Pressure Volume Temperature
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Heriot-Watt University DEPARTMENT OF PETROLEUM ENGINEERING
PVT
Pressure Volume Temperature Adrian C Todd
PVT - Scope
Reservoir fluid analysis provides key data to the petroleum engineer. Quality of the testing is important to ensure realistic values used in design. Sample quality is the first quality issue.
PVT Analysis
Provides data for field evaluation and design
Reservoir calculations
Well flow calculations
Surface facilities
PVT Analysis
Correlation between pressure and volume at reservoir temperature. Various physical constants in reservoir calculations; viscosity, density, compressibility.
Effect of separator conditions on Bo & GOR. etc.
Chemical composition of the volatile components.
PVT Analysis
Scope of the analysis depends on the nature of the fluid.
Dry gas:
composition, specific gravity, Bg, z, and viscosity
Wet gas:
as above plus information on liquid drop out, quantities and compositions.
Oil system:
Bubble point pressure, composition of reservoir and produced fluids, Bo, GOR, Bt and viscosity. All as function of pressure. Co. Below Pb considerations.
PVT Analysis
Gas condensate:
Reflect wet gas and oil.
Dew point pressure
Compressibility above Pd.
Impact of dropping below Pd
Sampling
Clearly the sample has to representative of the reservoir contents or the drainage area. Desirable to take samples early in the life of the reservoir. Either sub-surface or surface sampling.
Sub-Surface Sampling
Can only be representative when pressure at sampling point is above or equal to the saturation pressure. At pressure close to saturation pressure serious possibility of sample integrity being lost. In recent years considerable advance in downhole fluid sampling
Surface Sampling
Samples of oil and gas taken from separator connected with the well. Fluids recombined in the laboratory on the basis of the produced GOR
Vertical and Horizontal Separators
Separator Gas Sampling
Separator Liquid Sampling by Gas Displacement
Separator Liquid Sampling by Water Displacement
Wellhead sampling
A low cost option. Only possible for very undersaturated sytsems. Still single phase at wellhead.
Sampling Wet Gas and Gas Condensate Systems
Use and value of any PVT study dependant on the quality of the sample collected. Sampling wet gas and gas condensate fluids can give rise to errors. During sampling procedure it is possible to alter the conditions so that samples are not representative. An important consideration is the phase behaviour.
Phase Behaviour.
Fluids uniquely described by phase diagram. Within the phase diagram system is two phase. Whereas outside the phase envelope single phase
Single phase
Two phase
Phase Behaviour.
The separation of oil and gas as predicted by the phase diagram results in each phase having its own phase diagram. The oil exists at its bubble point . The gas exists at its dew point. This behaviour has important implications on well sampling
Sampling Wet Gas and Gas Condensate Potential locations Reservoirs for reservoir sampling
Sampling Wet Gas and Location Gas Condensate 1. Reservoir Reservoirs
For.
2.Bottom-hole
3.Well Head
4. Separator
Against
1. Ideal
1. Impossible
2. 1-phase
2. Representative ? Technology, Cost, Handling.
3. Cost
3. 2-phase ? Representative ?,
4.Gas/liquid volumes,, separator conditions, 4. Cost, 1-phase, buffer,sampling volume Representative ?,
Sampling Wet Gas and Gas Condensate Reservoirs-Flowing well.
Well behaviour can significantly influence nature and characteristics of fluids produced. Flowing well gas condensate - mist flow
Sampling Wet Gas and Gas Condensate Reservoirs-Shut in well after flow.
Well acts as a separator Liquid rains down and accumulates at bottom of well. Pressure builds up in the well and disturbed formation. Some gas goes back into solution.
Well flow after shut in.
Large variations in compositions of produced fluids. Early period lean gas produced. High GOR When fluids produced from bottom of well. Liquids much lower GOR. Then fluids from disturbed reservoir zone Eventually fluids from undisturbed reservoir
Sampling Wet Gas and Gas Condensate Reservoirs
In assessing quality of samples important to know how long it will take for unrepresentative samples to be displaced from reservoir. Volumetrics required on wells, facilities and near well volumes
Separator Sampling Points
A very practical issue is the location of sampling points on separators. Often located for convenience for accessibility.
