SPE-171241-MS

New Approach to PVT Correlation Selection

R. Khabibullin, M. Khasanov, A. Brusilovsky, A. Odegov, and D. Serebryakova, Gazpromneft NTC; V. Krasnov,Rosneft

Copyright 2014, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Russian Oil and Gas Exploration and Production Technical Conference and Exhibition held in Moscow, Russia,1416 October 2014.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contentsof the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflectany position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the writtenconsent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations maynot be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

This paper attempts to provide engineer with GOR PVT correlation selection rules. Rules based oncomparison of standard PVT correlations with calculations based on Equation of State, which believed tohave wider applicability range and be more general from physical point of view.

To make that comparison synthetic oils data bank where generated. Ranges for synthetic oils propertieswere chosen to cover all the real oils of interest (mainly from Western Siberia). All synthetic oils whereanalyzed using Peng Robinson equation of state with shift parameter and widely spread PVT correlations.

Results provided as GOR PVT correlation Applicably Maps and New Procedure calculation GOR byselected PVT correlation. Result was checked during PVT study of few Western Siberia oil fields.

IntroductionCorrect PVT properties have critical importance for engineering calculations and decision-making on allstages of oil field life. However, often comprehensive PVT data unavailable or inconsistent for specificconditions.

Common practice in a lack or unavailability of PVT measurements for the entire range of pressure andtemperature has been the use of PVT correlations. Correlations provide qualitatively correct descriptionof the basic properties of oil, but require adjustment to specific fields.

There are a number of PVT correlations. They build for different oil fields and regions. Nevertheless,often ranges of initial parameters for different correlation intersecting and correlations shows similarresults for similar input data. This lead to situation for ordinary oil fields then PVT correlation selectiondo not influence on calculation results much. As a result, engineers often do not pay attention tocorrelation selection and use default software options. This approach, applied to more complex fields, forexample with high GOR, can lead to sufficiently incorrect results. At the same time, there is no widespread rules for PVT correlation selection for wide range of conditions.

This paper attempts to provide engineer with GOR PVT correlation selection rules.The paper is organized as follows. The section 2 describes the basic methods for PVT-properties

estimation by using Black oil (correlations) and composition model (Equations of State). The section3 focuses upon solution gas-oil ratio at the pressure below bubble-point. There is review of correlation for

GOR and real oil tends in 3 section. In the section 4 GOR PVT correlation Applicable Maps are derived.The section 5 gives New Procedure calculation GOR by selected PVT correlation using Applicable Maps.There are examples of calculating GOR by given New Approach in the 5 section.

Oil PVT propertiesOil PVT properties are:

Pb bubble-point pressure Rsb, Rs (P) solution gas-oil ratio at the bubble-point pressure and below Bob, Bo (P) oil formation volume factor at the bubble-point pressure and below

There are two widely spread approache for PVT simulation:

1. Black oil (two pseudocomponents: gas, liquid; calculation by PVT correlations)2. Composition model (the oil composition is specified; calculation by Equations of State, EoS)

Figure 1Relations: Pressure - Gas-Oil Ratio and Pressure - Oil Formation Volume Factor

Figure 2Bubble-point pressure (a) and oil formation volume factor at the bubble-point pressure (b), calculated by by EoS without adaption,compare favorably with measured data (69 data points).

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The PVT correlations advantages:

1. PVT correlations are required minimum data2. The calculation by correlations takes less time then the calculation by EoS

For bubble-point pressure Pb and oil formation volume factor Bob correlations initial data usually isfollowing:

T - temperature g - gas specific gravity (air1) o - oil specific gravity Rsb solution gas-oil ratio at the bubble-point pressure

Rearranged bubble-point pressure correlation Pb f(o,g, T, Rsb) usually is used for calculationGas-Oil Ratio below bubble-point pressure Rs(P)i.e.:

And oil formation volume factor at the pressure below bubble-point:

Figure 3Relations Pressure - Gas-Oil Ratio from different correlations in in the coordinates: Pressure, MPa - Gas-Oil Ratio, Sm3/m3 (fig. a) andPressure/Bubble-Point Pressure - Gas-Oil Ratio/Gas-Oil Ratio at the bubble-point pressure (fig. b).

