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SPE 111918

Improved Characterization of Heptanes-Plus Fractions Of Light CrudesIsehunwa, 0 Sand Falade, G.K.

Department of Petroleum Engineering, University of Ibadan

Copyright 2007. Society of Petroleum Engineers Inc

This paper was prepared for presentation at the 31st Annual SPE InlemationalTechnical Conference and Exhibition in Abuja, Nigeria, August 6-8,2007.

This paper was selected for presentation by an SPE Program Committee followingreview of information contained in an abstract submitled by the author(s) Contentsof the paper, as presented, have not been reviewed by the Society of PetroleumEngineers and are subject to correction by the author(s). The material, as presented,does not necessarily reflect any position of the Society of Petroleum Engineers, itsofficers, or members. Papers presented at SPE meetings are subject to publicationreview by Editorial Committees of the Society of Petroleum Engineers Electronicreproduction. distribution. or storage of any part of this paper for commercia:purposes withou.t the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restncted to an abstract of not morethan 300 words, illustrations may not be copied. The abstract must containconspicuous acknowledgement of where and by whom the paper was presentedWrite librarian. SPE. POBox 833836. Richardson. TX 75083-3836. USA. fax 01-972-952-9435

AbstractHeptanes plus fractions have strong effects on thephysical properties and phase behaviour of petroleumfluids. It is therefore very important to properlycharacterize plus fractions. A step to achievingimproved characterization is to obtain more realisticmolecular weights. Most of the current methods ofheptanes plus characterization assume their molecularweights are accurate. However, what is commonlymeasured in the laboratory is the molecular weight ofthe complete fluid; the molecular weight of the heptane-plus fraction is then estimated using Kay's mixing rule.Unfortunately, physical properties like molecularweight obtained using 'equivalent fluid' principles bymixing pure components, do not give the same valueswith actual measurements. Therefore, while a veryaccurate estimate of the molecular weight ofa reservoiroil could be available, that of the heavy fractions,which is 'inferred' could be unreliable, because of themixing rule.

A simple technique has been formulated to achieveimproved characterization ofpetroleum fluids and theheavy fractions. We suggest fine tuning' Kay's mixingrule in order to achieve a match between actualmeasured molecular weight and the 'equivalent fluid'.Experimental data from over 4()O PVT reports from

over 100 fields in the Niger Delta were collected andstudied A correlation was established between oilgravity and molecular weight and compared with othercommonly used correlations. Statistical error analysiswas undertaken. Heptanes plus molecular weightswhich were generally estimated using Kay's mixingrule were found to be generally high and hence finetuned using a simple technique.

The results of this study show that the well-knownCragoe's and Standing correlations gave absoluteaverage deviation of 126.8 and 53.3 respectively forlight crudes, compared to 2.5 obtained in this studyFurthermore, better description of heavy fractions wasachieved with more accurate molecular weight.

It is concluded that the proposed technique perhapsprovides a theoretical basis for the usual 'tuning' ofheptane-plus properties during fluid modelling. It isalso concluded that a more accurate correlation forestimating the molecular weight of light crudes hasbeen developed

IntroductionAccurate description of reservoir fluid systems is a keyelement of petroleum reservoir characterization andmanagement. The aim is often to predict the propertiesand behaviour of fluids at any temperature andpressure, to promote best engineering and reservoirmanagement practices. Basic information for-reservoirfluid characterization may be obtained from laboratoryanalysis, use of mathematical models or approximatecorrelations. Experimental data, however are veryexpensive, time consuming and difficult to reproducefor near critical fluids. The use of mathematical modelscould be fast, but they require some degree of expertise.They also often require some experimental data forfine-tuning or calibration. Correlations, on the otherhand, often require little expertise to use and could be

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2 Improved Characterization of heptane- plus fractions of Light Crudes SPE

very useful as quick surveillance tools in new fields andin areas with sparse experimental data.

As noted by Dindoruk and Christman!' I, PYT propertycorrelations could be generic based on randomlyselected data sets, or they could be specializedrepresenting a type of oil or geographic area.

A number of techniques are avai lable for accuratemeasurement of molecular weights of hydrocarbons.These are largely based on the principles of boilingpoint elevation, freezing point depression, vapour phaseosmosis or chromatography, with the freezing pointdepression method preferred as the most accuratetechnique. However, the molecular weight of the heavyfractions can only be inferred from the measured valuesfor the complete fluid.

Theoretical FrameworkThe physical properties of fluids can be related to theirintermolecular forces and molecular size i.e.

e = f(intermolecular forces, molecular size)

It is perhaps with this in view that Riazi and Daubert(21

proposed a generalized physical property correlation inthe form:

( 1 )

Where, e is any physical property such as molecularweight, Ts is the normal boiling point and a, b, and care empirical constants.

