Gas Formation Volume Factor
The gas formation volume factor can be obtained by relating the gas volume under reservoir
conditions to the gas volume under standard conditions by means of the real gas equation of state.
By substituting for the gas volume using the real gas equation of state, the formation volume factor
Bg can be expressed as:
(GFVF1)
Where:
Vp,T = Volume of gas at pressure p and temperature T, ft3
Bg = gas formation volume factor, rcf/scf
z = Gas compressibility factor
T = Reservoir Temperature, oF
p = Reservoir Pressure, psia
The subscript ‘sc’ indicates standard conditions.
By assuming standard conditions of 14.7 psia and 60oF (520oR), this equation reduces to:
(GFVF2)
Gas Viscosity
The correlation proposed by Lee, Gonzalez & Eakin can be used to calculate the viscosity of natural
gases. This semi-empirical correlation is given as:
[
]
(GasVisc1)
Where:
µg = Gas viscosity, cp
ρg = Gas density at the reservoir pressure and temperature, g/cc
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The factors K, X and y are given as:
(
)(
)
)
(
(GasVisc2)
(GasVisc3)
(
)
Where:
T = Reservoir temperature, oF
Mwa = Apparent molecular weight of the natural gas mixture
The above method is capable of predicting viscosities with standard and maximum deviations of
2.7% and 8.99% respectively. This method is however not applicable for sour gases and is less
accurate for higher specific gravity gases.
Gas Density
The density of natural gas can be calculated from:
(GasDens1)
Where:
ρgas = Gas density, lb/ft3
p = Reservoir pressure, psia
T = Reservoir Temperature, oR
Z = Gas compressibility factor
γ = Gas specific gravity
References:
Gas Viscosity:
Lee, A.L., Gonzalez, M.H., and Eakin, B.E.: “The Viscosity of Natural Gases,” JPT (August 1966),
997-1000
Gas Formation Volume Factor/Gas Density:
Ahmed T, Reservoir Engineering Handbook, 4th Edition, © 2010 Elsevier Inc.
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