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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
• Vo• Liquid
• 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