Page 121 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 60

In contrast to the flash expansion, after each stage of the differential liberation, the total
amount of gas liberated during the latest pressure drop is removed from the PV cell by
injecting mercury at constant pressure, fig. 2.3. Thus, after the pressure drop from
2700 to 2400 psia, table 2.2, column 2, indicates that 0.0466 volumes of gas are
withdrawn from the cell at the lower pressure and at 200°F. These gas volumes v g are
measured relative to the unit volume of bubble point oil, as are all the relative volumes
listed in table 2.2. After each stage the incremental volume of liberated gas is
expanded to standard conditions and re−measured as V g relative volumes. Column 4 is
simply the cumulative amount of gas liberated below the bubble point expressed at
standard conditions, in relative volumes, and is denoted by F = Σ V g. Dividing values in
column 3 by those in column 2 (V g/v g) gives the gas expansion factor E defined in
Chapter 1, sec. 6. Thus the .0466 relative volumes liberated at 2400 psia will expand to
give 6.9457 relative volumes at standard conditions and the gas expansion factor is
therefore 6.9457/.0466 = 149.05. Knowing E, the Z−factor of the liberated gas can be
determined by explicitly solving equ . (1.25) for Z as

p T 1 p
Z = × sc × = 35.37
p T E ET
sc

and for the gas sample withdrawn at 2400 psia

2400
Z = 35.37 × = 0.863
149.05 660
×
These values are listed in column 6 of table 2.2.

Finally, the relative oil volumes, v o, are measured at each stage of depletion after
removal of the liberated gas, as listed in column 7.

Before considering how the laboratory derived data presented in table 2.2 are
converted to the required field parameters, B o, R s and B g, it is first necessary to
compare the physical difference between the flash and differential liberation
experiments and decide which, if either, is suitable for describing the separation of oil
and gas in the reservoir and the production of these volumes through the surface
separators to the stock tank.

The main difference between the two types of experiment shown in fig. 2.9(a) and (b) is
that in the flash expansion no gas is removed from the PV cell but instead remains in
equilibrium with the oil. As a result, the overall hydrocarbon composition in the cell
remains unchanged. In the differential liberation experiment, however, at each stage of
depletion the liberated gas is physically removed from contact with the oil and
therefore, there is a continual compositional change in the PV cell, the remaining
hydrocarbons becoming progressively richer in the heavier components, and the
average molecular weight thus increasing.

If both experiments are performed isothermally, in stages, through the same total
pressure drop, then the resulting volumes of liquid oil remaining at the lowest pressure
will, in general, be slightly different. For low volatility oils, in which the dissolved gas