Page 124 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 63

What is required for field use is B o expressed in rb/stb. In the differential liberation data
the corresponding parameter is v o (rb/rb b), that is, reservoir barrels of oil per unit barrel
at the bubble point. But from the flash data it is known that one reservoir barrel of oil, at
the bubble point, when flashed through the separators yields c stb. Therefore, the
f b
conversion from the differential data to give the required field parameter B o is

rb v rb rb
B o = o b
stb c f b stb rb b

Similarly, the solution gas oil ratio required in the field is R s (scf/stb). The parameter in
the differential liberation data from which this can be obtained is F (cumulative gas vol
at sc/oil vol at p b = stb/rb b). In fact, F, the cumulative gas liberated from the oil, must be
proportional to R si f − R (scf/stb), which is the initial solution gas oil ratio, as determined
s
in the flash experiment, minus the current solution gas oil ratio at some lower pressure.
The exact relationship is

scf stb scf 1 rb
(R si − R ) = F × 5.615 × b
s
f
stb rb b stb c f b stb
Finally, the determination of the third parameter B g can be obtained directly from the
differential parameter E as

rb 1 rcf 1 rb
B g = ×
scf E scf 5.615 rcf

Thus the laboratory differential data can be transformed to give the required field PVT
parameters using the following conversions

Laboratory Required
Differential Field Conversion
Parameter Parameter

v rb
v o (rb/ rb b) B o B = o (2.5)
o
c stb
f b
5.615 F rb

F (stb/rb b) R s R s = R si f − c b f stb (2.6)

1 rb
E (scf/rcf) B g B = (2.7)
g

5.612 E scf
EXERCISE 2.2 CONVERSION OF DIFFERENTIAL LIBERATION DATA TO GIVE
THE FIELD PVT PARAMETERS B o, R s AND B g

Convert the laboratory differential liberation data presented in table 2.2 to the required
PVT parameters, for field use, for the optimum separator conditions listed in table 2.3.