Page 146 - PVT Property Correlations

Volatile Oils Chapter | 6 123

Estimation of initial producing GOR is also more difficult in volatile oils.
The initial producing GOR is sensitive to separator conditions. Ibrahim et al.
(2011) reports field cases that show significant variation in producing GOR
when separator conditions change. These cases were reported for wells pro-
ducing above the bubble point pressure (i.e., the producing GOR is not
affected by free gas production from the reservoir). The total initial solution
GOR is needed as input to the correlations. In case the stock-tank GOR is
not measured, it needs to be estimated from the correlations presented in the
Black Oil Chapter. Fig. 6.3 shows the GOR for the stock-tank and low pres-
sure separator for many oil samples (covering the range of volatile oils, black
oils, and low GOR oils). The total surface GOR is higher for more volatile
oil (low C7 1 mole percent oil). The amount of gas separated in low pres-
sure separator and stock-tank can be considerable in volatile oil. When esti-
mating the total solution GOR in volatile oils, care should be taken to use
the oil correlations. The low pressure separator and stock-tank GOR can
change significantly and are strong functions of separator conditions.
An alternative approach for volatile oil PVT prediction is the MBO
approach. This approach has the advantage of estimating the vaporized
oil gas ratio. The MBO correlations for volatile oil can be applied to calcu-
late the four MBO PVT functions (oil and gas formation volume factors,
solution GOR, and vaporized oil gas ratio). The MBO correlations for vola-
tile oil are listed in Table 6.1, and the forms of the correlations are given in
Appendix A, Oil Correlations Formulae.

FIGURE 6.3 Stock-tank and low pressure separator GOR. GOR, gas oil ratio.