Experimental investigation of shale/hydrogen geochemical interactions

Underground Hydrogen Storage (UHS) is an effective technique that provides safe storage and recovery of the stored hydrogen for long-term energy storage. Shale caprocks are regarded as effective seals of the stored gas, however, one of the key challenges is the in-situ geochemical shale/hydrogen reactions that can impact the seal integrity during injection/production of hydrogen. To address this issue, this work investigates the geochemical reactions between hydrogen and shales, and evaluates the possible alterations in shales’ petro-physical properties. Calcite-rich shale samples are collected from EagleFord (EFS) and Wolfcamp (WCS) shale formations and exposed to pure hydrogen for 108 days under 9.65 MPa at 348 K. Analytical methods including X-ray diffraction, X-ray fluorescence and Scanning Electron Microscope analysis are performed. The results indicate calcite precipitation after hydrogen treatment in both samples due to the presence of organic contents. Sulfate and pyrite reduction reactions with hydrogen are not observed as hydrogen did not all react with sulfate minerals. Consequently, minor changes in the surface pore structure and pore spaces are observed after the treatment, confirming the low reactivity of calcite-rich shale rocks with hydrogen. Gas chromatography analysis shows neither considerable hydrogen loss nor the generation of other gases such as CH₄ or CO₂ occurred during the experiments. The obtained results suggest that the integrity of shale-caprocks are expected to remain strong during the injection and production of hydrogen, which is favorable for UHS. This work can be a suitable experimental framework to assist in evaluating the hydrogen reactivity to different types of shales with various mineralogy, and assessing the suitability of shale rock for UHS.