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Subsurface porous rocks have the potential to store large volumes of hydrogen(H$_2$) required for transitioning towards a H$_2$-based energy future.Understanding the flow and trapping behavior of H$_2$ in subsurface storagesystems, which is influenced by pore-scale heterogeneities inherent tosubsurface rocks, is crucial to reliably evaluate the storage efficiency of ageological formation. In this work, we performed 3D X-ray imaging and flowexperiments to investigate the impact of pore-scale heterogeneity on H$_2$distribution after its cyclic injection (drainage) and withdrawal (imbibition)from a layered rock sample, characterized by varying pore and throat sizes. Ourfindings reveal that even subtle variations in rock structure and propertiessignificantly influence H$_2$ displacement and storage efficiency. Duringdrainage, H$_2$ follows a path consisting of large pores and throats, bypassingthe majority of the low permeability rock layer consisting of smaller pores andthroats. This bypassing substantially reduces the H$_2$ storage capacity.Moreover, due to the varying pore and throat sizes in the layered sample,depending on the experimental flow strategy, we observe a higher H$_2$saturation after imbibition compared to drainage, which is counterintuitive andopposite to that observed in homogeneous rocks. These findings emphasize thatsmall-scale rock heterogeneity, which is often unaccounted for inreservoir-scale models, can play a vital role in the displacement and trappingof H$_2$ in subsurface porous media.

Subsurface hydrogen storage controlled by small-scale rock heterogeneities