Q913 rfp w2 lec 6

Reservoir Fluid Properties Course (1st Ed.)

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http://www.about.me/AlamiNia

mailto:[email protected]

http://www.about.me/AlamiNia

1. Formation Volume FactorA. 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


CME Experiment

Names: Constant-mass expansion (CME)

Constant-composition expansion (CCE)

Flash vaporization (FV)

Flash Liberation

Flash Expansion


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.



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.

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.



Flash Liberation P-V Diagram

Step 4. A plot of the cell pressure-total hydrocarbon volume is

constructed as shown in Figure.

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.


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


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.


Constant-Composition Experiment for a Gas Condensate


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.


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.


Calculations of the Experiment for a Gas CondensateThe 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: 𝑽𝒓𝒆𝒍 =𝑽𝒕𝒐𝒕

𝑽𝒔𝒂𝒕

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:

This liquid volume is often referred to as the liquid dropout.


%𝑳𝒊𝒒𝒖𝒊𝒅 𝒅𝒓𝒐𝒑𝒐𝒖𝒕 = 100 ∗𝑽𝒍𝒊𝒒

𝑽𝒔𝒂𝒕

Primary Results From the Experiment on a Gas Condensate MixturePrimary Results from a Constant–Mass Expansion

Experiment Performed on a Gas Condensate Mixture.Relative volume

Liquid volume

Z-factor (Only reported above saturation point)



Example of Results of the Experiment

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


Liquid Dropout and Relative Volume Curve

Liquid dropout curve (circles, full-drawn line and left y-axis) and

relative volume (triangles, dashed line and right y-axis) for constant-

mass expansion experiment at 155 °C on the gas condensate mixture.

Calculations of the Experiment for Oil MixturesFor oil mixtures, the isothermal compressibility is

recorded above the saturation point:

In this expression, V is the oil volume.

Below the saturation point, the Y-factor is recorded:

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.


𝒄𝒐 = −1

𝑽

𝜕𝑽

𝜕𝑷𝑻

𝒀 − 𝒇𝒂𝒄𝒕𝒐𝒓:

𝑷𝒔𝒂𝒕 − 𝑷𝑷

𝑽𝒕𝒐𝒕 − 𝑽𝒔𝒂𝒕

𝑽𝒔𝒂𝒕

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 single-phase 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.


Results of the Experiment at 97.5 ° C for the Oil Mixture



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 y-axis) for constant mass expansion

experiment at 97.5 ° C on the oil mixture

Primary Results From the Experiment on an Oil MixtureBelow 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)


Constant-Volume Depletion Experiment introductionAs 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.



Schematic Representation of the Experiment

Schematic Representation of a Constant-Volume Depletion

Experiment

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.


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).


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.


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.


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


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.


Notes about Differential Liberation ExperimentThe 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.



Schematic Representation of a Differential Depletion Experiment

Psat=Pb

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.


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.


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.


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.


1. Differential Liberation Experiment: Data set

2. Separator Experiment

3. Swelling Experiment

4. Other Experiments


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Q913 rfp w2 lec 6