4.3.2. Real World Cases

The excess of electricity produced [32,33] in Germany could be stored in underground gas storage by converting the power energy into gas (Power-to-Gas). PtG is a process of generation of a gas with high energy density through the electrolysis of water. The first intermediate product is power-to-hydrogen, which can be converted into synthetic methane gas power-to-methane in a subsequent methanation process that requires injection of CO2. In this way, the same seasonal underground gas storage can also be used as a battery for excess energy in a calendar year. Figure 8 shows Germany's data on excess electricity produced in calendar year 2017. It can be seen from the data that, during the summertime (from March till August), electricity produced from renewable sources, such as wind and solar, increases enormously. The combined wind energy (onshore and offshore) shows high variation during the first and fourth quarters of the year, meaning this high variation of

electricity production from renewable sources can be stored (power-to-gas) and reused (gas-to-power) in cases of excesses and shortages of electricity, respectively.

**Figure 8.** Data show the consumption of total electricity in Germany, along with electricity produced from renewable energy sources (such as wind, either offshore or onshore, and solar) in calendar year 2017 [32,33].

These scenarios have been performed on well X6. The schedule is based on the data shown in Figure 8. The baseline of 10 GW is the shut-in period. Greater than the 10-GW baseline is considered to have an excess of electricity that can be used as injection periods for power-to-gas storage, and less than the limit of 10 GW is a shortage of electricity. These periods have been considered for the production of gas for gas-to-power conversion. These data [32,33] on excess electricity from Germany have been used to conduct two short-term real world case schedule cases: one with limited water cut-off and one without limited water cut-off.

## *4.4. Case C*

This scenario has been performed on vertical well X6. The schedule timeline of one year has been adopted from Figure 8. The well water production rate (WWPR) is restricted to 5 m3/day (Figure 9c). The FPR eventually increases in this case to up to 15.72 MPa with slight variations during the gas production cycles. Gas injection succeeds to maintain the pressure, and it increases from ~15.39 MPa and reaches approximately ~15.72 MPa at the end of the one-year period (Figure 9a). The variability in WBHP is directly proportional to the gas production cycles. As shown in Figure 9a, as the gas production increases, the well bottom-hole pressure decreases even as the gas injection continues. The maximum WBHP reaches a value of 18.8 MPa, the minimum WBHP reaches a value of 14.4 MPa, and these maximum and minimum pressure values represent injection and production cycles (Figure 9b). The well gas injection rate (WGIR) varies during the year, having a minimum injection rate of approximately 32,000 m3/day and a maximum rate of about 66,000 m3/day. In contrast, the well gas production rate (WGPR) has minimum and maximum values of 4300 m3/day and 19,000 m3/day, respectively (Figure 9d). The lower WGPR compared to the WGIR is due to the limited WWPR, which does not allow the well to produce at a higher WGPR.

**Figure 9.** The fluctuation in electricity produced in Germany from renewable sources in 2017 is modelled into a future scenario testing case with limited water cut-off (5 m3/day). The excess of energy can be stored in UGS and can be used when needed. This schedule is helpful to understand in which month of the year energy can be stored as gas in UGS and in which month of the year this energy can be utilized when shortage occurs: (**a**) field pressure (FPR) profile for these cycles; (**b**) well bottom hole pressure (WBHP) of well X6; (**c**) well water production rate (WWPR) for well X6; and (**d**) well gas injection rate (WGIR) and well gas production rate (WGPR) for well X6.

### Results

The modelling results of this case are presented in the form of the pore pressure and effective stress changes in the top layer of the reservoir. Two time steps have been selected for the conclusion of the results for this model. Time step t1 is the starting point of the schedule case, i.e., 1 January 2020, and t2 is the end schedule point (31 December 2020). The pore pressure at the well X6 location is about 15.2 MPa at t1 and increases to about 15.8 MPa at t2; simultaneously, the effective stresses at well X6 is about 28.6 MPa, and it decreases to about 28.0 MPa at time steps t1 and t2 (Figure 10). There is an increase of 0.6 MPa in pore pressure and decrease of 0.6 MPa in effective stress at the top surface of the reservoir layer near well X6 from t1 to t2.

**Figure 10.** *Cont*.

**Figure 10.** Pore pressure (*Pp*) and effective stress (*Sef f ec*) changes from t1 (1 January 2020) to t2 (31 December 2020) in the short-term case with one well (X6) with a water cut-off rate of 5 m3/day and a random schedule. The arrows show the location of the maximum observed fluctuation in *Pp* and *Sef f ec* from t1 to t2. The color scale is in MPa. (**a**) is pore pressure at time t1; (**b**) is pore pressure at time t2; (**c**) is effective stress at time t1; (**d**) is effective stress at time t2.

