Page 138 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 123

possible to encounter retrograde condensation for some hydrocarbon composi-tions.
The P-T diagram also applies to temperature and pressure changes in a
wellbore. In the case of wellbore flow, the fluid moves from relatively high
reservoir temperature and pressure to relatively low surface temperature and
pressure. As a result, it is common to see fluids that are single-phase in the
reservoir become two-phase by the time they reach the surface.
Figure 13-2 is a P-T diagram that compares two-phase envelopes for four
types of fluids. A reservoir fluid can change from one fluid type to another
depending on how the reservoir is produced. A good example is dry gas injec-
tion into a black oil reservoir. Dry gas injection increases the relative amount
of low molecular weight components in the black oil. The two-phase envelope
rotates counter-clockwise in the P-T diagram as the relative amount of lower
molecular weight components increases. Similarly, dry gas injection into acon-
densate can make the phase envelope transform from one fluid type to another.
Thus, the way the reservoir is operated has a significant impact on fluid be-
havior in the reservoir and at the surface.

Temperature
Figure 13-2. Typical two-phase P-T envelopes for
different fluid types.
Table 13-2 shows different compositions for typical fluid types. Dry gas
usually contains only the lower molecular weight components. Gas condensates
start to add higher molecular weight components. Volatile oils continue to add
higher molecular weight components. The addition of higher molecular weight
components and the reduction of lower molecular weight components eventually