1. 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
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2. PVT Experiments
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3. 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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4. 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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5. 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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6. 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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7. 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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8. 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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9. Measured CCE Data Example
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10. 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 Z-factor from Standing correlation.
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11. 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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12. 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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13. Measured DLE Data Example
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14. Measured DLE Data Example
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15. Measured DLE Data Example
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16. Measured DLE Data Example
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17. 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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18. 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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19. 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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20. Measured MST Data Example
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21. Measured MST Data Example
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22. 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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23. 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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24. 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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25. Measured CVD Data Example
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26. Recovery
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27. 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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28. 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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29. TBP-Distillation Example of data from a TBP-distillation
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30. 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. Katz-Firoozabadi Generalized properties often assumed. Estimated molecular weight used to convert from mass to moles.
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31. 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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32. 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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33. 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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34. Measured Swelling Data Example
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35. 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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36. 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
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37. Measured Multi-Contact Data Example
Example of data from forward MCV experiment
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38. Data Quality Check Consistency check
Compare the stage phase properties, K-values together
Material balance check
Backward
Forward
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39. 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 1.2 PV of gas injection
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40. Slim-Tube Displacement (MMP)
At MMP or a higher pressure the oil 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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41. Measured Slim-Tube Data Example
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42. 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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43. “Representative” Fluid Samples
Determine initial fluids in place
Fluid model development
Representative Sample
Insitu-representative: Represents the original fluid(s) in the depth interval drained by the well during sampling.
Reservoir representative: Any fluid (mixture) produced from the reservoir! I x
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44. “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 low-permeability 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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45. 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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46. 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)
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47. 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
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48. 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
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49. 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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50. 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
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51. Iso-Kinetic Sampling When is Iso-Kinetic sampling recommended?
Lean gas condensate tested with high rates (liquid carry-over).
Method:
Get oil-free gas by sampling in same direction as the flow.
Sample against the direction of flow to get gas containing carry-over oil.
Comparing the two compositions,
carryover can be determined.
Sampling rate?
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52. Sampling-Summary
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53. 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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54. 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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55. 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)
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56. 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
Constant Volume Depletion (CVD)
Viscosities (not important for High Perm GC reservoirs)
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57. 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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58. 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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59. 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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60. Measured DLE Data Example
DLE Matbal
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61. 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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62. Reservoir Oil Density Calculation (Compare with DLE and/or CCE)
Field Units
Metric Units
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63. Measured SEP Data Example
DLE Matbal
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64. 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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65. 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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66. Measured CVD Data Example
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67. 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
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68. Class Evaluation Project Design of Fluid Sampling and PVT Experiments
Reservoir A:
Reservoir C:
Under-saturated oil reservoir
Medium sized reservoir
Depletion and/or water injection
Highly under-saturated gas condensate reservoir.
High permeability.
Primary depletion.
Reservoir B:
Reservoir D:
Saturated oil reservoir with initial gas cap
Large reservoir
Gas injection in gas cap, followed by depletion
Saturated gas condensate reservoir.
Low permeability.
Potential gas injection followed by depletion.
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69. Reservoir A – Sampling & PVT Measurements
Multi-Stage Separator Experiments (SEP)
Constant Composition (CCE)
Differential Liberation (DLE)
Oil Viscosity Measurements (VISC)
Reservoir A:
Under-saturated oil reservoir
Medium sized reservoir
Depletion and/or water injection
Key Data:
Bo and GOR from SEP experiment.
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).
Sampling:
Bottom-hole samples
Separator sample
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70. Reservoir B – Sampling & PVT Measurements
Multi-Stage Separator Experiments (SEP Oil & Gas)
Constant Composition (CCE Oil and Gas)
Differential Liberation (DLE Oil)
Constant Volume Depletion (CVD Gas)
Backward multi-contact experiments (Oil and Injection Gas).
Oil Viscosity Measurements (VISC Oil)
ECM experiment (Oil and Gas)
TBP (From reservoir oil)
Reservoir B:
Saturated oil reservoir with initial gas cap
Large reservoir
Gas injection in gas cap, followed by depletion
Sampling:
Separator sample (Gas & Oil)
Bottom-hole (Oil)
ECM Sampling
Key Data: (Addition to Res A)
Vaporization of oil
CVD gas compositions
Equilibrium oil and gas composition at GOC (ECM oil and gas).
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71. Reservoir C – Sampling & PVT Measurements
Constant Composition (CCE)
Constant Volume Depletion (CVD)
Multi-Stage Separator Experiments (not required for lean systems).
Reservoir C:
Highly under-saturated gas condensate reservoir.
High permeability.
Primary depletion.
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)
Sampling:
Separator sample (Always!)
Bottom-hole sample (check production GOR consistency)
Isokinetic sampling (evaluate for lean gases)
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72. Reservoir D – Sampling & PVT Measurements
Constant Composition (CCE)
Constant Volume Depletion (CVD)
Multi-Stage Separator Experiments
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)
Reservoir D:
Saturated gas condensate reservoir.
Low permeability.
Potential gas injection followed by depletion.
Sampling:
Separator sample.
Key Data (Addition to Res A)
Oil vaporization (injection)
Relative oil volume (blockage)
Oil viscosity (blockage)
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