Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information
Calculating gas properties
This page provides a number of examples that illustrate the mathematical calculations behind the different fundamental gas properties.
Calculating properties of natural gas
- γg = 0.7
- H2S = 7%
- CO2 = 10%
- p = 2,010 psia
- T = 75°F.
The density is calculated from Eq. 3 in Gas formation volume factor and density:
The formation volume factor is calculated from Eq. 2 in Gas formation volume factor and density:
- First, the viscosity for Mg = (0.7)(28.967) = 20.3 at p = 1 atm and T = 75°F is read from Fig. 2.
- This gives 0.0102 cp, but corrections are needed for the acid gases. The correction for 10% CO 2 is 0.0005 cp, and the correction for 7% H2S is 0.0002 cp. Hence, this gives μga = 0.0109 cp.
- Next, the ratio of μg/μga is read from Fig. 4, which gives μg/μga = 1.55.
- Hence, μg = (1.55) (0.0109 cp) = 0.0169 cp.
The compressibility is determined by first reading Figs. 1-2 in Isothermal compressibility of gases for the previously calculated values of pr = 3.200 and Tr = 1.500 to give crTr = 0.5. Because Tr = 1.500 then cr = 0.5/1.5 = 0.3333. Because cr = cg ppc,
Calculating the relative density (specific gravity)
Calculate the relative density (specific gravity) of natural gas with the following composition (all compositions are in mol%):
First, calculate the apparent mole weight from the information presented in Table 1.
where the molecular weight of air, Ma, is 28.967.
Calculating actual density
Calculate the actual density of the same mixture at 1,525 psia and 75°F
The density is calculated from
- p = 1,525 psia
- Mg = 20.424
- R = 10.7316 (psia-ft3)/(lbm mol°R)
- T = 75°F + 459.67 = 534.67°R
- z must be obtained from Fig. 2 in Real gases
1. Calculate zg from the known composition in Table 2.
Using Kay’s rules, we obtain from the known gas composition:
Tpc =ΣyiTi = 393.8°R,
Tpr = 534.67/393.8 = 1.3577,
ppc =Σyipci = 662.88 psia,
ppr = p/ppc = 1,525/662.88 =2.301,
and from Fig. 1, zg = 0.71.
From Fig. 2 in Real gases, we obtain zg = 0.745.
3. Using the Piper et al. method, we first calculate J and K using
The details of the calculations are found in Table 2.
Finally, looking up the z-factor chart (Fig. 2 in Real gases) gives z = 0.745.
Even though the Sutton correlation and the Piper et al. correlation gave slightly different critical properties, the z factors from those two methods are the same. Kay’s rule gives a value that is 4.6% lower, but the result using Sutton’s correlation and the Piper et al. correlation has been shown to be more accurate. The density is then given by
Calculating the z factor for a reservoir fluid
Calculate the z factor for the reservoir fluid in Table 3 at 307°F and 6,098 psia.
The experimental value is z = 0.998.
Using the Piper et al. method, we first calculate J and K using
The details of the calculation are in Table 4.
Finally, looking up the z-factor chart (Fig. 2 in Real gases) gives z = 1.02. This represents a 2% error with the experimental value.
|J||=||parameter in the Stewart et al. equations, K•Pa–1|
|K||=||parameter in the Stewart et al. equations, K•Pa–1/2|
|Ma||=||molecular weight of air|
|=||molecular weight of C7+ fraction|
|Mg||=||average molecular weight of gas mixture|
|n||=||number of moles|
|p||=||absolute pressure, Pa|
|pc||=||critical pressure, Pa|
|ppc||=||pseudocritical pressure of a gas mixture, Pa|
|R||=||gas-law constant, J/(g mol-K)|
|T||=||absolute temperature, K|
|Tc||=||critical temperature, K|
|Tci||=||critical temperature of component i in a gas mixture, K|
|Tpc||=||corrected pseudocritical temperature, K|
|z||=||compressibility factor (gas-deviation factor)|
|ρpc||=||relative density of C7+ fraction|
|μg||=||viscosity of gas, Pa•s|
|Bg||=||gas formation volume factor (RB/scf or Rm3/Sm3)|
|μga||=||viscosity of gas mixture at desired temperature and atmospheric pressure, Pa•s|
|cg||=||coefficient of isothermal compressibility|
|cr||=||dimensionless pseudoreduced gas compressibility|
|J||=||parameter in the Stewart et al. equations (Eqs. 5.9 and 5.10), K•Pa–1|
|K||=||parameter in the Stewart et al. equations (Eqs. 5.9 and 5.10), K•Pa–1/2|
|ρg>||=||density of gas, kg/m3|
- Carr, N.L., Kobayashi, R., and Burrows, D.B. 1954. Viscosity of Hydrocarbon Gases Under Pressure. J Pet Technol 6 (10): 47-55. SPE-297-G. http://dx.doi.org/10.2118/297-G
- Kay, W.B.: "Density of Hydrocarbon Gases at High Temperature and Pressure," Ind. Eng. Chem. (September 1936) 28, 1014–1019.
- Sutton, R.P.: "Compressibility Factors for High-Molecular-Weight Reservoir Gases," paper SPE 14265 presented at the 1985 SPE Annual Technical Conference and Exhibition, Las Vegas, Nevada, 22–25 September.
- Piper, L.D., McCain, W.D. Jr., and Corredor, J.H.: "Compressibility Factors for Naturally Occurring Petroleum Gases," paper SPE 26668 presented at the 1993 SPE Annual Technical Conference and Exhibition, Houston, 3–6 October. Cite error: Invalid
<ref>tag; name "r4" defined multiple times with different content
- Stewart, W.F., Burkhardt, S.F., and Voo, D.: "Prediction of Pseudocritical parameters for Mixtures," presented at the 1959 AIChE meeting, Kansas City, Missouri, 18 May. 
Noteworthy papers in OnePetro
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro