Pore-scale visualization of hydrogen storage in a sandstone at subsurface pressure and temperature conditions: Trapping, dissolution and wettability

Hypothesis Underground hydrogen (H-2) storage is a potentially viable solution for large-scale cyclic H-2 storage; however, the behavior of H-2 at subsurface pressure and temperature conditions is poorly known. This work investigates if the pore-scale displacement processes in H-2-brine systems in a porous sandstone can be sufficiently well defined to enable effective and economic storage operations. In particular, this study investigates trapping, dissolution, and wettability of H-2-brine systems at the pore-scale, at conditions that are realistic for subsurface H-2 storage. Experiments We have performed in situ X-ray imaging during a flow experiment to investigate pore-scale processes during H-2 injection and displacement in a brine saturated Bentheimer sandstone sample at temperature and pressure conditions representative of underground reservoirs. Two injection schemes were followed for imbibition: displacement of H-2 with H-2-equilibrated brine and with non-H-2-equilibrated brine. The results from the two cycles were compared with each other. Findings The sandstone was found to be wetting to the brine and non-wetting to H-2 after both displacement cycles, with average contact angles of 54 degrees and 53 degrees for H-2-equilibrated and non-H-2-equilibrated brine respectively. We also found a higher recovery of H-2 (43.1%) when displaced with non-H-2-equilibrated brine compared to that of H-2-equilibrated brine (31.6%), indicating potential dissolution of H-2 in the unequilibrated imbibing brine at reservoir conditions. Our results suggest that underground H-2 storage may indeed be a suitable strategy for energy storage, but considerable further research is needed to fully comprehend the pore-scale interactions at reservoir conditions.