Page 78 - Introduction to Petroleum Engineering

62 PROPERTIES OF RESERVOIR FLUIDS

Reduce Reduce Reduce Reduce
pressure pressure pressure pressure
Gas
Gas
V B > V A Oil V C = V sat > V B Oil V D < V sat Oil Oil
V A
Oil

Piston V E < V D <V sat

Step A Step B Step C Step D Step E
P A >> P sat P B > P sat P C = P sat P D < P sat P E << P sat
FIGURE 3.8 Constant composition expansion.

3.7.2 Differential Liberation
The DL test is used to determine the liberation of gas from live oil, that is, oil con-
taining dissolved gas. A live oil sample is placed in a PVT cell at reservoir tempera-
ture and BP pressure as in Figure 3.9. The pressure is lowered in small increments
and the evolved gas is removed at each stage. The volume of the evolved gas and the
volume of oil remaining in the cell are recorded. Oil viscosity is typically measured
in a DL test.

3.7.3 Separator Test
The separator test is used to study the behavior of a fluid as it flashes from reservoir
to surface conditions. A flash is the one‐step change from a relatively high‐pressure
and high‐temperature environment to a relatively low‐pressure and low‐temperature
environment. The primary difference between a flash and a differential process is the
magnitude of the pressure differential between stages. The pressure differential is
generally much smaller in the differential process than in the flash process.
Figure 3.10 illustrates a multistage flash where the input stream goes through
multiple flashes. Each flash has a different temperature and pressure relative to the
preceding flash.
The PVT cell in a separator test is charged with a carefully measured volume of
reservoir fluid at reservoir temperature and saturation pressure. The cell pressure and
temperature are then changed. Each change in pressure and temperature corresponds
to a separator stage, and one or more stages may be used in the test. The volume of
gas from each separation stage and the volume of the liquid remaining in the last
stage are measured.