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PVT Short Course for Cairo UniversitySPE Student Chapter

Ahmed Muaz

Khalda Petroleum Company

April 23, 2011


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Contents

Why Study PVT?

Objectives of PVT Analysis

Types of Reservoir Fluids

Differentiation Between the Different Types

of

Reservoir Fluids

Fluid Sampling

Laboratory Experiments

PVT Reports


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Why Study PVT?

Transport

.Sep

Gas

StockTank

Oil

.Diff Flow Regime


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Objectives of PVT Analysis

Need to understand PVT analysis for:

Determination of reservoir fluid type

Knowledge of physical properties of reservoir fluids

Match an EOS to the measured data

Creation of PVT models

Ultimate recoveries of different components

Amounts and composition of liquids left behind

Determination of well stream composition


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Objectives of PVT Analysis Cont.

Completion design

Separator/NGL plant specifications

Product values Vs. time

Prediction compositional gradient


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The Five Reservoir Fluids

Black Volatile Retrograde Wet Dry

Oil Oil Gas Gas Gas


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Reservoir Fluid Type Identification

Analyzing a representative sample in the lab

Production data

1.Initial GOR

2.API

3.Oil color

4.C7+


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Black Oil

An initial GOR, 1750 SCF/STB or less

A 45 API initial stock tank oil gravity or less

A dark color of stock tank oil

C7+ > 20 mole %

Initial Bo of 2 res bbl/STB or less


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Black Oil Phase Diagram

T h e in itia l re se rvo ir

( & )con d ition s P T are

m u ch lo w e r th a n

th e critica l

co n d itio n s

T h e h e a vy m o le cu le s

co n ce n tra tio n is

h ig h


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Volatile Oil

An initial GOR, 1750 SCF/STB or greater

A 51.2 API initial stock tank oil gravity

A medium orange stock tank oil color

C7+ concentration between 19 and 22 mole %

Bo 2.0 res bbl/STB or greater


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Volatile Oil Phase Diagram

The initial reservoir

conditions (P&T)

are close to the

critical conditions

Have fewer heavy

molecules than

black oil


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Retrograde Gas Condensate

An initial GOR > 3200 SCF/STB

API > 45 initial stock tank oil gravity

A light stock tank oil color

C7+ concentration


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Retrograde Gas Phase Diagram

The initial reservoir

conditions (P&T) arehigher than thecritical conditions

Have fewer heavy

molecules thanvolatile oils

The overallcomposition of the

reservoir fluidbecomes heavier asthe lighter gas isproduced and theheavier condensate

remains behind


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Wet Gas

An initial GOR > 15,000 SCF/STB

API up to 70 initial stock tank oil gravity

Water white of tank oil color

C7+ concentration


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Wet Gas Phase Diagram

Reservoir

Temperature aboveCricondentherm(pressure path notenter phaseenvelope)

The reservoir fluid isgas throughout thelife of the reservoir

The separatorcondition is twophases

Gravity of stock tankliquid same as

retrograde gas

Temperature

% Liquid

2

1

ressure pathn reservoir

e t g as

Criticalpoint

Bubble

point

line

Separator

150

Dewp

oint

line


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Dry Gas

An initial GOR > 1000,000 SCF/STB

Almost no liquid

C7+ concentration < 0.7 mole %


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Dry Gas Phase Diagram

Primarily methane and

some intermediates

The reservoir fluid isgas throughout thelife of the reservoir

No liquids formedeither in reservoir orat surface

Temperature

% Liquid

2

1

ressure pathn reservoir

r y g as

Separator

Dewp

o in t

l in e

150


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R e t r o g r a d ee t r o g r a d eR e g i o ne g i o n

w o P h a s e R e g io nwo P h a s e R e g i o nTemperatureTemperature

Pressur

e

Pressur

e

I LI L G A Slack Oil Volatile Critical Critical Gas Wet

Oil Oil Gas

Cricondenterm

ew PointLociubblePointLoci

r it ic al P oi ntricondenbar

A

B

C

eservoir Fluids Phase DiagramWindow


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C7+ & GOR for Reservoir Fluids


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Classification


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Composition of Reservoir Fluids

Differences Between Black and


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Differences Between Black andVolatile Oil

Black Oil

The evolved gas is a dry gas.

The solution gases remain gas phase in the

reservoir, tubulars and separator.

As reservoir pressure decreases, the gas leaving

solution, becomes richer in intermediate

components.

API gradually decreases during the reservoir life.

