Day1 – Part2:  Pvt Measurements  Fluid Sampling Methods  Fluid Properties  Representative Fluid Samples 

Day1 – Part2 

PVT measurements



Fluid sampling methods



Fluid properties – What is important and why?



Representative Fluid Samples

 Examples    



Backward DLE Material Balance & Separator Density Check Separator Recombination CVD check material balance (forward, backward) Estimate Gas and Oil Recovery from a Gas condensate reservoir produced by depletion. Design of a sampling and PVT program

Course in Advanced Fluid Phase Behavior. © Pera A/S 1

PVT Experiments Laboratory Analysis Bottomhole Sample Recombined Composition Constant Composition Expansion (CCE, CME) Differential Liberation Experiment (DLE) Multistage Separator Experiment (MSF) Constant Volume Depletion(CVD) C7+GC Simulated Distillation C7+ TBP Destillation

Oil

Gas Condensates N

N

PVT Experiments Designed for Gas Injection Multistage Swelling Experiment Multistage Contact Experiment Slimtube Experiments Standard Can be Performed N Not Recommmened

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Bottom Hole Sample BHS 

Flash Sample to standard conditions



Measure Volume of surface gas and surface oil



Determine the normalized weight fractions wgi and woi of surface sample using gas chromatograph



Measure surface-oil molecular weight (Mo), and specific gravity

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Example of a BHS  

The weight fractions are converted to mole fractions Recombining mathematically the surface oil and gas sample to obtain the composition of the bottom hole sample. Using the equations below:

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Recombined Separator Sample 

Recombined separator samples is the most common fluid sample type from gas condensate reservoirs. It is also often used for saturated and near-saturated oil reservoirs.



The separator gas composition is usually found by injecting a gas sample straight into a GC.



The separator oil composition is obtained using the same procedures as for bottom hole samples.

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Recombined Separator Composition 

The wellstream composition is found by mathematically recombination of the separator oil and gas compositions.



The laboratory recombination GOR is calculated using the following equation.



The reported separator GOR should be used for recombination

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Checking recombined separator samples 

The Hoffmann et al method can be used to check if the separator oil and gas compositions are in equilibrium.



A plot of LOG(KiP) vs Fi shall follow a straight line (C7+ might be a bit “low”).

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Constant Composition Expansion (CME, CCE) 

This is the most common PVT experiment.



CME is conducted both for oil and gas condensates



Measured Properties:  Total volume  Bubble point/Dew point pressure  Bubble point density  Undersaturated gas Z-factor  Isothermal compressibility for under saturated oil  Condensate volume below the dew point pressure (gas)

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Measured CCE Data Example

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Quality Checking of the Reported Data  Check the Y-function vs. pressure for oil (smoothing of total volume below saturation pressure)

 Gas Z-factor should be compared with calculated Zfactor from Standing correlation. Course in Advanced Fluid Phase Behavior. © Pera A/S 10

Differential Liberation Experiment 

Design to approximate the depletion process for an oil reservoir.



A blind PVT cell is usually used for the conventional measurement



Viscosity apparatus needed  Roiling ball, capillary tube, and etc.



After converting to stock tank basis, the measured oil PVT data can be used to generate black oil PVT tables

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Measured and Calculated Properties 

Measured Properties:  Bubble point oil volume  Removed gas volume, specific gravity and (compositions)  Remaining oil volume  Residual oil volume, specific gravity, and (compositions)  Oil viscosity



Calculated Properties:  Bubblepoint oil density  Differential solution gas/oil ratio  Differential oil FVF  Oil density and gas Z-factor

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Measured DLE Data Example

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Measured DLE Data Example

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Measured DLE Data Example

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Measured DLE Data Example

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Quality Checking of the Reported Data

 Material balance check when compositions are available  Backward MB  Start from residual oil and add the released gases.  Compared with the initial reservoir oil composition

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Converting to Stock-Tank Basis (Including Surface Process) 

Traditionally the data from an DLE and a MSF experiment has been combined to calculate black oil PVT data for the reservoir oil. The black oil PVT data is calculated using the following equations:



The most common method today is to tune an EOS to the measured DLE data and multi-stage separator. The tuned EOS model is then used to simulate the surface process by passing oil and gas at each stage through the surface process.



