Res Eng PP Ch14 - Petroleum engineering PDF

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Petroleum engineering...

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Heriot -Watt University Heriot-Watt DEPARTMENT OF PETROLEUM ENGINEERING

PVT Pressure Volume Temperature Adrian C Todd

PVT - Scope 

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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

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Provides data for field evaluation and design

 Reservoir calculations  Well flow calculations  Surface facilities

PVT Analysis 

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Correlation between pressure and volume at reservoir temperature. Various physical constants in reservoir calculations; viscosity, density, compressibility.

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Effect of separator conditions on Bo & GOR. etc.

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Chemical composition of the volatile components.

PVT Analysis 

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

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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

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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 

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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  

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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  

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A low cost option. Only possible for very undersaturated sytsems. Still single phase at wellhead.

Sampling Wet Gas and Gas Condensate Systems 

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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. 

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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

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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 Reservoirs

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Potential locations for reservoir sampling

Sampling Wet Gas and Gas Condensate Reservoirs

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For.

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Location

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1. Reservoir

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2.Bottom--hole

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3.Well Head

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4. Separator 

Against

1. Ideal

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1. Impossible

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2. 1-phase

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2. Representative ? Technology, Cost, Handling. Handling

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3. Cost

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3. 2-phase ? Representative ?,

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4. Cost, 1-phase,  4.Gas/liquid volumes,, separator conditions, buffer,sampling volume Representative ?,

Sampling Wet Gas and Gas Condensate Reservoirs-Flowing well.  

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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. 

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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. 

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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 

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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

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A very practical issue is the location of sampling points on separators.

Separator Sampling Points Liquid in gas line

Often located for convenience for accessibility. 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

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Date and time

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Cylinder identification

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Location of sampling points.

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Temperature and pressure

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GOR in separator

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Any special details ( H2S in sample, etc

Equipment for PVT Analysis 

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Apparatus for transfer and recombination of separator oil and gas samples. Apparatus for measuring gas and liquid volumes

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Apparatus for performing separator tests

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PVT cell and displacing pumps.

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High pressure viscometer

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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

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To provide physical property data for well flow calculations

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For surface facility design

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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.

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Differential vaporisation test.

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Separator tests.

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Viscosity measurements.

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Compositional measurements.

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Special studies: e.g. Interfacial tension.

Simple layout of a PVT Facility

Flash Vaporisation ( Relative Volume ) Test 

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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 throughout 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, β =

V2 − V1

V2 ( T2 − T1 )

V1 = volume at T1 , V2 = volume at T2

Flash Vaporisation (Relative Volume ) Test 

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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 

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Below bubble point in reservoir gas liquid separation separation in in the the reservoir reservoir is is aa constant constant changing changing system. system. A test has been design to attempt to simulate this this process. process. In the differential vaporisation test liberated gas gas is is removed removed from from the the cell cell step step wise. wise. At each step below bubble point, volumes densities densities ,, gas gas expansion expansion and and compressibility compressibility determined. determined. Bubble point starting point.

Differential Vaporisation

Flash liberation process

Differential liberation process

Differential Vaporisation

Differential Vaporisation

Differential Vaporisation

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8- 10 pressure reduction steps at reservoir temperature.

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Final step to 60oF.

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Remaining oil Residual Oil

Differential Vaporisation vs.Flash Vaporisation 

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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  

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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

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V res

Viscosity 

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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 

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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  

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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 Temperature  

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Different tests used: 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.

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Compressibility coefficient.

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Coefficient of thermal expansion.

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Relative total volume of oil and gas, Vt

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Cumulative relative volume of gas. Vg

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Cumulative relative volume of oil. V o

Summary of results provided by an oil sample PVT test. 

Gas formation volume factor or gas expansion factor

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Gas compressibility factor.

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Specific gravity of gas

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Liquid density

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Viscosity of liquids as a function of pressure.

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Oil formation volume factor

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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 

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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

σ=

(

gd e2 ρL − ρV l

) σ=

(

gde2 ρL − ρV l

l = shape factor a function of ds/de

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Interfacial Tension Measurements

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For very low IFT size of tube too small to suspend drop.

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Thin wire can be used

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Light scattering has been used.

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Heriot -Watt method -rising film method

Retrograde Condensation  

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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

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Purpose to determine z value above dew point.

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Determine dew point pressure

Dew point observed as drops on window

Gas Condensate- Constant Volume Depletion

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Carried out to simulate condition below dew point

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Series of pressure expansions

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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.

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Provides much of black oil information.

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Material balance equation basis for report.

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PVT report provides main data for MB equation.

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Both flash and differential separation assumed.

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Specific to a particular fluid

Example PVT report.

Example PVT report.

Example PVT report.

Example PVT report.

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

Boo above above bubble bubble point point Bo =

Bo =

vol. reservoir oil vol. stock tank oil

vol. bubble point oil vol. reservoir oil × 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 ρo = = vo vob v rel

Above bubble point 

Compressibility above bubble point can be obtained from relative volume test

1 ⎛ δv ⎞ Co = − ⎜ ⎟ v ⎝ δp ⎠ T 1 Co = − v avg

⎛ ∆v ⎞ ⎜ ⎟ ⎝ ∆p ⎠T

Total Formation Volume Below Bubble Point 

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Total formation volume factor, Bt Of little significance, but sometimes used in MB based calculations

Bo

BT

Total Formation Volume Below Bubble Point If we multiply B ob x vrel . we get Bt over the  

total pressure range above and below the bubble point pressure.

Differential liberation tests

Differential liberation tests 

Volume chan...