interrelationships between asphaltene precipitation

Interrelationships between Asphaltene PrecipitationInhibitor Effectiveness, Asphaltenes Characteristics, andPrecipitation Behavior during n-Heptane (Light Paraffin

Hydrocarbon)-Induced Asphaltene Precipitation

Hussam H. Ibrahim and Raphael O. Idem*

Process and Petroleum Systems Engineering Laboratory, Faculty of Engineering,University of Regina, 3737 Wascana Parkway, Regina, SK, Canada S4S 0A2

Received August 21, 2003. Revised Manuscript Received April 7, 2004

Three carefully chosen chemicalssdodecylbenzenesulfonic acid (DDBSA), nonyl phenol (NP),and tolueneswere studied for their asphaltene precipitation inhibition effectiveness during light-paraffin-hydrocarbon-induced asphaltene precipitation of three Saskatchewan crude oils, as wellas to evaluate possible interrelationships between their inhibition effectiveness, asphalteneprecipitation behavior (in terms of kinetics and equilibrium), and crude oil/asphaltene charac-teristics. Results showed that asphaltene precipitation rate dependence on asphaltene content(m) was a strong function of the content of heteroatoms (nitrogen (N), sulfur (S), and oxygen (O))of both the crude oil and asphaltenes, as well as the aromatic carbon fraction and degree ofbranching of the alkyl side chain of the asphaltene molecules. On the other hand, the asphalteneprecipitation rate dependence on the amount of n-heptane (i.e., light paraffin hydrocarbon) added(n), the frequency factor (k0), and the activation energy for asphaltene precipitation (Ea) werestrong functions of the paraffin fraction of the asphaltenes and the propensity of the asphaltenemolecules for aggregation. Furthermore, the equilibrium parameter (onset point) increased asthe paraffin fraction of the asphaltene molecules increased but decreased as the iron content ofthe oil increased. DDBSA was more effective with the least-aromatic medium oil, in terms of thekinetic parameters m and n, whereas it was more effective with the more-aromatic oil, in termsof the equilibrium parameter. A significant benefit obtained with NP and toluene was the drasticreduction of the rate constant (k), which resulted in a decrease in the overall rate of asphalteneprecipitation. NP exhibited the maximum inhibition efficiency (∼10%), in terms of the onset pointon the most-stable oil with the lowest iron content, and highest average number of carbons peralkyl side chain (i.e., high paraffin fraction) of the asphaltene molecules.

1. Introduction

Many light and medium reservoirs are subjected toCO2 or hydrocarbon flooding, for enhanced oil recoveryafter the initial water flooding. For example, 60 activemiscible CO2 projects were in operation in the UnitedStates in 1996, whereas in Canada, hydrocarbon mis-cible floods are more common and there are ∼40 suchactive projects.1 In Saskatchewan, Canada, most of thelight oil reservoirs have reached their economic limitsof production by water flooding2 and have thus becomesuitable candidates for other methods of flooding.3However, flooding processes cause several changes inthe flow and phase behavior of the reservoir fluids andcan significantly alter the formation properties with theresultant increase in the propensity for precipitation oforganic solids, mainly asphaltenes.4 Asphaltene pre-

cipitation can change the wettability of the reservoirmatrix and consequently affect the flood performance.5It can also cause formation damage and wellboreplugging, requiring expensive treatment and clean-upprocedures.6-10

According to von Albretch,11 the most effective as-phaltene-precipitation preventive action is reservoirpressure maintenance above the asphaltene precipita-

* Author to whom correspondence should be addressed. Fax: (306)585-4855. E-mail address: [email protected].

(1) Moritis, G. New Technology, Improved Economics, Boost EORHopes. Oil Gas J. 1996, 94, 39.

(2) Saskatchewan Energy & Mines Reservoir Annual Report, Re-gina, Saskatchewan, Canada, 1993.

(3) Huang, S. S.; Dyer, S. B. Miscible Displacement in the WeyburnReservoir A Laboratory Study. J. Can. Pet. Technol. 1993, 32, 42.

(4) Kokal, S. L.; Sayegh, S. G. Asphaltenes: The Cholesterol ofPetroleum. Presented at the Middle East Oil Show, Bahrain, March11-14, 1995, Paper No. SPE 29787.

(5) Buckley, J. S. Asphaltene Precipitation and Crude Oil Wetting.SPE Adv. Technol. Ser. 1995, 53.

