Page 126 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 65

liberation in the reservoir and flash expansion to stock tank conditions. Therefore if,
during the producing life of the reservoir, the separator conditions are changed, then

the fixed differential liberation data will have to be converted to give new tables of B o
and R s using values of c and R appropriate for the altered separator conditions.
f b si f
This combination of differential liberation in the reservoir and flash expansion to the
surface is generally regarded as a reasonable approximation to Dodson's PVT analysis
4
technique . In this form of experiment a differential liberation is performed but after
each pressure stage the volume of the oil remaining in the PV cell is flashed to stock
tank conditions through a chosen separator combination. The ratio of stock tank oil
volume to original oil volume in the PV cell prior to flashing gives a direct measure of
B o, while the gas evolved in the flash can be used directly to obtain R s. The process is
repeated taking a new oil sample for each pressure step, since the remaining oil in the
PV cell is always flashed to surface conditions. This type of analysis, while more
accurately representing the complex reservoir-production phase separation, is more
time consuming and therefore more costly, furthermore, it requires the availability of
large samples of the reservoir fluid. For low and moderately volatile crudes, the manner
of deriving the PVT parameters described in this section usually provides a very good
approximation to the results obtained from the Dodson analysis. For more volatile
crudes, however, the more elaborate experimental technique may be justified.

2.5 ALTERNATIVE MANNER OF EXPRESSING PVT LABORATORY ANALYSIS
RESULTS

The results of the differential liberation experiment, as listed in table 2.2, provide an
absolute set of data which can be modified, according to the surface separators used,
to give the values of the PVT parameters required for field use. In table 2.2 all volumes
are measured relative to the unit oil volume at the bubble point. There is, however, a
more common way of representing the results of the differential liberation in which
volumes are measured relative to the volume of residual oil at stock tank conditions.
This volume is obtained as the final step in the differential liberation experiment by
flashing the volume of oil measured at atmospheric pressure and reservoir
temperature, to atmospheric pressure and 60°F. This final step is shown in table 2.2 in
which 0.8296 relative oil volumes at 14.7 psia and 200°F yield 0.7794 relative oil
volumes at 14.7 psia and 60°F. This value of 0.7794 is the shrinkage factor for a unit
volume of bubble point oil during differential liberation to stock tank conditions and is
denoted by c . The value of c ,is not dependent on any separator conditions and
b
b
d
d
therefore, relating all volumes in the differential liberation to this value of c , which is
b
d
normally referred to as the "residual oil volume", will provide an alternative means of
expressing the differential liberation results.
It should be noted, however, that the magnitude of c is dependent on the number of
b
d
pressure steps taken in the differential experiment. Therefore, the differential liberation
results, in which all volumes are measured relative to c b d do not provide an absolute
set of data such as that obtained by relating all volumes to the unit volume of oil at the
bubble point.