Page 73 - Introduction to Petroleum Engineering

OIL PROPERTIES 57
Oil Formation Volume Factor. The FVF of oil at the BP pressure (B in
ob
RB/STB) can be estimated with the following correlation in terms of solution
GOR (R in SCF/STB), gas gravity (γ ), oil specific gravity (not API gravity), and
g
s
temperature (T in °F):
+
.
.
B = 0980 00012 A 12 . (3.17)
ob
with
 γ  05 .
A = R s  g  +125. T (3.18)
 γ o 

Example 3.3 Oil Formation Volume Factor
Calculate formation volume factor for the same conditions as Example 3.2,
that is, reservoir temperature of 220°F, oil gravity of 35°API, gas gravity of
0.68, and 350 SCF/STB of dissolved gas.

Answer
First, convert 35°API to oil specific gravity, and then use Equations 3.17 and
3.18 with the appropriate values:

141 5 . 141 5 .
γ = ° API +131 5 . = 35 +131 5 . = 085
.
o
.
.
A = (350 SCF/STB)  068   05 +125 220. ( ) = 588

 085. 
(
+
B = 0980 00012 588 ) 12 . = 123 RB/STB
.
.
.
ob
The plot in Figure 3.5 shows results for the correlation of Equations 3.17 and 3.18
with the properties of the previous example. We consider R varying from near 0 up
s
to 350 SCF/STB.
Oil FVF decreases because of compression of the oil above BP pressure. Oil FVF
B at pressure p above BP pressure p is calculated as
o b
Bp p )
B = B +δ p ( − b (3.19)
o
o
ob
where B is oil FVF at BP pressure and δ B is the change in oil FVF above
o
p
ob
BP pressure due to increasing pressure. The value of δ B for the oil FVF shown in
p
o
−5
.
Figure 3.6 is approximately −14 ×10 RB/STB/psi for pressures greater than
BP pressure. The slope δ B is negative since oil FVF decreases as pressure increases
p
o
for pressures greater than BP pressure.