Multianalytical Assessment of Shale Behavior after Medium-Pressure/Temperature Hydrogen Exposure

Abstract

Underground hydrogen (H2) storage (UHS) in siliciclastic porous reservoirs relies critically on the long-term integrity of shale caprocks. However, the influence of H2 on shale microstructure, elastic properties, and mechanical strength remains poorly constrained. This study investigates H2–rock interactions in shales from the Draupne Formation (i.e., Kimmeridge equivalent), a regionally important sealing unit on the Norwegian Continental Shelf. Samples from two wells representing contrasting shale facies were exposed for 7 days to 170 ± 1 bar and 60 ± 1 °C under either H2 or inert Ar atmospheres. Pre- and postexposure characterization employed X-ray micro-CT, scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), ultrasonic through-transmission acoustic wave velocity measurements, punch testing, and gas chromatography (GC). The two facies exhibit strong compositional and structural heterogeneity, with Draupne Shale 1 (DS1) dominated by carbonate-rich, relatively homogeneous layers, and DS2 characterized by organic-rich and laminated microstructures. SEM-EDS and GC analyses revealed no measurable evidence of mineralogical or chemical alteration, indicating that at these pressure and temperature (PT) conditions H2 does not react measurably with shale constituents. Acoustic velocities display small but inconsistent changes after gas exposure, which are best explained by sample heterogeneities and/or P-induced microcrack changes rather than H2-specific effects. In contrast, punch tests reveal distinct facies-dependent mechanical responses: DS2 shale shows reduced peak shear strength and more brittle postpeak behavior after H2 exposure, whereas Ar-treated samples remain more ductile. These results suggest that H2 may contribute to weakening along pre-existing heterogeneities in more compliant shale facies, even in the absence of detectable chemical reactions. The study demonstrates that microstructural variability governs shale behavior during H2 exposure and highlights the need for facies-specific assessment in UHS applications.