PVT_Dr M Idrees_Parts 1 and 2

8/11/2019 PVT_Dr M Idrees_Parts 1 and 2

1/49

PVTPressure Volume Temperature

(Parts 1 & 2)

Reservoir Engineering I

(PCB2023)


2/49

Outcomes

To describe various tests under PVT study

To relate oil physical properties generated

from PVT study for MBE applications

To determine gas physical properties from

PVT study


3/49

Importance of PVT Analysis

Provides data for field evaluation and design

Reservoir calculations

Well flow calculations

Surface facilities


4/49

Scope of PVT Analysis Scope of the analysis depends on the nature of the fluid.

Oil systems: Black oil and volatile oil

Bubble point pressure, composition of reservoir and produced fluids, Bo, GOR, oil

viscosity, Co.

Below Pbconsiderations: Bg, Bt, Z, gas viscosity.

Properties are measured as functions of pressure.


5/49

Scope of PVT Analysis Dry gas:

composition, specific gravity, Bg, z, and viscosity Wet gas:

as above plus information on liquid drop out, quantities and compositions.


6/49

Scope of PVT Analysis

Gas condensate:

Reflect wet gas and oil.

Dew point pressure

Compressibility above Pd. Impact of dropping below Pd


7/49

Sampling

Clearly the sample has to representative of

the reservoir contents or the drainage area.

Desirable to take samples early in the life ofthe reservoir.

Either sub-surface or surface sampling.


8/49

Sub-Surface Sampling

Can only be

representative whenpressure at sampling

point is above or equal

to the saturation

pressure.

At pressure close to

saturation pressure

serious possibility of

sample integrity beinglost.

In recent years

considerable advance in

downhole fluid sampling


9/49

Surface Sampling

Samples of oil and gas taken from a specialseparator connected with the well called the

test separator.

Fluids recombined in the laboratory on thebasis of the produced GOR.


10/49

Surface Sampling The separation of oil

and gas as predicted

by the phase diagramresults in each phase

having its own phase

diagram.

The oil exists at itsbubble point .

The gas exists at its

dew point.

This behavior has

important implications

on well sampling


11/49

Equipment for PVT Analysis

Apparatus for transfer and recombination ofseparator oil and gas samples.

Apparatus for measuring gas and liquid

volumes

Apparatus for performing separator tests

PVT cell and displacing pumps.

High pressure viscometer

Gas chromatograph or equivalent.


12/49

Main PVT Tests

Quality check of surface samples

Compositional measurements

Flash vaporization (Constant composition

expansion, CCE) or relative volume test

Differential vaporization test

Separator tests

Density measurements

Viscosity measurements

Special studies: e.g. Interfacial tension


13/49

Quality check of Surface samples

Samples received in the laboratory are evaluated for theirintegrity, primarily by measuring the opening pressure and

comparing with the reported sampling conditions.

This may be examined by heating the sampling bottles to

the sampling temperature.

Any leakage from a sampling bottle containing a gas-liquid

mixture will change the sample composition.


14/49

Compositional measurements

An important test on all reservoir fluid samples is the

determination of the fluid composition.

The most common method of compositional analysis of high

pressure fluids is to flash a relatively large volume of the

fluid sample at the atmospheric pressure to form generallytwo stabilized phases of gas and liquid. The gas and liquid

phases are commonly analyzed by gas chromatography and

distillation, respectively.


15/49

Flash Vaporization (CCE or Relative Volume) Test

Determination of the correlation between pressure

and volume at reservoir temperature.

The system never changes during the test.

The gas remains in equilibrium with the oil through

out the test.

The behavior below the bubble point does not

reflect reservoir behavior, where gas has greater

mobility than the oil.

This test determines the Bubble Pointpressure

corresponding to the reservoir temperature.


16/49

Flash Vaporization (CCE or

Relative Volume ) Test

Liberated gas remains in equilibrium with oil


17/49

Flash Vaporization (CCE or Relative Volume ) Test

By plotting P versus V, a break in the slope is obtained at

the Bubble Point pressure.


