Time-resolved 2D and 3D imaging of hydrogen and brine displacement processes in porous Clashach sandstone.

Hydrogen (H) storage in porous geological formations offers a promising means to balance supply and demand in the renewable energy sector, supporting the energy transition. Important unknowns to this technology include the H fluid flow dynamics through the porous medium which affect H injectivity and recovery. We used time-resolved X-ray computed microtomography to image real-time unsteady and steady state injections of H and brine (2 M KI) into a Clashach sandstone core at 5 MPa and ambient temperature. In steady state injections, H entered the brine-saturated rock within seconds, dispersing over several discrete pores. Over time, some H ganglia connected, disconnected and then reconnected from each other (intermittent flow), indicating that the current presumption of a constant connected flow pathway during multiphase fluid flow is an oversimplification. Pressure oscillations at the sample outlet were characterized as red noise, supporting observations of intermittent pore-filling. At higher H fractional flow the H saturation in the pore space increased from 20-22 % to 28 %. Average Euler characteristics were generally positive over time at all H flow fractions, indicating poorly connected H clusters and little control of connectivity on the H saturation. In unsteady state injections, H displaced brine in sudden pore-filling events termed Haines jumps, which are key to understanding fluid dynamics in porous media. Our results suggest a lower H storage capacity in sandstone aquifers with higher injection-induced hydrodynamic flow and suggest a low H recovery. For more accurate predictions of H storage potential and recovery, geological models should incorporate energy-dissipating processes such as Haines jumps.