Liquid in gas line
Important to recognise that the gas and liquid in a separator are at their saturation pressure. Small changes will result in liquid drop out and gas being produced
Iso kinetic sampling
Gas in liquid line
Sampling Details
Important to record and keep note off following. These records to go with samples
Date and time
Cylinder identification
Location of sampling points.
Temperature and pressure
GOR in separator
Any special details ( H2S in sample, etc
Equipment for PVT Analysis
Apparatus for transfer and recombination of separator 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.
Equipment for PVT Analysis-Subsurface Samples
Equipment for PVT Analysis-Surface Samples
Equipment for PVT Analysis-Gas Condensate Samples
PVT Tests
To provide data for reservoir calculations
To provide physical property data for well flow calculations
For surface facility design
The reservoir calculations are the main driving force for the various tests.
Over recent years reservoir simulation capability has generated the need to extend compositional description from C7+ to in some cases C29+. PVT report provides source of all reservoir engineering properties for behaviour over exploration, development and production
Main PVT Tests
Flash vaporisation or relative volume test.
Differential vaporisation test.
Separator tests.
Viscosity measurements.
Compositional measurements.
Special studies: e.g. Interfacial tension.
Simple layout of a PVT Facility
Flash Vaporisation ( 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 behaviour below the bubble point does not reflect reservoir behaviour, where gas has greater mobility than the oil. This test determines the Bubble Point pressure corresponding to the reservoir temperature.
Flash Vaporisation (Relative Volume ) Test
Liberated gas remains in equilibrium with oil
Flash Vaporisation (Relative Volume ) Test By plotting P versus V, a break in the slope is obtained at the Bubble Point pressure.
Flash Vaporisation (Relative Volume ) Test
Tests at constant pressure and varying temperature enables thermal expansion coefficient to be obtained for well flow calculations.
Thermal expansion,
V2 V1
V2 T2 T1
V1 volume at T1 , V2 volume at T2
Flash Vaporisation (Relative Volume ) Test
Above bubble point compressibility of oil at reservoir temperature can be determined. No free gas
V2 V1 c V2 P1 P2 V2 =volume at pressure P2 V1 =volume at pressure P1
Flash Vaporisation (Relative Volume ) Test
Main objectives: Reservoir bubble point pressure. Together with information from separator tests, formation volume factor above bubble point.
Differential Vaporisation
Below bubble point in reservoir gas liquid separation in the reservoir is a constant changing system. A test has been design to attempt to simulate this process. In the differential vaporisation test liberated gas is removed from the cell step wise. At each step below bubble point, volumes densities , gas expansion and compressibility determined. Bubble point starting point.
Differential Vaporisation
Flash liberation process
Differential liberation process
Differential Vaporisation
Differential Vaporisation
Differential Vaporisation
8-10 pressure reduction steps at reservoir temperature.
Final step to 60oF.
Remaining oil Residual Oil
Differential Vaporisation vs.Flash Vaporisation
Flash liberation considered to take place between reservoir and surface. Differential liberation considered to be representative of the process in the reservoir below bubble point pressure. Differential tests carried out to obtain oil formation volume factors and GOR’s to predict behaviour below bubble point pressure.
Separator Tests
Objective to determine impact of separator conditions on Bo, GOR, and produced fluid physical properties. Not the interest of facility designers. Carried out to give an indication of oil shrinkage and GOR when fluids produced to surface. There are not uniques values for Bo & GOR. They depend on separator conditions.
Starting point for the test is the bubble point pressure.
Fluid produced at surface conditons. Stock tank oil
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
V res
Viscosity
Measured at different pressures above and below bubble point pressure. Below bubble point pressure carried out under differential conditions. Rolling ball or capillary tube methods of measurement
Hydrocarbon analysis
Analysis from C1 to an upper C number based on paraffin series. Historically C6 & C7+. Much higher analysis capability. C+ characterised by specific gravity and apparent molecular weight. Latter by depression of freezing point. Higher C+ characterisation helpful to process engineers re. solid phase formation.
Wax and Ashphaltenes
Solid phase formation series concern. Heavy components at low temperatures can form solid phases. Wax in transfer lines and process facilities. Ashphaltene are larger molecules of hydrogen and carbon plus sulphur, oxygen or nitrogen. Ashphaltenes do not dissolve in oil but are dispersed as colloids.
Wax Crystallization Different tests used: Temperature
Filtering and measuring resistance to flow at different temperatures. Appearance temperature is considered to be affected by super cooling. The disappearance temperature is considered to be the equilibrium value.
