## 3 Results and Discussion
### Potential Replication of Hydrogen Storage  
Following the principles set out in Section 2.1.3 and in order to simulate the geological hydrogen storage process in the reservoir under study, the UNDERGY project obtained interesting results. Firstly, the geomechanical simulations, represented in Figure 6, demonstrated that the reservoir is stable up to pressures of 95 bar, with a sufficient safety margin, as the operating range will be below 90 bar, as required by regulations. Figure 5: Potential biomethane production from WWTP per province in GWh/year. Figure 6: Geomechanical simulation at 100 bars. Prepared by the authors using Flac3D within the framework of the UNDERGY project. In addition, the geochemical simulations, represented in Figure 7, showed that the different gases present (hydrogen, methane, and nitrogen) would behave in a practically immiscible manner, with hydrogen located at the top of the reservoir, ensuring efficient recovery. As for the characteristics of the reservoir, in the case study of the project, the rock material is porous sandstone, which, although slightly carbonated, is free from problematic compounds such as sulphur or cyanobacteria, making it ideal for hydrogen storage. The sealing material is a highly impermeable and non-reactive marl, which ensures that gas does not escape from the reservoir, creating the perfect conditions for safe long-term storage. As discussed above, the reservoir gas behaviour modelled in the simulations shows that hydrogen is positioned at the top of the reservoir, whereas pre-existing gases, such as methane and nitrogen, act as cushion gases that move to the lower layers of the reservoir. This immiscible behaviour of gases ensures efficient separation and precise control over the amount of hydrogen stored. In addition, the simulation of the aqueous phase of the brine at the bottom of the reservoir, also carried out in the framework of the UNDERGY project, indicated that there is no evidence of halite precipitation, which is relevant to avoid the reduction of sandstone porosity and permeability, two essential factors to maintain storage efficiency. In summary, the project has demonstrated the feasibility of storing hydrogen in geological reservoirs, such as old natural gas fields, with the possibility of harnessing surplus renewable energy to produce hydrogen and store it on a large scale, providing Europe with a sustainable and efficient solution for its energy transition. Furthermore, following the methodology described in Section 2.1.3, Spain's potential for Underground Hydrogen Storage (UHS) across three main geological formations was  
Figure 7: Geochemical simulation of gas conditions in tanks at 90 bars and 60 \({}^{\circ}\)C, developed with TOUGH+ as part of the UNDERGY project. analysed. The application of the calculation methods, considering lithostatic pressure and operational volume explained in Section 2.1.3, was used to estimate the potential storage capacity in Spain. Figure 8 shows the total potential storage estimated, taking into account all geological formations, and in Appendix C, the storage per type of geological formation and province is shown. Results show a total potential of hydrogen storage is 2865 TWh. Depleted hydrocarbon reservoirs dominate this capacity landscape, accounting for 1864 TWh (65.1% of the overall potential). This significant capability is geographically concentrated, with just three provinces hosting the major installations. The Gaviota facility in Bizkaia stands as the cornerstone of Spain's UHS infrastructure, with a capacity of 857.9 TWh (46% of the total from depleted reservoirs). The remaining capacity flows through installations in Guadalajara (Yela, 640 TWh), Huesca (Serrapablo, 320 TWh), and Huelva (Marismas, 47 TWh). While depleted reservoirs offer concentrated storage potential, aquifers present a more distributed alternative, contributing 999 TWh (34.9% of the total capacity).