Formation Volume Factor (Bg)

Introduction

The gas formation volume factor is used to relate the volume of gas, as measured at reservoir conditions, to the volume of the gas as measured at standard conditions, that is, 60°F and 14.7 psia. This gas property is then defined as the actual volume occupied by a certain amount of gas at a specified pressure and temperature, divided by the volume occupied by the same amount of gas at standard conditions. In equation form, the relationship is expressed as:

Where:

B_g = gas formation volume factor, ft³/scf

V_P,T = volume of gas at pressure P and temperature T, ft³

V_S = volume of gas at standard conditions

Applying the real gas equation , and subsituting for Volume V gives:

Where:

z_SC = Z-factor at standard conditions (or 1.0)

P_SC = standard pressure, often assumed to be 14.7

T_SC = standard temperature, degree 520 ^oR (or 60 ^oF)

Subsituting the values for standard conditions gives:

B_g = gas formation volume factor ft³/scf

B_g = gas formation volume factor bbl/scf

A plot of Gas B_g is shown below using the Dranchuk, Purivs, and Robinson z-correlation at a gas gravity of 0.65, and a reference temperature of 120^oF.

References

• Ahmed, Tarek, Equations of Stateand PVT Analysis: Applications for Improved Reservoir Modelling , 2007, Gulf Publishing.

• McCain, Willliam D., The Propertise of Petroleum Fluids, 1990, PennWell Books