Core Laboratories
Wax Crystallization Temperature
Core Laboratories
Summary of results provided by an oil sample PVT test.
Saturation pressure, -bubble point.
Compressibility coefficient.
Coefficient of thermal expansion.
Relative total volume of oil and gas, Vt
Cumulative relative volume of gas. Vg
Cumulative relative volume of oil. Vo
Summary of results provided by an oil sample PVT test.
Gas formation volume factor or gas expansion factor
Gas compressibility factor.
Specific gravity of gas
Liquid density
Viscosity of liquids as a function of pressure.
Oil formation volume factor
Solution gas- oil ratio. Shrinkage of separator oil to tank oil
Hydrocarbon analysis of reservoir and produced fluids
Volumetric relationship of fluids in an oil PVT test Reference point bubble point
Volumetric relationship of fluids in an oil PVT test Reference point Stock Tank Conditions
Volumetric relationship of fluids in an oil PVT test
Interfacial Tension, IFT
Impact of IFT now considered an important aspect particularly for gas condensates IFT has a significant impact on the behaviour of residual condensate saturation and associated relative permeability. IFT is very low as critical point approached
Interfacial Tension Measurements
Most common method pendant drop
gd e2 L V l
gd e2 L V l
l = shape factor a function of ds/de
Interfacial Tension Measurements
For very low IFT size of tube too small to suspend drop.
Thin wire can be used
Light scattering has been used.
Heriot-Watt method -rising film method
Retrograde Condensation
Saturation pressure is the dew point pressure. Gas condensate cells have a window to visualise dew point. Not possible to determine by change of slope of compressibilities of gas and liquid..
Gas Condensate
Main aspects of PVT study:
Constant mass expansion
Constant volume depletion
Specialised tests ( IFT)
Compositions of oil & gas
Compositions of fluids are generally made by blowing down samples and recombining the resultant liquid and gas phase compositions.
Gas Condensate- Constant Mass Study
No fluids removed from the cell
Purpose to determine z value above dew point.
Determine dew point pressure
Dew point observed as drops on window
Gas Condensate- Constant Volume Depletion
Carried out to simulate condition below dew point
Series of pressure expansions
Volume of cell returned to original volume
Gas Condensate- Constant Volume Depletion
Liquid volume produced below dew point generates a liquid drop out curve.
Gas Condensate-Special tests IFT and Full Compositional data Hg
Gas Density cell
Stirrer
High pressure sampling Hg Condensate
Gas Condensate-Special tests IFT and Full Compositional data Pendant drop
Hg
Gas Density cell
High pressure sampling Hg
Hg
Interface Condensate
Rising film method
Vapour 1 cm
Liquid
Rising film thickness
Rising film method - near critical point
Vapour 1 cm
Liquid Very thin meniscus height
Understanding PVT Rep[orts
Purpose of the PVT report: Although can be used for applications from reservoir to surface facilities. Reservoir engineering provides the main basis.
Provides much of black oil information.
Material balance equation basis for report.
PVT report provides main data for MB equation.
Both flash and differential separation assumed.
Specific to a particular fluid
Example PVT report.
Example PVT report.
Example PVT report.
Example PVT report.
Separator Test
Separator Test
Separator Test READ THE FOOTNOTES
Fluid Properties above bubble point
Relative Volume Test - Flash Vaporisation Test
Relative Volume Test Flash Vaporisation Test
P Pb Vsat
V
Bo above bubble point vol. reservoir oil Bo vol. stock tank oil
vol. bubble point oil vol. reservoir oil Bo vol. stock tank oil vol. bubble point oil
From separator test
Above bubble point
Density above bubble point obtained by combining data from separator test and relative volume tests.
1 1 o vo v ob v rel o
1 1 vo v ob v rel
Above bubble point
Compressibility above bubble point can be obtained from relative volume test
1 v Co v p T 1 v Co v p T
1 Co vavg
v p T
Total Formation Volume Below Bubble Point Total formation
volume factor, Bt
Of little significance, but sometimes used in MB based calculations
Bo
BT
Total Formation Volume If we multiply Bob x vrel.Below we get BBubble t over the total Point pressure range above and below the bubble
point pressure.
