Differential Liberation PVT fluid test

Reservoir Fluid Laboratory

Analysis Differential Liberation Test

Faculty of Petroleum & Mining Engineering

Suez University

By: Ahmed M. Saad

2013

Differential Liberation = Differential Vaporization

A PVT process has been designed in an attempt to provide

a means of in part simulating the changing systems as

separation occurs within the reservoir below the bubble

point.

The differential vaporization differs from the flash in that the liberated gas is removed from the cell stepwise.

It can be performed only for crude oil samples.

At each step below the bubble point the quantity of gas,

oil volume, density, gas expansion and gas compressibility

are determined.

The objectives of the differential test therefore are to

generate PVT data for conditions below the bubble point.

By Eng/Ahmed M. Saad Faculty of Pet.& Min Eng. Suez University Email : [email protected]

The following figure indicates the differential process :

Steps of the test :

The bubble point Pb is the starting pressure for the

differential test.

The next step is to reduce the pressure in the PV cell by

expansion of the PV cell volume .The reduction of pressure

causes the system to go two phase.

All the gas phase is removed at constant pressure .


By reducing the cell volume as gas is withdrawn before establishing equilibrium with the liquid phase and The

volume of the remaining oil is then determined.

The cell pressure is then again dropped by expansion of the PV cell and the above process repeated until the cell

pressure has been dropped to atmospheric pressure.

The pressure steps for the tests cover a range of around 8-

10 steps. All the above steps have taken place at reservoir temperature.

The final stage is to reduce the cell to 60°F keeping the pressure at atmospheric pressure .The final oil volume is measured.

This remaining all is termed residual oil to distinguish it from stock tank oil which although at the same pressure and temperature conditions has got there by a different process.

The cumulative weight of the amounts of gas withdrawn

are used in the calculation of the densities of the oil phase in the differential vaporization process .These densities can

also be determined directly if a pressure pycnometer is available.

Differential liberation is considered to be representative of

the gas-liquid separation process in the reservoir below the bubble point pressure.


Flash liberation is considered to take place between the reservoir and through the separator.

Differential liberation tests are carried out therefore to obtain oil formation volume factors and GOR’s that can be used to predict the behavior of a reservoir when the

pressure has dropped below the bubble point pressure.

Determination of Gas Composition , Z-factor, Specific Gravity : Composition of gas is determined from removed gas . Z-factor is determined from the following equation :

Where: V = volume of liberated gas in the PVT cell at P

and T.

Vsc = Volume of removed gas at standard condition.


Specific gravity of gas is determined from removed gas. The following figure is a plot for Gas Gravity against

pressure :

Determination of Oil density :

At each step oil volume is measured and from cumulative

weight of gas oil density is determined.

From Differential Liberation test

From Flash Vaporization test


Determination of Differential Solution Gas Oil Ratio (scf/STB) :

The removed gas at each step is flashed to standard

conditions and its volume is measured and from the cumulative volume of liberated gas measured at standard conditions is used to calculate gas oil ratio.

The following is an example from a real PVT report for

samples from Western Desert :


The following figure is a plot for Sol. Gas Oil Ratio :

Determination of Gas Formation Volume

Factor (βgd) :

It needs no correction to separator conditions.


The following is a plot for Gas Formation Volume Factor

against pressure :

Determination of Relative Oil Volume

(Diff. Oil FVF) βod :

The differential oil formation volume factors βod at all the

pressure levels are calculated by dividing the recorded oil

volumes V by the volume of residual oil Vsc .

The following Figure is a plot for Oil FVF :


Determination of Total Formation

Volume Factor βt :

where Rsb = gas solubility at the bubble-point pressure,

scf/STB

Rs= gas solubility at any pressure, scf/STB

βO= oil formation volume factor at any

pressure, bbl/STB

βg= gas formation volume factor, bbl/scf


The following figure illustrates the increase of βg with the

reduction of pressure :


Differential Liberation PVT fluid test

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