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Evaluation of proposed correlation equations for coefficients
of isothermal compressibility of oil at reservoir pressures
greater than bubblepoint pressure
Several correlation equations reported in the petroleum literature
were evaluated with the data set summarized in table 3-11. The
3,051 lines of data were all at or above the bubblepoint pressures of
each of the 399 laboratory reports. The underlying samples were of
worldwide origin.
Table 3-11. Statistical summary of database used to evaluate oil compressibility correlations at
pressures greater than bubblepoint pressure (3,051 lines of data from 399 reservoir fluid studies
with worldwide origins). Data within 200 psi of the bubblepoint pressure were eliminated due to
extremely high errors caused by round-off errors for pressures this close to bubblepoint pressure.
Laboratory measurement
Minimum
Median
Mean
Maximum
Reservoir pressure,
p
, psia
414.7
3,714.7
3,786.5
14,915.0
Bubblepoint pressure,
p
b
, psia
120.7
2,085.7
2,346.3
7,854.0
Reservoir temperature,
T
R
, ºF
70.0
190.0
192.7
320.0
Stock-tank oil gravity,
API
, ºAPI
11.6
37.7
36.4
57.7
Separator gas specific gravity,
γ
gSP
0.561
0.791
0.804
1.798
Solution gas-oil ratio at
p
b
,
R
sb
, scf/STB
12.0
567.0
636.9
1,808.0
Oil compressibility,
c
ofb
,1/psi x 10
6
3.6
11.9
13.4
50.3
Often the authors of proposed oil compressibility correlation equations
do not specify whether the resulting values were
c
o
or
c
ofb
(the assumption
is that none were
c
oi
). If so, both cases were tried in the evaluation
process, and the one with the lowest ARE and AARE was selected for each
correlation. The results of the evaluation comparing calculated results
with the data described in table 3-11 are given in table 3-12.
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