(6) Barker, K. M.; Germer, J. W.; Lesile, M. P. Removal andInhibition of Asphaltene Deposition on Formation Minerals. Presentedat the SPE International Petroleum Conference and Exhibition ofMexico in Villahermosa, Tabasco, Mexico, March 5-7, 1996, Paper No.SPE35342.

(7) Kamath, V. A.; Yang, J.; Sharma, G. D. Effect of AsphalteneDeposition on Dynamic Displacements of Oil by Water. Presented atthe Western Regional Meeting, Anchorage, Alaska, May 26-28, 1993,Paper No. SPE 26046.

(8) Novosad, Z.; Costain, T. G. Experimental and Modeling Studiesof Asphaltene Equilibria for a Reservoir Under CO2 Injection. Pre-sented at the SPE Annual Technical Conference and Exhibition, NewOrleans, LA, September 23-26, 1990, Paper No. SPE 20530.

(9) Leontaritis, K. J.; Mansoori, G. A. Fast Crude-Oil Heavy-Component Characterization Using Combination of ASTM, HPLC, andCPC Methods. J. Pet. Sci. Eng. 1989, 2, 1.

1038 Energy & Fuels 2004, 18, 1038-1048

10.1021/ef0340460 CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 06/03/2004

tion onset. In many cases, however, this condition isquite difficult or even impossible to accomplish, becauseof reservoir depletion and/or inconvenience in gas injec-tion. In these cases, asphaltene dispersants and pre-cipitation inhibitors constitute a good alternative.12-14

These products can keep the small initially formedasphaltene particles suspended into the oil matrix (inthe case of dispersants) or prevent their formation(when inhibitors are used). These methods can be moreeffective and cost reductive, because plugging andproduction losses are prevented.

Extensive research has been done to obtain insightsin inhibition mechanisms and the correlations betweencrude oil composition, structural characteristics of as-phaltenes,15,16 and the effectiveness of chemical addi-tives as asphaltene precipitation inhibitors.17,18 How-ever, no studies that investigate possible interrelation-ships between these parameters, in regard to their effecton light hydrocarbon or CO2 flooding, have been re-ported in the literature. Also, inhibition or dispersioneffectiveness of a chemical additive is typically mea-sured in terms of a delay in the onset point of asphalteneprecipitation (i.e., the equilibrium parameter).11,19 How-ever, because asphaltene precipitation can be consideredto be a rate process,20-24 the effectiveness of the chemi-cal additives could also be evaluated, in terms of thekinetic parameters. Therefore, it would be essential, as

well as informative, to correlate the effectiveness of thechemical additives directly with the overall asphalteneprecipitation behavior involving both the equilibriumand the kinetics.

At the present time, very limited kinetic evaluationhas been performed for asphaltene precipitation duringhydrocarbon flooding. Some of the reported kinetics ofprecipitation have focused on studying the criticalmicelle concentration (CMC), as a function of concentra-tion and time, using asphaltene samples that wereredissolved in toluene and/or heptane.20,21 The kineticsof n-heptane-induced asphaltene dissolution has alsobeen the subject of more recent studies.22,23 However,the kinetics of asphaltenes precipitation from actualcrude oil samples has not been reported previously inthe open literature, to the best of our knowledge,possibly because of the difficulty involved with usingthe conventional techniques to evaluate the kinetics.

In a recent study24 we developed a technique basedon a molar programmed titration of the oil with n-heptane to simulate light hydrocarbon flooding, toevaluate the kinetics of light-paraffin-hydrocarbon-induced precipitation of asphaltene from three Sas-katchewan crude oilssnamely L-O, M1-O, and M2-Osin which the kinetic data were fitted to an empiricalpower law model. This power law was of the form

where rA is the rate of n-heptane-induced precipitationof asphaltenes from crude oil (measured in terms ofmoles of asphaltene per mole of crude oil per minute),dNAp/dNnC7 represents the asphaltene precipitation asa function of n-heptane added, â is the molar pro-grammed rate of titration or addition of n-heptane (â )dNnC7/dt), Mo is the number of moles of crude oil chargedto the sample cell, K0 is the pre-exponential constant(units are dependent on the values of m and n), Ea isthe activation energy (in J/mol), R is the universal gasconstant (8.314 J mol-1 K-1), NAo is the number of molesof asphaltene in the oil at any time, NnC7 is the numberof moles of n-heptane in the oil at any time, m repre-sents the rate dependence of asphaltene precipitationon asphaltene content in the oil, and n is the ratedependence of asphaltene precipitation on n-heptanecontent in the oil (introduced during flooding).

