1. Reservoir Fluid Properties Course (1st Ed.)

2. 1. Constant-mass expansion Experiment
2. Constant-Volume Depletion Experiment
3. Differential Liberation Experiment: Procedure
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3. 1.
2.
3.
4.
Differential Liberation Experiment: Data set
Separator Experiment
Swelling Experiment
Other Experiments
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5. Experimental Data of
Differential Liberation Test
The experimental data obtained from the test include:
a. Amount of gas in solution as a function of pressure
b. The shrinkage in the oil volume as a function of
pressure
c. Properties of the evolved gas including the
composition of the liberated gas, the gas
compressibility factor (Z=PV/RT), and the gas specific
gravity
d. Density of the remaining oil as a function of pressure
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6. Obtaining Gas Gravity
The gas gravity is defined as the average molecular
weight of the gas divided by the average molecular
weight of atmospheric air:
𝑮𝒂𝒔 𝒈𝒓𝒂𝒗𝒊𝒕𝒚 =
𝑴𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒈𝒂𝒔
𝑴𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒂𝒕𝒎𝒐𝒔𝒑𝒉𝒆𝒓𝒊𝒄 𝒂𝒊𝒓
The molecular weight of atmospheric air is usually
assumed to be equal to 28.964 g/mol.
By expressing the molecular weight relative to that
of atmospheric air, the gas gravity becomes a
measure of the low-pressure density of the gas
relative to that of atmospheric air.
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7. Obtaining Bg
The volume at standard conditions of the liberated
gas is measured.
 This enables calculation of the gas formation
volume factor, Bg:
𝑩𝒈=
𝑮𝒂𝒔 𝒗𝒐𝒍𝒖𝒎𝒆 𝒂𝒕 𝒄𝒆𝒍𝒍 𝒄𝒐𝒏𝒅𝒊𝒕𝒊𝒐𝒏𝒔
𝑮𝒂𝒔 𝒗𝒐𝒍𝒖𝒎𝒆 𝒂𝒕 𝒔𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒄𝒐𝒏𝒅𝒊𝒕𝒊𝒐𝒏𝒔
Cell conditions refer to the pressure and temperature in the cell
at the pressure stage at which the gas was depleted.
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8. Obtaining Bo
A differential depletion experiment is usually continued
down to atmospheric pressure before cooling off the
cell to 15°C (or standard).
If the oil volume at stage N is VNoil, Bo is for stage N
defined as
𝒐𝒊𝒍
𝑩𝒐 𝑵 =
𝑽𝑵
𝒐𝒊𝒍
𝑽 𝒔𝒕𝒅
Vstdoil, The volume of the cell content at atmospheric
(standard) conditions is reported as the residual (or standard)
oil volume.
The liquid volumes at the remaining pressure stages are
reported relative to the residual oil volume through the
oil formation or shrinkage factor, Bo.
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9. Obtaining Rs
The solution gas/oil ratio, RS, is another important quantity
measured in a differential liberation experiment.
 The gas/oil ratio of the oil at a given stage in a differential
liberation experiment is calculated by adding the standard
volumes of the gas liberated in each of the subsequent stages and
then dividing this sum of gas volumes by the residual oil volume.
For the oil at stage N in a differential liberation experiment with a
total of NST pressure stages, RS is given by (Differential liberation
(or gas in solution) gas/oil ratio=Rs)
𝑹𝑺 𝑵 =
𝒈𝒂𝒔
𝑵𝑺𝑻
𝑽 𝒔𝒕𝒅,𝒏
𝒏=𝑵+1
𝒐𝒊𝒍
𝑽 𝒔𝒕𝒅
The volume at standard conditions of the gas liberated from stage
N in a differential liberation experiment is in the following
referred to as Vstd, Ngas.
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10. Differential Liberation Experiment
Differential Liberation Experiment in
a P–T Diagram
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11. Equivalent Gas Volume
When the gas is flashed to standard conditions a
small liquid dropout will usually be seen.
This volume is added to the gas volume entering
into Equation
𝑹𝑺 𝑵 =
𝒈𝒂𝒔
𝑵𝑺𝑻
𝒏=𝑵+1 𝑽 𝒔𝒕𝒅,𝒏
𝒐𝒊𝒍
𝑽 𝒔𝒕𝒅
As an equivalent gas volume.
(The volume the liquid would take up had it been in
gaseous form)
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12. Notes about Bo
The oil at standard conditions is often referred to as a stable
oil to indicate that it can be transported at standard
conditions without further release of gas.
The Bo-factor, is a measure of how much the oil shrinks
during production.
 If the oil volume at a given reservoir pressure P x equals
VOL x and the oil at the pressure P x has a Bo-factor of Bo, x,
the oil will have a volume of VOLx/Bo, x after depletion to
atmospheric conditions.
The Bo-factor will in general be greater than 1, expressing
that the oil shrinks during production. It shrinks
Because it releases gas when the pressure decreases and
 Because of thermal contraction with decreasing temperature.
