## 2 Study Area
### Three-Dimensional Geological Model Construction  
A 3D geological model of the target aquifer was constructed based on seismic survey and exploration well data obtained by the Korea Institute of Geoscience and Mineral Resources (KIGAM) in 2005 [43,44] (Figure 3). The modeled area is 2.8 km by 2.2 km in the east-west and the south-north direction, respectively. As shown in Figure 3, the target  
Figure 1: Location of Pohang Basin, South Korea (left). The white rectangle indicates the target area for geological CO2 storage. Figure 2: (**a**) A schematic view of the target aquifers and (**b**) interpretation of lithology of the target aquifers based on well logging data obtained from the injection well. aquifers become shallower as the distance from the injection well increases to the eastern and western boundaries. The number of grids is, respectively, 95 \(\times\) 152 \(\times\) 56 in the x-, y-, and z-(vertical) directions, including the overlying caprock and the intervening intra seal layer. The target aquifers have 24 grid-layers with a vertical length of 1 m. To improve the simulation computing speed, 32 grid-layers for the intra seal were treated as null-blocks with a permeability of 0.001 md. According to the fault information interpreted from the seismic data, there are 6 normal faults in the target aquifers [45]. The faults are located west and east from the injection well, and were named WF1 and WF2, and EF1, EF2, EF3, and EF4, respectively. Faults WF1, EF3, and EF4 are closed boundaries that limit the crossing fluid flow. For the southern and northern boundaries of the model, no fluid flow boundary conditions were assumed for a conservative analysis in terms of CO\({}_{2}\) storage capacity and injectivity. The additional 3 normal faults (WF2, EF1, and EF2) are located closer to the injection well, intersecting aquifers A and B, the caprock, and the intra seal layer. In the western direction of the injection well, WF2 developed in the SE direction, and its closest point is 945.3 m away from the injection well. In the east direction, EF1 developed in the NNE-SSW direction, and its closest point is 450.3 m from the injection well. Fault EF2 is horizontally shorter than EF1, and is 530.2 m away from the injection well. The porosity and permeability distributions used in this model are as shown in Figure 4. The porosity distribution was obtained from well logs at the injection well. The permeability distribution was obtained from the porosity distribution, determined using the Kozeny-Carman equation, which correlates the relationship between porosity and permeability. Consequently, the average porosity and permeability of the aquifer A were obtained as 32% and 30 md, respectively; aquifer B had a lower porosity and permeability of 24% and 11 md. Relative permeabilities of CO\({}_{2}\) and water for drainage and imbibition processes were used in the simulations with the model from [46; 47], respectively. Figure 3: A geological model of the target aquifers in the Pohang Basin. The faults EF1 and EF2 are 450.3 m and 530.2 m away from the injection well (IW). The pore pressure and temperature conditions in the target aquifer were determined by the hydrostatic pressure (10.0 kPa/m) and local geothermal gradients (46.5 \({}^{\circ}\)C/km), respectively [48]. As listed in Table 1, the pore pressure and temperature at the top of the model were 7.5 MPa and 55.0 \({}^{\circ}\)C at 750.0 m, respectively. The salinity of the aquifer was assumed as 100,000 ppm based on the reported value from the target aquifer [49]. According to a previous study, the fracture pressure at the top of the target formation is 18.2 MPa, which is expected to generate a hydraulically induced fracture [40]. However, 14.0 MPa was chosen for the operating bottomhole pressure (BHP) during the injection process, which was the maximum capacity of the compressor installed at the surface treating facility [41]. The minimum operating BHP at the relief well of 7.5 MPa was adopted, based on the hydrostatic pressure at the top of the target aquifer.