Differential liberation tests
Differential liberation tests
Volume changes during differential liberation
854-763=91scf/bbl residual oil
Residual oil not the same composition as stock tank oil
Calculation of Gas-Oil Ratio Below the Bubble point
The GOR resulting from the separator tests and those from the differential test have different values.
796 ft3/B STO & 854 ft3/B residual oil The difference a result of the differential process of pressure draw down over the total pressure. In practise, reservoir pressure drop is a differential process but the pressure drop through the tubing and separator is a flash process.
Calculation of Gas-Oil Ratio The separator value is Below the Bubble point correct.
We need to complete GOR values below the bubble point. The GOR is made up of two elements, Differential in the reservoir. Flash in the wells and to surface. Differential liberation Flash liberation
The differential GOR is converted Calculation of Gas-Oil Ratio in the following manner: Below the Bubble point
R s diff R s diff
liberated gas-oil ratio by differential liberation
ft 3 bbl residual oil
Differential test
Separator test
ft 3 bbl residual oil ft 3 bbl residual oil bbl bubble point oil bbl bubble point oil ft 3 bbl bubble point oil ft 3 bbl bubble point oil bbl stock tank oil bbl stock tank oil
ft 3 R s flash bbl stock tank oil
R s R si R s flash
R s flash R sb flash R s diff
Bob Vb / Vresid
Calculation of Formation Volume Factor Below the Bubble point Formation volume factors between bubble point and surface also show a distinct difference between flash and differential.
Calculation of Formation Volume Factor Below the Bubble point
V/Vresidual = relative volume at pressure, B/B resid. bbl Saturated oil bbl Residual oil bbl Saturated oil bbl Residual oil bbl Bubble point oil bbl Bubble point oil Bo
V Bob Vresid . Vb / Vresid
bbl Saturated oil bbl Bubble point oil bbl Saturated oil Bo bbl Bubble point oil bbl Stock Tank oil bbl Stock Tank oil
Bob V Bo Vresid . Vb / Vresid
Viscosity Data
Pressures below bubble point match differential test
Viscosity
Composition of Reservoir Fluid
Composition of Separator Gas
Composition of Separator Gas
Gas Condensate PVT Report
Gas Condensate PVT Report
Liquid Drop Out Curve 44%
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
Pressure Volume Temperature Adrian C Todd
PVT - Scope
Reservoir fluid analysis provides key data to the petroleum engineer. Quality of the testing is important to ensure realistic values used in design. Sample quality is the first quality issue.
PVT Analysis
Provides data for field evaluation and design
Reservoir calculations
Well flow calculations
Surface facilities
PVT Analysis
Correlation between pressure and volume at reservoir temperature. Various physical constants in reservoir calculations; viscosity, density, compressibility.
Effect of separator conditions on Bo & GOR. etc.
Chemical composition of the volatile components.
PVT Analysis
Scope of the analysis depends on the nature of the fluid.
Dry gas:
composition, specific gravity, Bg, z, and viscosity
Wet gas:
as above plus information on liquid drop out, quantities and compositions.
Oil system:
Bubble point pressure, composition of reservoir and produced fluids, Bo, GOR, Bt and viscosity. All as function of pressure. Co. Below Pb considerations.
PVT Analysis
Gas condensate:
Reflect wet gas and oil.
Dew point pressure
Compressibility above Pd.
Impact of dropping below Pd
Sampling
Clearly the sample has to representative of the reservoir contents or the drainage area. Desirable to take samples early in the life of the reservoir. Either sub-surface or surface sampling.
Sub-Surface Sampling
Can only be representative when pressure at sampling point is above or equal to the saturation pressure. At pressure close to saturation pressure serious possibility of sample integrity being lost. In recent years considerable advance in downhole fluid sampling
Surface Sampling
Samples of oil and gas taken from separator connected with the well. Fluids recombined in the laboratory on the basis of the produced GOR
Vertical and Horizontal Separators
Separator Gas Sampling
Separator Liquid Sampling by Gas Displacement
Separator Liquid Sampling by Water Displacement
Wellhead sampling
A low cost option. Only possible for very undersaturated sytsems. Still single phase at wellhead.
Sampling Wet Gas and Gas Condensate Systems
Use and value of any PVT study dependant on the quality of the sample collected. Sampling wet gas and gas condensate fluids can give rise to errors. During sampling procedure it is possible to alter the conditions so that samples are not representative. An important consideration is the phase behaviour.
