Objectives Upon completion of this module the student should be able to: 1.
List the five types of reservoir fluids.
2.
Explain the difference between reservoir oils and reservoir gases.
3.
Explain the defining difference between black oils and volatile oils.
4.
Explain how to distinguish between black oils and volatile oils using initial production data, laboratory data, or production history.
5.
Explain how to distinguish between volatile oils and retrograde gas condensates using initial production data, laboratory data, or production history.
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Objectives Upon completion of this module the student should be able to: 6.
Locate the initial position of black oil, volatile oil and retrograde in a phase diagram. Show the shapes of liquid drop out curve vs pressure at pressures below the saturation pressure.
7.
Discuss wet gases, their occurrence in nature, the usefulness of the concept of wet gas in engineering calculations, and the identification of a wet gas using field data.
8.
Discuss the unique feature of dry gases.
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Reservoir Fluid Classification Reservoir Fluids are Classified by their Behavior During Depressurization at Reservoir Temperature and Subsequent Production •
Dry Gas – No Phase change in Reservoir or Separator
•
Wet Gas – No Phase Change in Reservoir but Liquids Form in the Separator
•
Retrograde Gas – Exhibits a Retrograde Dew Point in the Reservoir
•
Black Oil - Exhibits a Bubble Point in the Reservoir
•
Volatile Oil – Not Well Defined. It is a High Shrinkage Oil that Exhibits a Bubble Point but has a Formation Volume Factor of 2 or Greater.
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Phase Diagram of a Black Oil
Stock Tank
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Phase Diagram of a Volatile Oil
Stock Tank
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Phase Diagram of a Retrograde Gas
Retrograde
Stock Tank
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Phase Diagram of a Wet Gas
Stock Tank
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Phase Diagram of a Dry Gas
Stock Tank
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Three Gases - What Are the Differences? • Dry gas - gas at surface is same as gas in reservoir • Wet gas - recombined surface gas and condensate represents gas in reservoir • Retrograde gas - recombined surface gas and condensate represents the gas in the reservoir But not the total reservoir fluid (retrograde condensate stays in reservoir)
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The Five Reservoir Fluids
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Liq % Curve of Black Oil, Volatile Oil and Retrograde Condensate Bubble Point
Liquid %
Black Oil Volatile Oil
Retrograde Condensate Dew Point
Pressure 11 CH LAT James Jose
Phase Diagrams of Near Critical Fluids Estimated critical point
Pressure
Bubblepoint line
0% 5% 10% 15%
100 90 80 70 60
15%
50 40 35 30 25 20 15 10 5
10%
5%
Temperature 12 CH LAT James Jose
Dewpoint line
Liquid % Curve of Near Critical Fluid Bubble Point
Liquid %
Volatile Oil
Retrograde Condensate
Dew Point
Pressure 13 CH LAT James Jose
McCain’s Generalizations for Fluid Types Black oil
Volatile Oil
Retrograde Gas
Wet Gas
Dry Gas
Initial producing GOR, scf/STB
<1,750
1,750 – 3,200
>3,200
>15,000*
100,000*
Initial stock-tank liquid gravity, o API
<45
>40
>40
up to 70
No liquid
Color of stocktank liquid
Dark
Colored
Lightly colored
Water white
No liquid
Bubble point
Bubble point
Dew Point
No phase change
No phase change
Heptane-plus, mole %
> 20
20 – 12.5
<12.5
<4*
<0.7*
Oil FVF at bubble point
<2.0
>2.0
-
-
-
Phase change in reservoir
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McCain’s Primary Production Trends
Time
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Time
Time
Time
No liquid
Time
° API
° API
Time
Dry Gas GOR
GOR
Wet Gas
Time
° API
Time
° API
° API
Time
Retrograde Gas GOR
Volatile Oil GOR
GOR
Black Oil
No liquid
Time
Exercise 1 One of the wells in the Merit field, completed in December 1967 in the North Rodessa formation, originally produced 54°API stock-tank liquid at a gas/oil ratio of about 23,000 scf/STB. During July 1969, the well produced 1987 STB of 58°API liquid and 78,946 Mscf of gas. By May 1972, the well was producing liquid at a rate of about 30 STB/d of 59°API liquid and gas at about 2,000 Mscf/d. What type of reservoir fluid is this well producing?
