## 5 Results and ANN Model Validation
### Optimization for Various Scenarios  
In order to generate sufficient training and testing datasets, optimization processes were performed. For given injection periods, the location of the relief well and the operating conditions of both the injection and relief wells were optimized to achieve the maximum cumulative mass of injected CO\({}_{2}\). After the injection period, the pore pressure distributions and trapping mechanisms were monitored. A total of 300 years of injection and monitoring periods were selected to investigate the long-term storage stability and storage efficiency. The injection scenarios of 10 years and 30 years were chosen to generate the training datasets. Additional optimization process was also performed with a 20-year injection period to validate the developed ANN model. Figure 16 shows the optimized location of the relief well for each injection period. When the CO\({}_{2}\) was injected for 10 years, the optimum location of the relief well was located 596.6 m north-east from the injection well (321.6 m and 502.5 m in the x- and y-direction, respectively) for the maximum mass of cumulative CO\({}_{2}\) injection. The reason for this was that the injection period was relatively short, and the injection rate was highly optimized and then pressure build-up occurred rapidly during the injection period. Thus, in order to manage the pore pressure due to CO\({}_{2}\) injection, the location of the relief well was optimized near the injection well. In addition, it was found that the relief well was located deeper (52.8 m) than the position of the injection well to prevent CO\({}_{2}\) breakthrough. For the cases of 20- and 30-year injection, the location of the relief well was located at 800.5 m and 767.5 m away from the injection well, 623.1 m and 502.5 m in the x- direction and 502.5 m and 542.7 m in the y- direction, respectively. The reason for this was that the location of the relief well was optimized for areas with high permeability to facilitate water production. Figure 15: Pore pressure distributions in the top layer of aquifer A (**a**) before the injection process, (**b**) at the end of injection (6.85 years) without a relief well and (**c**) at the end of injection (7.84 years) with a relief well. In general, a higher permeability resulted in a higher water production efficiency, which means that pressure control through the water production is easier [67]. The injection rates during the injection periods are shown in Figure 17a. The solid, dashed and dotted curves indicate the rates for 10-, 20- and 30-year injection periods, respectively. The optimized injection rate for the 10-year injection scenario is 60 tons/day, while an injection rate of 40 tons/day is expected to be optimum for the 20- and 30-year injection periods. It was found that the CO\({}_{2}\) was injected as planned for the 10- and 20-year injection scenarios. When the 30-year injection scenario was adopted, the injection was unexpectedly terminated at 22 years (red arrow in Figure 17a). In order to identify what caused the unexpected termination, occurrence of the fault reactivation and the CO\({}_{2}\) breakthrough phenomenon were closely investigated. Figure 17b indicates the pore pressure at fault EF1 (black curves) and the CO\({}_{2}\) production rate at the relief well (blue curve). Since the pore pressure build-up is most severe at the top layer of aquifer A, that particular location was taken into account for the analysis. According to the pore pressure behaviors, it appears that the pressure did not reach the fault reactivation pressure for all the injection scenarios. However, it was found that the injected CO\({}_{2}\) started to be produced at the relief well at 22 years when the 30-year injection scenario was chosen (blue dotted curve in Figure 17b). The cumulative mass of injected CO\({}_{2}\) for the 10-, 20-, and 30-year injection periods are 218, 292, and 332 kton. As the injection period extended, the cumulative injection mass increased.