spe_78538.1

7/30/2019 SPE_78538.1

1/7

Copyright 2002, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 10th

Abu Dhabi International Petroleum

Exhibition and Conference.

This paper was selected for presentation by an SPE Program Committee following review of

information contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect any

position of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper

for commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than 300words; illustrations may not be copied. The abstract must contain conspicuous

acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract

A detailed physical characterization of tar from acarbonate reservoir in Saudi Arabia was made toevaluate its mobility and ways of establishing contactbetween the lighter oil and its aquifer. Density andviscosity measurements were carried out on several tar

samples, under simulated reservoir conditions ofpressure and temperature. Other physical parameterssuch as simulated distillation, pour point and penetrationindex were also experimentally determined.

Tar physical properties were found to vary withdepth and area within the same field. The obtainedexperimental results showed a gradual increase indensity and viscosity from the tar/oil contact towards thetar/water contact. This increase was much morepronounced in the neighborhood of the tar/water contact.Density and viscosity of tar diluted with toluene were in

excellent agreement with those of pure tar.

The density of non preserved tar varied between

0.956 g/cc at 200F and 1.008 g/cc at 76F while that of

preserved tar varied between 0.944 g/cc at 200F and 0.

991 g/cc at 76F. The tar samples analyzed appear tobehave as Newtonian fluids.

Introduction

The present paper discusses detailed physicacharacterization of several extracted and RFT bottomhole tar samples obtained from a carbonate reservoir in

Saudi Arabia. The chemical aspect has already beenpresented elsewhere [1]. Tar is defined as extra heavy oil

with a gravity ranging between 29 and 9API albeit thedistinction between heavy oil and tar is rather shady.

Tar mat is present in abundance in the MiddleEast, Africa and elsewhere. In recent publicationsKaufman et al. [2] mentionned the presence of a tar zoneat the water/oil contact in Burgan field. This has beenknown for a long time. However, to ascertain lateral andvertical delineation of the tar zones appear to be stilunresolved. Thick tar zones identified through visua

observation and Latroscan analyses of weathered coresamples were reported in the Raudhatain field [3].

A tar mat is generally a thick column layingbetween an aquifer underneath and a much lighter oilreservoir above. This peculiar location poses a host ofchallenging problems for an efficient management of thelighter oil and ultimately for a proper management of thetar zone itself [4].

In this study, density of tar was measured with adigital Anton Parr densiometer having a maximum

temperature range of 300 F and a pressure limit of6000psi. Based on these measurements, specific gravity andAPI gravity were determined.

A rolling ball viscometer was used to measuretars viscosity at elevated pressures and varioustemperatures. The effect of visbreaking or permanentviscosity reduction due to thermal alteration has alsobeen examined.

SPE 78538

Characterization of Tar From a Carbonate Reservoir in Saudi Arabia: PhysicalAspects

Harouaka A. S, B. Mtawaa and W. A. Nofal, SPE, KFUPM/RI

7/30/2019 SPE_78538.1

2/7

2 [Harouaka A. S, B. Mtawaa and W. A. Nofal]

The pour point, which measures the lowesttemperature at which tar is observed to flow whencooled and examined under prescribed conditions, wasdetermined following ASTM standard procedure: ASTMD97-66.

Tar does not change from its semisolid state tothe liquid state at any definite temperature, but itgradually becomes softer as the temperature rises. Forthis reason, the determination of the softening point hasbeen carried out following the standard proceduredescribed in IP 58/83 [5]. The penetration index andflash point have also been determined by standardprocedures.

Distillation of tar has been conducted using theprocedure described in ASTM D402-73. The sample isdistilled at a controlled rate to a temperature of 680 F,and the volumes of distillate obtained at the specifiedtemperatures are measured. The residue and thedistillates can then be tested as required.

Experimental procedures and background

Modifications of existing oil characterizationmethods were necessary to facilitate experimentaldetermination of the physical properties of tar. Onetypical modification is to dilute tar with a suitableorganic solvent prior to testing. Measurements are thenmade using different dilution ratios. The Physicalproperties of pure tar are determined by extrapolation to

zero dilution ratio.