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Exercise 1 Solution •
Initial GLR over 3,200 scf/STB and producing gasliquid ratio and stock tank oil gravity increasing with production, thus fluid is retrograde gas condensate
•
Initial GLR over 15,000 scf/STB, thus fluid can be treated as wet gas
•
GLR apparently started increasing immediately, indicating that pi = pd and the possibility of an oil zone
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Exercise 2 A field in north Louisiana discovered in 1953 and developed by 1956 had an initial producing gas/oil ratio of 2,000 scf/STB. The stock-tank liquid was “medium orange” and had a gravity of 51.2°API. Classify this reservoir fluid. During the producing history of this field the stocktank liquid gravity steadily increased to 63°API, and the producing gas/oil ratio increased to a maximum of 29,000 scf/STB. Does this information confirm your classification? Why or why not?
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Exercise 2 Solution
•
Initial producing GLR greater than 1,750±250 scf/STB and less than 3,200 scf/STB, thus volatile oil
•
Both producing GLR and stock-tank oil gravity increasing indicates volatile oil
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Dry Gas- Basic PVT • • • •
Samples can be taken at BH, WH or Separator. Quality check: Opening pressure, contaminants Select Best sample for further study Composition to C12+ (more than enough as heavy components are not expected). • Constant Composition Expansion at Tr • Compressibility Factor (Z) Measurement • Report should include RV with respect to reservoir pressure, Z, Bg and calculated Gas viscosities at various pressures
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Wet Gas- Basic PVT • • • • • • • • •
Samples can be taken at BH, WH if there is no liquid production or Separator. Quality check: Opening pressure, contaminants Select Best sample/ samples for further study Recombination in case of separator samples Composition to C12+ (more than enough as heavy components are not expected). Constant Composition Expansion at Tr Compressibility Factor (Z) Measurement Simulated separation tests at various pressures Report should include RV with respect to reservoir pressure, Z, Bg and calculated Gas viscosities at various pressures, Simulated Separation tests
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Retrograde Gas (Condensate)- Basic PVT • • • • • • • • •
Samples can be taken at BH, WH if there is no liquid production (single phase) or Separator. Quality check: Opening pressure, contaminants, Dew points for BH or WH Samples, Bubble point for Separator liquids. Select Best sample/ samples for further study Recombination in case of separator samples Composition to C30+ Constant Composition Expansion at Tr Constant Volume Depletion at Tr Simulated separation tests at various pressures Report should include RV with respect to Pd, Fluid densities, Retrograde liquid dropout (RLD), Z, Cumulative production, Bg, produced fluid composition, calculated Gas viscosities at various pressures, Simulated Separation tests and amount of plant products expected.
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Volatile (Critical) Oil- Basic PVT •
Samples can be taken at BH, WH if they are in single phase or Separator.
•
Quality check: Opening pressure, contaminants, Bubble points for BH, WH and Separator liquids.
•
Select Best sample/ samples for further study
•
Recombination in case of separator samples
•
Composition to C30+
•
Constant Composition Expansion at Tr
•
If high shrinkage below Pb then Constant Volume Depletion at Tr
•
Low shrinkage : Differential vapourisation / Liberation
•
Combination of CVD and DV also is done in few cases
•
Separation tests: Single and Multistage
•
Viscosity of the fluid from Pr to atmospheric pressure.
•
Report should include RV with respect to Pb, Fluid densities, Liquid shrinkage below Pb, Z, Cumulative production, Bg, produced fluid composition, calculated Gas viscosities at various pressures (for CVD)
•
Bo, Rs, Liquid Density, produced gas properties like Bg, Z, Gas Density, Gas viscosity etc (for DV/DL)
•
Separation Test and Viscosity results should also be included in the report.
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Black Oil- Basic PVT • • • • • • • • • •
Samples can be taken at BH, WH (if they are in single phase) or Separator. Quality check: Opening pressure, contaminants, Bubble points for BH, WH and Separator liquids. Select Best sample/ samples for further study Recombination in case of separator samples Composition to C30+ Constant Composition Expansion at Tr Differential vaporization / Liberation (DV/DL) Separation tests: Single and Multistage Viscosity of the fluid from Pr to atmospheric pressure. Report should include RV with respect to Pb, Bo, Rs, Liquid Density, produced gas properties like Bg, Z, Gas Density, Gas viscosity etc, Separation Test and Viscosity
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