Some PVT laboratory provide the converted DLE data to stock tank basis

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Multi-Stage Separator Test 

Originally designed for reservoir oil to provide a basis for converting DLE data from residual oil to stock tank oil by including the effect of surface process.



Also apply to high liquid-yield gas condensate



Flash equipment is used for MST experiment



Measured Properties:  Initial volume at saturation pressure (or higher)  Separator oil volume at each stage  Release gas volume, specific gravity, and (compositions)  Residual oil volume, density, and (composition)



Calculated Properties:  Formation volume factor (FVF)  Gas-Oil ratio (GOR)

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Measured MST Data Example

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Measured MST Data Example

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Quality Checking of the Reported Data 

Check the calculated properties from the raw data



Hoffman K-values for the primary separator when compositions are available



Backward material balance check compositions are available  Start form the residual oil  Add released gases back  Compare the back calculated reservoir fluid composition with the measured.

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Constant Volume Depletion 

CVD experiment is the most important PVT experiment for gas condensate reservoirs.



Designed to provide volumetric and compositional data for gas condensate and volatile oil reservoirs producing by pressure depletion.



Extract reservoir engineering quantities:  Recovery vs. reservoir pressure  Produced well stream composition (surface product) vs. pressure  Average oil saturation in the reservoir during depletion.

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Constant Volume Depletion  Measured Properties  Bubble point/Dew point pressure  Liquid and gas volumes at each pressure stage  Gas composition and gas volume produced at each stage  Gas Z-factor  Volume, compositions,and density of the residual oil  The PVT Data provided from this experiment can be used directly to calculate oil and gas recovery for a GC-reservoir produced by depletion.

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Measured CVD Data Example

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Recovery 600 Surface Gas

CVD (Reservoir) Pressure, bara

500

Surface Oil

400

300

  p / Z d N  n p  1  rsi  Cog           p / Z n k 1  d  k 1  rsk  Cog  i 

RFgD

  p / Z d  1   p / Z i 

RFoD

  p / Z d  p / Z d N  n p    1         p / Z p / Z k 1  n d i  i 

200

rs 

 1/ rsi  C og     1/ r  C  sk og k

z7 1 R Tsc 7  Cog   1  z7 Cog Psc M7

100

0 0

10

20

30 40 50 60 70 Surface Gas and Surface Oil Recovery Factors

80

90

100

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Quality Checking of the Reported Data  Backward material balance  Start from residual oil and add the released gases back  Compare final back calculated composition with the initial measured gas composition  Not as sensitive to errors in composition as the forward MB

 Forward material balance  Start from the initial reservoir gas  Calculate the oil in the PVT cell at each stage  Compare calculated last stage oil with the measured residual oil composition  Check K-values at each stage  Easy to fail with lean gas condensate at initial stages

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TBP-Distillation 

In a TBP distillation the separation of fractions are done by vaporization. Each distillation cut has a range of boiling points.



For each of the distillation cuts the following physical properties are measured.  Molecular weight  Specific gravity (density)



The physical properties of the last fraction is often from material balance.

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TBP-Distillation  Example of data from a TBP-distillation

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Gas Chromatography (GC) Simulated Distillation 

GC distillation is based on selective separation of components as temperature is increased ion a capillary tube.



The mass distribution of the fluid produced out of the capillary tube is measured. The mass is “converted” to components based on the temperature (time) known pure components are produced.



No information about the physical properties are provided. KatzFiroozabadi Generalized properties often assumed. Estimated molecular weight used to convert from mass to moles.

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Gas Chromatography (GC) Simulated Distillation 

The GC-apparatus require proper maintenance to be reliable. It is specially sensitive to shift in baseline.

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Swelling Experiment 

The most common PVT experiment used in connection with gas injection



This experiment provide useful PVT data for swelling of under saturated oil by injection gas and for tuning of the EOS.



No vapor or liquid is removed.