18/49

Flash Vaporization (CCE or

Relative Volume ) Test

Tests at constant pressure

and varying temperature

enables thermal expansion

coefficient to be obtained for

well flow calculations.

12

2 2 1

1 1 2 2

V VThermal expansion,V T T

V volume at T , V volume at T


19/49

Flash Vaporization (CCE or Relative Volume )

Test

Above bubble point compressibility ofoil at reservoir temperature can be

determined.

No free gas

2 1

2 1 2

2 2

1 1

V Vc

V P P

V =volume at pressure P

V =volume at pressure P


20/49

Exercise 1 Flash vaporization

The data from a flash vaporization on a black oil at 220 oF are

given. DeterminePband prepare a table of relative volume forthe reservoir fluid study. (data in example 10-1, Mc Cain)

Pressure (psig) Total Volume (cc)

5000 61.030

4500 61.435

4000 61.866

3500 62.341

3000 62.8662900 62.974

2800 63.088

2700 63.208

2605 63.455

2591 63.576

2516 64.291

2401 65.532

2253 67.400

2090 69.901

1897 73.655

1698 78.676

1477 86.224

1292 95.050

1040 112.715

830 136.908

640 174.201

472 235.700


21/49

Differential Vaporization

Differential liberation process

Flash liberation process


22/49


The test has been designed to simulate gas-

liquid equilibrium system in oil reservoirs atpressures below the bubble point pressure.

The test starts from the bubble point pressure.

By this test it can be determined: solution gas-oil ratio, relative oil volume, total solution gas-

oil ratio at the bubble-point pressure, Z factor,

gas formation volume factor, and relative total

volume.


23/49

Differential Vaporization 8-10 pressure reduction steps at reservoir temperature.

Final step to 60oF.

Remaining oil Residual Oil


24/49


1. Solution gas oil ratio RsD

OUTPUTSfrom Differential Vaporization test

2. Relative Oil Volume, BoD

Volume of oil at each pressure divided by volume of oil at std conditions

(14.7 psia & 60 oF)


25/49


OUTPUTSfrom Differential Vaporization test

3. Total solution gas oil ratio at Pb, RsD

4. Z factorRscsc

scRR

TpV

TpVz

5. Gas formation volume factor,p

zTBg 0282.0

6. Relative Total Volume,BtD

)( sDsDbgoDtD RRBBB


26/49

Exercise 2: Differential Vaporization

The data from a differential vaporization on a black oil at 220 oF

are given. Prepare a table of solution gas-oil ratios, relative oilvolumes, and relative total volumes by this differential process.

Also include z-factors and formation volume factors of the

increments of gas removed.


27/49


Solution:

E i 2 Diff ti l V i ti


28/49


Solution:

E i 2 Diff ti l V i ti


29/49


Solution:



30/49

Exercise 2: Differential VaporizationSolution:



31/49

Exercise 2: Differential VaporizationSolution:


32/49

Separator TestsObjectives To determine impact of separator conditions on Bo, GOR, and

produced fluid physical properties. To determine the optimum operating conditions of the separator

Procedure


33/49

Separator Tests

Separator volume factor = L1/L2

PVT Cell pressure kept atbubble point

Separator volume factor = L1/L2


34/49

Separator TestsCalculations


35/49

Exercise 3: Separator Test

Data from a separator test on a black oil are given. Note that the volume

of separator liquid was measured at separator pressure andtemperature before it was released to the stock tank. Calculate the

formation volume factor and solution gas oil ratio.

Volume of oil at Pb and Tres = 182.637 cc

Volume of separator liquid at 100 psig and 75 oF = 131.588 cc

Volume of stock-tank oil at 0 psig and 75 oF = 124.773 cc

Volume of stock-tank oil at 0 psig and 60 oF = 123.906 cc

Volume of gas removed from separator = 0.52706 scf

Volume of gas removed from stock tank = 0.07139 scf

SG of stock tank oil = 0.8217

SG of stock separator gas = 0.786

SG of stock tank gas = 1.363


36/49

Solution of Exercise 3


37/49

Selection of Separator ConditionsThe optimum operating pressure is identified from the separator

tests as the separator pressure which results in a minimum of

total gas-oil ratio, a minimum in formation volume factor of oil (atbubble point), and a maximum in stock-tank oil gravity (API).