However, for a quick-look description of molecularweight, we can simplify equation (I), by setting b = 0 toobtain

(2)

Eqn. (2) is the basis of the correlation developed bysuch authors as Cragoe", Kesler-Lee'I'and Standing'r".

The Molecular weight M is composition dependent.Hence,

(3)

Where,x, = Mole fraction of components i.

For a multi-component hydrocarbon system, equation(3) is normally expressed using Kay's mixing rule as:

(4)

Where,

M, = Molecular weight of the pure components.

As noted by Kreglewskif" and several other workers,equation (4) is not very accurate. It can be improvedupon by use ofa better mixing rule.

Therefore, using a method similar to the way Equationsof state parameters are normally adjusted, we propose'matching' the actual experimental values with the'equivalent fluid' obtained from the mixing of purecomponents. Thus, we propose an expression of theform:

(5)

Where,M = Actual molecular weight measuredX, = Mole fraction of component i.M, = Molecular weight of component i'f' = Substance-dependent adjustment factor

Expanding the equation above, we have:

(6)

Or,

From the fractional compositional data and measuredmolecular weights, the actual molecular weight of theheptane plus fractions can be established.

However, where compositional data are not available,molecular weight can be established from surface oilgravity data.

Data CollectionLaboratory data on composition and molecular weightsfrom 400 PYT reports and about 100 fields cuttingacross the entire Niger Delta were collected andanalyzed after normal quality checks.

Data range is as shown in Table I, with depths rangingbetween 4100 ft and 12,400 ft-ss, and reservoirpressures ranging between 1768 and 8356 psia. Meanvalues are as shown in Table 2.

Results and DiscussionIt is known that unless an accurate mixing rule isemployed, a property such as molecular weight of an'equivalent fluid' obtained from mixing purecomponents is not the same as the actual property of a

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3 Improved Characterization of heptane- plus fractions of Light Crudes SPE

multi-component fluid (6). This is demonstrated inTables 3 and 4, where Kay's rule appl ied to the criticalproperties of binary mixtures did not repl icateaccurately the measured values obtained fromHorstman et al (7). Maximum deviation is obtained atabout 0.5 moles of each fraction.

For Niger Delta crude oil, the adjustment factor'+' wasestablished at 0.15.

Accurate estimation of the molecular weight of heavyfractions should make tuning of equation of stateparameters less necessary. Whitson (8,9) hasdemonstrated that the tuning of equation of stateparameters is theoretically same as changing themolecular weight of heptanes plus fractions.

Where compositional data are not available, thecorrelation for specific gravity was established as:

M = 9260.1 (APlyl 2894 (8)

Where,API= 141.5/Yo-131.5 (9)

And,Yo= Oil Specific gravity at 60/60 OF

Where it is desired to estimate the molecular weight ofthe heptanes fraction using only oil gravity, equation 10was developed for Niger Delta fluids:

M7+ = 459.75 - 16.67 API + 0.1778 API2

(10)

Where, API is the API gravity of the whole fluid at60/60.

The true boiling point for the heptanes plus fraction canbe estimated using an approximation of a straight chainhydrocarbon:

Ts = 25.525 M7+05739 (II )

These correlations were found to be valid for APIgravity less than 55. Above this value, Cragcesformula should be used. Table 5 shows the comparisonbetween this study with Cragoe and Standingcorrelations. Cragoe 's and Standing correlations gaveabsolute average deviation of 126.8 and 53.3respectively for light crudes, compared to 2.5 in thisstudy. This is illustrated in Figure I. Figure 2 shows therelationship between molecular weight and GaR in theNiger Delta, while Figures 3 and 4 show the effects ofC7+ fractions on molecular weight and on GaR. Theyconfirm the observation by Wu and Fish .0O), thatheptanes plus fraction controls several properties of

hydrocarbon fluids. Figures 3 and 4 suggest that thepercentage cut of the heavy fraction present is perhapsmore important that the molecular weight of the heavyfractions.

Conclusions

Improved correlations for predicting the molecularweight of Niger Delta crude oils and the heptanes plusfractions have been developed.

The correlations employ simple relationships, whilethere results were more accurate for the Niger Delta oilsthan other correlations that used data from other regionsof the world.

Nomenclature

a, b,c Empirical constants defined by equation I

API American Petroleum Institute (gravity)

GaR Gas - Oil Ratio

M Molecular weight

P Pressure

T Temperature

x Mole fraction

Y Specific gravity

Adjustment factor

Subscripts

B Boiling point

C Critical

Component

a Oil

AcknowledgmentsThis paper was supported through research grant fromthe Shell Petroleum Development Company to the ShellChair in Petroleum Engineering, University of Ibadan.