This work enabled us to determine n-heptane (i.e.,light paraffin hydrocarbon)-induced asphaltene precipi-tation behavior, in terms of equilibrium (i.e., the onsetpoint of precipitation, w) and kinetic (i.e., k0, m, n, andEa) parameters of the tertiary oil production processinvolving hydrocarbon flooding.

Also, in a more recent work,25 we evaluated theinterrelationships between asphaltene precipitationinhibitor effectiveness, asphaltene characteristics, andprecipitation behavior during CO2 miscible flooding,using three carefully chosen chemicals as additives forasphaltene precipitation inhibition. In the present work,we are evaluating the inhibition effectiveness of these

(10) Leontaritis, K. J.; Amaefule, J. O.; Charles, R. E. A SystematicApproach for the Prevention and Treatment of Formation DamageCaused by Asphaltene Deposition. Presented at the Symposium onFormation Damage Control, Lafayette, LA, February 26-27, 1992,Paper No. SPE 23810.

(11) von Albretch, C.; Diaz, B.; Salathiel, W. M.; Nierode, D. E.Stimulation of Asphaltic Deep Wells and Shallow Wells in LakeMaracaibo, Venezuela. Proc. 10th Pet. Congr. 1980, 3, 55.

(12) Broaddus, G. Well- and Formation-Damage Removal withNonacid Fluids. J. Pet. Technol. 1988, 40 (6), 685.

(13) Carbognani, L.; Espidel, J.; Izquierdo, A. Characterization ofAsphaltenic Deposits from Oil Productions and Transportation Opera-tions. In Asphaltenes and Asphalts 2. Development in PetroleumScience; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science B.V.:Amsterdam, 2000; p 335.

(14) Acosta, A. Efecto de las Resinas en la Deposicion de Asfaltenos,Trabajo Especial de Grada. Escuela de Ingenieriay Ciencias Aplicadas,University de Oriente: Venezuela, 1981.

(15) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L.Structural Characterization of Asphaltenes of Different Origins. EnergyFuels 1995, 9, 225.

(16) Yen, T. F. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1979, 24,901.

(17) Gonzalez, G.; Middea, A. Peptization of Asphaltene by VariousOil Soluble Amphiphiles. Colloids Surf. 1991, 52, 207.

(18) Chang, C. L.; Fogler, H. S. Stabilization of Asphaltenes inAliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effectof the Chemical Structure of Amphiphiles on Asphaltene Stabilization.Langmuir 1994a, 10, 1749.

(19) Escobedo, J.; Mansoori, G. A. Viscometric Determination of theOnset of Asphaltene Flocculation: A Novel Methodology. SPE Prod.Facil. 1995, 10 (May), 115.

(20) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura,M. Structure and Reactivity of Petroleum-Derived Asphaltene. EnergyFuels 1999, 13, 287.

(21) Priyanto; S.; Mansoori, G. A.; Suwono, A. Structure andProperties of Micelles and Micelle Coacervates of Asphaltene Macro-molecule. Presented at 2001 AIChE Annual Meeting, Session 90sNanotools.

(22) Permsukarome, P.; Chang, C.; Foggler, H. S. Kinetic Study ofAsphaltene Dissolution in Amphiphile/Alkane Solutions. Ind. Eng.Chem. Res. 1997, 36, 3960.

(23) Kaminski, T. J.; Fogler, H. S.; Wolf, N.; Wattana, P.; Mairal,A. Classification of Asphaltenes via Fractionation and the Effect ofHeteroatom Content on Dissolution Kinetics. Energy Fuels 2000, 14,25.

(24) Ibrahim, H. H.; Idem, R. O. A Method for Evaluating theKinetics of n-Heptane-Induced Asphaltene Precipitation from VariousSaskatchewan Crude Oils during Light Hydrocarbon (n-Heptane)Flooding, submitted to Fuel, 2004.

