PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 55

a decrease in oil volume due to the release of solution gas) plus the
increase of volume of the released solution gas in the reservoir.
Thus, the defining equations for this independent variable are
different depending on whether the reservoir pressure is above or below
bubblepoint pressure. This is the only oil fluid property that has a
discontinuity at the bubblepoint pressure. Just below the bubblepoint
pressure, the value of oil compressibility could be three to four times
greater than the value just above the bubblepoint pressure.
Coefficients of isothermal compressibility of oil at reservoir
pressures greater than bubblepoint pressure
The defining equations for coefficients of isothermal compressibility
of oils (oil compressibilities), shown here as equation (3.9), are very
simple. They are simply the fractional change of volume or density as
pressure changes at constant temperature.
(3.9)
However, there are some complexities in application. Three different
applications for this property are found in petroleum engineering
calculations:
1. Use of oil compressibilities in estimating certain fluid
properties at pressures above bubblepoint pressure
2. Use of oil compressibilities in the black oil material balance
equation at pressures above bubblepoint pressure
3. Use of oil compressibilities in single-phase (oil) pressure
transient analysis
Application 1 can be illustrated with the coefficient of isothermal
compressibility equation written in terms of oil density.
(3.10a)
(3.10b)