*6.2. Results*

The results of this section are based on the model setup explained in the above section. The methodology follows the different cases in which the initial reservoir pore pressure is multiplied by a factor of 1, 1.15, 1.25, and 1.5, corresponding to pore pressure values of ~16.2 MPa, ~18.6 MPa, ~20.25 MPa and ~24.3 MPa, respectively. The results display the oblique view of the topmost layer of the reservoir (Figure 15). The Mohr circles correspond to the well X6 location near the main fault (Figure 15). Two time steps have been considered for all the cases: t1 (1 January 2020) is the starting time step, and t2 (1 January 2021) is the end time step of the schedule year.

**Figure 15.** The computation of fault reactivation for initial pore pressure multiplied by a factor of 1 (**a**), 1.15 (**b**), 1.25 (**c**), and 1.5 (**d**) is shown on the left side. White boxes show the cells with stress states exceeding the failure criterion (arrows show northward direction). On the right side of the figure are the shear stress *Ss* vs. normal stress *Sn* diagrams, showing the Mohr–Coulomb failure criterion at the well X6 location (which is nearest the main fault of the reservoir). Time steps t1 and t2 correspond to the starting (1 January 2020) and final (1 January 2021) time steps of the tested cases, respectively. The increase in pore pressure leads to a decrease in effective stresses, causing the corresponding Mohr circle to shift to the left. If the failure line is finally touched, plastic straining and—in case of a fault zone—fault reactivation occur.

Fault reactivation is observed already at *Pp* X 1.25 (~20.25 MPa) (Figure 15b), at which the corresponding Mohr circle has just touched the tensile failure line. The amount of failure in the cells becomes more prominent as pore pressure increases to greater than the pore pressure factor of 1.25 (i.e., *Pp* X 1.5), and it causes the corresponding Mohr circles to move further left (Figure 15c,d). The increase in pore pressure causes decreases in effective stress, causing the Mohr circles to move to the tensile failure line. Thereafter, the material enters the plastic regime; therefore, in case of failure, fault reactivation occurs.

#### *6.3. Safe Injection Rate for Safe Storage Capacity*

The pore pressures for fault reactivation for the different scenarios calculated in the numerical modelling analysis provide an estimate of the pressure at different injection rates. The injection rate (in terms of volume rate) to achieve a perfect history match (16.2 MPa) is about 100,000 m3/day, and fault reactivation is already observed at *Pp* times 1.25, i.e., about 20.25 MPa at an injection rate of 240,000 m3/day. With the determination of the critical pore pressure, it is also possible to derive an upper limit for the injection rate to be selected for injection processes. By considering the highest safety margin, the injection rate between 100,000 m3/day and 150,000 m3/day would be the considered safe injection rate for safe storage for the case study reservoir. A gas injection rate greater than this threshold value can have a significant impact on the risk management and operational setup of underground gas storage.

#### *6.4. Storage Capacity of Power-to-Gas and Gas-to-Power*

Regarding the storage capacity of power-to-gas technologies, the case study reservoir can store 881,600 kWh/d up to maximum of 1,322,400 kWh/d of power from renewable or other resources with respect to the conversion of a natural gas volume of 100,000 m3/day to a maximum of 150,000 m3/day, respectively. Power-to-gas and gas-to-power convertible units are summarized in Table 3.

**Table 3.** Power-to-gas and gas-to-power convertible units.


#### **7. Discussions**
