## 3 Results and Discussion
### Influencing Factors
#### 3.4.1 Storage Time  
Because of the lack of data, the storage time has been chosen based on the equipment lifetime of 30 years [51]. To show the influence of longer storage times on the system environment, in this scenario a longer storage time of 300 years is modeled. With increasing storage time, the dissolved hydrogen has more time to diffuse into higher regions of the cap rock. After 300 years, the non-reactive  
Figure 4: Diffusion of a non-reactive hydrogen tracer through the cap rock over (a) 30 years and (b) 300 years. tracer diffuses 10 m through the cap rock (Figure 4b), whereas the reactive hydrogen affects just the first 5 m of the cap rock. When considering longer storage times, it becomes clear that the mass of generated water is increased to 1.03 kg (1 kg + 0.3 kg = 1.03 kg water in total) in the reservoir rock because the water-producing processes, BSR and methanogenesis (Equations (2) and (3)), are time-dependent. In the reservoir rock, the pH is increased to 8.7. The porosity loss from 10% to 9.19-9.99% is greater than that after 30 years of storage. The reason for the greater decrease is the intensified dissolution of quartz and illite. Calcite is completely consumed in the hotspot areas, the contact areas of the cap rock to the reservoir rock and of the underlying rock to the reservoir rock. In all other cells, calcite is still available, but it is only partly dissolved. On the other hand, the albitization process is intensified in the hotspot areas and more kaolinite is formed. The reactive amounts of anhydrite and goethite are completely consumed. The only sulfate source for BSR is delivered by diffusion from the underlying rock and the cap rock into the reservoir rock. The mineralogical changes in the cap rock are minimal and show that even after 300 years of hydrogen storage, the effects of diffusion of aqueous hydrogen are small. When the storage time is increased to 300 years (under the assumption that no new gas mixture of H\({}_{2(\mathrm{g})}\) and CO\({}_{2(\mathrm{g})}\) is injected), the total amount of newly generated CH\({}_{4(\mathrm{g})}\) is increased to an average of 0.25 mol L\({}^{-1}\) of gas and the H\({}_{2}\)S\({}_{(\mathrm{g})}\) concentration is increased to an average of 0.006 mol L\({}^{-1}\) of gas (Figure 2b). After 300 years, the total amount of stored H\({}_{2(\mathrm{g})}\) is consumed. In the case of storage cycles, including gas injection, storage, and production phases, the newly injected hydrogen gas is again accompanied by a new amount of CO\({}_{2(\mathrm{g})}\), which is available as an electron acceptor for methanogenic bacteria. This aspect is modeled in a separate transport model. The total loss of hydrogen by conversion to CH\({}_{4(\mathrm{g})}\) is higher if the storage cycle has a higher frequency, because CO\({}_{2(\mathrm{g})}\) is co-injected and is available for methanogenesis. With a total storage time of 30 years and a dwelling time of 6 months, a maximum of 60 injections of \"fresh gas\" (composed of H\({}_{2(\mathrm{g})}\) and CO\({}_{2(\mathrm{g})}\)) can be stored in the reservoir. This leads to a total generation of 6.72 mol L\({}^{-1}\) of gas CH\({}_{4(\mathrm{g})}\) after 30 years. However, the loss of hydrogen after 6 months, the shorter storage period, is of course smaller (0.112 mol L\({}^{-1}\) of gas in 6 months) than after 30 years of storage. Consequently, a shorter storage period leads to a lower amount of generated CH\({}_{4(\mathrm{g})}\) (and a lower loss of hydrogen) because methanogenesis is a time-dependent process. However, over the total time, the loss of hydrogen sums to a higher amount. This holds true only for methanogenesis. For bacterial sulfate reduction, the amount of generated H\({}_{2}\)S\({}_{(\mathrm{g})}\) depends not only on the amount of available sulfate but also on the time-dependent kinetic rate constant. To summarize, it is clear that longer storage times increase the risk of loss of hydrogen.