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Coefficients of isothermal compressibility, tangent at reservoir
pressures above bubblepoint pressure,
c
o
, can be estimated with
equations (3.15a)-(3.15c).
(3.15a)
The derivative in equation (3.15a) may be obtained using equation
(3.13), and the value of Z for equation (3.15b) should be calculated using
equation (3.13b).
(3.15b)
(3.15c)
Coefficients of isothermal compressibility of oil at reservoir
pressures less than bubblepoint pressure
Martin defined the coefficient of isothermal compressibility at
pressures below the bubblepoint pressure as equation (3.16a).
26
(3.16a)
This equation takes into account the change in the volume of oil as
well as the change in the volume of free gas, both referred to oil volume,
as pressure changes.
Equation (3.16a) can be written in the form of equation (3.16b),
which is convenient for use in calculating values of
c
o
from laboratory
data, since a plot of
B
o
vs.
R
s
is linear, resulting in a derivative that is
constant for a particular reservoir fluid study.
(3.16b)
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