Figure 4Different characters of Solution Gas-Oil Ratio of different reservoir oils

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For 389 data from the oil fields bubble-pointpressure and oil formation volume factor at thebubble-point pressure were calculated with the var-ious correlations. Ranges of data and calculationresults are in Appendix A.

Creating adequate PVT-model of natural hydro-carbon system is requied adaptation to the labora-tory results, among which bubble-point pressureand oil formation volume factor (Brusilovsky,

2004). The equation of state (EoS) has a lot of configurable parameters (critical pressure and criticaltemperature, acentric factor of fractions C7, binary interaction coefficients, etc.). Without adaptiondeviations of bubble-point pressure and oil formation volume factor may be recieved. For example seefigures 2 and Apendix A as results calculation bubble-point pressure and oil formation volume factor (for69 real data set) without adaption by EoS Three-parameter Peng-Robinson at the following parameters:Kesler-Lee correlations for critical pressure, critical temperature and acentric factor of fractions C7,binary interaction coefficients by Katz-Firoozabadi.

Thus calculation bubble-point pressure and oil formation volume factor by EoS for synthetic oils databank have no practical value. However trend of solution gas-oil ratio at the pressure below bubble-pointmay be analyzed by using EoS. Solution gas-oil ratio is required for calculation by correlation oilformation volume factor at the pressure below bubble-point. The gas-oil ratio at the pressure belowbubble-point is very important parameter in oil production. For example, it is limiting factor for oilextraction by ESP. For these reasons Solution gas-oil ratio is considered in the following.

Solution Gas-Oil RatioA review PVT correlation for GOR

Widely spread approache for calculation Gas-Oil Ratio Rs(P) is derived from rearranging a bubble-pointpressure correlation Pb f(o,g, T, Rsb).

A lot of correlations for bubble-point pressure have the following form:

where , and a1 a8 constants distinguished for different authors correlations.

For the first time this form was proposed by Standing (1947).But other bubble-point pressure correlation forms exist. For example correlation of Lasater (1958),

Glaso (1980), Al-Marhoun (1992) and other.Cardinally other approache for calculation Gas-Oil Ratio Rs(P) was proposed Velarde, Blasingame &

McCain (1997):

where

Table 1Ranges PVT-properties of oils from Synthetic oils data bank

Parameter

Tr, K 300-390

0 0.76-0.91

g 0.55-1.13

Pb, MPa 1 60

Rsb, Sm3/m3 8-400

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Figure 5Applicable Maps for GOR PVT-correlation. Various cases of oil type: a paraffinic oils, b-d naphtenic, e aromatic (division by criterionKw). Tr360 K, C2

Figure 6Applicable Maps for GOR PVT-correlation. Various cases of reservoir temperature and relationship C2-C3. Kw11.75

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So for one given composition o, g, T, Rsb may be received difference GOR-curved using differentcorrelations (see fig. 3a).

Types of Solution Gas-Oil Ratio curves for real oilsDifferent reservoir oils below the bubble-point differ character of relative gas evolution. This fact wasobserved by Cronquist (1972), Namiot (1976). Types of Solution Gas-Oil Ratio curves for real oils are onthe figure 4.

Comparison of Solution Gas-Oil Ratio curvesAnalize of Solution Gas-Oil Ratio curves is not clearly because real bubble point of reservoir oil andcalculated by any correlation often differ (fig. 3a). For this reason conversion curves to relativecoordinates (P/Pb,Rs/Rsb) is possible way for further omparison of curves. Examples see fig. 3b.

Comparison of curves in relative coordinates led to the conclusion to review only corelations ofStanding, Glaso and Velarde, Blasingame & McCain in this work.

Area of the figures W between curves is calculated for accurate comparison of curves in relativecoordinates. Area of the figures W is calculated by trapezium method. Thus the different correlations maybe compared with other data, for example real data from the labs. In this work, correlations were comparedwith calculations based on Equation of State.

Applicable Maps for GOR PVT-correlationsCreating MapsFor creating Applicable Maps for GOR PVT-correlation the following work was done:

1. Synthetic oils data bank was created by generation different molar compositions Zi, C7 propertiesand reservoir temperature. Ranges for synthetic oils properties were chosen to cover all the realoils of interest (mainly oil fields from Western Siberia).

2. All synthetic oils were analyzed using Peng Robinson equation of state (EoS) with shift parameter.As a result o, g, Rsb, Rs(P) were received. Ranges of received PVT-properties see table 1.

