Page 32 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

40 Reservoir Engineering

where y is a constant that depends on temperature, gas gravity and oil gravity.
Values of y are provided in Table 5-4.
Additional correlations suitable for use with programmable calculators are
discussed in a later section of this chapter.
Water Formation Volume Factor. The factors discussed that affected Bo also
affect the water formation volume factor, Bw. However, gas is only slightly soluble
in water so evolution of gas from water has a negligible effect on Bw. Expansion
and contraction of water due to reduction of pressure and temperature are slight
and offsetting. Hence, Bw is seldom greater than 1.06 [18] and is usually near
unity (see Table 5-5).
Several correlations for Bw are available, including the effect of gas saturation
in pure water and the effect of salinity [23], and the effect of natural gas on
Bw as a function of pressure and temperature [24]. However, since Bw is not
greatly affected by these variables, only a simplified correction is presented [MI:
Bw = (1 + AVT)(l + AVWJ (5-1 1)
where AVwp and AVwT are the volume changes caused by reduction in pressure
and temperature, respectively. Values of these corrections are given in Figures
5-16 and 5-17.

Fluid Compresslbilitles
Gas Compresslbility. The compressibility of a gas, which is the coefficient of
expansion at constant temperature, should not be confused with the com-
pressibility factor, z, which refers to the deviation from ideal gas behavior. From
the basic gas equation (see Equation 5-2), Muskat [25] provided an expression
€or the coefficient of isothermal compressibility:

Table 5-4
Values of Constant for Equation 5-10
Oil gravity T = 120°F T = 140°F
"API Gas gravity: 0.7 0.8 0.9 0.7 0.8 0.9
26 0.0494 0.0577 0.0645 0.0481 0.0563 0.0632
30 0.0568 0.0660 0.0737 0.0550 0.0635 0.0721
34 0.0654 0.0755 0.0842 0.0630 0.0736 0.0823
38 0.0752 0.0864 0.0962 0.0720 0.0841 0.0939
42 0.0865 0.0989 0.1099 0.0824 0.0961 0.1071
Oil gravlty T = 160°F T = 180°F
"API Gas gravity: 0.7 0.8 0.9 0.7 0.8 0.9
26 0.0453 0.0519 0.0591 0.0426 0.0481 0.0543
30 0.0522 0.0597 0.0677 0.0492 0.0557 0.0629
34 0.0601 0.0686 0.0775 0.0567 0.0645 0.0728
38 0.0692 0.0788 0.0887 0.0654 0.0747 0.0842
42 0.0797 0.0906 0.1 01 6 0.0755 0.0865 0.0975
From Reference 21.