1. Gas Properties & PVT Tests
Lecturer: Marianna Dmitrieva
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2. Standing and Katz compressibility factors chart.
Gas compressibility factor
Standing and Katz compressibility factors chart.

3. Gas formation volume factor
Rate = 15,000 ft^3/day
Pres = 2,000 psi
Gas flow rate in scf/day - ?
Tres = 120 °F

4. Gas as compressibility factor (direct calculation)
The Hall – Yarborough Method
Y – the reduced density that can be obtained as the solution
of the following equation
The Dranchuk – Abu – Kassem Method
The Dranchuk – Purvis – Robinson Method

5. Coefficient of isothermal compressibility of gases
For real gases:
dz/dP can be obtained from the slope of the z vs P curve.

6. Viscosity of paraffin hydrocarbon gases at one atmosphere.
Viscosity of gases
Viscosity of Mixtures
Viscosity of paraffin hydrocarbon gases at one atmosphere.

7. Methods of calculating the viscosity of gases
The Carr-Kobayashi-Burrows Correlation Method
Carr’s viscosity ratio correlation.
Carr’s atmospheric gas viscosity correlation.
“corrected” gas viscosity at one atmospheric pressure
and reservoir temperature, cp
- viscosity corrections due to the presence of N2
- viscosity corrections due to the presence of CO2
- viscosity corrections due to the presence of H2S
- uncorrected gas viscosity, cp

8. Specific gravity of a gas
Example:
Rate = 1.1 MMscf/day
a. Apparent molecular weight of the gas
Tres = 150 °F
Pres = 1500 psi
b. Gas density at reservoir conditions

9. Phase diagrams for reservoir fluids8

10. Phase diagrams for multicomponent systems9

11. Relative positions of phase envelopes10

12. Phase diagram for an oil reservoir with a gas cap11

13. PVT analysis PVT- Scope PVT Analysis
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
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.

14. 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.

15. 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.

16. 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

17. 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 behavior over exploration, development and production.

18. Main PVT tests
Flash vaporization or relative volume test
Differential vaporization test
Separator test
Viscosity measurements
Compositional measurements
Special studies: e.g. Interfacial tension

19. Liberated gas remains in equilibrium with oil.
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 throughout the test
The behavior below the bubble
point does not reflect reservoir
behavior, where gas has greater
mobility than oil
The test determines
the Bubble Point pressure
Liberated gas remains in equilibrium with oil.

20. Constant composition expansion (constant mass study)

21. Flash vaporisation (relative volume) test

22. Flash vaporisation (relative volume) test
By plotting P versus V, a break in the slope
is obtained at the Bubble Point pressure.
Above bubble point
compressibility of oil
at reservoir temperature
can be determined
No free gas

23. Constant composition expansion (constant mass study)
No fluids removed from the cell
Purpose to determine z value above dew point
Determine dew point pressure

24. Differential vaporization (liberation or expansion)
Below bubble point in reservoir gas liquid separation is a constant changing system
Applied to liquid/oil systems only
In the differential vaporization 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
Last pressure step will be a reduction to standard conditions - automatic

25. Differential vaporization (liberation or expansion)
Schematic Diagram of Differential Liberation

26. Differential vaporization (liberation or expansion)
Typical Experimental Results for DL

27. Separator test
Objective to determine impact of separator conditions on Bo, GOR and produced fluid physical properties
To give an indication of oil shrinkage and GOR when fluids produced to surface
There are not unique values for Bo & GOR. They depend on separator conditions
Starting point for the test is the bubble point pressure
Fluid produced at surface conditions. Stock tank oil

28. Density of reservoir oil
Separator test
Formation
Volume Factor
Gas Oil ratio
Density of reservoir oil

29. Constant volume depletion (constant mass study)
Specify a temperature (below cricodentherm) and a series of pressures
Applied to both oil and condensate systems
Vapor removed to restore cell to original volume
Relative volume reported is the fraction of the cell filled with liquid after the gas is removed

30. Constant volume depletion (constant mass study)
Typical Experimental Results for CVD

31. Constant volume depletion (constant mass study)
Comparison of Liquid Saturations from CCE and CVD
Liquid drop-out behavior of the North Sea gas condensate at 394 K in CCE and CVD tests

32. THANK YOU!
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