Phase Diagram of Black Oil and


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Phase Diagram of Black Oil andAssociated Gas

Production Processes for a Black


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Production Processes for a BlackOil

Differences Between Black and


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Differences Between Black andVolatile Oil

Volatile Oil

The evolved gas is a retrograde gas.

The evolved retrograde gases release a large

amount of. condensate at surface conditions.

Often over one half of the stock tank liquid produced

during the reservoirs life.

API steadily increases with time.

Phase Diagram of Volatile Oil and


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Phase Diagram of Volatile Oil andAssociated Gas

Production Processes for a Volatile


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Production Processes for a VolatileOil


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Analysis and Prediction Tool

Black Oil Mbal

Assume free gas in the reservoir remains gas in the

separator

Treat a multi component black oil mixture as a two-

component mixture: gas and oil

Volatile Oil Mbal

Treat mixture as a multi-component mixture

Total composition of the production stream is known

Effect of Using Black Oil Mbal for


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Effect of Using Black Oil Mbal forVolatile Oil

Differentiation between Volatile and


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Differentiation between Volatile andRetrograde

A G O R o f 3 2 0 0

/S C F S TB is a g oo d

-cu t o ff

.A va lu e o f 1 2 5 m o le

% +of C7 is a

u se fu l d iv id in g lin e

Differences between Volatile Oil


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Differences between Volatile Oiland Retrograde Gas

Retrograde Gas GOR increases with time as condensate dropout

API increases with time

Compositional Mbal should be used in reservoircalculations

Conventional gas Mbal can be used above dew point

Also, it can be used below dew point if two phase Z-

factors are used

Differences between Retrograde


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Differences between Retrogradeand Wet Gas

A n in itia l G O R o f

, /1 5 0 0 0 S C F S T B

ca n b e a cu to ff

%A va lu e o f 4 m o le

o r le ss ca n b e

u se fu l d iv id in g lin e


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of

Reservoir Fluids

R i Fl id S li


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Reservoir Fluid Sampling

Open Hole Sampling:

RFT, small sample volume and often contaminated

MDT, allows controlled drawdown and multiple

sample chambers

Optical MDT, allows to identify the type of fluid

being sampled

Cased Hole Sampling:

Surface Sampling

Subsurface Sampling

f b f li


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Surface & Subsurface Sampling

W ll C di i i d i S li


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Well Conditioning during Sampling

Small perforation is preferable

Limiting drawdown

Large tubing diameter in case of high rates

Better well cleaning

Stable production

No liquid slugging

Small amount of produced fluid prior to sampling

L b T


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Laboratory Tests

Primary tests

Routine laboratory tests

Special laboratory PVT tests

P i T t


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Primary Tests

API

Gas specific gravity

Separator gas composition

GOR

Routine Laboratory Tests


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Routine Laboratory Tests

Constant-Composition Expansion - CCE

Differential Liberation

Constant-Volume Depletion CVD

Separator Test

Constant Composition Expansion -


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Constant Composition Expansion CCE Test

The test is conducted for the purposes of

determining:

Saturation pressure (bubble-point or dew-point

pressure)

Isothermal compressibility coefficients of the single-

phase fluid in excess of saturation pressure

Compressibility factors of the gas phase

Total hydrocarbon volume as a function of pressure

CCE T t P d


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CCE Test Procedures

GasLiquid

Hg

SecondStep

Liquid

Hg

LiquidLiquid

Hg

FirstStep

Hg

ThirdStep

Hg

FourthStep

Liquid

GasVtLiquid

Vt

LiquidVtVt LiquidVt

pb

CCE Test Data


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CCE Test Data

Diff ti l Lib ti T t


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Differential Liberation Test

The experimental data obtained from the test

include:

Amount of gas in solution as a function of pressure

The shrinkage in the oil volume as a function of

pressure

Properties of the evolved gas including the

composition of the liberated gas, the gascompressibility factor, and the gas specific gravity

Density of the remaining oil as a function of

pressure

erent a erat on xpans on est


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Vo Liquid

Hg

Gas

Hg

Liquid

Gas

First step

Hg

Liquid

GasLiquid

Hg

Vo

Gas

Vo

Hg

Liquid

pb

erent a erat on xpans on estProcedures

Differential Liberation Test Data


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Differential Liberation Test Data

Separator Test Procedures


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Separator Test Procedures

LiquidHg

Liquid

Stocktank

Gas

Gas

Liquid

Se

pa

ra

tor

Hg

pb

resbbl

STB

resbbl

STBBob =

scf

scf

Rsb =scfSTB

Separator Test Data

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