## 3 Results and Discussion
### Influencing Factors
#### Pressure and Temperature Conditions in Gas Reservoirs  
Each depleted gas field has specific pressure and temperature conditions. To analyze the influence of pressure and temperature on the modeling results of hydrogen storage, the pressure and temperature conditions are varied.  
The higher pressure and temperature conditions chosen are 161 atm and 80 \({}^{\circ}\)C. At even higher temperature conditions, the sulfate-reducing bacteria and methanogenic bacteria are mostly no longer active. The maximum temperature for SRB and methanogenic bacteria is 80 \({}^{\circ}\)C [19; 24]. At higher pressure and temperature conditions, thermochemical sulfate reduction, as known from deeply buried hydrocarbon reservoirs, must be considered.  
At 161 atm and 80 \({}^{\circ}\)C (at invariable kinetic rate constants for BSR and methanogenesis), the amount of consumed H\({}_{\text{2(g/aq)}}\) is slightly lower (75%) than that in the reference scenario (76%; modeled at 40 atm and 40 \({}^{\circ}\)C). The increased pressure results in a decreased gas volume and in higher gas concentrations. The H\({}_{\text{2}}\)S\({}_{\text{(g)}}\) concentration is increased to an average of 0.009 mol L\({}^{-1}\) of gas and the CH\({}_{\text{4(g)}}\) concentration is increased to an average 0.54 mol L\({}^{-1}\) of gas after 30 years (Figure 2c).  
The porosity loss (from initial 10% to 9.52-9.86%) in the reservoir rock is, on average, higher than in the reference scenario. This can be explained by the stronger albitization at this higher pressure and temperature conditions and the pyrite formation increases as well. On the other hand, the dissolution of quartz, calcite, and illite is higher. However, the amount of precipitated minerals is higher than the amount of dissolved minerals and this causes the decrease in porosity. The pH in the reservoir rock brine is 7.5-8.4, which is lower than in the reference scenario. In addition, the amount of the aqueous sulfide sulfur (S(-II)) in the reservoir brine is decreased at these conditions. The mass of generated water is, despite BSR and methanogenesis, decreased from 1 kg to 0.96 kg. A reason for this could be the stronger albitization process, binding the water.  
The changed pressure and temperature conditions affect the equilibrium constants of the mass action laws of gases, minerals, and aqueous species. Here, a modeling limitation is achieved because the phreeqc.dat database does not contain pressure dependencies for the equilibrium constants for all mineral phases that are used in the model. The pressure dependency of the following mineral phases in the model are included: calcite, dolomite, anhydrite, barite, quartz, and halite.  
As in the reference scenario, the temperature and pressure conditions change along the diffusive pathway of the aqueous species through the cap rock according to the geothermal gradient of 33.3 \({}^{\circ}\)C km\({}^{-1}\) depth and 100 atm km\({}^{-1}\) depth under hydrostatic conditions, starting with 80 \({}^{\circ}\)C and 161 atm at the reservoir depth. Each cell is defined by a specific temperature and pressure condition. The influence of these changing pressure and temperature conditions on the cap rock is minor. The concentration of aqueous sulfate sulfur S(+VI) is higher and aqueous sulfide sulfur (S(-II)) is lower than in the reference scenario. Different from the reference scenario, albitization occurs at these higher pressure and temperature conditions even in the cap rock, and dawsonite precipitation and halite dissolution are slightly increased.  
To summarize, at higher pressure and temperature conditions (while the kinetic rate constants are not changed), the concentration of CH\({}_{\text{4(g)}}\) and H\({}_{\text{2}}\)S\({}_{\text{(g)}}\) is increased due to a lower gas volume. Furthermore, the loss in porosity in the reservoir rock is increased. However, the kinetic rate constant is pressure- and temperature-dependent; therefore, this influence is tested in a further scenario (Section 3.4.3).