Ostwald ripening leads to less hysteresis during hydrogen injection and withdrawal: A pore-scale imaging study

Abstract

This study explores hydrogen storage in Bentheimer sandstone using high-resolution 3D X-ray imaging during hydrogen injection and brine flooding cycles. The results reveal a reduction in hysteresis during hydrogen injection and withdrawal, attributed to Ostwald ripening–the transport of dissolved hydrogen in the aqueous phase to balance local capillary pressure. Hydrogen saturation reached 82% after injection, while the residual saturation decreased significantly from 40% in the first cycle of brine injection (imbibition) to less than 18% in the third. End-point capillary pressure during gas injection was directly measured as 20, 9, and 3 kPa in the first, second, and third cycles, respectively. During the imbibition steps, the gas saturation decreased consistently, reflecting reduced trapping effects and improved gas connectivity. After 16 h of rest, a single large connected ganglion formed, further reducing the Euler characteristic per unit volume from -10 ${\mathrm{mm}}^{-3}$ to -23 ${\mathrm{mm}}^{-3}$ during the third drainage cycle. These findings highlight that traditional hysteresis models that ignore the effect of Ostwald ripening over-estimate the amount of residual trapping in hydrogen storage.