Measured Properties:  Total volume  Bubble point/Dew point pressure  Bubble point density  Liquid volume below the saturation pressure (CME experiment at each injection point)

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Measured Swelling Data Example 

It is recommended to have 5-6 injection stages. The mixtures should cover the following fluids  1-2 Volatile oils  1 Near critical oil  1 Near critical gas condensate  1-2 Rich gas condensates



Perform a CME experiment at each injection stage



Occasionally, the compositions are determined for some stages in the swelling test (known as Equilibrium Phase Split experiment).

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Measured Swelling Data Example

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Backward and Forward Multi-Contact Experiment 

The forward and backward multi-contact experiments provide useful phase and volumetric data for gas injection study and tuning of the EOS.



Forward multi-contact: developed gases contact with original oil  Gas Injection in oil reservoirs at miscible or near-miscible conditions



Backward multi-contact: original gas contacts with altered oil  Can be used for revaporization of retrograde condensation  Gas Injection in oil reservoirs or gas condensate reservoirs below the dew point.

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Measured and Calculated Properties Backward MCV

 Measured Properties (Backward and Forward):  Injection and removal volumes  Vapor phase and liquid phase saturation  Density and viscosity for each phase  Composition analysis for each phase

 Calculated Properties:  K values at each injection stage Course in Advanced Fluid Phase Behavior. © Pera A/S 36

Measured Multi-Contact Data Example Example of data from forward MCV experiment

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Data Quality Check  Consistency check  Compare the stage phase properties, K-values together  Material balance check  Backward  Forward

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Slim-Tube Displacement (MMP) 

Slim-tube displacement yield the most reliable information for defining true multi-contact miscibility (MMP)



The following data are measured:  Cumulative oil production  Gas-oil ratio  Gas and oil density (standard conditions)



The MMP are interpolated from oil recovery after @ 1.2 PV of gas injection

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Slim-Tube Displacement (MMP) 

At MMP or a higher pressure the oil recovery @ 1.2 PV of gas injection should be >95%.



Above MMP the oil recovery is independent on pressure.



Slim-tube experiments run at 5-6 pressures is usually sufficient to determine MMP.



Minimum Enrichment pressure (MME) is found using enrichment as the variable instead of pressure (varying injection gas composition at a constant pressure).

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Measured Slim-Tube Data Example

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Fluid Sampling

 Fluid sampling methods      

Bottom hole sampling Separator sampling MDT-RFT sampling Wellhead sampling(Thornton Sampling) Isokinetic sampling Sampling Summary



Example: Gas condensate fluid sampling

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“Representative” Fluid Samples - Determine initial fluids in place

- Fluid model development

Representative Sample 1. Insitu-representative: Represents I the original fluid(s) in the depth interval drained by the well during sampling.

x 2. Reservoir representative: Any fluid (mixture) produced from the reservoir!

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“Representative” Fluid Samples Insitu-representative Samples: 







Represents the original fluid(s) in the depth interval drained by the well during sampling. May vary as a function of depth, from one fault block to another, and between non-communicating layers. May be difficult to measure directly, due to near-wellbore multiphase behavior in saturated, slightly undersaturated, and lowpermeability reservoirs Accurate insitu-representative samples are used to determine the initial hydrocarbons (oil and gas) in place.

Reservoir-representative Samples: 

Represents any fluid produced from the reservoir.



Are easily obtained.



May be used to create estimates of the insitu-representative fluids!



All reservoir-representative samples (having reliable PVT data and compositions) should be used in developing an EOS fluid characterization.

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Bottom hole Sampling

When is bottom hole sampling recommended?  Undersaturated oils  Flowing BHP higher than saturation pressure When is bottom hole sampling not recommended?  Gas Condensates  Foaming oils  Highly viscous oils Quality check:  BH pressure during sampling  Production conditions prior to sampling  Perforation interval  Characterization Factor (Multiple samples)

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Separator Sampling When is separator sampling recommended?  Oil reservoirs  Gas and gas condensate reservoirs When is separator sampling not recommended?  Waxy crude oils may require heating Quality check:  Production conditions prior and during to sampling  Recombination GOR  Perforation interval  Hoffman et. Al Kp-F plot  Characterization Factor (Multiple samples) Course in Advanced Fluid Phase Behavior. © Pera A/S 46