Phase Behaviour.
Fluids uniquely described by phase diagram. Within the phase diagram system is two phase. Whereas outside the phase envelope single phase
Single phase
Two phase
Phase Behaviour.
The separation of oil and gas as predicted by the phase diagram results in each phase having its own phase diagram. The oil exists at its bubble point . The gas exists at its dew point. This behaviour has important implications on well sampling
Sampling Wet Gas and Gas Condensate Potential locations Reservoirs for reservoir sampling
Sampling Wet Gas and Location Gas Condensate 1. Reservoir Reservoirs
For.
2.Bottom-hole
3.Well Head
4. Separator
Against
1. Ideal
1. Impossible
2. 1-phase
2. Representative ? Technology, Cost, Handling.
3. Cost
3. 2-phase ? Representative ?,
4.Gas/liquid volumes,, separator conditions, 4. Cost, 1-phase, buffer,sampling volume Representative ?,
Sampling Wet Gas and Gas Condensate Reservoirs-Flowing well.
Well behaviour can significantly influence nature and characteristics of fluids produced. Flowing well gas condensate - mist flow
Sampling Wet Gas and Gas Condensate Reservoirs-Shut in well after flow.
Well acts as a separator Liquid rains down and accumulates at bottom of well. Pressure builds up in the well and disturbed formation. Some gas goes back into solution.
Well flow after shut in.
Large variations in compositions of produced fluids. Early period lean gas produced. High GOR When fluids produced from bottom of well. Liquids much lower GOR. Then fluids from disturbed reservoir zone Eventually fluids from undisturbed reservoir
Sampling Wet Gas and Gas Condensate Reservoirs
In assessing quality of samples important to know how long it will take for unrepresentative samples to be displaced from reservoir. Volumetrics required on wells, facilities and near well volumes
Separator Sampling Points
A very practical issue is the location of sampling points on separators. Often located for convenience for accessibility.
Liquid in gas line
Important to recognise that the gas and liquid in a separator are at their saturation pressure. Small changes will result in liquid drop out and gas being produced
Iso kinetic sampling
Gas in liquid line
Sampling Details
Important to record and keep note off following. These records to go with samples
Date and time
Cylinder identification
Location of sampling points.
Temperature and pressure
GOR in separator
Any special details ( H2S in sample, etc
Equipment for PVT Analysis
Apparatus for transfer and recombination of separator 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.
Equipment for PVT Analysis-Subsurface Samples
Equipment for PVT Analysis-Surface Samples
Equipment for PVT Analysis-Gas Condensate Samples
PVT Tests
To provide data for reservoir calculations
To provide physical property data for well flow calculations
For surface facility design
The reservoir calculations are the main driving force for the various tests.
Over recent years reservoir simulation capability has generated the need to extend compositional description from C7+ to in some cases C29+. PVT report provides source of all reservoir engineering properties for behaviour over exploration, development and production
Main PVT Tests
Flash vaporisation or relative volume test.
Differential vaporisation test.
Separator tests.
Viscosity measurements.
Compositional measurements.
Special studies: e.g. Interfacial tension.
Simple layout of a PVT Facility
Flash Vaporisation ( 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 behaviour below the bubble point does not reflect reservoir behaviour, where gas has greater mobility than the oil. This test determines the Bubble Point pressure corresponding to the reservoir temperature.
Flash Vaporisation (Relative Volume ) Test
Liberated gas remains in equilibrium with oil
Flash Vaporisation (Relative Volume ) Test By plotting P versus V, a break in the slope is obtained at the Bubble Point pressure.
Flash Vaporisation (Relative Volume ) Test
Tests at constant pressure and varying temperature enables thermal expansion coefficient to be obtained for well flow calculations.
Thermal expansion,
V2 V1
V2 T2 T1
V1 volume at T1 , V2 volume at T2
Flash Vaporisation (Relative Volume ) Test
Above bubble point compressibility of oil at reservoir temperature can be determined. No free gas
V2 V1 c V2 P1 P2 V2 =volume at pressure P2 V1 =volume at pressure P1
Flash Vaporisation (Relative Volume ) Test
Main objectives: Reservoir bubble point pressure. Together with information from separator tests, formation volume factor above bubble point.
