Microfluidic investigation of pore-scale flow behavior and hysteresis in underground hydrogen storage in sandstones

Pore-scale investigation of hydrogen flow behavior in porous media is crucial for developing reliable hydrogen storage models. However, there is a lack of experimental pore-scale studies in the literature focusing on both injection and production processes. To address this gap, we developed a quasi-2D microfluidics system to investigate hydrogen-water flow dynamics, displacement, trapping mechanisms and hysteresis in cyclic hydrogen storage and production. The results are interpreted using in-situ wettability measurements, Euler characteristic calculation and sweep efficiency estimation. The behavior of hydrogen-brine system is also compared with that of CO2-water system. The results clearly show that hydrogen-water system is preferentially water-wet. Hydrogen-water front during the injection operation is far from piston-like displacement as it is affected by both capillary trapping and fingering. This results in underutilization of the pore space for hydrogen storage. The residual hydrogen saturation also increases from 38.7% to 42.8 % over the cycles, which is different from that of carbon dioxide-water system. Due to the higher dissolution of CO2, the residual trapped CO2 is lower than that of the H-2. Hydrogen storage is injection flow-rate dependent, with a favorable capillary number similar to 10(-8) that reduces the adverse effects of both capillary and viscous fingering. The Euler characteristic reflects significant hysteresis between the drainage and imbibition stages, consistent with the recovery factors during the injection-production cycles. It is also found that the residual gas saturation is correlated to the initial gas saturation in each cycle, consistent with Land trapping model. These findings contribute to further understanding of underground hydrogen storage in aquifers.