Separator Sampling - Hoffman et. Al Kp-F plot Used for checking consistency of reported separator gas- and oil compositions. Standing Low-pressure K-values:

Should be reasonable for psp < 1000 psia, and Tsp < 200 F Course in Advanced Fluid Phase Behavior. © Pera A/S 47

Separator Sampling – Characterization Factor Used for checking consistency of STO density and molecular weight Watson Characterization Factor

Kw sould be constant for a given field (common fluid system) within +/- 0.01 Paraffinic: Kw = 12.5 – 13.5 Naphtenic: Kw = 11 – 12.5 Aromatic: Kw = 8.5 – 11.0 Course in Advanced Fluid Phase Behavior. © Pera A/S 48

MDT-RFT Fluid Samples When is MDT/RFT sampling recommended?  Oil reservoirs  Gas and gas condensate reservoirs  Layered reservoirs with different fluid  Compositional grading reservoirs When is separator sampling not recommended?  Highly viscous crude  Low permeability reservoirs  Carefully when oil based drilling mud (OBM) has been used Quality checks:  Saturation pressure and  Sampling transfer  Volume of sample

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Mini Laboratory Thornton Sampling When is Thornton sampling recommended?  Lean gas condensate.  Separator rates are not needed. When is separator sampling not recommended?  Quite expensive – careful separator sampling, potentially with isokinetic liquid-carryover control, is usually sufficient. Quality checks:  Production conditions prior and during to sampling  Recombination GOR Course in Advanced Fluid Phase Behavior. © Pera A/S 50

Iso-Kinetic Sampling When is Iso-Kinetic sampling recommended?  Lean gas condensate tested with high rates (liquid carry-over). Method: 1. Get oil-free gas by sampling in same direction as the flow. 2. Sample against the direction of flow to get gas containing carryover oil. Comparing the two compositions, carryover can be determined. Sampling rate?

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Sampling-Summary Produced Fluids

Well Position

Ref.

Producing Reservoirs or New Reservoirs or New Reservoirs or depleted Undepleted Zones Undepleted Zones zones Producing New Reservoirs Reservoirs or depleted zones

Volatile Oil or doubtful cases

Gas Reservoirs

Oil Reservoirs

3.21

Sampling Possibilities and Procedure Surface Sampling MDT/RFT Sampling Stable "drawdown". Be GOR=GORi=Consta aware of OBM nt Pwsi>Pb Well flowing with Pwf>pb Stabilized flow with Pwf>Pb contamination. Reservoir and Flow Bottomhole Sampling Characterisitcs

4.1

Smallest possible stable flow Stable low "drawdown" rate. If GOC is close, Watch up for OBM Progressive reduction Perforate close but below contamination. Good GOR> GORi of flow rate. Well GOC and produce at high (additional) sampling Psi = Pb closed until stabilized rate to allow gas coning. method in reservoirs with Saturated Reservoirs conditions Sample during gas coning. compositional gradients. GOR=GORi=Consta nt Pwsi>Pb Same procedure as in 3.21 GOR> GORi Accurate of as Insitu Psi > Pb Same estimate procedure in 3.22 GOR> GORi No insitu representative GOC oil and gas composition No insitu representative sampling possible can be achieved using ECM sampling possible Psi < Pb Smallest possible stable flow rate. Let GOR and seprator Stable low "drawdown". Be conditions stabilize. aware of OBM In Reservoirs with a saturated contamination. Good GOC near perfect samples additional sampling method GOR=GORi or close can be achieved using ECM in reservoirs with to GORi Not advisable (see point 3.22) compositional gradients.