(25) Ibrahim, H. H.; Idem, R. O. CO2 Miscible Flooding for ThreeSaskatchewan Crude Oils: Interrelationships between AsphaltenePrecipitation Inhibitor Effectiveness, Asphaltenes Characteristics, andPrecipitation Behavior. Energy Fuels 2004, 18, 743-754.

rA ) âMO

dNAp

dNnC7

) kO exp(-Ea

RT)NAo

m NnC7

n (1)

n-Heptane-Induced Asphaltene Precipitation Energy & Fuels, Vol. 18, No. 4, 2004 1039

three chemicals during hydrocarbon flooding for threeSaskatchewan crude oils and their relationships withasphaltene characteristics and precipitation behavior.The results are presented and discussed in this paper.

2. Experimental Section

2.1. Crude Oil Samples and Their CorrespondingAsphaltenes. Three crude oil samples (one light oil (L-O) andtwo medium oils (M1-O and M2-O)) from three differentreservoirs were used for this study. Their identities have beendescribed in detail in our earlier work.24,26

Asphaltene samples were extracted from each oil sample,and the extracted samples were then thoroughly characterized.Details on the extraction procedure are given elsewhere.25 Inthe case of characterization, all the crude oils and theirn-heptane-derived asphaltenes were characterized using amultitechnique approach including carbon (C), hydrogen (H),nitrogen (N), sulfur (S), and oxygen (O) (i.e., CHNS-O)analysis, metal content, average molecular weight, Fouriertransform infrared (FTIR) spectroscopy, proton nuclear mag-netic resonance (1H NMR), carbon nuclear magnetic resonance(13C NMR), and gated spin-echo (GASPE) spectroscopy, asdescribed by Ibrahim and Idem.26

2.2. Inhibitors and Stabilizers. Two nonaromatic-basedsurfactants or inhibitorssnamely, dodecylbenzenesulfonic acid(DDBSA) and nonyl phenol (NP)sin addition to a well-knownaromatic-based solvent (toluene) were evaluated for theirability to peptize (i.e., stabilize) asphaltenes in the asphalt-ene-oil system. All the chemicals are commercially available,and they were used as received. The selection of thesechemicals was based on four different criteria: (i) environ-mentally friendly inhibitors (DDBSA and NP) versus non-environmentally friendly solvent (toluene), (ii) functionalgroups (SO2-OH in DDBSA, OH in NP, and the benzene ringin toluene), (iii) alkyl side-chain length, and (iv) polarity(DDBSA > NP > toluene). According to the literature,27,28 atypical range of concentrations of inhibitors used is 500-3000ppm. We decided to use 1000 ppm for all our runs.

2.3. Effectiveness of Additives as Asphaltene Precipi-tation Inhibitors or Stabilizers during HydrocarbonFlooding. The effectiveness of the chemicals as inhibitors orstabilizers for asphaltene precipitation was evaluated bycomparing the asphaltene precipitation behavior during lighthydrocarbon flooding for the three oils with and withoutchemical additives or inhibitors.

2.3.1. Equipment. Asphaltene precipitation behavior duringlight hydrocarbon flooding was evaluated under isothermaland isobaric conditions for both noninhibited and inhibitedcrude oils for the three crude oils. Light hydrocarbon floodingwas simulated using a molar n-heptane programmed titrationtechnique24 in a solid detection system (SDS) obtained fromDB Robinson & Manufacturing Limited (Edmonton, Canada).This equipment consisted of a mercury-free, variable-volume,fully visual, JEFRI PVT cell retrofit with fiber-optic lighttransmission probes (source and detector). The SDS enabledus to conduct experiments using field crude oil samples ratherthan preprecipitated and toluene-redissolved asphaltenesamples that are usually used in most reported experimentsto circumvent the problem of opaqueness of the oil samples.6,27

2.3.2. Procedure for Nonstabilized Oil (i.e., without Inhibi-tor). A known quantity of a crude oil sample was fed to thePVT cell from a high-pressure cylinder using a JEFRI dis-placement pump (JDP). When the required amount of sample

(0.069 mol for L-O, 0.064 mol for M1-O, and 0.67 mol for M2-O) was in place, the sample cell was then completely isolated.The n-heptane pressure was increased to 17.2 MPa, and theremaining oil in the tubing was considered to be dead volume.The oil sample and the n-heptane were then respectivelyallowed to equilibrate overnight for ∼15 h at the desiredpressure and temperature in the PVT cell and the 1000-mLhigh-pressure solvent transfer cylinder. Before introducingn-heptane from the solvent cylinder to the PVT (sample) cell,the pressures in the lines connecting the n-heptane cylinderto the sample cell were equilibrated to avoid any backflash.The system was then opened to the backpressure regulator(BPR) to maintain the desired constant pressure needed forthe duration of the experiment. The SDS works by transmit-ting a laser beam through the sample in the PVT sample cell.The beam transmittance was recorded as an output powersignal, the variation of which was inversely proportional tothe solids content of the sample cell.

