1. Lecture 9
Liquid Properties
10/10/2014

2. Liquid Properties
The density of liquid at reservoir conditions allows for assessment of oil formation factor, leading to production decisions
Ideal liquid solutions
Liquid w/ known composition
2 methods to compute density (hard & easy)
Liquid & gas w/ unknown composition
Correlations for formation volume factors
Corrections for pressures above bubble point
Viscosity of liquids

3. Definition Recollection
Oil Formation Volume Factor Bo = vol. oil + dissolved gas at reservoir conditions volume oil entering stock tank barrel at s.c. Solution gas-oil Ratio Rs= (volume of dissolved gas at r.c.) (volume of oil entering stock tank at s.c.) B0
Reservoir Pressure
Pbp
PSC
1.0 Rs
Reservoir Pressure
Pbp
PSC

4. Ideal Liquid Solution V1 V V2
If “ideal”, volume of mixture is: And density is: V V1 V2

5. Reservoir Conditions Not Ideal
C1-C2are not liquids at standard conditions
Partial vaporization of oil at standard conditions
Consequences:
Composition changes as a function of pressure and time
Density at SC difficult to determine because no liquid density available for lighter components

6. Apparent Liquid Density
Use an apparent liquid density (ALD) for methane and ethane
ALD: density C1 and C2 if they were liquid at standard conditions
ALD represents contribution of methane and ethane to
density of the liquid mixture
ALD is determined by:
1. Measuring densities at elevated p and T for mixtures
of methane & heavier, and ethane & heavier
2. Using compressibility and thermal expansion factors to
go back to s.c.
3. Subtracting contribution by heavier components

7. ALD Chart
This chart gives the ALD as a function of the density of the complete system at SC The density of the complete system is called the pseudoliquid density of the system McCain Fig Standing and Katz, Trans., AIME, 146, 159.

8. ALD Chart
This chart gives the ALD as a function of the density of the complete system at SC The density of the complete system is called the pseudoliquid density of the system McCain Fig Standing and Katz, Trans., AIME, 146, 159.
1 g/cc = 62.4 lb/cu. ft

9. ALD Chart
This chart gives the ALD as a function of the density of the complete system at SC The density of the complete system is called the pseudoliquid density of the system McCain Fig Standing and Katz, Trans., AIME, 146, 159.
1 g/cc = 62.4 lb/cu. ft
Cannot use this chart without having a prior estimate of the density at SC

10. Isothermal Compressibility Chart
This chart gives necessary compressibility corrections McCain Fig Standing, Volumetric and Phase Behavior of Oild Field Hydrocarbons, SPE, Dallas, 1951.

11. Isothermal Compressibility Chart
This chart gives necessary thermal expansion corrections Approx. independent of pressure McCain Fig. 11-4

12. Procedure
If we want liquid RC, then we need: pseudoliquid SC RC
Pseudoliquid correction charts

13. Procedure
If we want liquid RC, then we need: pseudoliquid SC BUT If we want pseudoliquid SC, then we need: ALD of methane and ethane RC
Pseudoliquid correction charts
ALD Chart

14. Procedure
If we want liquid RC, then we need: pseudoliquid SC BUT If we want pseudoliquid SC, then we need: ALD of methane and ethane HOWEVER If we want the ALD of C1 and C2, Then we need: pseudoliquid SC RC
Pseudoliquid correction charts
ALD Chart

15. Procedure
If we want liquid RC, then we need: pseudoliquid SC BUT If we want pseudoliquid SC, then we need: ALD of methane and ethane HOWEVER If we want the ALD of C1 and C2, Then we need: pseudoliquid SC RC
Pseudoliquid correction charts
ALD Chart

16. Iterative Approach
1. Guess the pseudoliquid density at SC 2. Determine corresponding apparent liquid densities 3. Compute corresponding pseudoliquid density. If same as guess, stop. 4. Repeat for a second initial guess 5. Plot guesses and computed pseudoliquid densities 6. Determine point on graph for which y = x

17. Example
Compute the density of a reservoir liquid at its bubble point at 3280 psia and 218 F with the following composition of well stream. Initial Guess: 47 lb/cu ft

18. Example

19. Example – Guess 1

20. Example

21. Example – Guess 2

22. Example

23. Compressibility and Thermal Expansion
Compressibility correction = +0.9
Thermal expansion correction = -4.4
Density at reservoir conditions is = 47.0 lb/cu ft

24. Simpler Method
1. Compute weight fraction W1 of methane in the mixture 2. Compute weight fraction W2 of ethane in ethane+ mixture 3. Read density ratio from Fig in McCain 4. Multiply density ratio by density of propane+ which gives pseudoliquid density Then: find density at reservoir conditions using compressibility and thermal expansion charts

25. Simpler Method
The weight fraction of C1 in the mixture is: The weight fraction of C2 in C2+ is: Figure 7-5 gives a density ratio of approx The density of C3+ is: which leads to a pseudoliquid density of 50.0 lb/cu ft (note a slightly different result than before).

