PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 56

(3.10c)
The integration is from the bubblepoint pressure to some selected
pressure higher than bubblepoint pressure. This will permit values of oil
densities at higher pressures to be calculated from a known value of oil
density at the bubblepoint pressure.
Placing the oil compressibility, c o , outside the integral sign in
equation (3.10b) implies that it is constant as pressure changes. This
is not correct, as can be seen in figure 3-8. Performing the integration
as indicated in equation (3.10b) implies that the oil compressibility is
some weighted average of oil compressibilities from the bubblepoint
pressure to the higher pressure. This oil compressibility will be called
c ofb , i.e., weighted-average oil compressibility from bubblepoint pressure to a
higher pressure of interest.
Application 2 can be illustrated with the material balance equation
for undersaturated oil reservoirs. 21
(3.11a)
(3.11b)
(3.11c)
The pressure changes in both equations (3.11a) and (3.11c) are from
initial reservoir pressure, p i , to some lower pressure of interest; both
pressures above the bubblepoint pressure. Thus, this oil compressibility
is some weighted-average from initial reservoir pressure to some selected
lower pressure. This oil compressibility will be called c oi , i.e., average oil
compressibility from initial pressure to a lower pressure of interest.
Application 3 can be illustrated with the continuity equation for
radial flow, which is the starting point of the derivation of equations for
pressure transient analysis. 22
(3.12a)