1. TPG 4140 NTNU Natural Gas Properties
Professor Jon Steinar Gudmundsson
Department of Petroleum Engineering and Applied Geophysics
Norwegian University of Science and Technology
Trondheim
September 18, 2013

2. Outline Properties used in natural gas calculations
Phase diagrams and terminology (CCP & CCT)
Real gas law, z-factor, density and FVF
Corresponding states
Compressibility factor (z-factor)
Example calculation (GPA gas composition)
Heat capacity and heat capacity ratio
Viscosity of natural gas
Calorific values (GCV & NCV)
Summary

3. Properties Used Density, need z-factor and molecular weight
Flow in wells, need z-factor and viscosity
Pressure drop in pipelines, need density and viscosity
Temperature in pipelines, need heat capacity
Compressor power and exhaust temperature, need molecular weight and heat capacity ratio
Molecular weight, need relative density (gravity)
Reynolds number, need density and viscosity
Wobbe index, need gross calorific value and relative density

4. PHASE DIAGRAM
Pedersen et al. (1989) Properties of Oils and Natural Gases, Gulf Publishing Company

6. Rojey & Jaffret (1997)

7. Rojey & Jaffret (1997)

8. TERMINOLOGY
Natural gas, C1-C5+, water, inert gases
NGL (Natural Gas Liquids), under pressure
LPG (Liquefied Petroleum Gas), propan+butan, -42 C
LNG (Liquefied Natural Gas), -162 C, 1 atm
CNG (Compressed Natural Gas), bar
Condensate (liquid), C4-C7, transition gas-to-oil
Oil, C6 and heavier fractions

9. TYPICAL SPECIFICATIONS
Transport Specification
Hydrocarbon dew point, 5-10 C below ambient
Water dew point, about 5 C below HC dew point
Temperature, C
Pressure, depends on receiving terminal
Sales Specification (in addition to above)
Heating value (GHV = Gross Heating Value), MJ/Sm3
Wobbe Index (WI = GHV/(specific density)0,5
Removal of non-HC gasser (inert gases)

10. TYPICAL SPECIFICATIONS

11. COMPOSITION
Generalization
Non-Associated (dry gas) methane > 90 volume %
Associated gas (wet gas) methane < 90 volume %
Søtt gass (sweet gas) CO2 < 2 volume %
Surt gass (sour gas) CO2 > 2 volume %
Søtt gass (sweet gas) H2S < 1 volume %
Surt gass (sour gas) H2S > 1 volume %
Rojey & Jaffret (1997) fra Valais (1983)

12. Volume Rate at s.c. to Mass Rate
Compress 7 MSm3/d from 80 to 160 bara
Inlet temperature 30 C
Molecular weight M = kg/kmol
Atm. Pressure bara
At s.c., ρ = kg/Sm3 (T = 15 C)
Mass flow rate 7∙0.864 = 6.05∙106 kg/d
Per second m = 70 kg/s
M and ρ from Excel sheets

13. Real Gas Law

14. Density and FVF

15. Finding Real Gas Factor (z-factor)
Diagram based on corresponding states, reduced pressure and temperature for single components and pseudo-reduced pressure and temperature for natural gas.
Empirical equations matched to z-factor diagram for natural gas. Uses many constants and coefficients and in some cases iteration.
Equation Of State (EOS) such as Peng-Robinson, Redlich-Kwong and Benedict-Webb-Rubin. Implemented in many different computer programs.
EOS implemented in many commercial computer packages such as Hysys, Prosper and PVTsim.

16. Real Gas Factor vs. Pressure at 25 C
Fletcher (1993)

17. Real gas factor with reduced p & T
Fletcher (1993)

18. Reduced p & T
Kay’s Rule

19. Corresponding States
When pressure and temperature are normalized using critical pressure and temperature, then all properties become the same/similar, irrespective of composition.
Normalized pressure or temperature are called reduced pressure or temperature in one component systems.
Normalized pressure or temperature are called pseudo-reduced pressure or temperature in multi-component systems.
Commonly used when gas properties (natural gas and other gases) are to be correlated and/or presented.

20. GPA (1998)

21. Rojey o.a. (1997)

22. Equations and Gravity
Thomas o.a. (1970), fra Rojey o.a. (1997)

23. Physical Constants of Natural Gas
Campbell (1984)

24. Example Calculation for Natural Gas
GPA (1998)

25. GPA (1998) Gas Composition (Excel sheet)
Components
Molecular weight
Mole fraction
Tci
Pci
g/mole yi oR K
psia
Mpa
Methan, CH4
16,042
0,8319
343
190
667,8
4,61
Ethan, C2H6
30,07
0,0848
549,8
305
708
4,88
Propan, C3H8
44,10
0,0437
665,7
370
616
4,25
i-Butane, C4H10
58,12
0,0076
734,7
408
529
3,65
n-Butane, C4H10
0,0168
765
425
551
3,80
i-Pentane C5H12
72,15
0,0057
829
460
491
3,39
n-Pentane C5H12
0,0032
845
469
489
3,37
Hexane C6H14
86,18
0,0063
913
507
437
3,01
Heptane C7H16
100,21
972
540
397
2,74
Hydogen, H2
2,02 60 33
187
1,29
Nitrogen, N2
28,01
227,4
126
492
Oxygen, O2
32,00
277,8
154
731
5,04
Carbon dioxid, CO2
44,01
547,6
304
1071
7,38
Hydrogensulfid, H2S
34,08
672,4
373
1306
9,01
Dihydrogenoksid, H2O
18,02
1165
647
3199
22,06
Σ Mole fraction
1,0000
Total molecular weight gas
20,43

