. Hence, there have to be effective H2-consuming processes operating on geologic time scales, e.g. microbial activity or abiotic redox reactions with minerals. The observation in natural geologic systems raises the question, whether these depletion pathways can be of significance for H2 underground storage operations. Here, we aim to identify and quantify the rate of abiotic redox reactions of H2 under underground storage conditions. We investigate the reactivity of H2 towards several iron minerals that are common accessory minerals in potential storage rocks or cap rocks. Gas-fluid-mineral reactions were investigated in gold capsules in high-pressure autoclaves at 120°C and 20 MPa. The reaction progress is monitored by several GC methods to determine the consumption of H2 and the formation of e.g. H2O or H2S. Dry mineral - H2 experiments revealed that only hematite and pyrite oxidize significant amounts of H2 within 14 days. Pyrite reduction by H2 leads to the release of the corresponding amount of H2S, which could be an undesirable issue for gas quality. Ongoing H2-mineral experiments of different duration will provide kinetic data of hematite and pyrite reduction by H2.