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

Abstract

Hypothesis: Underground hydrogen (H₂) storage is a potentially viable solution for large-scale cyclic H₂ storage; however, the behavior of H₂ at subsurface pressure and temperature conditions is poorly known. This work investigates if the pore-scale displacement processes in H₂-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 H₂-brine systems at the pore-scale, at conditions that are realistic for subsurface H₂ storage.

Experiments: We have performed in situ X-ray imaging during a flow experiment to investigate pore-scale processes during H₂ 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 H₂ with H₂-equilibrated brine and with non-H₂-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 H₂ after both displacement cycles, with average contact angles of 54° and 53° for H₂-equilibrated and non-H₂-equilibrated brine respectively. We also found a higher recovery of H₂ (43.1%) when displaced with non-H₂-equilibrated brine compared to that of H₂-equilibrated brine (31.6%), indicating potential dissolution of H₂ in the unequilibrated imbibing brine at reservoir conditions. Our results suggest that underground H₂ 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.

Keywords: 3D X-ray visualization; Hydrogen wettability; In situ flow experiment; Underground hydrogen storage.