## 3 Results and Discussion
### Hydrogeochemical Effects of Hydrogen Storage on Reservoir Rock and Cap Rock  
Storage of hydrogen in depleted gas fields can lead to dissolution and precipitation of minerals, which in turn could change the porosity of the reservoir rock and cap rock. An increase in the cap rock porosity would decrease the sealing capacity and increase the risk of pathways for the stored hydrogen to reach, for example, overlying aquifers. On the contrary, a decrease of the cap rock porosity would increase its sealing capacity. [PERSON] et al. [57] argued that the influence of hydrogen on sulfur-bearing minerals like framboidal pyrite is high but the effect on other minerals available in claystones like clay minerals, quartz and calcite is minor at \"low temperatures\". By contrast, [PERSON] et al. [2] stated that the addition of hydrogen causes abiotic reactions of hydrogen with minerals of the reservoir and cap rocks which leads to dissolution of sulfate and carbonate-bearing minerals, clay minerals of the chlorite group and feldspars and to precipitation of iron-sulfide bearing minerals, illite, and pyrrhotite. However, both studies agree that pyrite is a potential oxidant for hydrogen and is reduced to pyrrhotite (FeS1 + x) [2; 57]. The 1 DRMT modeling results of the reference scenario of this study show that at 40 \({}^{\circ}\)C and 40 atm the storage of hydrogen induces K-feldspar, kaolinite, and dolomite precipitation in the reservoir rock. Quartz, calcite, and illite are dissolved. The reactive amounts of barite, goethite, and anhydrite are completely consumed during the storage time of 30 years. After the complete consumption of reactive anhydrite, the only sulfate source comes from the cap rock and the underlying rock by diffusion and consequently this limits the loss of hydrogen by bacterial sulfate reduction. High concentrations of S(-II)(aq) species in the reservoir brine can be observed from the modeling results. The assumed reactive amount of goethite is completely consumed in less than 3 years. The goethite dissolution buffers the effect of H2S(g) generation because the amount of Fe(+II), which has been reduced from Fe(+III), reacts with the aqueous sulfide to form pyrite (a potential secondary phase in the model) so that aqueous sulfide is no longer available for H2S(g) generation. The modeling results show also that the higher pH conditions (pH increases from initial 6.4 to 8.2-8.7) and the high sodium concentrations in the brine induce weak albitization in the hotspot areas (the contact area of the reservoir rock and the cap rock and the contact area of the reservoir rock and the underlying rock). Except for pyrite and albite, the other potential secondary phases, dawsonite, mackinawite, nahcolite, and sulfur, are not formed under reservoir conditions. Elemental sulfur does not form, even if mackinawite and pyrite are not considered as potential secondary phases. Small amounts of calcite are precipitated because the dissolution of anhydrite contributes Ca2\({}^{+}\)(aq) into the reservoir brine. However, at the same time, calcite is dissolved and delivers CO2 for methanogenesis so that the total amount of calcite decreases. The volume changes of the mineral phases are shown in Figure 3. The changes in porosity are calculated from the PHREQC modeling results. The porosity in the reservoir rock decreases from initially 10% to 9.79-9.95%. This means that the available pore space for hydrogen storage decreases over 30 years such that (i) the same amount of hydrogen is movedto greater distances from the bore hole or (ii) less hydrogen can be stored in future injection phases (max. 0.2%). The mineralogical changes in the cap rock, induced by diffusion of aqueous H\({}_{\text{2(aq)}}\) from the reservoir rock, are minimal and cause no changes to the initial porosity. The reasons for that are the short storage time of 30 years and the slow diffusion process. Furthermore, the hydrogen reacts with the mineralogical assemblage of the reservoir rock and brine and it is converted by BSR and methanogenesis.