Ostwald Ripening Leads to Less Hysteresis during Hydrogen Injection and Withdrawal: A Pore-Scale Imaging Study

We performed three cycles of hydrogen injection (drainage) followed by brineinjection (imbibition) combined with high-resolution three-dimensional X-ray imaging on asample of Bentheimer sandstone. After each injection, the sample was imaged initially andafter waiting 16 hours. Capillary pressure was measured from the differential pressure acrossthe rock once the injection has stopped and from estimating the meniscus curvatures from theimages. In addition, the gas saturation, pore occupancy, Euler characteristic and interfacial areaswere measured. There was a significant rearrangement of the gas in the pore space after injection stopped,which we hypothesise is caused by Ostwald ripening, namely the transport of dissolved hy-drogen in the aqueous phase to equilibriate local capillary pressure. This rearrangement led tothe formation of larger, more connected gas ganglia. The capillary pressure displayed char-acteristics that cannot be explained by traditional hysteresis models. While the direct andcurvature-based measurements agreed to within experimental uncertainty, and corresponded toindependent measurements in the literature on initial displacement, Ostwald ripening allowedless trapping and less hysteresis (a smaller difference between drainage and imbibiton capillarypressure) than measured previously on mercury and oil/water systems where Ostwald ripeningdid not occur. The results imply that for gas storage applications it is not appropriate to use hysteresismeasurements based on mercury or hydrocarbon systems. Instead, in local capillary equi-librium, there is less trapping and less difference between drainage and imbibition capillarypressure.