Micro-CT imaging of drainage and spontaneous imbibition for underground hydrogen storage in saline aquifers

• UHS in saline aquifers was studied using a hydrophilic porous plate to mimic an aquifer source for displacement. • Over 40% of the hydrogen initially in place was displaced by the end of spontaneous imbibition. • Water injection after spontaneous imbibition recovered 16% of the remaining hydrogen. • Micro-CT imaging revealed significant trapping by snap-off at the end of spontaneous imbibition • Gas rearrangement by Ostwald ripening during storage increased gas connectivity. . This study experimentally investigates hydrogen-brine displacement dynamics in Bentheimer sandstone, with a focus on spontaneous imbibition and its role in underground hydrogen storage in saline aquifers. The displacement is considered in two steps: (1) spontaneous imbibition, where gas is connected and capillary pressure decreases during withdrawal, and (2) brine flooding, where most of the gas is disconnected and the capillary pressure can become negative. The experiments were conducted using high-resolution micro-CT imaging at 3.1 µm/voxel resolution under 4 MPa and 23°C conditions. A water-wet porous plate was placed at the outlet to mimic an aquifer source to perform multiple drainage displacements to anchor the irreducible water saturation, followed by spontaneous imbibition, where capillary pressure was reduced incrementally. After reaching P c = 0, the pressure was maintained for 48 hours to observe gas rearrangement via Ostwald ripening at the end of spontaneous imbibition, followed by brine injection to evaluate the gas recovery. The results showed that spontaneous imbibition led to significant gas snap-off below P c = 5 kPa, and over 40% of the initial gas was displaced when P c reached 0; the gas saturation was 0.51. After the storage time, the initially disconnected large gas clusters became connected across most of the sample’s length. Subsequent brine injection led to some additional gas displacement, with the final gas saturation reaching 0.43. In situ contact angle measurements at P c = 0 and after brine injection showed an average of 40°, indicating water-wet conditions, while the H 2 -brine interfacial curvature was low, consistent with a local capillary pressure of approximately only 1 kPa. Pore occupancy analysis showed gas was initially displaced from narrow pores, with residual gas ganglia trapped in the largest pores, as expected in a water-wet rock. These findings demonstrate that spontaneous imbibition alone can account for a significant fraction of gas displacement above the gas-water contact and should be incorporated into capillary pressure-saturation models.