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Constant-Composition Test

This test involves measuring the pressure-volume relations of the reservoir fluid at reservoir temperature with a visual cell. This usual PVT cell allows the visual observation of the condensation process that result from changing the pressures. The experimental test procedure is similar to that conducted on crude oil systems. The CCE test is designed to provide the dew-point pressure pd at reservoir temperature and the total relative volume Vrel of the reservoir fluid (relative to the dew-point volume) as a function of pressure. The relative volume is equal to one at pd. The gas compressibility factor at pressures greater than or equal to the saturation pressure is also reported. It is only necessary to experimentally measure the z-factor at one pressure p1 and determine the gas deviation factor at the other pressure p from:

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constant composition test


z = gas deviation factor at p

Vrel = relative volume at pressure p

(Vrel)1 = relative volume at pressure p1

If the gas compressibility factor is measured at the dew-point pressure, then:

gas compressibility factor


zd = gas compressibility factor at the dew-point pressure pd

pd = dew-point pressure, psia

p = pressure, psia

Constant-Volume Depletion (CVD) Test (Pressure depletion terst)

Constant-volume depletion (CVD) experiments are performed on gas condensates and volatile oils to simulate reservoir depletion performance and compositional variation. The test provides a variety of useful and important information that is used in reservoir engineering calculations:

Step 1.

A measured amount of a representative sample of the original reservoir fluid with a known overall composition of zi is charged to a visual PVT cell at the dew-point pressure pd (“a” in Figure 3-12). The temperature of the PVT cell is maintained at the reservoir temperature T throughout the experiment. The initial volume Vi of the saturated fluid is used as a reference volume.

Step 2.

The initial gas compressibility factor is calculated from the real gas equation

constant volume depletion

constant volume

dew point pressure


pd = dew-point pressure, psia

Vi = initial gas volume, ft3

ni = initial number of moles of the gas = m/Ma

R = gas constant, 10.73

T = temperature, °R

zd = compressibility factor at dew-point pressure

Step 3.

The cell pressure is reduced from the saturation pressure to a predetermined level P. This can be achieved by withdrawing mercury from the cell, as illustrated in column b of Figure 3-12. During the process, a second phase (retrograde liquid) is formed. The fluid in the cell is brought to equilibrium and the gas volume Vg and volume of the retrograde liquid VL are visually measured. This retrograde volume is reported as a percent of the initial volume Vi which basically represents the retrograde liquid saturation SL:

gas compressibility factor

Step 4.

Mercury is reinjected into the PVT cell at constant pressure Pwhile an equivalent volume of gas is simultaneously removed. When the initial volume Vi is reached, mercury injection is ceased, as illustrated in column c of Figure 3-12. This step simulates a reservoir producing only gas, with retrograde liquid remaining immobile in the reservoir.

Step 5.

The removed gas is charged to analytical equipment where its composition yi is determined, and its volume is measured at standard conditions and recorded as (Vgp)sc. The corresponding moles of gas produced can be calculated from the expression:

analytical equipment


np = moles of gas produced

(Vgp)sc = volume of gas produced measured at standard conditions, scf

Tsc = standard temperature, °R

psc = standard pressure, psia

R = 10.73

Step 6.

The gas compressibility factor at cell pressure and temperature is calculated from the real gas equation-of-state as follows:

moles of gas

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