Page 107 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 46

Fig. 2.2 depicts the situation when the reservoir pressure has fallen from its initial value
p i to some lower value p, which is still above the bubble point. As shown in the P−T
diagram (inset) the only fluid in the reservoir is undersaturated liquid oil. When this oil is
produced to the surface each stock tank barrel will yield, upon gas oil separation, R si
standard cubic feet of gas. Since the oil is undersaturated with gas, which implies that it
could dissolve more if the latter were available, then the initial value of the solution gas
oil ratio must remain constant at R si (scf/stb) until the pressure drops to the bubble
point, when the oil becomes saturated, as shown in fig. 2.5(b).

Figure 2.2 also shows, in accordance with the definitions of B o and R s, that if R si scf of
gas are taken down to the reservoir with one stb of oil, then the gas will totally dissolve
in the oil at the reservoir pressure and temperature to give a volume of B o rb of oil plus
dissolved gas. Figure 2.5(a) shows that B o increases slightly as the pressure is reduced
from initial to the bubble point pressure. This effect is simply due to liquid expansion
and, since the compressibility of the undersaturated oil in the reservoir is low, the
expansion is relatively small.

Typical values of B o and R s above the bubble point are indicated in fig. 2.5, these are
the plotted results of the laboratory analysis presented in table 2.4. The initial value of
the oil formation volume factor B oi is 1.2417 which increases to 1.2511 at the bubble
point. Thus initially, 1.2417 reservoir barrels of oil plus its dissolved gas will produce
one stb of oil. This is a rather favourable ratio indicating an oil of moderate volatility
and, as would be expected in this case, the initial solution gas oil ratio is also relatively
low at 510 scf/stb. Under less favourable circumstances, for more volatile oils, B oi can
have much higher values. For instance, in the Statfjord field in the North Sea, B oi is
2.7 rb/stb while the value of R si is approximately 3000 scf/stb. Obviously the most
favourable value of B oi is as close to unity as possible indicating that the oil contains
hardly any dissolved gas and reservoir volumes are approximately equal to surface
volumes. The small oil fields of Beykan and Kayaköy in the east of Turkey provide good
examples of this latter condition having values of B oi and R si of 1.05 and 20 scf/stb
respectively.

Below the bubble point the situation is more complicated as shown in fig. 2.3.