Page 26 - PVT Property Correlations

Introduction Chapter | 1 7

Hydrocarbon vapor Hydrocarbon liquid
Water phase
phase (gas phase) phase (oil phase)
Surface gas X X
Stock-tank oil (surface oil) X X
Surface water X

FIGURE 1.5 Relation of fluids produced on surface and reservoir phases for MBO models.
MBO, modified black-oil.

simulation, nodal analysis, and artificial lift design) and for most common
reservoir-depletion processes (e.g., primary and secondary recovery).

Modified Black-Oil (MBO) Models
The MBO model (also called extended black oil) is similar to black-oil mod-
els in most respects; it differs, however, in the way it handles oil produced
on surface (stock-tank oil) (Spivak and Dixon, 1973). The oil produced on
surface is assumed to come from both the reservoir gas and oil phases. The
MBO model is therefore capable of handling the oil produced from the gas
phase. This capability is important for modeling both volatile oil and gas
condensate fluids, as the oil produced from the gas phase can be significant.
Fig. 1.5 shows the relation of fluids produced on surface (surface gas and
stock-tank oil) and the phases existing in the reservoir (gas and oil phases).
Four PVT functions are required for the MBO model in addition to oil
and gas viscosity. Viscosities are defined and obtained in much the same
way as in black-oil models. The four functions are (1) oil formation volume
factor, (2) solution gas oil ratio, (3) gas formation volume factor, and
(4) vaporized oil gas ratio. The definitions of these functions are given in
the following equations:
Reservoir oil volume at T and P
B o 5 ð1:1Þ
Stock tankoil from reservoir oil
Surface gas from reservoir oil
R s 5 ð1:2Þ
Stock tankoil from reservoir oil
Reservoir gas volume at T and P
B g 5 ð1:3Þ
Surface gas volume from reservoir gas
Stock tankoil volume from reservoir gas
R v 5 ð1:4Þ
Surface gas volume from reservoir gas
The first three functions have the same definitions for both black-oil and
MBO models. The partitioning of how much surface gas comes from the