2.3.3. Procedure for Stabilized Oil (i.e., with Inhibitor). Inthe case for inhibited oil, a known amount of inhibitor (1000ppm) was added to the crude oil under ambient conditions.The resulting solution was then stirred vigorously using amagnetic stirrer at 1200 ppm to ensure that all the inhibitordissolved in the oil under an argon blanket. A test sample wascharged to the high-pressure transfer cylinder from which thesample was fed into the PVT cell using a JDP. Apart fromthis prior addition of the inhibitor to the crude oil, theprocedure to evaluate the asphaltene precipitation behaviorfor inhibited oil was the same as that already described fornonstabilized oil.

2.3.4. Typical Experimental Run. Each experimental runinvolved a programmed addition of pressure- and temperature-equilibrated n-heptane transferred from the solvent cylinderto the crude oil sample contained in the sample cell. Thecontents of the sample cell were vigorously stirred at a rate of2400 rpm, to prevent the settling of any solids formed duringn-heptane titration or addition in the cell. The addition ofn-heptane continued beyond the point where asphaltenestarted to precipitate from the sample in the cell, and furtheruntil there was no net asphaltene precipitation. This wasindicated when further additions of n-heptane did not resultin any significant decrease in the recorded power output signal.The inverse proportionality between the power output signaland the solids content of the cell was used in conjunction withknowledge of the asphaltene contents obtained from saturates,aromatics, resins, and asphaltenes (SARA) analysis to quantifythe amount of asphaltene precipitated as a function of timeor the amount of n-heptane added. A programmed n-heptaneflow rate of 0.5 mL/min was used in the presence of aninhibitor in the oil matrix at a temperature of 338 K in variousexperimental runs for the three crude oils. Also, a constantpressure of 17.2 MPa was maintained throughout all theexperimental runs. Details of the experimental procedure, aswell as the typical run, are as described in our previous work,24

which also outlines the procedure for evaluating the precipita-tion behavior parameters (i.e., when and how fast asphalteneis precipitated) for noninhibited and inhibited oil for light-hydrocarbon-induced asphaltene precipitation.

3. Results and Discussion

3.1. Relationship between Asphaltene Charac-teristics and Precipitation Behavior for Non-inhibited Oils. The characteristics of the crude oils arepresented in Table 1, whereas the characteristics oftheir n-heptane-derived asphaltene are presented inTable 2.26 Characteristics in these tables include the

(26) Ibrahim, H. H.; Idem, R. O. Structural and Molecular Charac-teristics of Asphaltenes from Various Saskatchewan Crude Oils,submitted to Energy Fuels, 2003.

(27) Aquino-Olivos, M. A.; Buenrostro-Gonzalez, E.; Anderson, S.I.; Lira-Galeana, C. Investigations of Inhibition of Asphaltene Precipi-tation at High-Pressure Using Bottomhole Samples. Energy Fuels2001, 15 (1), 236.

(28) Garcia, M. C.; Carbognani, L. Asphaltene-Paraffin StructuralInteractions. Effect on Crude Oil Stability. Energy Fuels 2001, 15 (5),1021.

1040 Energy & Fuels, Vol. 18, No. 4, 2004 Ibrahim

elemental composition, molecular weight, viscosity,density, specific gravity, metals content, SARA analysis,the average number of carbons per alkyl side chain (n),the methylene-to-methyl ratio (CH2/CH3), the aromaticcarbon fraction (fa), and the degree of condensation (Cb/Cbn). The equilibrium and kinetic parameters for n-heptane-induced asphaltene precipitation for three non-

inhibited crude oils are summarized in Table 3A,24

whereas similar parameters for inhibited oils are givenin Table 3B. The relationships given in Figures 1-11were derived by relating the characteristics given inTables 1 and 2 to the asphaltene precipitation behaviorgiven in Table 3.

