## 4 Conclusions  
The simulation results show that the underground storage of hydrogen in depleted gas fields entails the risk of hydrogen loss (and related energy loss) by bacterial conversion to CH4(g) and H2S(g) and gas-water-rock interactions, which in turn lead to changes in the porosity of the reservoir rock. The modeling results of the one-dimensional reactive mass transport model identify the factors that control the loss of hydrogen:  
* The loss of hydrogen by bacterial conversion to CH4(g) via methanogenesis is limited mainly by the amount of co-injected CO2(g), the reaction kinetics, and the connected maximal storage time of 30 years. Less co-injected CO2(g) will reduce H2(g) loss but cannot inhibit the conversion to CH4(g) if further CO2 sources are available in the form of residual gas and carbonate-bearing minerals. The generation of CH4(g) by methanogenesis where CO2 is only delivered by the dissolution of carbonate-bearing minerals is slower because the dissolution process limits the conversion to CH4(g). * The loss of hydrogen by bacterial conversion to H2S(g) via bacterial sulfate reduction is limited mainly by the amount of available sulfate in the reservoir. After complete consumption of reactive anhydrite, the only sulfate source comes from the cap rock and the underlying rock by diffusion and consequently limits the loss of hydrogen by bacterial sulfate reduction because the process of diffusion is slow. * The mass of generated water, as a product of BSR and methanogenesis, increases the pressure in the system and diffusion can be intensified. * The mineralogical changes in the reservoir rock result in a small decrease in porosity. As a consequence, the available pore space for hydrogen storage decreases over 30 years such that (i) the same amount of hydrogen is moved to greater distances from the bore hole or (ii) less hydrogen can be stored in future injection phases. * The loss of aqueous hydrogen by diffusion and related effects on the cap rock mineralogy is negligibly small, with a storage time of 30 years, because hydrogen storage causes gas-water-rock interactions in the reservoir rock and brine and is converted by BSR and methanogenesis to a greater extent. Furthermore, the process of diffusion is slow. * A longer storage period increases the loss of the stored hydrogen. Shorter storage periods lead to less hydrogen loss for each period, but over the total time the summed loss of hydrogen is higher because new CO2(g) is available for methanogenesis after each gas injection. * At higher pressure and temperature conditions, the concentrations of CH4(g) and H2S(g) increase due to a lower gas volume. Furthermore, the loss in porosity in the reservoir rock increases as well. * Knowledge of kinetic rate constants for bacterial sulfate reduction and methanogenesis at elevated levels of pressure and temperature are required as accurately as possible. It is recommended to choose depleted gas fields for hydrogen storage where the residual gas has low CO2(g) concentrations. The mineralogical composition of the reservoir rocks should contain low amounts of sulfate- and carbonate-bearing minerals but high amounts of reactive iron-bearing minerals. Reservoirs with low pressure and temperature conditions are recommended as well as storage gas compositions with low amounts of CO2(g). From the modeling results, it seems reasonable to implement a multicomponent transport model and to conduct a joint variation of several parameters as a next step. The following are available online at [[http://www.mdpi.com/2076-3417/8/11/2282/s1](http://www.mdpi.com/2076-3417/8/11/2282/s1)]([http://www.mdpi.com/2076-3417/8/11/2282/s1](http://www.mdpi.com/2076-3417/8/11/2282/s1)), Input file SI for the transport model to calculate the reference scenario (for PHREQC Version 3.1.4-8929; ready to copy, paste, and run). Conceptual model, C.H. ; modeling, C.H. and W.v.B. ; validation of results: C.H. and W.v.B. ; analysis of results, C.H. and W.v.B. ; writing--original draft preparation: C.H. ; writing--review and editing: C.H.