Example of selecting optimum separator conditions for Good Oil

Co. Well No. 4.


38/49

Flash vaporization is used to characterize reservoir fluid above and below reservoir

bubble point pressure.

Differential vaporization considered to be representative of the process in the

reservoir below bubble point pressure.

Separator test considered to be representative of the process from the bottom of the

well to the stock tank when the reservoir pressure is equal or less than Pb.

COMPARISON BETWEEN THE

THREE TESTS

Under these assumptions, fluid properties abovebubble

point pressure can be estimated by a combination of Flashvaporization and separator test.

Fluid properties belowbubble point pressure can be simulated

by a combination of differential vaporization and separator test.


39/49

OIL FORMATION VOLUME FACTOR

FOR MBE & RESERVOIR STUDIES

At pressures above bubble-point pressure, oil formation volume factors

are calculated from a combination of flash vaporization data and

separator test data.

P Pb

At pressures below the bubble-point pressure, oil formation

volume factors are calculated from a combination of

differential vaporization data and separator test data.

P Pb


40/49

Example of Oil Formation Volume Factor Data

Oil Formation Volume Factor at 200 F

1.000

1.100

1.200

1.300

1.400

1.500

1.600

0 1000 2000 3000 4000 5000

Pressure, psig

OilFormationVo

lumeFacto

bbl/stb

exp sim

SOLUTION GAS OIL RATIO


41/49

SOLUTION GAS OIL RATIO


Solution gas-oil ratio at pressures above bubble-point pressure is a

constant equal to the solution gas-oil ratio at the bubble point.

@ P Pb

Solution gas-oil ratios at pressures below, bubble-point pressure are

calculated from a combination of differential vaporization data and

separator test data.

@ P < Pb


42/49


43/49

GAS FORMATION VOLUME FACTOR


Gas formation volume factors are calculated with z-factors measured

with the gases removed from the cell at each pressure step during

differential vaporization.

TOTAL FORMATION VOLUME FACTOR

Total formation volume factors may be

calculated as

If relative total volumes, Btare reported as a part of the results of the

differential vaporization, total formation factors can be calculated as:


44/49

COEFFICIENT OF ISOTHERMAL

COMPRESSIBILITY OF OIL

The following Equation may be used with the flash vaporization data to

calculate oil compressibility at pressures above the bubble point.

When the pressure is below the bubble point pressure, the following

equation can be used to calculate the Co



45/49


Oil Density at 200 F

0.700

0.720

0.740

0.760

0.780

0.800

0.820

0.840

0 1000 2000 3000 4000 5000

Pressure, psig

OilDensity,g

/cc

exp sim

Density of oil at reservoir temperature and different pressures

can be measured by an instrument attached to the PVT cell.

Example of Density Data


46/49

Viscosity measurements


47/49

Oil and Gas Viscosity

0.000

0.500

1.000

1.500

2.000

2.500

0 1000 2000 3000 4000 5000

Pressure, psig

Viscosity

,cP

Oil viscosity Gas viscosity

Example of Viscosity Data


48/49

Special Studies of Reservoir Fluids:

Different methods of enhanced oil recovery (EOR)

require different reservoir fluid studies.

Examples:

EOR by miscible gas injection requires measuring the

minimum miscibility pressure.

EOR by surfactant flooding requires measuring the interfacial

tension.

EOR by foam flooding requires measuring foaming ability ofthe surfactant used in presence of reservoir fluids.

High Pressure / High Temperature, HP/HT


49/49

High Pressure / High Temperature, HP/HT

Fluids

Recent years exploration activity has moved deeper.

High pressure and temperature accumulations found

Conventional PVT facilities do not enable testing

these fluids. Ranges 250oC and 20,000 psi.

At these conditions role of water cannot be ignored.

PVT_Dr M Idrees_Parts 1 and 2

Documents