Page 175 - PVT Property Correlations

Black Oils Chapter | 7 149

mobility than oil, the producing GOR is expected to increase when the
reservoir pressure decreases below the bubble-point pressure. Other PVT
properties also undergo significant changes when the reservoir pressure
passes through the bubble-point pressure, as will be shown in the next
sections.

Formation Volume Factor of Oil

Oil formation volume factor is defined as the volume of oil (and dissolved
gas) at reservoir pressure and temperature required to produce one stock tank
barrel of oil at the surface. The oil formation volume factor is expressed in
units of reservoir volume over standard volume (usually rbbl/STB). At sur-
face, the oil loses the dissolved gas it contained under high pressure and tem-
perature in the reservoir. Three things happen to the reservoir oil (with
dissolved gas) when it moves from reservoir conditions to surface:
1. The oil loses mass due to the loss of dissolved gas on the oil trip from
the reservoir to the surface.
2. The oil shrinks slightly due to the reduction in temperature.
3. The oil expands slightly due to the reduction in pressure.
The volume reduction due to temperature and the volume increase due to
pressure usually have minimal effect on the oil and tend to cancel each other.
The mass loss due to liberation of dissolved gas represents the major effect
and is primarily responsible for the shape of oil formation volume factor
below the bubble point. Fig. 7.2 shows that oil formation volume factor
increases with reduction in pressure until the oil reaches the bubble-point
pressure. The volume increase at pressures above the bubble point is due to
the expansion of oil (with its dissolved gas). Below the bubble point, and
with the continued reduction in pressure, the oil formation volume factor is
reduced primarily due to mass loss with the additional release of dissolved
gas. For black oils, the highest value for formation volume factor is usually
less than 2 rbbl/STB (McCain, 1993).

Solution Gas Oil Ratio
Solution GOR (also called gas in solution) is defined as the gas dissolved in
oil at any pressure and temperature. At standard pressure and temperature,
dissolved gas is completely released from the oil, and therefore the oil con-
tains no gas. The gas liberated from the oil will be released in the form of
free gas. Oil at any pressure and temperature has a certain capacity to dis-
solve gas. In general, the more volatile the oil (due to the presence of higher
amounts of lighter hydrocarbons), the greater the capacity of oil to dissolve
gas. Therefore, volatile oils often have a higher solution GOR initially and at
any pressure, than do black oils. The bubble-point pressure at a particular