Q913 rfp w2 lec 6

Reservoir Fluid Properties Course (1st Ed.)

1. Formation Volume Factor
A. Oil
B. Total (two phase)

2. Property Constants

2013 H. AlamiNia

Reservoir Fluid Properties Course: PVT Experiments (CME & CVD & DL)

2

1. Constant-mass expansion Experiment
2. Constant-Volume Depletion Experiment
3. Differential Liberation Experiment: Procedure

2013 H. AlamiNia


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CME Experiment
Names:
Constant-mass expansion (CME)
Constant-composition expansion (CCE)
Flash vaporization (FV)
Flash Liberation
Flash Expansion

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CME Experiment (Cont.)
The constant-mass (or constant-composition)
expansion experiment is sketched for a gas
condensate mixture in next slide, but it may also be
performed on oil mixtures.
A fixed amount of a reservoir fluid is transferred to
a closed cell in which the temperature is kept
constant, often at the reservoir temperature.
The volume of the cell may be varied. This may be
accomplished, by moving a piston up and down.
 The maximum volume is typically around 400 cm
3.
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A Schematic Diagram of
the Flash Liberation Test

In the flash liberation process, the
gas which is liberated from the oil
during a pressure decline remains in
contact with the oil from which it
was liberated.

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Flash Liberation for Oil Mixture
The process, involves the following steps:
Step 1. The reservoir fluid sample is charged to a PVT cell
which is maintained at reservoir temperature
throughout the experiments.
Step 2. The cell pressure is elevated at a pressure higher
than the saturation pressure.
Step 3. The cell pressure is lowered in small increments.
The total volume of the hydrocarbon system is recorded
at each pressure.

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Flash Liberation P-V Diagram

Step 4. A plot of the cell pressuretotal hydrocarbon volume is
constructed as shown in Figure.

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Flash Liberation for Oil Mixture (Cont.)
Step 5. When the cell pressure reaches the bubble-point
pressure of the hydrocarbon system, a sign of formation
of a gas phase is noted.
This stage is marked by a sharp change in the pressure-volume
slope.

Step 6. As the pressure level is reduced below the
bubble-point pressure, the liberated gas is allowed to
remain in contact and reach an equilibrium state with
the oil phase. This thermodynamic equilibrium is assured
by agitating the cell.
Step 7. The equilibrium pressure level and the
corresponding hydrocarbon total volume is recorded.
Step 8. Steps 6 and 7 are repeated until the capacity of
the cell is reached.
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Flash Liberation Experimental Data
The experimental data obtained from the flash
liberation test include:
The bubble-point pressure

The isothermal compressibility coefficient of the
liquid phase above the bubble-point pressure
c. Below the bubble point, the two-phase volume is
measured as a function of pressure

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Flash Liberation Simulation
The foregoing process simulates the gas liberation
sequence, which is taking place in the reservoir at
pressures immediately below the bubble-point
pressure. This can be justified by the fact that the
liberated gas remains immobile in the pores and in
contact with oil until the critical gas saturation is
reached at a certain pressure below Pb·
The flash liberation process best represents the
separator type liberation. When entering the separator,
the reservoir fluids are in equilibrium due to the
agitation occurring in the tubing. In the separator, the
two phases are brought to equilibrium and the oil and
gas are separated. This behavior follows the flash
liberation sequence.
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Constant-Composition Experiment for
a Gas Condensate

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Initial Step of the Experiment
 A constant-mass expansion experiment gives
information
About the saturation pressure at the reservoir temperature
and
 About the relative volumetric amounts of gas and oil in the
reservoir at various stages of the lifetime of the reservoir.

The experiment is started at a pressure higher than the
saturation point.
For a gas condensate mixture this means the experiment is
started at a pressure above the dew point pressure, and
For an oil mixture it is started at a pressure above the bubble
point pressure.

 The initial mixture volume is recorded.
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Other Steps of the Experiment
 The mixture volume is increased stepwise. At each
step
The mixture volume and
The cell pressure
Are measured. Furthermore, the saturation point is
recorded.

