Subsurface hydrogen storage controlled by small-scale rock heterogeneities
arXiv:2310.05302
Abstract
Subsurface porous rocks have the potential to store large volumes of hydrogen (H) required for transitioning towards a H-based energy future. Understanding the flow and trapping behavior of H in subsurface storage systems, which is influenced by pore-scale heterogeneities inherent to subsurface rocks, is crucial to reliably evaluate the storage efficiency of a geological formation. In this work, we performed 3D X-ray imaging and flow experiments to investigate the impact of pore-scale heterogeneity on H distribution after its cyclic injection (drainage) and withdrawal (imbibition) from a layered rock sample, characterized by varying pore and throat sizes. Our findings reveal that even subtle variations in rock structure and properties significantly influence H displacement and storage efficiency. During drainage, H follows a path consisting of large pores and throats, bypassing the majority of the low permeability rock layer consisting of smaller pores and throats. This bypassing substantially reduces the H 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 saturation after imbibition compared to drainage, which is counterintuitive and opposite to that observed in homogeneous rocks. These findings emphasize that small-scale rock heterogeneity, which is often unaccounted for in reservoir-scale models, can play a vital role in the displacement and trapping of H in subsurface porous media.