Chirinos M. L. et al. [6] studied the effect of

temperature (ambiant to about 194F) and percentage ofadded diluent on the density and viscosity of nearly 100crude oils obtained from the Venezuelan Orinico OilBelt. An Atomix mixer, a Fisher specific gravityhydrometer and a Haak Rotovisco RV3 viscometer wereused to perform this study. These crudes and theirmixtures with diluents behave as Newtonian but not asplastic fluids. Expressions to estimate the density andkinematic viscosity of these oils and their mixtures with

diluents at any temperature were also presented.

Tobey et al. [7] reported that when core plugsobtained from tar zones near the oil-water contact in theArab-D formation in Saudi Arabia were extracted with aseries of solvents (naphtha, toluene, and methylenechloride or trichloroethane) for at least 72 hours; theirporosity improved and their permeability showed only a

small change after the initial cleaning with naphtha. Itwas hypothesized that macro pores were easily accessedby the solvents and hence cleaned right after the naphthaand toluene extractions. The micro pores and the smallerpore-throats remained completely or partially plugged bytar that differ from one location to another within thesame reservoir.

Puttagunta V. R. et al. [8] developed ageneralized viscosity correlation to predict, with goodaccuracy, the viscosity of any Alberta bitumen's orheavy oils, which are known to vary widely from deposito deposit, over a wide range of pressure andtemperature, usually encountered in in-situ recoverymethods. This correlation requires only one viscosity

measurement at 86F and atmospheric pressure.

Erno B. P. et al. [9] measured the viscosity of oi

samples extracted at 194F using a Beckman 28/Mheated centrifuge from preserved cores obtained fromvarious depths and different wells in the LoweCretaceous Clear water "B" heavy oil reservoir in theCaribou lake area. They observed that the viscosityincreases consistently with depth and correlates welwith structural elevation. Simulated distillation datashowed that the viscosity variations are due tocompositional differences. Samples from other heavy oilreservoirs such as McMurray, the Wabiskaw and theWaseca sand at Pikes Peak in west central Saskatchewanwere found to exhibit similar trends.

Svrcek and Mehrotra [10] measured theviscosity, density and solubility of CO2, CH4 and N2 inbitumen samples obtained by toluene extraction from theAthabasca tar-sands. These measurements were carriedout over a temperature range of 77 to 212F and apressure up to 1450 psi. They observed that CO2 has thehighest solubility in Bitumen and reduces its viscositydrastically. On the other hand N2has quite low solubilityin the bitumen and less effect on its viscosity. They alsoreported that dead oil viscosity decreases drastically withthe increase of temperature and is less affected by thepressure while that of live oil decreases with pressureand temperature.

Following is a brief description of theexperimental procedures and conditions under which thephysical characterization of tar samples retrieved fromthe tar mat zone, in a carbonate reservoir from SaudiArabia, was determined.

7/30/2019 SPE_78538.1

3/7

[Paper Number] [Characterization of Tar From a Carbonate reservoir in Saudi Arabia: Physical Aspects ] 3

Density

Density measurements were carried-out using adigital Anton Paar densiometer (DMA-60). Themeasuring principle involves the period of oscillation ofa vibrating U-shaped sample tube filled with the sampleor through which the sample flows continuously. It wasequipped with a Ruska mercury injection pump tocharge the tar sample into the sample tube and pressurizeit to the desired pressure, and a Heto constanttemperature oil bath to maintain the sample tube at thedesired temperature.

Before being used, the densiometer wascalibrated using vacuum and a standard oil sample witha density close to that of the tar samples to be analyzed.The density is calculated once the sample is introducedinto the sample tube and its period of oscillation isstabilized at the desired pressure and temperature.