Differential Vaporisation
Below bubble point in reservoir gas liquid separation in the reservoir is a constant changing system. A test has been design to attempt to simulate this process. In the differential vaporisation test liberated gas is removed from the cell step wise. At each step below bubble point, volumes densities , gas expansion and compressibility determined. Bubble point starting point.
Differential Vaporisation
Flash liberation process
Differential liberation process
Differential Vaporisation
Differential Vaporisation
Differential Vaporisation
8-10 pressure reduction steps at reservoir temperature.
Final step to 60oF.
Remaining oil Residual Oil
Differential Vaporisation vs.Flash Vaporisation
Flash liberation considered to take place between reservoir and surface. Differential liberation considered to be representative of the process in the reservoir below bubble point pressure. Differential tests carried out to obtain oil formation volume factors and GOR’s to predict behaviour below bubble point pressure.
Separator Tests
Objective to determine impact of separator conditions on Bo, GOR, and produced fluid physical properties. Not the interest of facility designers. Carried out to give an indication of oil shrinkage and GOR when fluids produced to surface. There are not uniques values for Bo & GOR. They depend on separator conditions.
Starting point for the test is the bubble point pressure.
Fluid produced at surface conditons. Stock tank oil
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
Separator Tests PVT Cell pressure kept at bubble point
V res
Viscosity
Measured at different pressures above and below bubble point pressure. Below bubble point pressure carried out under differential conditions. Rolling ball or capillary tube methods of measurement
Hydrocarbon analysis
Analysis from C1 to an upper C number based on paraffin series. Historically C6 & C7+. Much higher analysis capability. C+ characterised by specific gravity and apparent molecular weight. Latter by depression of freezing point. Higher C+ characterisation helpful to process engineers re. solid phase formation.
Wax and Ashphaltenes
Solid phase formation series concern. Heavy components at low temperatures can form solid phases. Wax in transfer lines and process facilities. Ashphaltene are larger molecules of hydrogen and carbon plus sulphur, oxygen or nitrogen. Ashphaltenes do not dissolve in oil but are dispersed as colloids.
Wax Crystallization Different tests used: Temperature
Filtering and measuring resistance to flow at different temperatures. Appearance temperature is considered to be affected by super cooling. The disappearance temperature is considered to be the equilibrium value.
Core Laboratories
Wax Crystallization Temperature
Core Laboratories
Summary of results provided by an oil sample PVT test.
Saturation pressure, -bubble point.
Compressibility coefficient.
Coefficient of thermal expansion.
Relative total volume of oil and gas, Vt
Cumulative relative volume of gas. Vg
Cumulative relative volume of oil. Vo
Summary of results provided by an oil sample PVT test.
Gas formation volume factor or gas expansion factor
Gas compressibility factor.
Specific gravity of gas
Liquid density
Viscosity of liquids as a function of pressure.
Oil formation volume factor
Solution gas- oil ratio. Shrinkage of separator oil to tank oil
Hydrocarbon analysis of reservoir and produced fluids
Volumetric relationship of fluids in an oil PVT test Reference point bubble point
Volumetric relationship of fluids in an oil PVT test Reference point Stock Tank Conditions
Volumetric relationship of fluids in an oil PVT test
Interfacial Tension, IFT
Impact of IFT now considered an important aspect particularly for gas condensates IFT has a significant impact on the behaviour of residual condensate saturation and associated relative permeability. IFT is very low as critical point approached
Interfacial Tension Measurements
Most common method pendant drop
gd e2 L V l
gd e2 L V l
l = shape factor a function of ds/de
Interfacial Tension Measurements
For very low IFT size of tube too small to suspend drop.
Thin wire can be used
Light scattering has been used.
Heriot-Watt method -rising film method
Retrograde Condensation
Saturation pressure is the dew point pressure. Gas condensate cells have a window to visualise dew point. Not possible to determine by change of slope of compressibilities of gas and liquid..
Gas Condensate
Main aspects of PVT study:
Constant mass expansion
Constant volume depletion
Specialised tests ( IFT)
Compositions of oil & gas
Compositions of fluids are generally made by blowing down samples and recombining the resultant liquid and gas phase compositions.
Gas Condensate- Constant Mass Study
No fluids removed from the cell
Purpose to determine z value above dew point.
