# The Influencing Factors of CO2 Utilization and Storage Efficiency in Gas Reservoir
## 3 Numerical Simulations  
### One-Dimensional Simulations for Long Core Displacement  
The supercritical features of CO\({}_{2}\) and the difference in physical property between CO\({}_{2}\) and CH\({}_{4}\) were characterized by fluid property analyses which could provide references for factor study of the CO\({}_{2}\) displacement and storage. Whether the process of CO\({}_{2}\) flooding CH\({}_{4}\) is more inclined to the favorable \"piston displacement\" could be reflected by the differences in density and viscosity between CO\({}_{2}\) and CH\({}_{4}\), while the degree of supercritical CO\({}_{2}\) deviation from ideal gas could be reflected by compression factor (\(Z\)), which indicates whether the fluid property is more similar to \"gas\" or \"liquid\" under different temperatures and pressures. Therefore, the fluid differences and the appropriate temperature and pressure condition in the displacement process were quantified by calculating the density, viscosity and \(Z\)-factor values of CO\({}_{2}\) and CH\({}_{4}\) through PR equation. \[p=\frac{RT}{V-b}-\frac{aa(T)}{V(V+b)+b(V-b)} \tag{4}\] \begin{table}
\begin{tabular}{c c} \hline
**Measure Tools** & **Error** \\
**and Error Events** & **Value** \\ \hline Pore permeability instrument & Systematic error: \(\pm\)1\% \\ Flowmeter of syringe pump & Systematic error: \(\pm\)1\% \\ Gas flowmeter at outlet & Systematic error: \(\pm\)1\% \\ Gas chromatograph & Systematic error: \(\pm\)0.5\% \\ Annular pressure indicator & Systematic error: \(\pm\)0.08\% \\ Inlet pressure indicator & Systematic error: \(\pm\)0.08\% \\ Outlet pressure indicator & Systematic error: \(\pm\)0.08\% \\ \hline Fluid retention in pipeline & Systematic error: \(-\)1.5\% max recovery \\ Air doping in gas chromatograph & Systematic error: \(\pm\)1\% \\ Timing of stopping injection & Systematic error: \(\pm\)1\% for last point \\ \hline \end{tabular}
\end{table} 
Table 6: Measure tools and systematic error estimations. where \(R\) is the gas constant, 8.31 MPa-cm/(mol-K); \(T\) is the temperature, \(K\); \(P\) is the pressure, MPa; \(V\) is the gas molar volume, m\({}^{3}\)/kmol; \(a\) is the attraction parameter, kPa-m\({}^{3}\)/kmol; and \(b\) is the van der Waals volume, m\({}^{3}\)/kmol. To analyze the influences of reservoir temperature, pressure and injection CO\({}_{2}\) purity on permeation and diffusion during the displacement and storage process, the long core numerical model was established. Then, the displacement and storage process were simulated through CMG/GEM according to the long core experiments. Thus, the recovery and storage efficiency under different temperatures, pressures and injection gas purities were obtained, and the mechanisms and the influence laws were deepened. The one-dimensional compositional long core model was established along I direction which was based on the experimental long core with medium permeability (Figure 4a). Each grid corresponded to an actual short core according to the long core sequence (Table 3). The accuracy of the numerical model was ensured by using the actual core property parameters for the model grid. Moreover, the heterogeneity was reflected, and the model became closer to the real core by mesh refinement (Figure 4b). Each core (grid) was subdivided into four small grids; the porosities and the permeabilities were calculated by corresponding actual core properties and the actual reservoir variation coefficient. The refined long core model had a better fit to the experimental results (Figure 5), which demonstrated the necessity of considering core heterogeneity. The same displacement conditions were adopted in the long core simulations as the experiments. The temperature of basic long core simulation was set to 80 \({}^{\circ}\)C; the pressure was set to 8 MPa; and the injection rate was set to 0.2 mL/min. Furthermore, simulations (Table 7) of different temperatures, pressures and injected gas purities were designed to obtain the variation in recovery and storage efficiency under different parameters values.