1. Extracted tar

To measure the density of extracted tar, it mustbe injected into the densiometer's sample tube asexplained earlier. However, at atmospheric pressure anddifferent temperatures, it does not have the necessarymobility to be injected. This is why it has been diluted intoluene. The density of extracted tar-toluenehomogeneous mixtures was then measured at differenttoluene concentrations (weight %). The density of theextracted tar is simply taken as the value extrapolated to

zero toluene concentration.

2. Dead RFT tar

A tar sample taken out of the RFT bottom holesample transfer cell and saved in a container was leftopen to atmosphere for few days until the associated gaswas liberated. Its density was then measured followingthe same procedure used for the extracted tar.

3. Live RFT tar

The cylinder containing the live RFT tar wasconnected to the inlet port of the densiometer sampletube while another cylinder was connected to its outletport to let the tar sample flow continuously from thesource cell to the second cylinder via the sample tube.The sample cell was then heated to the desiredtemperature using a temperature regulator and displayalong with heating tape.

The sample was subsequently pressurized to thedesired pressure using a Ruska mercury injection pumpA second pump was used to maintain flow through thedensiometer sample tube. The density of the undisturbedRFT tar sample was then determined at differenpressures and temperatures.

Viscosity

Viscosity measurements of extracted and deadRFT tar were carried out at ambient pressure anddifferent temperatures using a highly sensitive torquemeasuring system made of a plate-cone type ContravesLow Shear-30 viscometer and a Haak-M regulatedconstant temperature oil bath.

Tar samples (0.50 cc) were heated to the desiredtemperature and poured into the measuring cup of theviscometer. Similarly, the viscosity of tar/toluene

homogeneous mixtures were measured at varioustoluene concentrations (volume %). Results wereextrapolated to zero toluene concentration to find thepure tar viscosity.

Viscosity measurements of extracted and liveRFT tar were also obtained at various high pressures andtemperatures using an ROP rolling-ball viscometeDT14001, along with a Haak-M constant-temperature oibath and a Ruska mercury injection pump. An importanfeature of this viscometer is the utilization of a magneticfield for detecting the rolling ball. Accurate ball location

can be detected even when the ball is completely coatedwith the high viscosity tar.

The viscometer was calibrated using a standardoil sample with a viscosity of 27 cp at 210F and 477 cpat 100F. Viscosity values were calculated using thefollowing relation:

= A *(?b-?t)*t + B

Where:

is the tar viscosity (cp),A is the slope of the calibration curve at the desiredangle,?b and ?t are the densities of the steel ball and tarrespectively,t is the measured time (sec.) andB is the intercept of the calibration curve at the sameangle.

7/30/2019 SPE_78538.1

4/7


Simulated distillation

There are several distillation methods coveringthe boiling range distribution of petroleum products. Theextended ASTM D2887-73 (standard test methods forboiling range distribution of petroleum fractions by gaschromatography) was followed for the distillation ofextracted and RFT tars. This method is mainlyapplicable to petroleum products and fractions with afinal boiling point of 1000F or less but extended tocover a higher boiling point range.

The apparatus used included a Hewlet-Packardlevel 4,5880A Gas Chromatograph, a flame ionizationdetector (FID), a liquid nitrogen cooling system, and acartridge tape unit. The column used was a 1/8 x 20inch stainless steel, 10% UC-W982 on 80/100 meshchromosorb P-AW.

The results obtained are shown in Table 1below. The initial boiling point (IBP) of extracted tar(552 F) was almost twice that of RFT tar (228F).However, at the end, their yields (around 1000F) arealmost equal 10-11% of the total sample. This indicatesthat their compositions are mainly the same.

The difference at low temperature may be due tothe fact that the extracted tar has lost its lightercomponents keeping in mind the cores were exposed toatmosphere for a long period of time.

Table 1. Distillation data of extracted and RFT tars.

Extracted tar RFT tar

% OFF Temperature F

IBP* 552 228

1 590 297

2 639 377

3 675 444

4 702 518

5 723 588

6 752 6627 790 736

8 828 811

9 871 892

10 916 999

11 975

*Initial boil ing point: The point at which a cumulative area count equalto 0.5% of the total area under the chromatogram is achieved.