4.2a

GOR = GORi

Not advisable

as for 4.1

4.2b

GOR>GORi

Not advisable

No insitu representative sampling possible

3.22 3.31 3.32(a) 3.32(b)

5

No possibility of getting any reservoir chraracteristics from Well test data Not advisable

as for 4.1

Remarks

Near perfect GOC oil and gas samples can be acheived from the separtor samples using the ECM (Equilibrium contact mixing method) (SPE 28829)

No insitu representative sampling possible Stable low "drawdown". Be aware of OBM contamination. Good additional sampling method in reservoirs with Sample representativty might be compositional gradients. known after PVT study

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Sampling-Summary 

Advantages of Subsurface sampling  Collect desired sample directly  Can maintain full pressure of sample  Avoids use of surface separators (surface metering uncertainties)  Avoids recombination errors  Less sampling information transmitted to PVT laboratory



Advantages of MDT/RFT samples  Collects the fluid sample directly the formation  Fluid sample from a very narrow depth interval  Not affected by fluid segregation in the well



Advantages of separator samples  Large fluid volumes can be taken  Easy, convenient and less expensive when surface separators are already on location  No tools in the borehole  Does not require single phase fluid in the well bore

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What is important and why? Gas Condensate Reservoirs  All production mechanisms  Gas Z-factor, solution OGR  Depletion drive  y6+ with pressure (from CVD experiments)  Saturation pressure  Z-factor with pressure  Gas injection  Multi contact experiment (backward)

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What is important and why? Oil Reservoirs  All production mechanisms  FVF, density (reservoir & STO), viscosity, GOR  Saturation pressure (important for tuning of EOS)  Depletion drive  Gas liberation (from DLE experiments)  Oil shrinkage  Gas injection  MMP (Near Miscible/Miscible gas injection)  Multi stage swelling experiment  Multi contact experiment (backward) Course in Advanced Fluid Phase Behavior. © Pera A/S 55

What Type of PVT Experiments are Required Depletion & Water Injection  Oil reservoir  Multistage Separator Experiment (MSF)  Differential Liberation Experiments (DLE)  Constant Mass Expansion (CME)  Oil Viscosity (DLE,CME)  Gas condensate/Near Critical fluids  Multistage Separator Experiment (MSF)  Constant Volume Depletion (CVD)  Viscosities (not important for High Perm GC reservoirs)

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What Type of PVT Experiments are Required in Addition for Gas/Wag Injection  Oil/Near critical fluids  Multistage Stage Swelling Experiments (MCV) through critical fluid (including oil & gas compositions)  Multi Contact Experiments (Forward/Backward)  Slimtube experiments (minimum 5-6 pressures)  Gas condensate (injection below dew point)  Multi Contact Experiments (Backward)

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Exercise: Quality Check Differential Liberation Experiment 

What’s measured?  Bubble point oil volume  Removed gas volume, specific gravity and (compositions)  Remaining oil volume  Residual oil volume, specific gravity, and (compositions)  Oil viscosity



What to check?  Stage oil densities  Gas z-factor  Also compare reservoir density with calculated reservoir density from multi-separator test.

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DLE Quality Check  Material balance check when compositions are available  Backward MB  Start from residual oil and add the released gases.  Compared with the initial reservoir oil composition

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Measured DLE Data Example

DLE Matbal Course in Advanced Fluid Phase Behavior. © Pera A/S 60

Exercise: Consistency Check of SEP Data 

What’s measured?  Initial volume at saturation pressure (or higher)  Separator oil volume at each stage  Release gas volume, specific gravity, and (compositions)  Residual oil volume, density, and (composition



What to check?  Reservoir oil densities (compare with DLE and/or CCE data).  GOR and Bo trend if you have several separator experiments.

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Reservoir Oil Density Calculation (Compare with DLE and/or CCE) Field Units

o 

62.4   o  0.0136   g  Rs tot Bo Metric Units

o 

 o  0.0012192 g Rs tot Bo

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Measured SEP Data Example

DLE Matbal Course in Advanced Fluid Phase Behavior. © Pera A/S 63

Exercise: Quality Check CVD Experiment 

What’s measured?  Bubble point/Dew point pressure  Liquid and gas volumes at each pressure stage  Gas composition and gas volume produced at each stage  Gas Z-factor  Volume, compositions,and density of the residual oil



What to check?  Material balance (forward and backward).

