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Compressibility Of Natural Gases

Knowledge of the variability of fluid compressibility with pressure and temperature is essential in performing many reservoir engineering calculations. For a liquid phase, the compressibility is small and usually assumed to be constant. For a gas phase, the compressibility is neither small nor constant. By definition, the isothermal gas compressibility is the change in volume per unit volume for a unit change in pressure or, in equation form:

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Cg = 1/ p – 1/Z (ƏZ / ƏP) at constant temp.

For ideal gas Z=1 and (ƏZ / ƏP) =0, therefore:

Cg = 1/ p

Gas Formation Volume Factor

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, i.e., 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 an equation form, the relationship is expressed as:

Bg = (Vp,T / V sc)


Bg = gas formation volume factor, ft3/scf

Vp,T = volume of gas at pressure p and temperature, T, ft3

Vsc = volume of gas at standard conditions, scf

Applying the real gas equation-of-state and substituting for the volume V, gives

Bg = (Psc / Tsc) (ZT/P)

Where Zsc = Z – factor at ideal condition= 1.0

Psc and Tsc are pressure and temperature at standard condition.

Assuming that the standard conditions are represented by psc =14.7 psia and Tsc = 520, the above expression can be reduced to the following relationship:

Bg = .0287 (zT/ P)


Bg = gas formation volume factor, ft3/scf

Z = gas compressibility factor

T = temperature, °R

Gas Viscosity

The viscosity of a fluid is a measure of the internal fluid friction (resistance) to flow. If the friction between layers of the fluid is small, i.e., low viscosity, an applied shearing force will result in a large velocity gradient. As the viscosity increases, each fluid layer exerts a larger frictional drag on the adjacent layers and velocity gradient decreases. The viscosity of a fluid is generally defined as the ratio of the shear force per unit area to the local velocity gradient. Viscosities are expressed in terms of poises, centipoises, or micro poises. One poise equals a viscosity of 1 dyne-sec/cm2 and can be converted to other field units by the following relationships:

1 poise = 100 centipoises

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