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Improved Characterization of heptane- plus fractions of Light Crudes SPE

ReferencesI. Dindoruk, B and Christman, P.G., "PVT properties

and viscosity correlations for Gulf of MexicoOils." Paper SPE 71633 presented at the AnnualTechnical Conference and Exhibition, NewOrleans, Lousiana, October 200 I .

2. Riazi, M. R. and Daubert, T.E., "Simplify PropertyPredictions", Hydrocarbon Processing, (March1980), I 15-1 16.

3. Cragoe, C.S., "Thermodynamic Properties ofPetroleum Products", Bureau of Standards, U. S.Department of Commerce (1929), MiscellaneousPublication 27.

4. Kesler, M. G. and Lee, B.L., "Improve Predictionof Enthalpy of Fractions", HydrocarbonProcessing, (March 1976), 153-158.

5. Standing, M. B., Volumetric and Phase Behaviourof Oil Field Hydrocarbon Systems, SPE, Dallas,1977.

6. Kreglewski, A. "A Semi-Empirical Treatment ofProperties of Fluid Mixtures", Proceedings of theThomas Alvin Boyd Lectures in ChemicalEngineering; Bulletin 20 I, Engineering ExperimentStation, Ohio State University, Columbus, Ohio.,April, 1967.

7. Horstmann, S., Fischer, K. and Gmehling, J.:"Experimental Determination of Critical Data ofMixtures and their relevance for the Developmentof thermodynamic Models". Chern. Eng. Sci.(2000),56,6905-6913.

8. Whitson, C.H., "Characterizing Hydrocarbon PlusFractions", Paper EUR 183, presented at theEuropec meeting, London, October 21 -24, 1980.

9. Whitson, C. H., "Effect of physical PropertiesEstimation on Equation of State Predictions." SPEJ(Dec. 1984),685-696.

10. Wu, R.S. and Fish, R.M., "C7+ Characterizationfor Fluid Properties Predictions". J. Can. Pet.Tech., 28, (1989), I 12-1 17

II. Ahmed, T. Hydrocarbon Phase Behaviour, GulfPublishing Company, Houston, 1989.

12. Danesh, A. PVT and Phase Behaviour ofPetroleum Reservoir Fluids, Elsevier Publishers,Amsterdam, 1998.

13. Bondi, A. Physical Properties of MolecularLiquids, Crystals and Glasses, Wiley, 1968.

14. Firrozabadi, A.: Thermodynamics of HydrocarbonReservoirs, McGraw Hill, (1999).

15. Kay, W. B., "Density of hydrocarbon Gases andVapor", Ind. Eng. Chern. Vol. 28, (1936), 1014-1019.

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Table 1: Ranges of Data used

PropertyCondensate

RangeOil

No offieldsNo of PYT ReportsStatic Reservoir Pressure (psia)Reservoir Depths (ft-ss)Reservoir Temperature CR)Gas-Oi I Ratio (scf/stb)Specific oil Gravity

1004001768 - 54994100 - 12400518 -72043 -74250.716-0.95

15252330 - 83564100 - 12400590 -7223,000 - 20,0000.36 - 0.78

Table 2: Mean Composition of petroleum fluids in the Niger Delta

Component Condensate(mole %)

N2CO2

CIC2

C3

iC4nC4i.c,n-C,C6C7+

Crude Oils(mole %)0.13±0.100.72 ± 0.6843.24 ± 11.404.22 ± 2.303.00 ± 2.751.24 ± 0.981.70± 1.421.00 ± 0.800.80 ± 0.701.70 ± 1.2342.00 ± 15.25

0.42 ± 0.401.47± 1.0082.52 ± 9.775.54 ± 1.912.89 ± 2.461.01 ± 0.531.01 ± 1.020.55 ± 0.390.43 ± 0.340.61±0.413.62 ± 2.89

TABLE 3: Deviation of Kay's Mixing Rule for Ethane-Propane System (7)

Ethane(1) -Propane(2)

x1000000.12020.23980.35980.48030.58070.66030.73890.82050.89971.0000

X21.00000.87980.76020.64020.51970.41930.33970.26110.17950.10030.0000

EXPERIMENT ALTc (K) Pc (mPa)369.70 4.250363.96 4.447358.00 4.647352.45 4.79834406 4.960337.91 503033305 5.061327.48 5076321.38 5.065314.10 5017305.41 4.881

CALCULATEDTc (K) Pc (mPa)369.7000 4.250000361.9723 4.325846354.2833 4.401314346.5685 4.477034338.8215 4.553069332.3668 4.616422327.2493 4.666649322.1961 4.716246316.9501 4.767736311.8583 4.817711305.4100 4.881000