Other Physical Parameters

1. Penetration index

The penetration index is the consistency of abituminous material expressed as the distance in tenthsof a millimeter a standard needle vertically penetrates a

sample of the material under known conditions ofloading, time, and temperature. Penetration tests werecarried out for the extracted and RFT tar samples at77oF according to ASTM D5-83 using the penetrationapparatus (Penetrometro-697) and its accessories(sample containers, timer, a water bath, needles, andweights). It was found that penetration index of theextracted and RFT tar samples were 67 and 196 units,respectively as shown in Table 2 below.

Table 2. Penetration index of extracted and RFT tar.

Tar sample Penetration* Extracted Tar 67

RFT tar 196

*One penetration unit = 0.1 mm, Time = 5 sec, needle weight with

spindle = 50 gms.

2. Flash point

The flash point is defined as the lowes

temperature, corrected to a barometric pressure of 760mmHg, at which application of a test flame causes thevapor of the sample to ignite under specified testingconditions. A Gallenkamp-Autoflash apparatus, a formof the Pensky-Martens closed tester, was utilized todetermine the flash point of the extracted and RFT tarsamples according to ASTM D93-80.

3. Pour point

The pour point is the lowest temperatureexpressed as a multiple of 5F at which the oil/tar is

observed to flow when cooled and examined undeprescribed conditions. The pour point tests wereconducted according to ASTM D 97 using a tesapparatus assembled in house (Brookfield Ex 200thermostatic bath, a test jar, and two certifiedthermometers). The obtained pour points were 163 oFfor the extracted tar and 89 oF for the RFT tar.

7/30/2019 SPE_78538.1

5/7


Results and discussions

This section summarizes the main objective ofthis study, the physical properties determination ofseveral tar samples including extracted and RFTbottomhole samples.

Extracted Tars: Various extracted tar/toluenemixtures (20, 40, 60, 80 and 100% by weight) wereprepared manually. Once homogenization wascompleted, densities were measured at atmosphericpressure and temperatures ranging from ambient to210oF to include the reservoir temperature, estimated at190 oF. At each temperature, the density of the mixturesbehaved as expected, giving a linear variation withsolvent concentration as shown in Figure 1. Eachstraight line was extrapolated to zero tolueneconcentration to determine the density of extracted tar atthat particular temperature.

It has been determined that within the range oftemperatures considered in these experiments, thedensities of tar/toluene mixtures follow an equation ofthe form:

m = Wd *(d - t) + t

Where,

mis the density of the mixture in g/cc,

d is the density of the diluent (toluene) in g/cc,

t is the density of tar in g/cc andWd is the weight fraction of diluent.

Experiments to evaluate the effect of pressure onthe density of extracted tar were performed at threedifferent temperatures (220, 230, and 240oF). They weregenerated maintaining a constant temperature whiledecreasing the pressure from 3500 to 1000 psig. Theresults obtained are shown in Figure 2.

As in the case of extracted tar, densities of RFTtar/toluene mixtures were measured at atmospheric

pressure and various temperatures (102, 120, 160, 180,190 and 200o F). Here also there is a linear relationshipbetween densities and solvent concentrations asindicated by the straight lines shown in Figure 3.Extrapolation to zero toluene concentration yields thedensity of RFT tar at the desired temperature.

When these densities are plotted against

temperature along with the densities of extracted tar asshown in Figure 4 one can see, rather cle arly, that atemperatures less than 180o F the density of the RFT taris lower than that of the extracted tar. However, beyond180o F the two curves coincide. This is expected since ahigher temperatures RFT tar loses its lighter componentsand has the tendency to become similar in compositionto the extracted tar.

Experiments to determine the effect of pressureon the density of RFT tar were also performed at threetemperatures (160, 190, and 220o F). The data weretaken while decreasing the pressure from 3500 to 1000psig and maintaining the temperature constant. Thedensity increases linearly, at constant temperature, withincreasing pressure within the specified pressure rangeas shown in Figure 5.

