Reservoir EngineeringPVT Properties
Gas Formation Volume Factor Formula
Gas formation volume factor converts gas volume between reservoir conditions and standard surface conditions using pressure, temperature, and gas z-factor.
How engineers use this formula
Use it in PVT, material balance, gas-in-place, and reservoir engineering calculations that require Bg.
Assumptions
- Gas z-factor is known or estimated separately.
- Pressure and temperature units match the equation form.
- Gas behavior is represented by the real-gas relationship used by the formula.
Limitations
- Does not calculate z-factor from gas composition.
- Does not handle condensate dropout or compositional phase behavior.
Common mistakes
- Using Fahrenheit instead of absolute temperature where required.
- Mixing psia and psig.
- Using an assumed z-factor outside its valid pressure-temperature range.
Default example
Using the default inputs, B_g equals 0.004806 ft³/SCF.
zdimensionless
0.85
T°R
600
Ppsia
3000
Inputs
z
dimensionlessGas Compressibility Factor
T
°RReservoir Temperature
P
psiaReservoir Pressure
Outputs
B_g
ft³/SCF
Gas Formation Volume Factor
z
dimensionless
Gas Compressibility Factor
T
°R
Reservoir Temperature
P
psia
Reservoir Pressure
Source and review
reviewedAhmed, T. 2007. Equations of State and PVT Analysis, Natural Gas Properties, Equation 3-52.
Source