3. Then for all synthetic oils solution GOR curves Rs(P) were formed using widely spread PVTcorrelations: Standing(1947), Glaso(1980), Velarde, Blasingame & McCain (1997)

4. Solution GOR curves Rs(P) from EoS (Constant Composition Expansion at reservoir temperature

Table 2Examples data

Parameter Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7

Tr, K 312 323 329 371 357 363 366

0 0.87 0.88 0.88 0.80 0.83 0.84 0.82

g 0.58 0.57 0.71 0.98 0.91 0.81 0.79

Pb, MPa 8.1 12.4 10.5 14.7 21.5 30 30.5

Rsb, Sm3/m3 30 41 42 165 238 244 356

Selected correlation(s) Gl, V, St Gl, V, St Gl V V V, Gl, St Gl, St, V

Table 3Results calcu lation for example 4

Rs at the P13.5 MPa. From EoS: Rs 152.0 Sm3/m3

Before After

Rs, Sm3/m3 Eror, % Rs, Sm3/m3 Error, %

Standing 137.2 -10 149.0 -2

Glaso 116.3 -23 149.1 -2

Velarde, Blasingame & McCain 145.0 -5 152.6 0

SPE-171241-MS 7

Figure 7Estimate solution gas-oil ratios by correlations before and after using New Approach.

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Figure 8Estimate solution gas-oil ratios by correlations before and after using New Approach.

SPE-171241-MS 9

were calculated) and from correlations were transformerd to relative coordinates (P/Pb, Rs/Rsb).Area of the figures W was calculated for each pair EoS-correlation curves.

5. In different ranges of g and Rsb best correlations were choosen by relying W calculation results(correlation is considerd as the best then its W is the lowest among others. Correlation isconsidered as applicable in case its W less then 0.045. If all correlations have W more then 0.045 select correlation with the lowest among others. In case of other correlation doesnt exceed 20%W of the best correlation it is considerd as applicable too). Examples of calculation resultsW seefigure 11.

ResultsResults calculation for synthetic oils data bank can be provided as Applicable Maps for GOR PVT-correlation. See fig. 5-6.

New Approach for GOR estimation by using Applicable Maps for PVTcorrelationProcedure calculationWe recommend next algorithm for estimate solution gas-oil ratio Rs(P) of reservoir oil at the pressure Pbelow bubble point pressure Pb:

Step 1. In accordance with T, g and Rsb (Kw and C2-C3-ratio in some case) select PVT correlationusing Applicably Maps (fig. 5-6).

Step 2. By selected correlation calculate Pbcorrf(o,g,T,Rsb)Step 3. Calculate

Step 4. By selected correlation calculate Rs(P)f(o,g,T,P=)

Examples of UseNew Approach were cheked during 77 data set from real data bank (Western Siberia oil fields) and briefinformation are in Appendix B. Results confirmed adequacy of New Approach.

See examples.Consider Example 4 in detail.Calculate Rs at the P 13.5 MPa

1. At The Applicably Maps for T 360 K find point Rsb 165 Sm3/m3 and g 0.98. Select

Figure 9Estimate solution gas-oil ratios by correlations before and after using New Approach.

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Velarde, Blasingame & McCain (1997) correlation.2. For this selected GOR correlation calculate Pbcorr using Valko and McCain(2003) correlation:Pbcorr 15.5 MPa

3. Calculate

4. By Velarde, Blasingame & McCain (1997) correlation at the P= 14.3 MPa: Rs 152.6 Sm3/m3Results calculation for this example 4 with using introduced New Approach see table 3 and figure 8b.Results calculation for other examples see figures 7-9.

ConclusionThe paper proposes the New Approach for GOR estimation by using widely spread PVT correlations.Applicable Maps for GOR PVT-correlations were provided. The applicability of correlation depends ongas specific gravity, solution gas-oil ratio at bubble-point pressure, temperature, oil type (paraffinic,naphtenic, aromatic), C2-C3-ratio.

New Approach was checked during PVT study of 77 Western Siberia oil fields. 7 examples of usagethe New Approach are described.

