## 2 Geology of the Ulleung Basin and Simulation Approach
### Reservoir Model  
Figure 4 shows a schematic of the reservoir model. The cylindrical coordinated system was used to model the gas hydrate layer with overburden and underburden. Because of the generally low permeability of the system and the relative abundance of clays, well spacing was restricted to 500 m, resulting in the 250 m outer radius of the cylindrical system [23]. The overburden thickness was 140 m, and the HBS thickness was 13 m. Underburden thickness was 300 m. In order to increase numerical convergence stability, discretization along the r-direction was not uniform, instead increasing logarithmically from the center of the system. The z-direction grid size was 10 m for the overburden and underburden, and 0.1 m for the HBS. Therefore, the system was discretized into 160 \(\times\) 1 \(\times\) 90 = 14,400 gridblocks. The vertical production well was located at the center of the cylindrical model, and the radius of the well was 0.1 m. The geomechanical grid system was the same as the reservoir grid system.  
Various previous simulation studies have utilized many assumptions, such as the simple reservoir model, correlation relative permeability curve model and permeability  
Figure 4: Cylindrical model used in the simulation.  
Figure 3: One-way and two-way coupling method.  
model [12; 18; 23; 25; 30; 31; 32]. Accordingly, the reservoir model of this study was constructed by using realistic field data, as shown in Figure 5 and Table 1. This model used the logging data of UBGH2-6, and the HBS of this model is composed of 14 sand layers and 13 shale layers [33]. The sand porosity is 45%, and mud porosity is 67%. The range of hydrate saturation is 38.8-86.2%. Moreover, we adopted experimental data of the relative permeability curve and permeability model, using a core sample from UBGH2-6, as illustrated in Figures 6 and 7[33; 34]. The experimental results of relative permeability were validated with results of X-ray CT (Computerized Tomography), and it represented good matching results [33]. Furthermore, even though the intrinsic permeability was different with each soil specimen, the permeability reduction trends with increasing hydrate saturation were similar for all samples, and the N values of Figure 7 represent the porosity [34].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Overburden thickness (m) & 140 \\ \hline Underburden thickness (m) & 300 \\ \hline Layer thicknesses and porosities & As in Figure 5 \\ \hline Hydrate saturation in HBL & As in Figure 5 \\ \hline Initial pressure at top layer (MPa) & 22.261 \\ \hline Initial temperature at top layer (\({}^{\circ}\)C) & 0.482 \\ \hline Dry thermal conductivity (W/m/K) & 1 \\ \hline Wet thermal conductivity (W/m/K) & 1.45 \\ \hline Bottomhole pressure (MPa) & 9 \\ \hline Intrinsic permeability (m\({}^{2}\)) & Overburden & Sand & Mud interlayer \\ \cline{2-3}  & 2.00 \(\times\)\(10^{-18}\) & 1.78 \(\times\)\(10^{-13}\) \\ \hline Porosity (\%) & 67 & 45 & 67 & 67 \\ \hline GH saturation (\%) & 0 & 38.8–86.2 & 0 & 0 \\ \hline Bulk density (kg/m\({}^{3}\)) & 2620 & 2650 & 2640 & 2660 \\ \hline Young’s modulus (MPa) & 14 & 40 (at S\({}_{\text{h}}\) = 0) & 18 & 20 \\ \cline{2-3}  & 1400 (at S\({}_{\text{h}}\) = 1) & & \\ \hline Poisson’s ratio & 0.35 & 0.25 & 0.35 & 0.35 \\ \hline Cohesion (MPa) & 0.030 & 0.035 & 0.030 & 0.040 \\ \hline Rock compressibility (1/Pa) & \multicolumn{3}{c}{\(1.0\times 10^{-8}\)} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Initial conditions and properties.  
Figure 5: Schematic diagram of UBGH2-6 [33].