Page 192 - PVT Property Correlations

166 PVT Property Correlations

the correct bubble-point pressure is known). The modified R s curve is to be
used in further calculations of the remaining PVT properties.

Sequence of Calculations
The following sequence of calculations for black-oil PVT properties from
correlations are in fact general steps for oil correlations and can be applied
to volatile oils as well.
The calculations first determine whether or not the user input for initial
solution GOR is the total solution GOR. If the user inputs the GOR based on
measuring gas rates at the separator and oil rates at the stock-tank (the usual
situation), it is necessary to use a correlation to estimate the stock-tank
GOR. Then the stock-tank GOR is added to separator GOR to obtain the
total solution GOR by the following equation:

tGOR 5 sGOR 1 STGOR ð7:6Þ
The second check is for bubble point. If the user inputs the bubble-point
pressure at the specified temperature, then the input bubble point is used in
further calculations. If bubble-point pressure is not available, it should be
calculated from any of the bubble-point correlations.
After the bubble-point pressure has been determined, dead oil viscosity is
calculated from any of the dead oil viscosity correlations. Dead oil viscosity is
the viscosity of the oil assuming no gas in solution. Other PVT properties for
oil (solution GOR, oil formation volume factor, oil compressibility, oil density,
and oil viscosity) are computed for all saturated pressure points (pressures
below the bubble-point pressure). The PVT property values are also calculated
at the bubble-point pressure. The following step is to calculate the same PVT
properties for undersaturated pressure points (pressures above the bubble-point).
The sequence of calculations is presented in the flow diagram of Fig. 7.13.

APPLICATIONS OF OIL PVT CORRELATIONS

In the absence of representative PVT samples, engineers still must use some
estimates of oil PVT properties. This section explains several critical points
to be taken into consideration for calculation of good estimates of PVT prop-
erties. The procedures to furnish the correct values to be used as input para-
meters in oil PVT correlations are illustrated with example calculations.

Problem 1—Estimating Input Data for Oil Correlations

An oil reservoir was discovered recently and has been producing with initial
reservoir pressure of 4100 psig and GOR of 600 scf/STB (GOR is measured
based on separator data). Primary separator conditions are 200 psig and

75 F. A sample from sales gas (separator gas) was taken to measure the