It is the pressure at which an additional phase
starts to form.
For a gas condensate this additional phase appears as a
liquid droplet, and
For an oil it will be seen as a gas bubble.
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Calculations of the Experiment for
a Gas Condensate
The term V sat is used for the saturation point
volume.
At each stage of the experiment the relative
volume is recorded, defined as the ratio between
 The actual volume and
The volume

At the saturation pressure:
𝑹𝒆𝒍𝒂𝒕𝒊𝒗𝒆 𝒗𝒐𝒍𝒖𝒎𝒆:

2013 H. AlamiNia

𝑽 𝒓𝒆𝒍

𝑽 𝒕𝒐𝒕
= 𝒔𝒂𝒕
𝑽


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Calculation of Z for the Experiment
For a gas condensate mixture, the gas-phase
compressibility factor Z (Z=PV/RT) is recorded above
the saturation pressure.
Below the dew point, the liquid volume, V liq, of a
gas condensate is recorded as the percentage of the
mixture volume at the dew point:
𝑽 𝒍𝒊𝒒
%𝑳𝒊𝒒𝒖𝒊𝒅 𝒅𝒓𝒐𝒑𝒐𝒖𝒕 = 100 ∗ 𝒔𝒂𝒕
𝑽

This liquid volume is often referred to as the liquid
dropout.
2013 H. AlamiNia


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Primary Results From the Experiment
on a Gas Condensate Mixture
Primary Results from a Constant–Mass Expansion
Experiment Performed on a Gas Condensate
Mixture.
Relative volume
Liquid volume
Z-factor (Only reported above saturation point)

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Example of Results of the Experiment

Results of the Experiment at 155 °C
for a Gas Condensate Mixture

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Liquid Dropout and Relative Volume Curve

Liquid dropout curve (circles, fulldrawn line and left y-axis) and
relative volume (triangles, dashed
line and right y-axis) for constantmass expansion experiment at 155 °
C on the gas condensate mixture.

2013 H. AlamiNia


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Calculations of the Experiment for
Oil Mixtures
For oil mixtures, the isothermal compressibility is
recorded above the saturation point:
In this expression, V is the oil volume.

1
𝒄𝒐 = −
𝑽

𝜕𝑽
𝜕𝑷

𝑻

Below the saturation point, the Y-factor is recorded:
𝑷 𝒔𝒂𝒕 − 𝑷
𝒀 − 𝒇𝒂𝒄𝒕𝒐𝒓: 𝒕𝒐𝒕 𝑷 𝒔𝒂𝒕
𝑽 − 𝑽
𝑽 𝒔𝒂𝒕
V tot is the total volume of the cell content.

The Y-factor is a measure of the ratio between the
relative changes in pressure and total volume in the
two-phase region.
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Y-Factor for the Experiment
As gas takes up more volume than liquid the
volumetric changes with decreasing pressure will be
larger in the two-phase region than in the singlephase region.
An oil that releases much gas with decreasing pressure
will have a small Y-factor,
 Whereas an oil that only releases small amounts of gas
with decreasing pressure will have a large Y-factor.

 A constant-mass expansion experiment is usually
stopped at a pressure somewhere in interval from
50 to 100 bar.
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Results of the Experiment
at 97.5 ° C for the Oil Mixture

2013 H. AlamiNia


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Y-Factor and Relative Volume for Constant
the Experiment on the Oil Mixture

Y-factor (circles, full-drawn line and
left y-axis) and relative volume
(triangles, dashed line and right yaxis) for constant mass expansion
experiment at 97.5 ° C on the oil
mixture

2013 H. AlamiNia


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Primary Results From
the Experiment on an Oil Mixture
Below is the list of the primary results from a
constant-mass expansion experiment performed on
an oil mixture.
Relative volume
Compressibility (Only reported above saturation point)
Oil density (Only above saturation point. Not reported
standard)
Y-factor (Only reported below saturation point)

2013 H. AlamiNia


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Constant-Volume Depletion
Experiment introduction
As with the constant-mass expansion experiment, a
fixed amount of reservoir fluid (gas condensate or
volatile oil) is transferred to a cell kept at a fixed
temperature, often the reservoir temperature.
The cell is constructed in the same manner as for a
constant-mass expansion experiment, but is
equipped with a valve on top allowing depletion of
gas during the experiment.

2013 H. AlamiNia


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Schematic Representation of the
Experiment

Schematic Representation of a
Constant-Volume Depletion
Experiment

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The Experiment Procedure
The experiment is started at the saturation point.
The saturation point pressure, P sat, and
 The saturation point volume, V sat, are recorded.

