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Isothermal compressibility of oil
Isothermal compressibility is the change in volume of a system as the pressure changes while temperature remains constant.
Isothermal compressibility of oil
The isothermal compressibility of undersaturated oil is defined as
which reflects the change in volume with change in pressure under constant temperature conditions. Below the bubblepoint pressure, oil isothermal compressibility is defined from oil and gas properties to account for gas coming out of solution. The corresponding saturated oil compressibility is
Above bubblepoint pressure, oil volume changes as a function of isothermal compressibility only. Tables 1 and 2 summarize the correlations developed to predict this property. Oil formation volume factors (FVFs) for undersaturated crude oil are determined as a function of bubblepoint FVF, isothermal compressibility, and pressure above bubblepoint from
Comparison of correlation methods
A total of 141 data points were available from the GeoMark PVT database.  Geographically, these samples were obtained from the Gulf of Mexico and the Gulf of Suez. Table 3 provides a summary of the data. This data was used to evaluate and rank the performance of the isothermal compressibility correlations. Table 4 provides the results. Data in the table have been sorted by absolute average relative error, which provides a means to rank the methods. Fig. 1 graphically shows isothermal compressibility vs. pressure.
Impact of gravity changes
Fig. 2 shows how isothermal compressibility changes with crude oil gravity. As oil gravity increases, isothermal compressibility should increase. Results predicted by Ahmed, Al-Marhoun,  De Ghetto et al.,  and Elsharkawy and Alikhan do not properly model the phenomena. De Ghetto et al. proposed a method that uses several equations covering various API gravity ranges. This technique results in discontinuities in predicted properties as the equations change.
Changes with solution GOR
Fig. 3 shows the change in isothermal compressibility with solution GOR. Varying this property also results in varying the bubblepoint pressure. To illustrate this effect, isothermal compressibility is determined at 1,000 psi above a variable saturation pressure. Results from methods proposed by Petrosky and Farshad,  Kartoatmodjo and Schmidt,  and Dindoruk and Christman are undefined for solution GORs of zero. Methods proposed by Ahmed, Al-Marhoun, and Kartoatmodjo produce unphysical results with changing GOR.
|co||=||oil isothermal compressibility, Lt2/m, psi-1|
|Bo||=||oil FVF, bbl/STB|
|Bg||=||gas FVF, ft3/scf|
|T||=||temperature, T, °F|
|Rs||=||solution GOR, scf/STB|
|p||=||pressure, m/Lt2, psia|
|Bob||=||oil formation volume at bubblepoint pressure, bbl/STB|
|pb||=||bubblepoint pressure, m/Lt2, psia|
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