Page 316 - Principles of Applied Reservoir Simulation 2E

Part V: Technical Supplements 301

most natural gases, the relationship \!B g versus pressure will be very
nearly linear, especially at moderate to high pressures. Plotting l/B g
versus pressure and extrapolating to PMAX should provide more realistic
values of B g at higher pressures. Interpolating B g using l/B g versus
pressure substantially improves material balance.

2. Once the B g versus P curve is fixed, R so versus P and B 0 versus P curves
must be extrapolated so as to avoid a negative oil compressibility being
calculated over any pressure increment. To ensure that negative oil
compressibilities will not be calculated by the program, the following test
should be used. For any pressure increment P, to P 2, where P 2 > P {, the
following relationship should hold:

0 * - - B ol) + .
v (B 02 ;
5.615
where the units of B 0, B g, and R so are RB/STB, RCF/SCF, and SCF/ STB,
respectively. Note that this test applies only to the saturated oil PVT data.

3 . The above concepts also apply to the water PVT data. However, for most
simulations, it can be assumed that R sw = 0.0, thus - AB W/B WAP approxi-
mates water compressibility.

28.5 Gas PVT Correlation Option

Basic Gas Properties
Following Govier [1975], real gas Z-factors are computed using the
Dranchuk, et al. [1974] representation of the Standing-Katz Z-factor charts
[1942]. This representation employs the Benedict- Webb-Rubin [1940] eight-
parameter equation of state to express the Z-factor as a function of pseudo-
critical temperature T r and pseudo-critical pressure P r, thus
Z = Z(P r, T r) (28.2)