1 Apr, 2018
Impact of CO2 on Natural Gas Density
Due to the importance of CO2 injection for enhanced oil recovery and the increasing interest in CO2 capture and sequestration, this study was undertaken to prepare simple charts for accurately estimating the density for hydrocarbon systems containing nil to 100% CO2.
The September 2008 Tip of the Month (TOTM)  evaluated the accuracy of Katz  and Wichert-Aziz  shortcut methods for predicting sour and acid gas density. The tip demonstrated that for binary mixtures of CH4 and CO2, the Wichert-Aziz method gives a more accurate result for CO2 content of between 10 and 90 mole percent.
The October 2008 TOTM  evaluated the accuracy of density calculations using two process simulation software packages, NIST REFPROP program , the GERG-2004 equation of state , and AGA 8 method  (in addition to the above shortcut methods) against experimental data. An experimental database was used for the basis of comparison. The sources of experimental data were GPA RR-138  and GPA RR 68 . Table 1 of the October TOTM indicated that REFPROP and GERG 2004 give equally the best results.
In continuing the September and October 2008 TOTMs, this study was undertaken to prepare simple charts for accurate estimation of the density of hydrocarbon systems containing nil to 100% CO2. The charts present density of CO2 + light hydrocarbons mixtures as a function of pressure and CO2 concentration for four isotherms of -50 °C, 0 °C, 50 °C, and 100 °C (-58 °F, 32 °F, 122 °F, 212 °F). The pressure range was from 0.5 MPa to 30 MPa (72.5 psia to 4350 psia) and the CO2 concentrations range was from 0 to 100 mole % (0, 10, 30, 50, 70, 90, 100 mole%). Table 1 presents the composition of systems studied. The default equations in REFPROP are used to calculate the phase boundaries and densities .
Table 1. Composition of CO2 + light hydrocarbons systems
The performance of REFPROP program extracted from Table 1 of the October 2008 TOTM  is presented in Table 2. This table indicates that the average absolute percent error (AAPE) and average percent error (APE) are 0.46 and 0.23, respectively. Due to the high accuracy of REFPROP program, this TOTM will use REFPROP to calculate the density of CO2 + light hydrocarbon systems.
Table 2. Summary of error analysis for the binary system of CH4 + CO2 density prediction by REFPROP program
We replotted the experimental density data reported in the GPA RR-138  and GPA RR 68  to demonstrate the accuracy of REFPROP program. The results of this evaluation are shown in Figures 1A through 5A (Appendix A), for CO2 content of 9.83 to 100 mole percent and the temperature and pressure ranges of Table 2...
Next, we plotted the calculated density by REFPROP as a function of pressure and CO2 content for four temperatures in Figures 1 through 4.
Figure 1. Variation of density with pressure and CO2 concentration at -50 °C (-58 °F)
Figure 2. Variation of density with pressure and CO2 concentration at 0 °C (32 °F)
According to REFPROP the dashed lines in Figure 1 present the two-phase region for all CO2 concentrations except for the case of 100 mole % which is in the gas phase. All solid lines present the liquid phase region. REFPROP indicated that in Figure 2:
►For CO2 concentrations of 0, 10, and 30 mole %, the dashed lines present gas phase for pressures up to 2.5 MPa (362.5 psia) and two-phase for pressures more than 2.5 MPa (362.5 psia). The solid lines present the supercritical region.
►For CO2 concentrations of 50, 70, and 90 mole %, the dashed lines present gas phase for pressures up to 2 MPa (290 psia) and two-phase for pressures more than 2 MPa (290 psia). The solid lines present the liquid phase.
►For CO2 concentration of 100 mole %, the dashed line presents gas phase. The solid line presents the liquid phase.
REFPROP also indicated that all dashed lines in Figures 3 and 4 present the gas phase region and all solid lines present the supercritical region.
Figure 3. Variation of density with pressure and CO 2 concentration at 50 °C (122 °F)
Figure 4. Variation of density with pressure and CO2 concentration at 100 °C (212 °F)
Based on the work done in this TOTM, the following can be concluded:
►CO2 concentration has a great impact on the mixture density. As CO2 concentration increases the mixture density increases.
►REFPROP is relatively accurate for density calculations of pure CO2 and mixtures of light hydrocarbons and CO2 (Table 2 and Figures 1A-5A).
►Simple density charts are presented for accurate estimation of a natural gas (relative density 0.65) as a function of pressure and CO2 concentration for four temperatures (Figures 1-4). These charts are composition specific (Table 2), similar charts should be developed for different compositions.
►Knowledge of phase boundaries and behavior is essential for density calculation.
To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Practical Computer Simulation Applications in Gas Processing), and G6 (Gas Treating and Sulfur Recovery) courses.
PetroSkills offers consulting expertise on this subject and many others. For more information about these services, visit our website at http://petroskills.com/consulting, or email us at consulting@PetroSkills.com.
By: Dr. Mahmood Moshfeghian
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- Moshfeghian, M., “How good are the shortcut methods for sour gas density calculations?,” PetroSkills tip of the month, Sep 2008
- Standing, M.B. and Katz, D.L.; “Density of Natural gas gases,” AIME Trans., 146, 140-49 (1942)
- Wichert, E. and Aziz, K., Hydr. Proc., p. 119 (May 1972).
- Moshfeghian, M., “How good are the detailed methods for sour gas density calculations?,” PetroSkills tip of the month, Oct 2008
- Lemmon, E.W., Huber, M.L., McLinden, M.O. NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP, Version 9.1, National Institute of Standards and Technology, Standard Reference Data Program, Gaithersburg, 2013.
- Kunz, O., Klimeck, R., Wagner, W., and Jaeschke, M. “The GERG-2004 Wide-Range Equation of State for Natural Gases and Other Mixtures,” GERG Technical Monograph 15 (2007)
- K. E. Starling, et al., “Self-Consistent Correlation of Thermodynamic and Transport Properties,” GRI/AGA Project No. Br-1111; OU-ORA Project No. 2036 156-716. Report: GR/AGA/BR-1111/77-36.
- Hwang, C-A., Duarte-Garza, H., Eubank, P. T., Holste, J. C. Hall, K. R., Gammon, B. E., March, K. N., “Thermodynamic Properties of CO2 + CH4 Mixtures,” GPA RR-138, Gas Processors Association, Tulsa, OK, June 1995
- Hall, K. R., Eubank, P. T., Holste, J., Marsh, K.N., “Properties of C02-Rich Mixtures Literature Search and Pure CO2 Data, Phase I,” GPA RR-68, A Joint Research Report by Gas Processor Association and the Gas Research Institute, Gas Processors Association, Tulsa, OK, June 1985
Figure 1A. REFPROP (solid line) and experimental (symbols)  density for binary mixture CH4 + CO2 (9.93 mole% CO2)
Figure 2A. REFPROP (solid line) and experimental (symbols)  density for binary mixture CH4 + CO2 (29.11 mole% CO2)
Figure 3A. REFPROP (solid line) and experimental (symbols)  density for binary mixture CH4 + CO2 (66.82 mole% CO 2)
Figure 4A. REFPROP (solid line) and experimental (symbols)  density for binary mixture CH4 + CO2 (90.11 mole% CO2)
Figure 5A. REFPROP (solid line) and experimental (symbols)  density for 100 mole% CO 2