3.1.1. Asphaltene Precipitation Rate Dependence onAsphaltene Content in the Oil (m). Figure 1 relates mto the heteroatom content and the average percentagesubstitution of peripheral aromatic carbons (As%).Figure 1 shows that m increases as As% increases,indicating the detrimental effect on n-heptane-inducedasphaltene precipitation kinetics of an increase in As%.On the other hand, the figure shows that m decreaseswith an increase in the total heteroatom content. Thislatter result shows that an increase in heteroatomscontent is actually beneficial to the kinetics of n-heptane(i.e., light paraffin hydrocarbon)-induced asphalteneprecipitation.

Some crude oil characteristics were considered forpossible direct relationships with precipitation behaviorparameters. One of such characteristics was the as-phaltenes content of the oil. According to Table 1, thiscontent increased in the order L-O < M1-O < M2-O,whereas Table 3a shows that m decreased in the orderL-O > M1-O > M2-O. A plot of asphaltene precipitationrate dependence on asphaltene content in the oil (m)with the asphaltenes content is given in Figure 2. Thisfigure shows that msand, therefore, the ratesdecreasesas the asphaltenes content of the oil increases. This isan interesting result for n-heptane-induced asphaltene

Table 1. Crude Oil Characteristics

Crude Oil

characteristic L-O M1-O M2-O

composition (wt %)carbon 85.4 85.1 84.5hydrogen 12.9 12.8 12.3nitrogen 0.1 0.2 0.2sulfur 1.5 2.2 3.0oxygen 0.4 0.4 0.4

heteroatom content (wt %) 2.0 2.8 3.6molecular weight (g/g-mol) 372.8 403.5 398.3viscosity @ 15 °C (cSt) 10.7 9.2 24.0density @ 15 °C (g/cm3) 0.9 0.9 0.9°API 33.9 32.7 28.3metal content (µg/g)

iron 2.0 16.0 0.6nickel 2.0 16.0 17.0vanadium 3.1 24.0 31.0total metals 7.1 56.0 48.6

saturates content (wt %) 45.5 37.3 N/Pa

aromatics content (wt %) 17.7 22.6 N/Pa

resins content (wt %) 8.6 17.2 N/Pa

asphaltenes content ( wt %)n-heptane-derived 1.2 3.2 4.8n-pentane-derived 1.8 3.8 5.2

a Not performed.

Table 2. Asphaltenes Characteristics

Asphaltenea

characteristic physical meaning L-O M1-O M2-O

composition (wt %)carbon 83.60 83.09 82.78hydrogen 6.95 7.40 7.20nitrogen 1.06 1.34 1.28sulfur 4.64 5.90 6.91oxygen 2.60 1.29 1.40

total heteroatoms (wt %) 8.30 8.53 9.59molecular weight (g/g-mol) 3345.7 4550.2 3380.4metal content (µg/g)

iron 79 260 62nickel 100 290 240vanadium 140 440 410total metals 319 990 712

nc average number of carbon per alkyl side chain, from FTIR 2.69 2.58 2.97CdO empirical index of carbonyl abundances, from FTIR 0.48 0.35 0.36RCH2/CH3 (by FTIR) molar ratio of CH2 and CH3 groups, from FTIR 2.34 2.46 2.62I3435/I3050 propensity for hydrogen bond formation (aggregation) 2.10 4.33 3.6S1H/4H ratio of intensities of aromatic C-H out-of-plane deformation with

one adjacent proton to four adjacent protons1.40 1.20 1.20

CH2/CH3 methylene-to-methyl group ratio 1.50 1.56 1.64fa aromatic carbon fraction 0.60 0.59 0.58nNMR average number of carbons per alkyl side chain, from NMR 4.48 4.06 4.46As (%) average percentage of substitution of aromatic carbon 18.62 18.24 17.69r number of substituent rings 0.86 0.80 0.85Cb/Cnb degree of condensation 1.83 1.72 1.70nGASPE average number of carbon per alkyl side chain, from GASPE 13C NMR 13.5 13.1 13.2NR average number of rings per molecule 0.68 0.66 0.65NB average number of branches per molecule 1.78 1.74 1.73C quaternary carbonCH methine group 0.132 0.133 0.131CH2 methylene group 0.688 0.683 0.685CH3 methyl group 0.180 0.184 0.184CH3/CH methyl-to-methine group ratio 1.360 1.386 1.404CH2/CH3 (by NMR) methylene-to-methyl group ratio 3.834 3.713 3.729

a n-Heptane-extracted asphaltenes.


precipitation and represents the first attempt at con-firming what influence the asphaltenes content of theoil has on the kinetics of asphaltene precipitation duringlight hydrocarbon flooding. Similar results and argu-ments are applicable to other oil characteristics suchas total heteroatom content and density of the crude oil.