26. Phase Envelope for GPA (1998) Gas

27. Beggs (1984)

28. Campbell (1984)

29. Heat Capacity

30. Smith o.a. (1996)

31. Heat Capacity Natural Gas Temperature, Pressure, Relative Density
b = 1.014
c =
d = 2.170
e = 1.015
f =
Moshfeghian, M. (2013)

32. Heat Capacity Ratio M = 20.43 kg/kmol T = 30 C = 86 F p = 80 bara
k = 1.26 (from diagram)
R = kJ/kmol.K
z = 0.783
k = 1.26 (from “old” equation = ideal gas)
k = 1.19 (from “new” equation = real gas)

33. Ratio of Specific Heat Capacity

34. Heat Capacity Ratio Hysys Estimation
GPA (1998) Associated Gas

35. Comparison of Heat Capacity Ratio GPA (1998) Gas at 80 bara & 30 Ck
Diagram
1,26
Ideal gas
Ideal gas with z-correction
1,19
Hysys ideal gas
1,158
Hysys
1,714

36. Ideal Gas Compression m (kg/s) 70 M (kg/kmol) 20,43 R (J/kmolK)
8314,34
T innløp C 30
k (Cp/Cv)
1,26
p innløp bara 80
p utløp bara
160
P (MW)
6,44
T utløp C 77

37. Real k Gas Compression m (kg/s) 7 M (kg/kmol) 20,43 R (J/kmolK)
8314,34
T innløp C 30
k (Cp/Cv)
1,19
p innløp bara 80
p utløp bara
160
P (MW)
6,33
T utløp C 65

38. Viscosity from Diagram
Diagram shows viscosity against temperature for gas components (methane, ethane, propane etc.) at atmospheric pressure.
Empirical equation (shown under, based on kinetic theory of gases) gives estimate of viscosity to natural gas (mixture of methane, ethane, propane etc.) at atmospheric pressure.
Diagram gives viscosity ratio to viscosity at atmospheric pressure against reduced pressure and temperature

39. Katz o.a. (1959), fra Rojey o.a. (1997)

40. Katz o.a. (1959), fra Rojey o.a. (1997)

41. Viscosity Correlation
Several correlations in literature, for example Carr et al. (1954), Lee et al. (1966) and Pedersen et al. (1987).
Lee et al. (1966) correlation has recently been evaluated by Jeje and Mattar (2004) and has the form shown below. K, X and y are given by empirical equations.
The correlation is available as spreadsheet.

42. Combined Cycle Power Plant

44. Combustion and Calorific Value

45. Summary
Several physical and thermodynamic properties of natural gas are used in natural gas calculations.
Real gas law and reduced pressure and temperature used in diagrams to obtain z-factor.
Empirical correlations used for transport properties, for example for viscosity.
Heat capacity at constant pressure can be obtained from figures and equations. Also as function of pressure, temperature and relative density.
Heat capacity ratio values uncertain.
Equation Of State (EOS) used in computer programs for pVT properties (also thermodynamic properties).

46. References
Beggs, H.D. (1984): Gas Production Operations, OGCI Publications, Tulsa, Oklahoma
Campbell, J.M. (1994): Gas Conditioning and Processing, Campbell Petroleum Series, Norman, Oklahoma.
Fletcher, P. (1993): Chemical Thermodynamics, Longman, Harlow, Essex.
Gas Processors Association (1998): Engineering Data Book, Tulsa, Oklahoma.
Gupta, N. (2011): Overview of Ormen Lange Project, Guest Lecture, TPG 4140 Natural Gas, NTNU.
Jeje. O. & Mattar, L. (2004): Comparison of Correlations for Viscosity of Sour Natural Gas, 5th Canadian International Petroleum Conference, Calgary, Alberta, June 8-10, Paper
Moshfeghian, M. (2013): Variation of Natural Gas Heat Capacity with Temperature, Pressure and Relative Density, J.M. Campbell & Co., (Internett February 2013).
Rojey, A. (1997): Natural Gas, Éditions Technip, Paris.
Smith, J.M., Van Ness, H.C. & Abbott, M.M. (1996): Introduction to Chemical Engineeering Thermodynamics, McGraw-Hill, New York.