PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 39

of stock-tank gas is rarely measured in the field. Apparently only one
publication has addressed the problem of estimating the specific
gravities, γ gST , of stock-tank gases. 4

(3.5a)

where

(3.5b)

and

(3.5c)

n VAR n C0 n C1 n C2 n C3 n C4 n

1 ln p SP -17.275 7.9597 -1.1013 2.7735 x 10 -2 3.2287 x 10 -3

2 ln R SP -0.3354 -0.3346 0.1956 -3.4374 x 10 -2 2.08 x 10 -3

3 API 3.705 -0.4273 1.818 x 10 -2 -3.459 x 10 -4 2.505 x 10 -6

4 γ gSP -155.52 629.61 -957.38 647.57 -163.26

5 T SP 2.085 -7.097 x 10 -2 9.859 x 10 -4 -6.312 x 10 -6 1.4 x 10 -8

Use of equation (3.5) requires knowledge of separator pressures and
temperatures. If this information is not available, equation (3.6) can
be used. 5

(3.6)

Evaluation of proposed correlation equations for weighted-average
surface-gas specific gravities
These two methods were evaluated with the data set described in
table 3-1, and the results are given in table 3-4. Only 618 of the lines
of data could be used because several of the laboratory reports did not
report the stock-tank gas specific gravity.

Table 3-4. Average errors in estimates of weighted-average surface-gas specific gravities are well
within expected experimental error.

Predicted weighted-average surface-gas specific gravities
ARE, % Correlation AARE, %

Valko-McCain (2003) equations (3.5), (3.2), and (3.4) -0.7 2.2

Valko-McCain (2003) equation (3.6) 0.0 3.8