# The Influencing Factors of CO2 Utilization and Storage Efficiency in Gas Reservoir
## 3 Numerical Simulations
### Three-Dimensional Simulations for Storage Mechanism Analyses  
In order to comprehensively analyze the CO\({}_{2}\) storage, distribution and the proportion of CO\({}_{2}\) storage volume with different storage mechanisms in gas reservoir, the three-dimensional numerical well group model was established to completely simulate each stage from depletion to storage, and the CO\({}_{2}\) content under different stages and different forms (supercritical, ionic and mineral state) was obtained. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Simulation** & **Sensitive** & **Simulation** \\
**Number** & **Parameter** & **Design** \\ \hline
1 & & 8 MPa, 35 \({}^{\circ}\)C \\
2 & Temperature & 8 MPa, 60 \({}^{\circ}\)C \\
3 & & 8 MPa, 80 \({}^{\circ}\)C \\ \hline
4 & & 7.5 MPa, 80 \({}^{\circ}\)C \\
5 & Pressure & 10 MPa, 80 \({}^{\circ}\)C \\
6 & & 15 MPa, 80 \({}^{\circ}\)C \\ \hline
7 & & 100\%CO\({}_{2}\) \\
8 & CO\({}_{2}\) purity & 90\%CO\({}_{2}\) + 10\%N\({}_{2}\) \\
9 & CO\({}_{2}\) purity & 80\%CO\({}_{2}\) + 20\%N\({}_{2}\) \\
10 & & 70\%CO\({}_{2}\) + 30\%N\({}_{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Long core simulation design. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Layer** & **Thickness** & **Porosity** & **Permeability** \\
**Number** & **(m)** & & **(10\({}^{-3}\)\(\mu\)m\({}^{2}\))** \\ \hline
1 & 15 & 0.1 & 20 \\
2 & 15 & 0.1 & 20 \\
3 & 15 & 0.15 & 40 \\
4 & 15 & 0.15 & 40 \\
5 & 15 & 0.2 & 60 \\
6 & 15 & 0.2 & 60 \\
7 & 15 & 0.25 & 80 \\
8 & 15 & 0.25 & 80 \\
9 & 15 & 0.3 & 100 \\
10 & 15 & 0.3 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Basic information of each layer in two-dimensional numerical model. The numerical well group model was cut from the actual gas reservoir model. The central depth of the model was 1750 m; the formation temperature was 66.8 \({}^{\circ}\)C; the initial pressure was 18 MPa; and the water saturation was 45%. The model was established by corner-point grids which amounted to 9000 (\(30\times 30\times 10\)), and the areal grid dimension was 50 m \(\times\) 50 m. Five-point vertical well pattern was adopted, and the well types (production or injection well) were set according to the development phase. The grid setting and well location of the three-dimensional model are shown in Figure 7a. According to the vertical heterogeneity and profile models simulation, the three-dimensional model was designed as a reverse rhythm model which had better porosity and permeability in the upper layers (Figure 7b). The influence of bottom water on supercritical CO2 displacement and the storage process of the gas reservoir could be characterized by setting the bottom layers as an aquifer. Supercritical storage, ionic storage and mineral storage were mainly considered in the simulations. Ionic storage: the supercritical CO2 was dissolved in water and turned into an ionic state (Equations (5)-(7)) when they met in the rock pore during displacement. Mineral storage: the ionic CO2 (HCO3 ions) reacted with calcite (Equation (8)), kaolinite (Equation (9)) and anorthite (Equation (10)) and turned into solid state (mineral precipitation). The reaction equation of different storage mechanisms were as follows:  
\[\text{CO}_{2}+\text{H}_{2}\text{O}=\text{H}^{+}+\text{HCO}_{3}{}^{-} \tag{5}\]  
\[\text{OH}^{-}+\text{H}^{+}=\text{H}_{2}\text{O} \tag{6}\]  
\[\text{CO}_{3}{}^{2}+\text{H}^{+}=\text{HCO}_{3}{}^{-} \tag{7}\]  
\[\text{CaCO}_{3}+\text{H}^{+}=\text{Ca}^{2+}+\text{HCO}_{3}{}^{-}+\text{H}_{2} \text{O} \tag{8}\]  
\[\text{Al}_{4}\text{[Si}_{4}\text{O}_{10}](\text{OH})_{8}+\text{12H}^{+}=10 \text{H}_{2}\text{O}+4\text{Al}^{3+}+4\text{SiO}_{2} \tag{9}\]  
Figure 6: Permeability distribution of numerical model. (**a**) Positive rhythm; (**b**) Reverse rhythm. Figure 7: Three-dimensional model. (**a**) Grid setting and well location; (**b**) Permeability distribution.