PVT Short Course for Cairo UniversitySPE Student Chapter

Ahmed MuazKhalda Petroleum Company

April 23, 2011

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

Why Study PVT?

Transport

Sep.

Gas

Stock Tank

Oil

Diff. Flow Regime

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

Objectives of PVT Analysis – Cont.

• Completion design

• Separator/NGL plant specifications

• Product values Vs. time

• Prediction compositional gradient

The Five Reservoir Fluids

Black Volatile Retrograde Wet Dry

Oil Oil Gas Gas Gas

Reservoir Fluid Type Identification

• Analyzing a representative sample in the lab

• Production data

1. Initial GOR

2. API

3. Oil color

4. C7+

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

Black Oil Phase Diagram

• The initial reservoir

conditions (P&T) are

much lower than the

critical conditions

• The heavy molecules

concentration is high

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

Volatile Oil Phase Diagram

• The initial reservoir

conditions (P&T) are close

to the critical conditions

• Have fewer heavy

molecules than black oil

Retrograde Gas Condensate

• An initial GOR > 3200 SCF/STB

• API > 45 initial stock tank oil gravity

• A light stock tank oil color

• C7+ concentration <12.5 mole %

Retrograde Gas Phase Diagram

• The initial reservoir conditions (P&T) are higher than the critical conditions

• Have fewer heavy molecules than volatile oils

• The overall composition of the reservoir fluid becomes heavier as the lighter gas is produced and the heavier condensate remains behind

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 <4 mole %

Wet Gas Phase Diagram

• Reservoir Temperature above Cricondentherm (pressure path not enter phase envelope)

• The reservoir fluid is gas throughout the life of the reservoir

• The separator condition is two phases

• Gravity of stock tank liquid same as retrograde gas -constant during life of field

Pres

sure

Temperature

% Liquid

2

1

Pressure pathin reservoir

Wet gas

Criticalpoint

Bubb

lepo

int

line

Separator

152530

Dew

poin

t lin

e

Dry Gas

• An initial GOR > 1000,000 SCF/STB

• Almost no liquid

• C7+ concentration < 0.7 mole %

Dry Gas Phase Diagram

• Primarily methane and some intermediates

• The reservoir fluid is gas throughout the life of the reservoir

• No liquids formed either in reservoir or at surface Pr

essu

re

Temperature

% Liquid

2

1

Pressure pathin reservoir

Dry gas

Separator

Dew

poin

t lin

e

1

50

RetrogradeRegion

Two Phase Region

Temperature

Pres

sure

OIL GAS

Heavy Oil Black Oil Volatile Critical Critical Gas Wet Dry Oil Oil Gas Condens. Gas Gas

Cricondenterm

Dew PointLoci

Bubble PointLoci

Critical PointCricondenbar

A

B

C

Reservoir Fluids Phase Diagram Window

C7+ & GOR for Reservoir Fluids

Reservoir Fluid Types Classification

Composition of Reservoir Fluids

Comp Name

Black Oil Volatile Oil

Gas Conden.

Dry Gas.

C1 43.83 64.36 87.07 95.85 C2 2.75 7.52 4.39 2.67 C3 1.93 4.74 2.29 0.34 C4 1.6 4.12 1.74 0.52 C5 1.15 2.97 0.83 0.08 C6 1.59 1.38 0.6 0.12 C7+ 42.15 14.91 3.8 0.42 100 100 100 100

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 Associated Gas

Production Processes for a Black Oil

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

reservoir’s life.

• API steadily increases with time.

Phase Diagram of Volatile Oil and Associated Gas

Production Processes for a Volatile Oil

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 Volatile Oil

Differentiation between Volatile and Retrograde

• A GOR of 3200 SCF/STB

is a good cut-off

• A value of 12.5 mole %

of C7+ is a useful

dividing line

Differences between Volatile Oil and Retrograde Gas

Retrograde Gas• GOR increases with time as condensate dropout

• API increases with time

• Compositional Mbal should be used in reservoir calculations

• 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 and Wet Gas

• An initial GOR of 15,000

SCF/STB can be a cutoff

• A value of 4 mole% or

less can be useful

dividing line

Field and Laboratory Identification ofReservoir Fluids

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

Surface & Subsurface Sampling

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

Laboratory Tests

• Primary tests

• Routine laboratory tests

• Special laboratory PVT tests

Primary Tests

• API

• Gas specific gravity

• Separator gas composition

• GOR

Routine Laboratory Tests

• Constant-Composition Expansion - CCE

• Differential Liberation

• Constant-Volume Depletion – CVD

• Separator Test

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 Test Procedures

GasLiquid

Hg

Second Step

Liquid

Hg

LiquidLiquid

Hg

First Step

Hg

Third Step

Hg

Fourth Step

Liquid

GasVtLiquid

Vt

LiquidVtVt LiquidVt

pb

CCE Test Data

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 gas compressibility factor, and the gas specific

gravity

• Density of the remaining oil as a function of pressure

VoLiquid

Hg

Gas

Hg

Liquid

Gas

First step

Hg

Liquid

Gas Liquid

Hg

Vo

Gas

Vo

Hg

Liquid

pb

Differential Liberation Expansion Test Procedures

Differential Liberation Test Data

Separator Test Procedures

LiquidHg

Liquid

Stock tank

Gas

Gas

Liquid

Sep

arat

or

Hg

pb

resbbl

STB

resbblSTB

Bob =

scf

scf

Rsb =scfSTB

Separator Test Data

CVD Test Procedures

Constant Volume Depletion – CVD Test

PVT Reports

Thank You