Reservoir Fluid Properties - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 19

Fig. 3.5 Phase diagram of a normal GOR well. Courtesy Schlumberger
Oil Reservoirs
Figure 3.5 shows a plot of pressure vs. temperature for an oil-producing reservoir.
At formation conditions, pressure and temperature are such as to place point A′,
representing the original conditions, above the critical pressure. The multicompo-
nent hydrocarbon therefore exists as an undersaturated liquid. As the pressure in the
reservoir is drawn down by the production process, point A—the bubble-point—is
reached. Some gas can now start to come out of solution. The path from the produc-
ing horizon up the production string to the separator is illustrated by the dashed line.
The final step to the stock tank leaves a point that falls on a line representing some
percentage of oil and some percentage of gas. Typically, 80-90 % of the original
fluid is recovered in the form of liquid (oil).
Condensate Reservoirs
Figure 3.6 illustrates a reservoir where the starting point A' is above the critical
temperature. Thus, all of the multicomponent system exists as a gas. During the
production process, however, the temperature and pressure fall sufficiently to place
point B, for example, back inside the bubble-point-line-dewpoint-line envelope.
Thus, although in the reservoir the hydrocarbon exists as gas, by the time it reaches
the separator, some of it exists as liquid oil. This process is known as retrograde
condensation. Typically, 25 % of the hydrocarbon may be recovered as oil at the
separator and somewhat less in the stock tank.