Page 112 - PVT Property Correlations

90 PVT Property Correlations

temperature. Dew point pressure usually refers to dew point pressure at res-
ervoir temperature, unless another temperature is specified.
Knowledge of dew point pressure (at reservoir temperature) is necessary
in production of gas condensate reservoirs. The composition of the produced
well stream changes when reservoir pressure falls below dew point pressure.
In general, the production stream becomes leaner with lower amounts of
heavier hydrocarbon components.
Several correlations are available to calculate dew point pressure from
basic information. These correlations can be grouped in three categories:
(1) correlations that depend on full composition information; (2) correlations
that depend on gas specific gravity and field information; and (3) correla-
tions that depend on downhole composition information. The next section
summarizes the use of these distinct correlation types.

Gas Deviation Factor (z-Factor)
Single-phase gas deviation factor is measured in the laboratory for gas con-
densate fluids (in the constant composition expansion procedure). Single-
phase gas deviation factor for gas condensates has the same definition as for
dry gases. For pressures above the dew point pressure in gas condensates,
z-factor is adequate for material balance (MB) calculations. The z-factor for
gas condensates can also be obtained in the same way as in dry gases if gas
composition is used. If gas specific gravity is used, the condensate drop out
on surface must be taken into consideration. The same calculations presented
in Chapter 4, Wet Gases, are followed to calculate the reservoir gas specific
gravity. Then, the standard procedure is followed to find the pseudo-critical
properties and the pseudo-reduced properties. The single-phase z-factor is
then calculated from any of the available z-factor correlations.

Other PVT Properties for Gas Condensates
As reviewed in Chapter 3, Dry Gases, the PVT properties dependent on
z-factor (e.g., gas formation volume factor, gas density, gas viscosity, and
gas compressibility) have the same definitions for gas condensates as dry
gases. These properties can be calculated from single-phase z-factor accord-
ing to the methods explained in Chapter 3, Dry Gases.

Two-Phase z-Factor

The two-phase z-factor is a quantity that is back-calculated from the constant
volume depletion (CVD) experiment procedure performed on gas condensate
fluids. The following equation is used to calculate two-phase z-factor in the
laboratory (Whitson and Brule, 2000).