26. Simpler Method
1. Compute weight fraction W1 of methane in the mixture 2. Compute weight fraction W2 of ethane in ethane+ mixture 3. Read density ratio from Fig in McCain 4. Multiply density ratio by density of propane+ which gives pseudoliquid density Then: find density at reservoir conditions using compressibility and thermal expansion charts
0.915

27. Simpler Method
The weight fraction of C1 in the mixture is: The weight fraction of C2 in C2+ is: Figure 7-5 gives a density ratio of approx The density of C3+ is: which leads to a pseudoliquid density of 49.7 lb/cu ft (note a slightly different result than before).

28. Compressibility and Thermal Expansion
Compressibility correction = +0.9
Thermal expansion correction = -4.4
Density at reservoir conditions is = 46.5 lb/cu ft

29. Unknown Compositions Reservoir liquid composition is commonly unknown
Typical knowns are:
solution gas-oil ratio, Rs
specific gravity of gas at SC
API of crude at SC(related to specific gravity)
(Fig in McCain) gives apparent liquid density of gas as function of gas specific gravity and °API of crude
Results usually within 5% of measured values

30. Apparent Liquid Densities of Natural Gas

31. Class Example Sep. 1 Sep. 2 Tank Total
Produced Rs in scf/STB
Specific gravity of gas
The tank oil has a gravity of 27.4 °API (or a specific gravity of 0.891).
The reservoir is at 3280 psia (bubble point pressure) and
218 °F
Compute the density of the oil in the reservoir.
Also, compute the formation volume factor of the oil.
5.615 cubic feet in 1 STB,
62.4 lb of water in 1 cu ft of SC
1lb mole of gas occupies a volume of 379 SC

32. Steps Calculate the specific gravity of the overall gas
Calculate the apparent liquid density of gas
Calculate the mass of the gas
Calculate the mass of the stock tank oil
Calculate the pseudoliquid volume of gas
Calculate the pseudoliquid density of reservoir SC
Calculate the density of the reservoir RC
Calculate the volume of the reservoir RC
Calculate the formation volume factor

33. Solution
Calculate the specific gravity of the overall gas

34. Solution Calculate the specific gravity of the overall gas
414.0 × × × 1.54
( )
= 0.726

35. Solution Calculate the specific gravity of the overall gas
414.0 × × × 1.54
( )
= 0.726
Calculate the apparent liquid density of gas
0.726 and API of 27.4 gives reading of 24.9 lb/cu ft at s.c.

36. Solution Calculate the mass of the gas
MW: × lb/lb mole = 21.0 lb/lb mole.
Vol. in scf: = scf.
Mass: scf/STB / 379 scf/lb mol × 21.0= 29.3 lb/STB

37. Solution Calculate the mass of the gas
MW: × lb/lb mole = 21.0 lb/lb mole.
Vol. in scf: = scf.
Mass: scf/STB / 379 scf/lb mol × 21.0= 29.3 lb/STB
Calculate the mass of the stock tank oil
0.891 × 62.4 lb/cu ft × cu ft/STB
= 312 lb/STB STB).

38. Solution Calculate the pseudoliquid volume of gas
29.3 lb/STB / 24.9 lb/cu ft = 1.18 cu ft/STB

39. Solution Calculate the pseudoliquid volume of gas
29.3 lb/STB / 24.9 lb/cu ft = 1.18 cu ft/STB
Calculate the pseudoliquid density of reservoir SC
Total volume: = 6.79 cu ft/STB
Total mass: = 341 lb/STB
Density: / 6.79 = 50.2 lb/cu ft

40. Solution Calculate the density of the reservoir oil @ RC
Compressibility = lb/cu ft
Thermal expansion = -4.8 lb/cu ft
= 45.9 lb/cu ft

41. Solution Calculate the volume of the reservoir oil @ RC
341 lb/STB / 45.9 lb/cu ft = 7.42 cu ft/STB
Calculate the formation volume factor
7.42 cu ft/STB / cu ft/bbl = 1.28 bbl/STB.

42. Example
Compute the density of a reservoir liquid at its bubble point at 3280 psia and 218 F with the following composition of well stream. Initial Guess: 49 lb/cu. ft. OR g/cc

43. Example

44. Example – Guess 1

45. Example
2nd Guess = 53 lb//cu ft

46. Example – Guess 2

47. Example

48. Compressibility and Thermal Expansion
Compressibility correction = +0.9
Thermal expansion correction = -4.4
Density at reservoir conditions is = 47.0 lb/cu ft