Formation Volume Factor and Density

The gas formation volume factor (B_g) relates the volume of gas in the reservoir to the volume on the surface (i.e., at standard conditions p_sc and T_sc). It is generally expressed in either cubic feet or barrels of reservoir volume per standard cubic foot of gas. Assuming a gas deviation factor of unity for the standard conditions, the reservoir volume of 1 std ft³ at reservoir pressure p and temperature T by Eq. (2.8) is

where p_sc is 14.7 psia and T_sc is 60°F:

The constants in Eq. (2.16) are for 14.7 psia and 60°F only, and different constants must be calculated for other standards. Thus for the Bell Field gas at a reservoir pressure of 3250 psia, a temperature of 213°F, and a gas deviation factor of 0.910, the gas volume factor is

These gas volume factors mean that 1 std ft³ (at 14.7 psia and 60°F) will occupy 0.00533 ft³ of space in the reservoir at 3250 psia and 213°F. Because oil is usually expressed in barrels and gas in cubic feet, when calculations are made on combination reservoirs containing both gas and oil, either the oil volume must be expressed in cubic feet or the gas volume in barrels. The foregoing gas volume factor expressed in barrels is 0.000949 bbl/SCF. Then 1000 ft³ of reservoir pore volume in the Bell Field gas reservoir at 3250 psia contains

G = 1000 ft³ ÷ 0.00533 ft³/SCF = 188 M SCF

Equation (2.8) may also be used to calculate the density of a reservoir gas. An expression for the moles of gas in 1 ft³ of reservoir gas pore space is p/zRT. By Eq. (2.6), the molecular weight of a gas is 28.97 × γ_g lb per mol. Therefore, the pounds contained in 1 ft³—that is, the reservoir gas density (ρ_g)—is

For example, the density of the Bell Field reservoir gas with a gas gravity of 0.665 is