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13. Notes about Bg
The Bg-factor (or gas formation volume factor) is a
measure of how much the gas volume increases
from reservoir to standard conditions.
The changes in gas volume during production are
larger than the changes in oil volume.
The gas volume increases approximately as much
as the pressure decreases.
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14. Notes about Rs
The definition of the solution gas/oil ratio (RS)
takes its starting point in a volume element of oil at
reservoir conditions.
RS expresses the ratio between the standard
volume of gas and standard volume of oil produced
from this particular volume element.
 The reservoir pressure will decrease during
production. From the time the pressure reaches the
saturation pressure, two phases will be present, an
oil phase and a gas phase.
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15. Bo-Factor as a Function of Pressure
Figure shows a plot of the Bo-factor
against pressure in the differential
liberation experiment at 97.5°C on
oil composition
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16. Bo Trend during Production
Owing to continuous liberation of gas, the amount
of gas dissolved in the oil will decrease with
decreasing pressure. This will result in decreasing
Bo-factors and gas/oil ratios with decreasing
pressure.
It is seen that the Bo-factor increases with
decreasing pressure above the saturation point.
This is because the oil expands with decreasing pressure
until it starts releasing gas.
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17. Rs as a Function of Pressure
Figure shows a plot of RS toward
pressure in the differential liberation
experiment at 97.5°C on oil
composition
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18. Rs Trend during Production
Above the saturation point, RS is constant
 Because the composition of the produced reservoir fluid
is constant until the saturation point is reached.
Below the saturation point, RS decreases with
decreasing pressure. The gas liberated from the oil
just below the saturation point primarily consists of
lighter gas components. As the pressure is further
decreased, the content of heavier compounds in
the gas will increase. This is reflected in an
increasing gas gravity with decreasing pressure.
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19. Primary Results from a Differential
Liberation Experiment
Primary results from a differential liberation
experiment performed on an oil mixture:
The quantities measured in a differential liberation
experiment are summarized as below
Bo (i.e., oil volume at actual pressure, divided by volume of
residual oil at standard conditions)
Rs (i.e., total standard volume of gas liberated at lower
pressure stages than the actual one, divided by the volume of
the residual oil at standard conditions)
Oil Density (Density of oil phase at cell conditions)
Bg (Gas formation volume factor defined as gas volume at the
actual pressure divided by the volume of the same gas at
standard conditions)
Z-factor gas (Refers to depleted gas at cell conditions)
Gas gravity
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20. Design Objectives
The differential liberation test is considered to better
describe:
The separation process taking place in the reservoir and
Is also considered to simulate the flowing behavior of
hydrocarbon systems at conditions above the critical gas
saturation.
As the saturation of the liberated gas reaches the
critical gas saturation, the liberated gas begins to flow,
leaving behind the oil that originally contained it. This is
attributed to the fact that gases have, in general, higher
mobility than oils. Consequently, this behavior follows
the differential liberation sequence.
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21. Idealized Comparison of Flash and
Differential Gas Solubilities vs. P
This relationship between the two
processes may occur as shown or in
reverse, depending upon the
composition of the hydrocarbon
system.
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24. Schematic Representation of
a Three-Stage Separator Experiment
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25. Separator Test Procedure
Separator experiments are carried out for both oil
and gas condensate mixtures.
The reservoir fluid is placed in a closed cell
(henceforth referred to as a separator) at a pressure
and temperature somewhat below the pressure
and temperature in the reservoir.
Typical conditions can be 70 bar (~1015 psi) and 50°C
(~120 °F), at which the reservoir fluid mixture separates
in a gas and a liquid phase.
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26. Separator Test Procedure (Cont.)
The gas is let out of the separator through the top
and is transferred to standard conditions, where its
volume is measured.
 As for the differential liberation experiment, liquid
dropping out from the gas is converted to an
equivalent gas volume at standard conditions.
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27. 2nd Stage of Separator Test
The liquid from the first separator is let into a
second separator at a lower pressure and
temperature than the first one, at which conditions
more gas will be liberated.
As with the gas from the first separator, this gas is
transferred to standard conditions. The oil from the
last separator at standard conditions is often called
stock tank oil, and the volume of this oil is called
stock tank oil volume.
The term stock tank signals that the oil can be stored at
atmospheric conditions without liberating gas.
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28. Purpose of the Experiment
The purpose of a separator experiment is to get a
first idea about the relative volumetric amounts of
gas and oil produced from a particular petroleum
reservoir.
