Reservoir Fluid Properties Course (1st Ed.)

1. Constant-mass expansion Experiment

2. Constant-Volume Depletion Experiment

3. Differential Liberation Experiment: Procedure

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1. Differential Liberation Experiment: Data set

2. Separator Experiment

3. Swelling Experiment

4. Other Experiments

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Experimental Data of Differential Liberation TestThe 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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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:

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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𝑮𝒂𝒔 𝒈𝒓𝒂𝒗𝒊𝒕𝒚 =𝑴𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒈𝒂𝒔

𝑴𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒂𝒕𝒎𝒐𝒔𝒑𝒉𝒆𝒓𝒊𝒄 𝒂𝒊𝒓

Obtaining Bg

The volume at standard conditions of the liberated gas is measured.

This enables calculation of the gas formation volume factor, Bg:

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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𝑩𝒈 =𝑮𝒂𝒔 𝒗𝒐𝒍𝒖𝒎𝒆 𝒂𝒕 𝒄𝒆𝒍𝒍 𝒄𝒐𝒏𝒅𝒊𝒕𝒊𝒐𝒏𝒔

𝑮𝒂𝒔 𝒗𝒐𝒍𝒖𝒎𝒆 𝒂𝒕 𝒔𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒄𝒐𝒏𝒅𝒊𝒕𝒊𝒐𝒏𝒔

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

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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𝑩𝒐 𝑵 =𝑽𝑵𝒐𝒊𝒍

𝑽𝒔𝒕𝒅𝒐𝒊𝒍

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)

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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𝑹𝑺 𝑵 = 𝒏=𝑵+1𝑵𝑺𝑻 𝑽𝒔𝒕𝒅,𝒏

𝒈𝒂𝒔

𝑽𝒔𝒕𝒅𝒐𝒊𝒍

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Differential Liberation Experiment

Differential Liberation Experiment in a P–T Diagram

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

As an equivalent gas volume.(The volume the liquid would take up had it been in

gaseous form)

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𝑹𝑺 𝑵 = 𝒏=𝑵+1𝑵𝑺𝑻 𝑽𝒔𝒕𝒅,𝒏

𝒈𝒂𝒔

𝑽𝒔𝒕𝒅𝒐𝒊𝒍

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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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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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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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

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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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

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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Primary Results from a Differential Liberation ExperimentPrimary results from a differential liberation

experiment performed on an oil mixture:The quantities measured in a differential liberation

experiment are summarized as belowBo (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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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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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.

Schematic Representation of a Three-Stage Separator Experiment

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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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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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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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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:

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𝑺𝒆𝒑𝒂𝒓𝒂𝒕𝒐𝒓 𝒈𝒂 𝒔 𝒐 𝒊𝒍 𝒓𝒂𝒕𝒊𝒐:𝑽𝑵,𝒔𝒕𝒅𝒈𝒂𝒔

𝑽𝒔𝒕𝒅𝒐𝒊𝒍

Primary Results From a Separator ExperimentBelow is a summary of the results reported from a

separator experiment Performed on an Oil or a Gas Condensate MixtureSeparator 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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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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Schematic Representation of Swelling Experiment

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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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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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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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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 ratioStandard volume of gas added per initial volume of oil

Saturation pressureSaturation pressure after addition of gas

Swollen volumeVolume of oil-injection gas mixture at saturation point per

initial volume of oil

DensityDensity of swollen mixture at saturation point

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PVT Experiments

Explain experiments goals

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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 rolling-ball 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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Viscosimeter

Courtesy IPE, Tehran, 2012

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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Schematic Representation of a Slim Tube Apparatus

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Slim Tube

Courtesy AUT, Tehran, 2011

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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.

Multiple Contact Experiment

Explain multiple-contact experiment.

What is the recovery in this experiment?

How could we find out MMP?

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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. General Notes about EoS

2. Ideal Gas EoS

3. Compressibility Factor

4. Van Der Waals EoS

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