You must log in to edit PetroWiki. Help with editing
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
Message: PetroWiki content is moving to OnePetro! Please note that all projects need to be complete by November 1, 2024, to ensure a smooth transition. Online editing will be turned off on this date.
Gas formation volume factor and density
Formation volume factor (FVF) is a useful relationship for relating gas volumes in the reservoir to the produced volume at standard conditions. Formation volume factor also enables the calculation of density. This page discusses calculation of FVF and density.
Formation volume factor
The formation volume factor of gas is defined as the ratio of the volume of gas at the reservoir temperature and pressure to the volume at the standard or surface temperature and pressure (ps and Ts). It is given the symbol Bg and is often expressed in either cubic feet of reservoir volume per standard cubic foot of gas or barrels of reservoir volume per standard cubic foot of gas. The gas deviation factor is unity at standard conditions; hence, the equation for the gas formation volume factor can be calculated using the real gas equation:
....................(1)
The n divides out here because both volumes refer to the same quantity of mass.
When ps is 1 atm (14.696 psia or 101.325 kPa) and Ts is 60°F (519.67°R or 288.71°K), this equation can be written in three well-known standard forms:
....................(2)
where rcf/scf = reservoir cubic feet per standard cubic feet, RB = reservoir barrels, and Rm3/Sm3 = reservoir cubic meters per standard cubic meters. The formation volume factor is always in units of reservoir volumes per standard volumes.
The three forms in Eq. 2 are for specific units. In the first two equation forms, the pressure is in psia and the temperature is in °R. In the third form, the pressure is in kPa and the temperature is in K.
Density
The density of a reservoir gas is defined as the mass of the gas divided by its reservoir volume, so it can also be derived and calculated from the real gas law:
....................(3)
Nomenclature
| Bg | = | gas formation volume factor (RB/scf or Rm3/Sm3) |
| mg | = | mass of gas, kg |
| Ma | = | molecular weight of air |
| Mg | = | average molecular weight of gas mixture |
| n | = | number of moles |
| p | = | absolute pressure, Pa |
| psc | = | pressure at standard conditions, Pa |
| R | = | gas-law constant, J/(g mol-K) |
| T | = | absolute temperature, K |
| Tsc | = | temperature at standard conditions, K |
| V | = | volume, m3 |
| VR | = | volume of gas at reservoir temperature and pressure, m3 |
| Vsc | = | volume at standard conditions, m3 |
| z | = | compressibility factor (gas-deviation factor) |
| zsc | = | compressibility factor at standard conditions |
| γg | = | specific gravity for gas |
| ρg | = | density of gas, kg/m3 |
Noteworthy papers in OnePetro
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
External links
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro
See also
Isothermal compressibility of gases