The separator gas/oil ratio equals the ratio
between the volume of the gas liberated from the
current stage taken to standard conditions and the
volume of the oil from the last separator stage,
which is at standard conditions. The separator
gas/oil ratio for separator number N becomes:
𝒈𝒂𝒔
𝑺𝒆𝒑𝒂𝒓𝒂𝒕𝒐𝒓 𝒈𝒂 𝒔 𝒐 𝒊𝒍 𝒓𝒂𝒕𝒊𝒐:
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𝑽 𝑵,𝒔𝒕𝒅
𝒐𝒊𝒍
𝑽 𝒔𝒕𝒅
Reservoir Fluid Properties Course: PVT Experiments (DL & Other Experiments)
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29. Primary Results From
a Separator Experiment
Below is a summary of the results reported from a
separator experiment Performed on an Oil or a Gas
Condensate Mixture
Separator gas/oil ratio (Volume of gas from actual separator
stage at standard conditions divided by the volume of the oil
from the last stage (at atmospheric conditions))
Gas gravity
Separator Bo (volume of oil at actual separator stage, divided
by volume of oil from last stage (at atmospheric conditions).
For oil mixture it is customary also to report Bo of the
saturated reservoir oil)
Gas compositions (Molar compositions of separator gas in
each stage)
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31. Swelling Test Procedure
A swelling test (or swelling experiment) starts with a
reservoir oil at its saturation point in a PVT cell kept at
the reservoir temperature.
A known molar amount of injection gas is transferred
into the PVT cell.
The pressure is increased, maintaining a constant
temperature until all the gas has dissolved. When the
last gas bubble disappears, the new cell mixture (oil +
injected gas) is at its saturation point.
The pressure and
The swollen volume are recorded.
 More gas is injected, and the pressure increased until
all gas is in solution in the oil.
This process is repeated for a number of stages.
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32. Schematic Representation of
Swelling Experiment
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33. The Experiment Objectives
A swelling experiment is carried out to investigate
how a reservoir fluid will react to gas injection.
To the extent the gas dissolves in the oil, the oil
volume will increase (the oil will swell) and the
saturation point of the oil will increase.
The increase in volume and saturation pressure are
key factors in determining whether gas injection
will result in an enhanced recovery.
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34. Gas injection Objectives
 Years back, when pipelines for transporting gas
from reservoir to consumer were rare, injection of
natural gas into a reservoir was primarily seen as a
way of getting rid of excess gas and to a less extent
as a way of enhancing the oil recovery.
Today, gas injection often means CO2 injection and
is seen both as a way of decreasing the release of
CO2 to the atmosphere and as a means of
enhancing the oil recovery.
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35. Results from a Swelling Experiment
The experiment primarily gives information on the
volume increase (swelling behavior) as a result of a
particular gas dissolving in the oil and on how large
a pressure is needed to dissolve all the injected gas.
The swelling gas/oil ratio is defined as the
cumulative volume of the injection gas at standard
conditions per initial oil volume and differs from
other definitions of gas/oil ratio.
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36. Results from a Swelling Experiment
Results from a swelling experiment performed on
an oil mixture:
Mole percentage gas
Cumulative mole percentage gas added per initial mole oil
Gas/oil ratio
Standard volume of gas added per initial volume of oil
Saturation pressure
Saturation pressure after addition of gas
Swollen volume
Volume of oil-injection gas mixture at saturation point per
initial volume of oil
Density
Density of swollen mixture at saturation point
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37. PVT Experiments
Explain experiments goals
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39. Viscosity Experiment
The purpose of a viscosity experiment is to measure the
oil viscosity at constant temperature, typically the
reservoir temperature, at decreasing pressure.
 One frequently used experimental setup is a rollingball viscosimeter, where the viscosity is related to the
time it takes for a ball of a given weight and diameter to
fall from the top to the bottom of a cell filled with the
oil under investigation.
Gas viscosities are often seen reported along with the
oil viscosity, but they are most often found from a gas
viscosity correlation.
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40. Viscosimeter
Courtesy IPE, Tehran, 2012
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41. Slim Tube Experiment
Gas is often injected into oil reservoirs with the
purpose of obtaining an enhanced recovery.
 The gas may be nitrogen, carbon dioxide, or natural gas.
An enhanced recovery can only be expected if the oil
and gas are miscible.
Miscibility can be achieved at the injection well, at the gas–oil
front or somewhere in between. Miscibility allows a complete
displacement of the reservoir fluid.
To avoid gas breakthrough it is of much interest to find
out whether a reservoir oil and an injection gas are
miscible at the actual reservoir pressure.
The lowest pressure at which miscibility is obtained is called
the minimum miscibility pressure, or MMP. This pressure may
be determined using a slim tube apparatus.
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42. Schematic Representation of
a Slim Tube Apparatus
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43. Slim Tube
Courtesy AUT, Tehran, 2011
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44. Multiple-Contact Experiment
The purpose of the multiple-contact
experiment is to develop an
understanding of the phase
equilibria near a well with gas
injection.
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45. Multiple Contact
Experiment
Explain multiple-contact experiment.
What is the recovery in this experiment?
How could we find out MMP?
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46. 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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47. 1.
2.
3.
4.
General Notes about EoS
Ideal Gas EoS
Compressibility Factor
Van Der Waals EoS
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