Geological storage of hydrogen in deep aquifers – an experimental multidisciplinary study

Abstract

Dihydrogen (H₂) is a promising source of energy in the field of energy transition. Similarly to natural gas, identifying storage solutions for large volumes of H₂ is essential. Geological storage of H₂ and methane mixtures in underground gas storage (UGS) such as deep aquifers is universally promising in particular in our current energy avid world. That said, interactions between water formation, reservoir rock, gas mixture and the microbial ecosystems remain poorly defined and further clarifications on this issue remain fundamental. Our study aims at identifying the effect of H₂ injection on the aforementioned milieu and at clarifying those interactions in UGS. The aquifer was reproduced experimentally in a reactor: water and rock phases sampled from the actual aquifer and a synthetic gas phase representing the gaseous mix to store. Since the beginning of the experiment (at a pressure of 85 bar methane – 1% carbon dioxide, 47 °C), sulfate was consumed continuously until its depletion from the liquid phase. As soon as H₂ was injected (10% H₂ at 95 bar), formate was produced in the aqueous phase and CO₂ was consumed from the gas phase. Once sulfate was depleted, the microbial activities were based on the consumption of H₂ and CO₂, indicating a switch in the microbial ecosystems towards Subsurface Lithoautotrophic Microbial Ecosystems (SLiMEs). Transcriptomic diversity analyses subsequently confirmed the increased activity of methanogens after H₂ injection. Moreover, methanogenic archaea became the majority in the ecosystem. Once the CO₂ was depleted in the gas phase, H₂ consumption and formate production instantly stopped. In less than 90 days, nearly 40% of injected H₂ transformed either into H₂S, formate and methane. This suggests that microbial life harbored in a deep aquifer has a major impact on the evolution of H₂ storage especially on sulfate, CO₂, calcite and H₂ concentrations in the system.