Determine dew point pressure
Dew point observed as drops on window
Gas Condensate- Constant Volume Depletion
Carried out to simulate condition below dew point
Series of pressure expansions
Volume of cell returned to original volume
Gas Condensate- Constant Volume Depletion
Liquid volume produced below dew point generates a liquid drop out curve.
Gas Condensate-Special tests IFT and Full Compositional data Hg
Gas Density cell
Stirrer
High pressure sampling Hg Condensate
Gas Condensate-Special tests IFT and Full Compositional data Pendant drop
Hg
Gas Density cell
High pressure sampling Hg
Hg
Interface Condensate
Rising film method
Vapour 1 cm
Liquid
Rising film thickness
Rising film method - near critical point
Vapour 1 cm
Liquid Very thin meniscus height
Understanding PVT Rep[orts
Purpose of the PVT report: Although can be used for applications from reservoir to surface facilities. Reservoir engineering provides the main basis.
Provides much of black oil information.
Material balance equation basis for report.
PVT report provides main data for MB equation.
Both flash and differential separation assumed.
Specific to a particular fluid
Example PVT report.
Example PVT report.
Example PVT report.
Example PVT report.
Separator Test
Separator Test
Separator Test READ THE FOOTNOTES
Fluid Properties above bubble point
Relative Volume Test - Flash Vaporisation Test
Relative Volume Test Flash Vaporisation Test
P Pb Vsat
V
Bo above bubble point vol. reservoir oil Bo vol. stock tank oil
vol. bubble point oil vol. reservoir oil Bo vol. stock tank oil vol. bubble point oil
From separator test
Above bubble point
Density above bubble point obtained by combining data from separator test and relative volume tests.
1 1 o vo v ob v rel o
1 1 vo v ob v rel
Above bubble point
Compressibility above bubble point can be obtained from relative volume test
1 v Co v p T 1 v Co v p T
1 Co vavg
v p T
Total Formation Volume Below Bubble Point Total formation
volume factor, Bt
Of little significance, but sometimes used in MB based calculations
Bo
BT
Total Formation Volume If we multiply Bob x vrel.Below we get BBubble t over the total Point pressure range above and below the bubble
point pressure.
Differential liberation tests
Differential liberation tests
Volume changes during differential liberation
854-763=91scf/bbl residual oil
Residual oil not the same composition as stock tank oil
Calculation of Gas-Oil Ratio Below the Bubble point
The GOR resulting from the separator tests and those from the differential test have different values.
796 ft3/B STO & 854 ft3/B residual oil The difference a result of the differential process of pressure draw down over the total pressure. In practise, reservoir pressure drop is a differential process but the pressure drop through the tubing and separator is a flash process.
Calculation of Gas-Oil Ratio The separator value is Below the Bubble point correct.
We need to complete GOR values below the bubble point. The GOR is made up of two elements, Differential in the reservoir. Flash in the wells and to surface. Differential liberation Flash liberation
The differential GOR is converted Calculation of Gas-Oil Ratio in the following manner: Below the Bubble point
R s diff R s diff
liberated gas-oil ratio by differential liberation
ft 3 bbl residual oil
Differential test
Separator test
ft 3 bbl residual oil ft 3 bbl residual oil bbl bubble point oil bbl bubble point oil ft 3 bbl bubble point oil ft 3 bbl bubble point oil bbl stock tank oil bbl stock tank oil
ft 3 R s flash bbl stock tank oil
R s R si R s flash
R s flash R sb flash R s diff
Bob Vb / Vresid
Calculation of Formation Volume Factor Below the Bubble point Formation volume factors between bubble point and surface also show a distinct difference between flash and differential.
Calculation of Formation Volume Factor Below the Bubble point
V/Vresidual = relative volume at pressure, B/B resid. bbl Saturated oil bbl Residual oil bbl Saturated oil bbl Residual oil bbl Bubble point oil bbl Bubble point oil Bo
V Bob Vresid . Vb / Vresid
bbl Saturated oil bbl Bubble point oil bbl Saturated oil Bo bbl Bubble point oil bbl Stock Tank oil bbl Stock Tank oil
Bob V Bo Vresid . Vb / Vresid
Viscosity Data
Pressures below bubble point match differential test
Viscosity
Composition of Reservoir Fluid
Composition of Separator Gas
Composition of Separator Gas
Gas Condensate PVT Report
Gas Condensate PVT Report
Liquid Drop Out Curve 44%
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.
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