Nomenclature

Bob, Bo(P) oil formation volume factor at bubble-point pressure and belowGOR gas-oil ratioP pressurePb bubble-point pressureRsb, Rs(P) solution gas-oil ratio at bubble-point pressure and belowT temperatureg gas specific gravity (air1)o oil specific gravityKw Whatson factorC2 molar fraction of C2H6 in reservoir oil compositionC3 molar fraction of C3H8 in reservoir oil compositionEoS Equation of State

ReferencesAl-Marhoun, M.A.: New Correlations For Formation Volume Factors of Oil And Gas Mixtures, J.

Cdn. Pet. Tech. (March 1992) 22.Al-Shammasi, A.A.: A Review of Bubblepoint Pressure and Oil Formation Volume Factor Corre-

lations, SPEREE (April 2001) 146.Brusilovsky A.I. 2004. Methodology of equation of state application for modeling of gas condensate

mixtures. Gazovaya promyshlennost, 4: 1619Chapman Cronquist Dimensionless PVT Behavior of Gulf Coast Reservoir Oils, J.Pet. (May 1973)De Ghetto, G., Paone, F., and Villa, M.: Pressure-Volume-Temperature Correlations for Heavy and

Extra Heavy Oils, paper SPE 30316 presented at the 1995 SPE International Heavy Oil Symposium,Calgary, 1921 June.

Glaso O. Generalized pressure-volume-temperature correlations, J.Pet. Tech., pp 785795 (May1980)

Kartoatmodjo, R.S.T. and Schmidt Z.: Large Data Bank Improves Crude Physical Property Corre-lations, Oil and Gas J. (4 July 1994) 51.

Khairy, M., El-Tayeb, S., and Hamdallah, M.: PVT Correlations Developed for Egyptian Crudes,Oil and Gas J. (4 May 1998) 114

SPE-171241-MS 11

Labedi, R.M.: Use of Production Data To Estimate The Saturation Pressure, Solution GOR andChemical Composition of Reservoir Fluids, paper SPE 21164 presented at the 1990 SPE Latin AmericanPetroleum Engineering Conference, Rio De Janeiro

Larry W. Lake Petroleum Engineering Handbook.L John R. Fanchi. General Engineering. Richard-son, Texas: SPE, 2006

Lasater, J.A.: Bubble Point Pressure Correlation, Trans., AIME (1958) 213, 379.Levitan, L.L. and Murtha, M.: New Correlations Estimate Pb, FVF, Oil and Gas J. (March 1999)Macary, S.M. and El-Batanoney, M.H.: Derivation of PVT Correlations for the Gulf of Suez Crude

Oils, Proc., 1992 EGPC Petroleum Exploration and Production Conference (1992) 374.W.D. McCain, Jr. The Properties of Petroleum Fluids. 2-nd Edition. - Tulsa, Oklahoma: PennWell

Publishing Company, 1990, 551 .Standing, M.B.: Volumetric and Phase Behavior of Oil Hydrocarbon Systems, ninth edition, SPE,

Dallas (1981).Namiot A.Y. Phase conversions in the development of oil and gas fields, M. Nedra. 1976Petrosky, G.E. Jr. and Farshad, F.F.: Pressure-Volume-Temperature Correlations For Gulf of Mexico

Crude Oils, SPEREE (October 1998) 416.Standing, M.B.: A Pressure-Volume-Temperature Correlation For Mixtures of California Oils and

Gases, Drill. and Prod. Prac., API (1947) 275.Valko P.P. and McCain W.D., Reservoir Oil Bubblepoint Pressures revisited; solution gas/oil ratios

and surface gas specific gravities, J. Pet. Sc. Eng., 37 pp 153169 (2003)Vazquez, M.E. and Beggs, H.D.: Correlations for Fluid Physical Property Prediction, JPT (June

1980)Velarde, J., Blasingame, T.A., and McCain, W.D. Jr.: Correlation of Black Oil Properties At

Pressures Below Bubble Point PressureA New Approach, paper 97-93 presented at 1997 Annual CIMPetroleum Soc. Technical Meeting, Calgary, 811 June

C.H. Whitson and M.R. Brul;. Phase Behavior, first edition, - Richardson, Texas: SPE, 2000

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Appendix A

Results calculation of Bubble-Point Pressure and Oil Formation Volume Factor for real oils data bank

Table 4Ranges data

Min Avr Max

Pb, Mba 3,0 15,8 34,2

T, K 289 354 393

0 0,772 0,841 0,945

Rsb, Sm3/m3 16 127 401

g 0,56 0,78 1,10

Bob 1,03 1,41 2,64

Figure 10Oil formation volume factor at the bubble-point pressure (a) and bubble-point pressure (b), calculated by correlations, compare favorablywith measured data (389 data points).

