Large-scale storage of hydrogen in deep underground porous formations has been proposed as a powerful means to manage the lag between energy production and demand in a decarbonized energy system based on renewable sources such as wind and solar [1]. Porous formations have been proposed as a powerful and flexible storage option, as they allow other geographical locations and offer adequate capacities, but important knowledge gaps still need to be addressed before being fully operational [2]. In porous formations, the injected hydrogen displaces the reservoir fluids (brine or residual hydrocarbon), which leads to complex multiphase flow behaviours and the creation of hydrogen finger structures. Diffusion and dispersion of the injected hydrogen in the heterogeneous host medium lead to the dissolution and mixing of the hydrogen phase into the reservoir fluids or cushion gas. We numerically investigate how the displacement of cushion gas and brine by hydrogen affects gas purity, capillary trapping, and hysteresis. We present a compositional Darcy-scale model of cyclic injection and extraction of hydrogen into/from a model formation (an anticlinal structure in the San Pedro belt in northern Spain [3]). We explore various operational scenarios, from its initial development from the current aquifer conditions to a scenario where a cushion gas is previously injected in the aquifer before hydrogen storage cycles begin. We analyse the evolution of produced gas composition and water cut, as well as the pressure operation regimes depending on the pressure support due to the strength of the surrounding aquifer and the impact of the nearby sealing faults. We discuss recommendations on the potential development and operation of an UHS facility in the San Pedro dome. REFERENCES [1] Carden, P.O., and Paterson, L. Physical, chemical and energy aspects of underground hydrogen storage. International Journal of Hydrogen Energy, 4: 559-569 (1979) [2] Heinemann, N., et al. Enabling large-scale hydrogen storage in porous media – the scientific challenges. Energy & Enviromental Science, 14: 853–864 (2021) [3] Sáinz-García A., Abarca, E., Rubi, V., Grandia, F. Assessment of feasible strategies for seasonal underground hydrogen storage in a saline aquifer. Int. Journal of Hydrogen Energy, 42:16657-16666 (2017)