Page 27 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Basic Principles, Definitions, and Data 15

Since one lb mole is equivalent to 379 ft3 at 60°F and 14.7 psia [15]:

1
- 10.73
X
B, = 379 % = 0.00504- zTi7 (5-3)
5.615 p P
In this expression, Bg will be in reservoir barrels per standard ft3 (RB/scf).
Gas formation volume factor can also be expressed in units of reservoir barrels
per stock tank barrel or ft3 of gas at reservoir conditions per ft3 of gas at
standard conditions:

B, = 0.02827(460+TR)Z (5-4)
P

Because the gas formation volume factor can be expressed in so many
different units (including the reciprocal of BB), caution should be exercised when
Bg is used. In much of the recent petroleum literature, notably SPE, B, is
expressed in RB/scf. If units of ft3/scf are given, Bg can be divided by 5.615 or
multiplied by 0.1781 to get RB/scf.
Gas formation volume factors can be estimated by determining the gas
deviation factor or compressibility factor, z, at reservoir pressure, p, and
temperature T, from the correlations of Standing and Katz [16] (Figure 5-13).
To obtain the z factor, reduced pressure, pr, and reduced temperature, T,
are calculated:

P
Pr =- (5-5)
PC
where p, is the critical pressure and

T
T, = -
Tc (5-6)
where TE is the critical temperature. The critical pressure and temperature
represent conditions above which the liquid and vapor phase are indistinguishable.
Compressibility factor and gas formation volume factor can be more con-
veniently estimated by the use of programs available for hand-held calculators.
These programs will be subsequently discussed.
011 Formatlon Volume Factor. The volume of hydrocarbon liquids produced
and measured at surface conditions will be less than the volume at reservoir
conditions. The primary cause is the evolution of gas from the liquids as
pressure is decreased from the reservoir to the surface. When there is a
substantial amount of dissolved gas, a large decrease in liquid volume occurs.
Other factors that influence the volume of liquids include changes in tem-
perature (a decrease in temperature will cause the liquid to shrink) and pressure
(a decrease in pressure will cause some liquids to expand). All of these factors
are included in the oil formation volume factor, Bo, which is the volume of oil
in reservoir barrels, at the prevailing reservoir conditions of pressure and