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Properties of Natural Gas
Karianne Vågenes
TPG4140 Natural Gas
November 17, 2011
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Content
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Terminology
Classification
Composition
Specification
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Real Gas Law, z-factor
Corresponding states
Viscosity
Heat capacity
Summary
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Terminology
• Natural gas, C1-C5+, water, inert gases
• NGL (Natural Gas Liquids), under pressure
• LPG (Liquefied Petroleum Gas),
propane + butane, -42 C
• LNG (Liquefied Natural Gas), -162 C, 1 atm
• CNG (Compressed Natural Gas), 200-300 bara
• Condensate (liquid), C4-C7, transition gas-to-oil
• Oil, C6 and heavier fractions
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Terminology of Natural Gas
Jahn et al. (1998)
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Classification
• Reservoir conditions
– Non-associated (“dry gas”), produced alone
(T > Tcricondemtherm)
– Associated gas (“wet gas”), produced with oil
• Well, pipeline and process conditions
–
–
–
–
–
Rich gas, from production platform
Dry gas, no liquid fraction
“Wet gas”, NGL, no gas phase present
Condensate (Tcritical < T < Tcricondemtherm)
Dense phase (p > pcritical)
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Phase envelopes for reservoir fluids. C is critical point
Pedersen et al. (1989) Properties of Oils and Natural Gases, Gulf Publishing
Company
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Pressure/temperature phase envelopes for main hydrocarbon types
Jahn et al. 1998
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Compositions, wellhead conditions
R
o
j
e
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Composition, generalization
Non-Associated (dry gas)
Associated gas (wet gas)
Methane > 90 volume %
Methane < 90 volume %
Sweet gas
Sour gas
CO2 < 2 volume %
CO2 > 2 volume %
Sweet gas
Sour gas
H2S < 1 volume %
H2S > 1 volume %
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Composition, Processed Gas
Molecule
Troll (1)
Norway
Sleipner (2)
Norway
Draugen (3)
Norway
Groningen (4)
Netherlands
Methane
Ethane
Propane
Iso-Butane
N-Butane
C5++
Nitrogen
Carbon-dioxide
93.070
3.720
0.582
0.346
0.083
0.203
1.657
0.319
83.465
8.653
3.004
0.250
0.327
0.105
0.745
3.429
44.659
13.64
22.825
4.875
9.466
3.078
0.738
0.720
81.29
2.87
0.38
0,15
0.04
0.06
14.32
0.89
100
100
100
100
(1) After processing at Kollsnes (on-shore processing plant), average for November 2000.
(2) After off-shore processing into off-shore pipelines, combination of Sleipner East and West, average November 2000.
(3) After off-shore processing into pipeline Åsgard Transport to Kårstø (on–shore processing plant) for further processing, average for December 2000.
(4) Into onshore grid in The Netherlands.
Source: K. Jakobsen, A/S Norske Shell
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Specifications
• Transport Specification
–
–
–
–
Hydrocarbon dew point, 5-10 C below ambient
Water dew point, 5-10 C below HC dew point
Temperature, 30-50 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)
http://www.ipt.ntnu.no/~jsg/undervisning/prosessering/forelesninger/05Produktspesifikasjoner.pdf
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Properties Used in Equations
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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 calorific value and relative density
Hydrates and water, need water vapour in natural gas
(diagram or PVT package)
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Real Gas Law
pV  znRT
 p  Tsc  1 
Vsc  V    
 psc  T  z 
 p  Tsc  1 
qsc  q    
 psc  T  z 
zsc  1
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Density and FVF
pV  znRT
M

V
n
  M i yi
i
pM
zRT
V  m3 
B   FVF  


Vsc  Sm 3 
 T  psc 
B     z
 Tsc  p 
q  qsc B
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Finding z-factor
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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, RedlichKwong and Benedict-Webb-Rubin. Implemented in many
different computer programs, such as HYSYS, Prosper and
PVTsim.
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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 pseudoreduced pressure or temperature in multi-component
systems.
• Commonly used when gas properties (natural gas and
other gases) are to be correlated and/or presented.
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Reduced P & T
p
pr  p pr  
pc
T
Tr  T pr  
Tc
pc   pci yi
i
Tc   Tci yi
i
Kay’s Rule
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Calculate z-factor
P=125 bara
T=49 C
Pc=25 bara
Tc=38 C
Pr=P/Pc=5
Tr=T/Tc=1,29
•Z=0,73
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Rojey et al. (1997)
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Calculate density
From gas law:
pV  znRT
m
m
V  ,n 

M
pM

zRT
111 kg/m3
Given:
P=125 bara
M=18,3 kg/kmol
Z=0.73
R=8314 J/kmol.K
T=49 C
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Viscosity from Diagram
• Diagram shows viscosity against temperature for gas
components (methane, ethane, propane etc.) at
atmospheric pressure.
• Empirical equation (shown under) 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

1/ 2
y
M

 i i i
1/ 2
y
M
 i i
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Katz et al. (1959), fra Rojey et al. (1997)
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Katz et al. (1959), fra Rojey et al. (1997)
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Heat Capacity
C p    T   T 2
Cp
R
 A  BT  CT 2
 Cp 
3
6 2
   1, 702  9, 08110 T  2,164 10 T
 R CH 4
R  8, 314 (kJ / kmol.K )
C 
p CH
4
 0, 2047 1, 092 103  0, 260310 6 (kJ / kmol.K)
Når per mol bruk molfraksjon for blanding
Når per masse bruk massefraksjon for blanding
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Campbell (1984)
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Beggs (1984)
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Summary
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Terminology uses English-derived abbreviations, for example
such products as LPG and LNG.
Classification expresses origin and physical condition of
natural gas in reservoirs, wells, pipelines and processing
plants.
Phase envelope shows physical state of oil and gas at all
reservoir conditions and all processing conditions.
Natural gases have different composition, from production to
processing. Non-HC included (water, inert gases).
Specifications state requirements for transport and sale,
mainly composition and heating value, but also pressure and
temperature.
Volume specifications vary from country to country. Norway
uses 1 atm and 15 C (USA uses 1 atm and 60 F).
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• Several physical and thermodynamic properties of
natural gas are used in course.
• Real gas law and reduced pressure and temperature
used in diagrams.
• Empirical correlations used for transport properties, for
example for viscosity.
• Heat capacity can be obtained from figures.
• Equation Of State (EOS) used in computer programs for
pVT properties (also thermodynamic properties).
• Hysys available at NTNU (see info. on home page).
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References
• Beggs, H.D. (1984): Gas Production Operations, OGCI Publications,
Tulsa, Oklahoma
• Campbell, J.S. (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.
• 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 2004-214.
• 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.