#### *4.5. Case D*

This scenario has been performed on vertical well X6. The schedule timeline of one year has been adopted from Figure 8. There was no water cut-off rate limit set in this scenario; hence, the maximum water production rate (WWPR) increases up to 43 m3/day and remains less than 20 m3/day throughout the production and injection period of one year, respectively (Figure 11c). The FPR is sustained in this case by gas injection and increases to up to a maximum value of 15.62 MPa with slight variation during the gas production cycles in the one-year period (Figure 11a). The alteration in WBHP is directly proportional to the gas production cycles without a water cut-off limit. As shown in Figure 11b, the WBHP reaches a maximum value of 18.8 MPa and a minimum value of approximately 13.8 MPa. The well gas injection rate (WGIR) varies during the year, having a minimum injection rate of approximately 32,000 m3/day and a maximum rate of about 60,000 m3/day, whereas the well gas production rate (WGPR) has minimum and maximum values of 16,000 m3/day and 40,000 m3/day, respectively (Figure 11d).

#### Results

The modelling results of this case are presented in the form of pore pressure and effective stress changes of the top layer of the reservoir. Two time steps have been selected for the conclusion of the results for this model. Time step t1 is the starting point of the schedule case, i.e., 1 January 20, and t2 is the end schedule point (31 December 2020). The pore pressure at the well X6 location is about 15.1 MPa at t1 and increases to about 15.7 MPa at t2, whereas the effective stress at well X6 is about 28.7 MPa, and it decreases to about 28.1 MPa at time steps t1 and t2. There is an increase of 0.6 MPa in pore pressure and a decrease of 0.6 MPa in effective stress at the top layer of the reservoir around well X6 from t1 to t2 (Figure 12).

**Figure 11.** The fluctuation of electricity produced in German from renewable sources in 2017 is modelled into a future scenario testing case without limited water cut-off. The excess of energy can be stored in UGS reservoirs and can be used when needed. This schedule is helpful to understand which month of the year's energy can be stored as gas in UGS and in which month of the year this energy can be utilized when shortages occur: (**a**) field pressure (FPR) profile for these cycles; (**b**) well bottom hole pressure (WBHP) of well X6; (**c**) well water production rate (WWPR) for well X6; and (**d**) injection (WGIR) and production (WGPR) gas rates for well X6.

**Figure 12.** Pore pressure (*Pp*) and effective stress (*Sef f ec*) changes from t1 (1 January 2020) to t2 (31 December 2020) in short-term case with one well (X6) without a water cut-off rate of 5 m3/day with a random schedule. The arrows show the location of the maximum observed fluctuation in *Pp* and *Sef f ec* from t1 to t2. The color scale is in MPa. (**a**) is pore pressure at time t1; (**b**) is pore pressure at time t2; (**c**) is effective stress at time t1; (**d**) is effective stress at time t2.

The summary of all the results is compiled in Table 2 to have better understanding of pore pressure and effective stress changes of all future testing cases with time (t1 to t2).

**Table 2.** Summary of results for all future test scenarios. The sign + in the pore pressure changes indicates a positive change or an increase in pore pressure from time step t1 to t2, while the sign—in the changes in effective stresses denotes a decrease in magnitudes of effective stresses for time step t1 to t2. These two quantities are inversely proportional to each other and are expressed in MPa and KPa for a better understanding of the changes.


### **5. Thermal Analysis**

The same dynamic model has been used for thermal analyses. Since long-term injection would impact the thermal changes in the reservoir significantly, a long-term seasonal case is used to analyse the temperature changes within the reservoir if a foreign gas is injected into it. Therefore, six months of gas injection and six months of gas withdrawal are considered in this modelling case. The initial reservoir temperature is ~45 ◦C, and the foreign gas temperature is 25 ◦C. Two cycles have been considered to analyse the temperature changes during these injection/production operations. Gas is injected into the reservoir for the first half year and produced in the second half of the year. Two wells, X2 and X6, are considered to analyse the temperature changes around the well bore vicinity. The bottom hole pressure (WBHP) for both wells is set to an upper limit of 18.8 MPa and a lower limit of 13.8 MPa in case of the injection and production phases, respectively. These pressure limits are set in place to avoid fault reactivation or fracture-inducing phenomena during the injection phase, as well as to avoid sand production or contraction of the reservoir during the production phase. The WGIR and WGPR are set to 100,000 m3/day for both wells.

Thermal stresses are the stresses that occur due to the change in temperature in the system, i.e., original temperature minus final temperature. If foreign gas is injected into the underground gas reservoir, the temperature in the reservoir changes, which causes thermal-related stress changes in the reservoir. The relationship of temperature changes and thermal stress is expressed by the following equation [34]:

$$S\_t = E \* \mathfrak{a}\_t (T\_f - T\_0) = E \* \mathfrak{a}\_t (\Delta T), \tag{11}$$

In the above equation, *St* is thermal stress, *E* is the Young's modulus, *α<sup>t</sup>* is the thermal coefficient, *Tf* is the final temperature of the reservoir, *T*<sup>0</sup> is the original temperature of the reservoir, and Δ*T* is the temperature difference in the reservoir. Less Δ*T* causes less thermal stress in the reservoir and vice versa.