DEVIATIONTc (K) Pc (mPa)o 000000 0 0000001.987658 0.1211543.716742 0.2456865.881542 0.3209665.238487 0.4069315.543203 0.4135785.800687 0.3943515.283881 0.3597544.429945 0.2972652.241713 0.1992890.000000 0.000000

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TABLE 4: Deviation of Kay's Mixing Rule for Ethane-n-Butane System (7)

Ethane(1) -n-Butane(2)

EXPERIMENT AL CALCULATED DEVIATIONPc

x1 X2 Tc (K) Pc (mPa) Tc (K) Pc (mPa) Tc (K) (mPa)0.0000 1.0000 424.92 3.790 424.9200 3.790000 0000000 0.0000000.1496 0.8504 415.72 4.285 4070413 3.953214 8.678696 0.3317860.2990 0.7010 403.82 4.810 389.1865 4.116209 14.63349 0.6937910.4402 0.5598 390.67 5.266 372.3117 4.270258 18.35830 0.9957420.5605 0.4395 377.54 5.598 357.9346 4.401506 19.60536 1.1964950.6601 0.3399 364.38 5.749 3460314 4.510169 18.34855 1.2388310.7407 0.2593 352.55 5.813 336.3989 4.598104 16.15106 1.2148960.8185 0.1815 340.15 5.701 327.1011 4.682984 13.04894 1.0180170.9095 0.0905 324.39 5.413 316.2257 4.782265 8.164345 0.6307361.0000 0.0000 305.41 4.881 305.4100 4.881000 0.000000 0.000000

Table 5: Comparison of this study with OthersEXPT THIS

SAMPLE API MW STUDY CRAGOE STANDING1 22.30 15908 169.04 370.88 189.932 23.99 231.20 153.85 336.23 186.213 45.38 50.60 67.66 154.12 139.184 45.38 63.80 67.66 154.12 139.185 45.38 46.33 67.66 154.12 139.186 1903 229.79 207.42 463.30 197.137 36.95 79.80 88.17 195.93 157.708 19.03 126.25 207.42 463.30 197.139 17.45 19304 232.02 526.87 200.62

10 38.98 73.71 82.29 183.91 153.2411 41.06 83.98 76.96 173.03 148.6712 41.06 68.07 76.96 173.03 148.6713 41.06 74.96 76.96 17303 148.6714 45.38 53.30 67.66 154.12 139.1815 20.65 18106 186.70 412.46 193.5716 34.97 95.68 94.66 209.28 162.0617 23.99 148.68 153.85 336.23 186.2118 34.97 91.34 94.66 209.28 1620619 36.95 96.45 88.17 195.93 157.7020 45.38 54.01 67.66 154.12 139.1821 22.30 120.67 169.04 370.88 189.9322 20.65 150.70 186.70 412.46 193.5723 1903 178.82 207.42 463.30 197.1324 22.30 161.70 169.04 370.88 189.9325 22.30 130.50 169.04 370.88 189.9326 31.14 139.10 109.92 241.01 170.4827 54.68 34.90 53.19 124.71 118.6928 38.98 84.60 82.29 183.91 153.2429 45.38 50.33 67.66 154.12 139.18

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0::<CI-...J::z:ae>W-...JWO~:E

API VERSUS MOLECULAR WEIGHT OFNIGERIAN CRUDES

600.00500.00400.00300.00200.00100.000.00

0.00

._--_._---------_._--,ir-··-

II_--------_~-~!-.~-~~-:------------------~~• ~ "t,~, ,10.00 20.00 30.00 40.00 50.00 60.00

API GRAVITY

• EXPTAL fiI THIS STUDY CRAGOE STANDING

Figure 1: API Gravity Correlation of Molecular Weight

Molecular Weight Variation with GOR

300.00l- •::z: 250.00o • •W 200.00~ ..~0::

150.00<C...J::l

100.00 .. ~(,..UW ~...J

50.00~ .... •0 ~

:E •000

0.0 500.0 1000.0 1500.0 2000.0 2500.0 3000.0 3500.0

GOR (SCF/STB)

Figure 2: Molecular Weight vs GOR in the Niger Delta

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C7+ Cut Vs GOR

80.0

~ 70.0

~ 60.0~ 50.0+ 40.01/1

~ 30.0C'OQ. 20.0Q)

::r: 10.00.0

o

•

500 1000 1500 2000 2500 3000GOR (SCF/STB)

3500

Figure 3: Heptanes plus cut Vs GOR in the Niger Delta

C7+ Cut Vs Molecular Weight of Crude Oil inNiger Delta

100.0 ••~o-

• +Q)

oE

+ 10.01/1Q)c::C'O•..C.Q)::r:

1.0o 100

Molecuf~PWeight 300

Figure 4: Heptanes Plus Cut Vs Molecular weight in the Niger Delta

400

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