Subsurface hydrogen storage controlled by small-scale rock heterogeneities

Subsurface porous rocks hold significant hydrogen (H₂) storage potential to support an H₂-based energy future. Understanding H₂ flow and trapping in subsurface rocks is crucial to reliably evaluate their storage efficiency. In this work, we perform cyclic H₂ flow visualization experiments on a layered rock sample with varying pore and throat sizes. During drainage, H₂ follows a path consisting of large pores and throats, through a low permeability rock layer, substantially reducing H₂ storage capacity. Moreover, due to the rock heterogeneity and depending on the experimental flow strategy, imbibition unexpectedly results in higher H₂ saturation compared to drainage. These results emphasize that small-scale rock heterogeneity, which is often unaccounted for in reservoir-scale models, plays a vital role in H₂ displacement and trapping in subsurface porous media, with implications for efficient storage strategies.