Geoscience Reference
In-Depth Information
Ramey suggested that the derivatives of B
o
and
R
s
for equation (3.16a)
be evaluated using the best available correlation equations for each
independent variable rather than using a separate correlation for oil
compressibility,
c
o
.
27
Only one correlation equation has been proposed
in the petroleum literature for this fluid property; McCain et al.
28
Evaluation of proposed correlation equations for coefficients
of isothermal compressibility at reservoir pressures less than
bubblepoint pressure
The data set described in table 3-9 was used to calculate values of
c
o
at
pressures below bubblepoint pressure for comparison with the McCain
et al. equation and the Ramey suggestion. Equation (3.16b) was used to
determine values of
c
o
from the data.
Equation (3.16a) was used to calculate values of
c
o
from the
correlations. The solution gas-oil ratio derivative, ∂
R
s
/∂
p
, was obtained
with equations (3.8a)-(3.8f). The formation volume factor derivative,
∂
B
o
/∂
p
, was obtained with equation (3.21b), with the necessary
derivative of reservoir oil density, ∂
ρ
oR
/∂
p
, coming from equations
(3.19a)-(3.19g). Oil formation volume factor,
B
o
, can be obtained with
equation (3.21b), and gas formation volume factor,
B
g
, can be obtained
with equation (2.12).
The results of the evaluations are given in table 3-13.
Table 3-13. Performance of correlation equations for coefficients of isothermal compressibility of
oils at reservoir pressures below bubblepoint pressure reveals the preferred procedure.
Predicted coefficients of isothermal compressibility of oils
ARE, %
Correlation
AARE, %
Equation (3.16a) with equations (3.17a)-(3.17h)
3.85
10.25
McCain et al. (1988)
16.02
18.41
Figures 3-13, 3-14, and 3-15 show how this procedure holds up
across the distribution of independent variables. The independent
variables were sorted and sliced into subsets with approximately 450
lines of data in each subset.
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