The following section describes the results, i.e., temperature changes with injection of colder foreign gas (25 ◦C) into the reservoir (i.e., about 45 ◦C) through space and time with seasonal cyclic injection/production phases. The top view of the reservoir surface is shown in the figure at different time steps (Figure 13). Two injection/production cycles with four-time steps have been selected to show thermal changes in the reservoir with injection and production phases. Time step t1 (1 January 2020) is the pre-operational history temperature of the reservoir at wells X2 and X6 (Figure 13a). Time step t2 (1 July 2020) represents the end of the injection time of colder foreign gas (25 ◦C), which is injected for the first half of 2020 (Figure 13b). Time step t3 (1 January 2021) is the end of the production period of the cycle (Figure 13c), and t4 (1 July 2021) is again the end of injection phase of the second cycle (Figure 13d).

**Figure 13.** Temperature changes around wells X2 and X6 by injecting colder foreign gas at different time steps (t1, t2, t3, and t4). Time steps t1, t2, t3, and t4 correspond to 1 January 2020, 1 July 2020, 1 January 2021, and 1 July 2021, respectively. The arrows show the exact location of significant temperature differences during injection/production phases. The colour scale is in ◦C, whereas the arrows with N indicate a northward direction. (**a**) is temperature at time t1; (**b**) is temperature at time t2; (**c**) is temperature at time t3; (**d**) is temperature at time t4.

The reservoir temperature is 45 ◦C at time step t1, which is the pre-operational temperature. The temperature decreases to about 43 ◦C at well X6 and 42.5 ◦C at well X2 at time step t2 after constant injection of colder gas (25 ◦C), with a well gas injection rate (WGIR) of 100,000 m3/day for six months. The temperature increases to about 43.5 ◦C and ~43 ◦C at wells X6 and X2, respectively, at time step t3. There is only about a 0.5 ◦C increase in temperature from t2 to t3. Temperatures at well X6 and X2 decrease to about 41.5 ◦C and 42 ◦C, respectively, again in the second cycle of injection at t4. Thermal changes are minor and occur only at the vicinity and around the well locations. The thermal effects on the stress are not significant in the reservoir even after injection of 100,000 m3/day of colder gas for about a half year. This outcome shows that the thermal changes in the short-term cases are negligible for analysing the geomechanical stresses on the reservoir in storage operations.

#### **6. Potential Fault Reactivation Analyses**

#### *6.1. Model Setup*

Fault reactivation is the possibility of failure in geomechanical assessment of the reservoir, which can risk operational safety, cause micro seismicity within and around reservoirs, and provide a leakage path for gas to escape. Fault reactivation occurs when the shear stress acting on the fault planes exceeds the shear strength of the fault. The Mohr–Coulomb failure criterion relationship of pore pressure and the principal stresses of this case study reservoir (lies in normal stress regime) are expressed by this equation [35]:

$$P\_{\mathcal{P}} = \frac{1}{a} [\frac{1}{2}(S\_{\mathcal{v}} + S\_{\text{lmin}}) + \frac{1}{2}(S\_{\mathcal{v}} - S\_{\text{lmin}})\cos 2\theta - \frac{1}{2}(S\_{\mathcal{v}} - S\_{\text{lmin}})\frac{\sin 2\theta}{\mu}],\tag{12}$$

where *α* is the Biot coefficient (assumed 1), *Sv* is the vertical and maximum principal stress, *Shmin* is the minimum horizontal stress, *θ* is the angle between the dip line of the fault and the *Shmin* direction, *Pp* is the critical pore pressure, and *μ* is the coefficient of friction.

These analyses include the calculation of the critical pore pressure with the aim of observing possible differences in pore pressure required for fault reactivation. The pore pressure derived from the history matching scenario is multiplied by a fixed factor controlled by gas rates until fault reactivation occurs. The upper limit of the BHP is removed to obtain a higher pore pressure. The factors used for this operation are 1.15, 1.25, and 1.5 (Figure 14). It is then possible to evaluate pressure changes in the reservoir required to reactivate the fault, as well as the safe storage capacity of the reservoir.

**Figure 14.** Pressure profiles for the history matching scenario and scenarios multiplied by fixed factors (*Pp*, *Pp* X 1.15, *Pp* X 1.25, and *Pp* X 1.5) controlled by gas injection rates until fault reactivation occurs.

Figure 14 shows different pressure profiles based on distinct gas injection scenarios. The *Pp* (history match pressure) curve exhibits a maximum value of ~16.2 MPa with gas injected at a rate of 100,000 m3/day for one year, whereas to reach a pressure value 1.15 times the actual history match pressure case, 175,000 m3/day of gas are injected for 1 year. The pressure increased up to 18.6 MPa. In case *Pp* X 1.25, 240,000 m3/day is injected to reach a pressure of about 20.25 MPa. Similarly, in case of *Pp* X 1.5, a gas volume of 560,000 m3/day is injected (for one year), which increases the pressure up to 24.3 MPa.
