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Green hydrogen, produced from surplus electricity during peak production, can be injected into subsurface reservoirs and retrieved during high-demand periods. In this study, X-ray tomography was employed to examine hysteresis resulting from repeated hydrogen injection and withdrawal. An unsteady state experiment was performed to evaluate the distribution of hydrogen and brine after drainage and imbibition cycles: images of the pore-space configuration of fluids were taken immediately once injection had stopped and after waiting for a period of 16 h with no flow. A Bentheimer sandstone sample with a length of 60 mm and diameter of 12.8 mm was used, and hydrogen was injected at ambient temperature and a pore pressure of 1 MPa. The gas flow rate was decreased from 2 ml/min to 0.08 ml/min over three cycles of gas injection followed by water flooding, while the brine injection rate was kept constant. The results showed the presence of capillary pressure hysteresis and hydrogen migration through Ostwald ripening due to the diffusion of gas dissolved in the brine. These phenomena were characterized through analysis of interfacial curvature, area, connectivity and pore occupancy. The hydrogen tended to reside in the larger pore spaces, consistent with water-wet conditions. 16 h after flow had stopped, the hydrogen aggregated into larger ganglia with a single large connected ganglion dominating the volume. Moreover, the Euler characteristic decreased after 16 h, indicating an improvement in connectivity. The work implies that Ostwald ripening – mass transport of dissolved gas – leads to less hysteresis and better connectivity than would be assumed ignoring this effect, as done in assessments of hydrocarbon flow and trapping. [Display omitted] • X-ray imaging investigated pore-scale hysteresis during hydrogen storage. • Hydrogen migration by Ostwald ripening was observed. • Ostwald ripening led to better connectivity of the gas phase. • The work implies less hysteresis for hydrogen than for hydrocarbons. [ABSTRACT FROM AUTHOR]

• Remobilization of residually trapped CO2 can occur as a result of reservoir pressure decline. • 3D X-ray microtomography was used to characterize CO2 remobilization under pressure depletion. • Remobilization was observed at a higher saturation than residual saturation. • Ostwald ripening enhanced remobilization by allowing local reconnection of trapped gas. A decrease in reservoir pressure can lead to remobilization of residually trapped CO 2. In this study, the pore-scale processes related to trapped CO 2 remobilization under pressure depletion were investigated with the use of high-resolution 3D X-ray microtomography. The distribution of CO 2 in the pore space of Bentheimer sandstone was measured after waterflooding at a fluid pressure of 10 MPa, and then at pressures of 8, 6 and 5 MPa. At each stage CO 2 was produced, implying that swelling of the gas phase and exsolution allowed the gas to reconnect and flow. After production, the gas reached a new position of equilibrium where it may be trapped again. At the end of the experiment, we imaged the sample again after 30 hours. Firstly, the results showed that an increase in saturation beyond the residual value was required to remobilize the gas, which is consistent with earlier field-scale results. Additionally, Ostwald ripening and continuing exsolution lead to a significant change in fluid saturation: transport of dissolved gas in the aqueous phase to equilibriate capillary pressure led to reconnection of the gas and its flow upwards under gravity. The implications for CO 2 storage are discussed: an increase in saturation beyond the residual value is required to mobilize the gas, but Ostwald ripening can allow local reconnection of hitherto trapped gas, thus enhancing migration and may reduce the amount of CO 2 that can be capillary trapped in storage operations. [ABSTRACT FROM AUTHOR]

We use high‐resolution three‐dimensional X‐ray imaging to study hydrogen injection and withdrawal in the pore space of Bentheimer sandstone. The results are compared with a replicate experiment using nitrogen. We observe less trapping with hydrogen because the initial saturation after drainage is lower due to channeling. Remarkably we observe that after imbibition, if the sample is imaged again after 12 hr, there is a significant rearrangement of the trapped hydrogen. Many smaller ganglia disappear while the larger ganglia swell, with no detectable change in overall gas volume. For nitrogen, the fluid configuration is largely unchanged. This rearrangement is facilitated by concentration gradients of dissolved gas in the aqueous phase—Ostwald ripening, We estimate the time‐scales for this effect to be significant, consistent with the experimental observations. The swelling of larger ganglia potentially increases the gas connectivity, leading to less hysteresis and more efficient withdrawal. Plain Language Summary: The supply of energy by hydrogen is a key component of a zero‐carbon economy; an essential part of global hydrogen use is underground storage at the gigatonne scale in porous rocks. However, the behavior of hydrogen in the subsurface is not well understood. Using high‐resolution three‐dimensional X‐ray imaging, we observe that in the pore‐space, there is a significant rearrangement of hydrogen trapped by water. Many smaller trapped bubbles disappear while the larger ones grow. The process is driven by differences in the amount of gas dissolved in the aqueous phase: this is called Ostwald ripening. We estimate the time‐scales effect as being important, which are consistent with the experimental observations. The work implies that there is less trapping for hydrogen injection and withdrawal compared to equivalent hydrocarbon systems, which makes the process efficient. Key Points: Hydrogen trapping and rearrangement in porous rock have been observed in situOstwald ripening is observed over timescales consistent with theoretical predictionsThe results imply less trapping and hysteresis than with completely insoluble gas [ABSTRACT FROM AUTHOR]