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Table 5Results calculation of Bubble-Point Pressure

ARE, % STD, % AARE, %

PVT-correlations for 389 labdata set

Standing (1947) 3,56 17,61 11,82

De Ghetto & Villa (1994) -1 -6,56 17,32 13,24

De Ghetto & Villa (1994) -2 11,71 20,32 15,35

GlasO (1980) -1 18,70 25,73 20,31

GlasO (1980) -2 -5,39 16,25 13,29

Kartoatmojo and Schmidt (1991) -1 -2,70 20,45 15,25

Kartoatmojo and Schmidt (1991) -2 13,34 25,04 17,43

Al-Marhoun (1992) 20,45 31,88 25,69

De Ghetto & Villa (1994) -3 13,82 36,35 26,20

Valko and McCain (2003) 6,35 16,19 12,39

Velarde, Blasingame & McCain (1997) 12,29 22,58 17,38

Lasater (1958) 17,10 24,22 20,23

Vasquez & Beggs (1980) -1 19,25 27,30 21,01

Vasquez & Beggs (1980) -2 10,59 22,65 14,98

Petrosky & Farshad (1993) 15,28 25,74 19,15

Al-Shammasi (1999) 1,89 17,50 12,94

Labedi (1990) (mod. Standing) 25,91 33,33 27,17

Labedi (1990) 5,42 22,30 17,78

Dindoruk and Christman (2001) 11,05 21,95 17,43

Hanafy and Macary (1997) 16,43 25,55 21,47

Levitan and Murtha (1999) 8,39 20,34 13,95

Khairy, El-Tayeb and Hamdallah (1998) 4,07 22,06 16,87

Dokla and Osman (1990) 8,41 22,24 16,60

EoS for 69 lab data set Three-parameter Peng-Robinson, Kesler-Lee correlations forcritical pressure, critical temperature and acentric factorof fractions C7, binary interaction coefficients byKatz-Firoozabadi

5,15 19,62 13,46

Table 6Results calculation of Oil Formation Volume Factor

ARE, % STD, % AARE, %

PVT-correlations for 389 labdata set

Standing (1947) 1,27 3,60 2,14

GlasO (1980) -0,98 3,27 2,22

Kartoatmojo and Schmidt (1991) 0,34 3,36 2,32

Al-Marhoun (1988) 0,13 3,48 2,44

Al-Marhoun (1992) 0,38 3,16 2,14

Vasquez & Beggs (1980) -1 0,06 4,49 3,28

Vasquez & Beggs (1980) -2 -2,69 7,52 5,46

Petrosky & Farshad (1993) 0,91 3,20 2,10

Al-Shammasi (1999) 0,61 3,26 2,28

Hanafy and Macary (1997) 10,62 11,81 10,62

Levitan and Murtha (1999) 1,06 4,02 2,97

Khairy, El-Tayeb and Hamdallah (1998) 3,28 6,44 4,56

Dindoruk and Christman (2001) 0,44 4,17 2,35

Dokla and Osman(1990) 0,46 3,80 2,49

EoS for 69 lab data set Three-parameter Peng-Robinson, Kesler-Lee correlations forcritical pressure, critical temperature and acentric factorof fractions C7, binary interaction coefficients byKatz-Firoozabadi

-2,56 5,59 3,91

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Appendix B

Some results received during creating Applicable Maps for GOR PVT-correlation

Figure 11Example calculation results W for case 360 K, C2 < C3, Kw 11.75 (fig.6 c).

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Figure 12Results of check-up calculation for 77 oils from real data bank by New Approach

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New Approach to PVT Correlation SelectionIntroductionOil PVT propertiesSolution Gas-Oil RatioA review PVT correlation for GORTypes of Solution Gas-Oil Ratio curves for real oilsComparison of Solution Gas-Oil Ratio curves

Applicable Maps for GOR PVT-correlationsCreating MapsResults

New Approach for GOR estimation by using Applicable Maps for PVT correlationProcedure calculationExamples of Use

Conclusion

References