The volume is increased, which will make the
pressure decrease, and two separate phases are
formed in the cell.

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The Experiment Procedure (Cont.)
The mixture volume is subsequently decreased to V sat
by letting out the excess gas through the valve on top,
maintaining a constant pressure.
The molar amount of gas depleted as a percentage of the gas
initially in the cell and
The liquid volume in the cell as a percentage of the saturation
point volume are recorded.
The compressibility factor (Z=PV/RT) at cell conditions and
The molar composition of the depleted gas are measured.

The volume is increased again, the excess volume is
depleted and so on until the pressure is somewhere
between 100 and 40 bar (~1450-580 psi).
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Design Objectives of the Experiment
The constant-volume depletion experiment has
been designed to gain knowledge about the
changes with time in PVT properties of the
produced well streams from gas condensate and
volatile oil reservoirs.
The reservoir is seen as a tank of fixed volume and
at a fixed temperature.
During production the pressure decreases because
material is removed from the field, while the
volume and temperature remain (almost) constant.
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Design Objectives of the Experiment
(Cont.)
 When the pressure reaches the saturation point, the
mixture splits into a gas and a liquid phase. If all the
production comes from the gas zone, the mixture
produced will have the same composition as the gas
removed from the cell in a constant volume depletion
experiment.
This gas will gradually become less enriched in heavy
hydrocarbons, and less liquid will be produced from the
topside separation plant.
The amount of reservoir fluid removed from the
reservoir from the time the pressure is P 1 until it has
decreased to P 2 corresponds to the amount of gas
removed through the valve on top of the PVT cell in the
depletion stage at pressure P 2.
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Primary Results from the Experiment
The primary results from a constant-volume
depletion experiment performed on a gas
condensate or volatile mixture are summarized
below:
Liquid volume
Percentage produced
Z-factor gas
Two-phase Z-factor
Viscosity of gas (Viscosity of the gas in cell (usually not
measured but calculated))
Gas compositions
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Differential Liberation Characterization
In the differential liberation process, the solution
gas that is liberated from an oil sample during a
decline in pressure is continuously removed from
contact with the oil, and before establishing
equilibrium with the liquid phase.
This type of liberation is characterized by a varying
composition of the total hydrocarbon system.

2013 H. AlamiNia


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Notes about Differential Liberation
Experiment
The differential liberation (or differential depletion)
experiment is only carried out for oil mixtures.
The experiment is started by transferring reservoir
fluid to a cell kept at a fixed temperature, often the
reservoir temperature.
As with the constant-volume depletion cell, the
differential liberation cell is equipped with a valve
on top allowing gas to be depleted during the
experiment.

2013 H. AlamiNia


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Schematic Representation of
a Differential Depletion Experiment

Psat=Pb

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The Experiment Procedure
Step 1. The reservoir fluid sample is placed in a PVT
cell at reservoir temperature.
Step 2. The cell is pressurized to saturation.
Step 3. The volume of the all-liquid sample is
recorded.
Step 4. The cell pressure is lowered.
Step 5. The liberated gas is removed from the cell
through the cell flow valve. During this process, the
cell pressure is kept constant by reinjecting mercury
(H2O) in the cell at the same rate as the gas
discharge rate.
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The Experiment Procedure (Cont.)
Step 6. The volume of the discharged gas is
measured at standard conditions and the volume of
the remaining oil is recorded.
Step 7. Steps 5 and 6 are repeated until the cell
pressure is lowered to atmospheric pressure.
Step 8. The remaining oil at atmospheric pressure is
measured and converted to a volume at 60°F. This
final volume is referred to as the residual oil.

2013 H. AlamiNia


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Purpose of the Experiment
The primary purpose of PVT experiments is to gain
experimental knowledge about the behavior of a
reservoir fluid at reservoir conditions.
The differential depletion experiment has a
secondary purpose of
Generating information on the volumetric changes
taking place with the well stream when produced at
standard conditions.

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1. Pedersen, K.S., Christensen, P.L., and Azeem,
S.J. (2006). Phase behavior of petroleum
reservoir fluids (CRC Press). Ch3.
2. Tarek, A. (1989). Hydrocarbon Phase Behavior
(Gulf Publishing Company, Houston). Ch4.

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1.
2.
3.
4.

Differential Liberation Experiment: Data set
Separator Experiment
Swelling Experiment
Other Experiments

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