Viscosity Measurements

Extracted Tars: The viscosity of extracted tarwas measured at different temperatures ranging from160 to 270 oF and atmospheric pressure. The resultsobtained are shown in Figure 6. The log-log plot of shearrate vs. shear stress depicted in Figure 7 shows a straighline with a slope of one indicating the extracted tarbehaves as a Newtonian fluid with some plastic behavioas the yield (intercept) is nonzero.

As in the case of extracted tar the viscosity ofRFT tar was also measured at atmospheric pressure and

the same temperatures as the extracted tar. The resultsobtained are shown in Figure 8. A comparison betweenFigures 6 and 8 indicates clearly that the RFT tar has alower viscosity than the extracted tar.

The RFT tar is also believed to behave like aNewtonian fluid as indicated in the log-log plot of shearrate vs. shear stress (Figure 9). The slope shown inFigure 9 is slightly less than one indicating apseudoplastic behavior. The yield on the other hand isessentially zero. One should keep in mind that theextracted tar is implicitly filtered during the extraction

process as most solid particles are separated from therecovered tar. The RFT tar, on the other hand, has notbeen filtered. Based on this analysis, it is believed thatthe RFT tar behaves like a Newtonian fluid underreservoir conditions.

The standard way to plot viscosity vstemperature is on ASTM viscosity vs. temperature chartfor liquid petroleum products, which for Newtonian

7/30/2019 SPE_78538.1

6/7


fluids generally yield a straight-line relationship betweenkinematic viscosity and temperature. The straight line isbased on a mathematical expression that allows thecalculation of viscosity, in centistokes (cSt) at anytemperature T in degree Rankin, as follows:

Log log (? + 0.7) = A - B log T.

Where, ? is the kinematic viscosity in centistokes,A is the intercept andB the slope.

Various tar toluene mixtures, ranging from nineto 32 % by weight of toluene, were prepared manuallyfor both extracted and RFT samples. Oncehomogenization was achieved, viscosity measurementsof tar/toluene samples were performed at 180 and 190oFand atmospheric pressure. The results are shown inFigures 10 and 11 respectively. At each temperature theviscosity of the tar/toluene mixtures exhibited a linearvariation with solvent concentration as shown in Figures10 and 11. Each straight line was then extrapolated tozero toluene concentration to find the viscosity ofextracted and RFT tar samples at that particulartemperature.

At any temperature the viscosity of tar/toluenemixtures appear to follow an equation of the form:Y = A-BC.

Where,

Y = log log (? + 0.7),A is the straight-line intercept,B is the slope of the straight line andC is the solvent concentration in volume %.

The straight-line equation, at 180oF, forextracted tar/toluene mixture viscosity is:

Y = 0.70651-0.015519*C.

The straight-line equations for RFT tar/toluenemixture viscosity at 180 and 190o F are respectively:

Y = 0.62468 - 0.01474 * C.Y = 0.59805 - 0.015742 * C.

Figure 12 shows a plot of kinematic viscosity vs.solvent concentration for both extracted and RFTtar/toluene mixtures at 180 and 190o F, and atmosphericpressure. One may observe from Figure 12 that an

extrapolation to zero solvent concentration givekinematic viscosities comparable to those for pure tar ata given temperature for both extracted and RFT tars.

The viscosity of RFT tar was also measured atdifferent temperatures (160 up to 230oF) and pressuresranging from 1000 to 3500 psig. The results are shownin Figures 13. It can be seen from Figure 13 that theviscosity of the RFT tar decreases with increasingtemperature and decreasing pressure, unlike that of thedead RFT, which decreases significantly withtemperature and remains more or less unchanged withpressure.

CONCLUSIONS

The following conclusions may be drawn fromthe detailed physical characterization of severaextracted and live RFT bottom hole tar samples obtained

from a carbonate reservoir in Saudi Arabia:

1. The physical properties of the analyzed tasamples were found to vary with depth and areawithin the same field.

2. A gradual increase in density and viscosity fromthe tar/oil contact towards the tar/water contacwas observed. This increase was much morepronounced in the neighborhood of the tar/watercontact.

