Page 125 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 64

EXERCISE 2.2 SOLUTION

The optimum separator pressure in table 2.3 is 150 psia since this gives the largest
value of the flash shrinkage factor c as 0.7993 (stb/rb b) and correspondingly, the
f b
lowest flash solution gas oil ratio R of 510 scf/stb. Using these two figures the
si
f
laboratory differential data in table 2.2 can be converted to give the field parameters B o,
R s and B g using equs. (2.5) − (2.7), as follows

Pressure B = v o R R − 5.615 F B 1
o
c s = f si c g = 5.615 E
f b f b
(psia) (rb/stb) (scf/stb) (rb/scf)
4000 (p i) 1.2417 (B ) 510 (R )
si
oi
f
f
3500 1.2480 510
1
3330 (p b) 1.2511 (B ob f = c ) 510 .00087
f b
3000 1.2222 450 .00096

2700 1.2022 401 .00107
2400 1.1822 352 .00119

2100 1.1633 304 .00137
1800 1.1450 257 .00161
1500 1.1287 214 .00196

1200 1.1115 167 .00249
900 1.0940 122 .00339

600 1.0763 78 .00519
300 1.0583 35 .01066
TABLE 2.4
Field PVT parameters adjusted for single stage, surface separation
at 150 psia and 80°F; c = .7993 (Data for pressures above 3330 psi
f b
are taken from the flash experiment, table 2.1)

The data in table 2.4 are plotted in fig. 2.5(a) − (c).
In summary of this section, it can be stated that the laboratory differential liberation
experiment, which is regarded as best simulating phase separation in the reservoir,
provides an absolute set of PVT data in which all volumes are expressed relative to the
unit oil volume at the bubble point, the latter being a unique volume. The PVT
parameters conventionally used in the field, however, are dependent on the nature of
the surface separation. The basic differential data can be modified in accordance with
the surface separators employed using equs. (2.5) − (2.7) in which c and R are
f b f b
determined by flashing unit volume of reservoir oil through the separator system. The
modified PVT parameters thus obtained approximate the process of differential