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CVD Material Balance  Backward material balance  Start from residual oil and add the released gases back  Compare final back calculated composition with the initial measured gas composition  Not as sensitive to errors in composition as the forward MB

 Forward material balance  Start from the initial reservoir gas  Calculate the oil in the PVT cell at each stage  Compare calculated last stage oil with the measured residual oil composition  Check K-values at each stage  Easy to fail with lean gas condensate at initial stages

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Measured CVD Data Example

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Class Exercise: Oil and Gas Recovery for a Gas Condensate produced by depletions • Calculate oil and gas recovery for the gas condensate fluid shown in Table 6.12 • Assume production by depletion • Neglect water and rock compressibility • Assume a surface process with C7+ as the STO • Assume a surface process with C6+ as the STO

GC-Rec Spreadsheet Course in Advanced Fluid Phase Behavior. © Pera A/S 67

Class Evaluation Project Design of Fluid Sampling and PVT Experiments Reservoir A:

- Under-saturated oil reservoir - Medium sized reservoir - Depletion and/or water injection

Reservoir B:

- Saturated oil reservoir with initial gas cap - Large reservoir - Gas injection in gas cap, followed by depletion

Reservoir C:

- Highly under-saturated gas condensate reservoir. - High permeability. - Primary depletion.

Reservoir D:

- Saturated gas condensate reservoir. - Low permeability. - Potential gas injection followed by depletion.

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Reservoir A – Sampling & PVT Measurements PVT Measurements:

Reservoir A:

- Multi-Stage Separator Experiments (SEP) - Constant Composition (CCE) - Differential Liberation (DLE)

- Under-saturated oil reservoir

- Medium sized reservoir - Depletion and/or water injection

- Oil Viscosity Measurements (VISC)

Key Data: - Bo and GOR from SEP experiment.

Sampling: - Bottom-hole samples - Separator sample

- Oil densities (CCE, DLE). - Oil viscosities (CCE, DLE). - Oil shrinkage and Solution GOR from DLE. - Gas Z-factor - Oil compressibility (CCE). - Bubble-point pressure (EOS tuning).

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Reservoir B – Sampling & PVT Measurements PVT Measurements:

Reservoir B:

- Multi-Stage Separator Experiments (SEP Oil & Gas) - Constant Composition (CCE Oil and Gas)

- Saturated oil reservoir with initial gas cap - Large reservoir - Gas injection in gas cap, followed by depletion

- Differential Liberation (DLE Oil) - Constant Volume Depletion (CVD Gas) - Backward multi-contact experiments (Oil and Injection Gas).

- Oil Viscosity Measurements (VISC Oil)

Sampling: - Separator sample (Gas & Oil)

- ECM experiment (Oil and Gas) - TBP (From reservoir oil)

- Bottom-hole (Oil)

Key Data: (Addition to Res A)

- ECM Sampling

- Vaporization of oil - CVD gas compositions - Equilibrium oil and gas composition at GOC (ECM oil and gas).

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Reservoir C – Sampling & PVT Measurements PVT Measurements:

Reservoir C:

- Constant Composition (CCE) - Constant Volume Depletion (CVD)

- Highly under-saturated gas condensate reservoir. - High permeability. - Primary depletion.

Sampling: - Separator sample (Always!) - Bottom-hole sample (check production GOR consistency) - Isokinetic sampling (evaluate for lean gases)

- Multi-Stage Separator Experiments (not required for lean systems).

Key Data: - Gas z-factor - Producing gas composition (amount of C7+ in gas). - Cumulative amount of gas produced from CVD. - Separator oil densities (C7+ characterization). - Dew-point pressure (only for EOS tuning)

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Reservoir D – Sampling & PVT Measurements PVT Measurements:

Reservoir D:

- Constant Composition (CCE) - Constant Volume Depletion (CVD)

- Multi-Stage Separator Experiments - Saturated gas condensate reservoir. - Low permeability. - Potential gas injection followed by depletion.

Sampling:

- Multi Contact Vaporization - Viscosity measurements (Separator Oil, at reservoir p, T) - Slim-tube Experiments (for rich, near critical fluid systems only). - TBP (Stock Tank Oil)

- Separator sample.

Key Data (Addition to Res A) - Oil vaporization (injection) - Relative oil volume (blockage) - Oil viscosity (blockage) Course in Advanced Fluid Phase Behavior. © Pera A/S 72