3. At each temperature, the density of tar/toluenemixtures behaved as expected, giving a linear

variation with solvent concentration.4. Density and viscosity of tar diluted with toluene

were in excellent agreement with those of puretar.

5. The viscosity of the RFT tar sample decreaseswith increasing temperature and decreasingpressure.

6. The viscosity of dead RFT decreasesignificantly with temperature and remains moreor less unchanged with pressure.

7. The extracted tar is believed to behave as aNewtonian fluid with some plastic behavior

8. The RFT tar is also believed to behave as Newtonian fluid with a pseudoplastic behavior.

ACKNOWLEDGEMENTS

The authors which to acknowledge the supportof the Research Institute of King Fahd University of

7/30/2019 SPE_78538.1

7/7


Petroleum and Minerals (KFUPM/RI). They also thankKFUPM/RI for their permission to publish this paper.The authors are grateful to A. A. Habelreeh for hisvaluable contribution to the final manuscript preparation.

REFERENCES

1. Harouaka A.S., H.K. Asar, A.A. Al-Arfaj, A.H. Al-Husaini and W.A. Nofal (1991). Characterizationof tar from a carbonate reservoir in Saudi Arabia:Part 1-Chemical aspect Paper SPE 21004presented at the SPE International symposium onoilfield chemistry held in Anaheim, California,Febr. 20-22.

2. Kafman R.L., H. Dashti, C.S. Kabir, J.M. Pederson,M.S. Moon, R. Quttainah and H. Al-Wael (1997).Characterizing the Greater Burgan Field: Use ofGeochemistry and Oil Fingerprinting Paper SPE27803 presented at the 10

thSPE Middle East Oil

Technical Conference and Exhibition, Bahrain,March 15-18.

3. Nemcsok S., N.H. Morisson, A. Carruthers and S.H.Abdullah (1998). Sedimentary Interpretation of aMultilayered clastic Oil Reservoir: Impact onDevelopment Plans For the Zubair Reservoir,Raudhatain field Paper SPE 48972 presented at theAnnual Technical Conference and Exhibition, heldin New Orleans, LA, Sept. 27-30.

4. Harouaka A.S. and H.K. Asar (1990). Tar matsevaluation- a resource and a nuisance First Saudisymposium on energy utilization and conservation

held in Jeddah, Saudi Arabia, March 4-7.5. Haines W.E. (1976). Extension of the US Bureau

of Mines- API Scheme For the Characterization ofHeavy Oils Erdoel Kohle-Ergas-Petrochem V. 29,No 9, August.

6. Chirinos, M.L., J. Gonzalez and I. Layrisse (1983)."Rheological Properties of Crude Oils from theOrinoco Oil Belt and their Mixtures with Diluents"Rev. Tech. Interp, July 1983, 103-115.

7. Tobey M. H., H.I. Halpern, G.A. Cole, J.D. Lynn,J.M. Al-Dubaisi and P.C. Sese (1993). Geochemical Study of Tar in the Uthmaniyah

Reservoir Paper SPE 25609 presented at the SPEMiddle East Oil Technical Conference andExhibition, Bahrain, April 3-6.

8. Puttagunta V. R., B. Singh, E. Cooper and B.Thunder (1988). A Generalized ViscosityCorrelation for Alberta Heavy Oils and Bitumens"Preprint Paper 158 presented at the fourthUNITAR/UNDP conference on heavy crude and tarsands, Edmonton, Alberta, Canada August 7-12.

9. Erno B. P., J.R. Chriest and R.C. Wilson (1991). Depth-Related Oil Viscosity Variation in CanadianHeavy Oil Reservoirs JCPT, Vo. 30, No. 3, MayJune, 36-41.

10. Svrcek W.Y. and A.K. Mehrotra (1982). GasSolubility, Viscosity and Density Measurements forAthabasca Bitumen JCPT, July-August, 31-38.

spe_78538.1

Documents