Figure 2 also presents the variation of m with thealkyl side-chain length of the asphaltene molecules(measured in terms of CH2/CH3 and RCH2/CH3 ratios).Because a high CH2/CH3 ratio is an indication of a lowdegree of branching, this figure shows that the longerand straighter the alkyl side-chain length is, the smallerthe effect m has on the kinetics. This assertion wasverified by making a plot of the variation of m with thedegree of branching of the alkyl side chain of the

asphaltene molecules (NB), as illustrated in Figure 3,which also contains relationships of m with otherasphaltene characteristics (aromatic carbon fraction (fa),average number of aromatic rings per molecule (NR),and degree of condensation (Cb/Cbn)). This figure showsthat m increases as NB, fa, NR, and Cb/Cbn each increase.In the case of m versus NB, the result confirms ourearlier assertion that m decreases as the amount ofnormal alkyl side chains (i.e., degree of straightness ofthe alkyl side chain) increases (see Figure 2), which,conversely, implies that m increases with the degree ofbranching of the alkyl side chain. Because the otherparameters (fa, NR, and Cb/Cbn) provide a generalmeasure of the aromaticity of the asphaltenes, resultsof their relation with m indicate that m increases asthe aromaticity of the asphaltenes increases.

Table 3. Estimates of the Values of Kinetic and Equilibrium Parameters for n-Heptane-Induced AsphaltenePrecipitation for (A) Noninhibited Crude Oils at Temperatures of 304-338 K and (B) Inhibited Crude Oils at a

Temperature of 338 K

Estimated Value

parameter inhibitor L-O M1-O M2-O

(A) Noninhibited Crude Oils, at 304-338 Kln k0 7.08 ( 5.26 60.55 ( 23.38 23.41 ( 6.94k ) k0 exp[-(E/RT)] 4.03 × 10-24 6.7 × 10-1 4.99 × 10-17

Ea (kJ/mol) 171.27 ( 27.15 61.01 ( 17.48 106.83 ( 20.46m 2.4 ( 0.3 1.5 ( 0.2 1.1 ( 0.1n 36.1 ( 6.1 46.9 ( 12.6 39.1 ( 6.4onset point (mL n-heptane) 28.45 25.51 30.30

(B) Inhibited Crude Oils, at 338 K

ln kDDBSAa -147.81 ( 19.63 -163.07 ( 2.37 -14.68 ( 0.005NPb -83.75 ( 3.54 -134.27 ( 17.79 -177.99 ( 30.81toluene -98.98 ( 14.92 -103.11 ( 38.79 -146.07 ( 93.16

mDDBSAa 4.38 ( 0.52 2.18 ( 0.023 4.53 × 10-5 ( 6.42 × 10-5

NPb 2.78 ( 0.09 1.91 ( 0.18 1.86 ( 0.23toluene 2.76 ( 0.33 1.83 ( 0.39 1.84 ( 0.66

nDDBSAa 75.59 ( 9.64 88.31 ( 1.27 -0.998 ( 0.0026NPb 43 ( 1.74 73.91 ( 9.61 117.37 ( 20.03toluene 53.91 ( 7.79 55.30 ( 20.82 90.46 ( 57.84

onset point (mL n-heptane)DDBSAa 30.37 25.65 30.35NPb 29.90 26.20 33.45toluene 30.50 25.85 31.10

a Dodecylbenzenesulfonic acid. b Nonyl phenol.

Figure 1. Variation of asphaltene precipitation rate depen-dence on asphaltene content (m) with (O) heteroatoms contentand (4) percentage substitution of peripheral C atoms ofasphaltenes (As%).

Figure 2. Variation of asphaltene precipitation rate depen-dence on asphaltene content (m) with (×) asphaltene contentand the (O) CH2/CH3 and (4) RCH2/CH3 ratios of the asphaltenemolecules.


3.1.2. Asphaltene Precipitation Rate Dependence onn-Heptane Content in the Oil (n). Figure 4 shows thevariation of n with the asphaltene molecular weight andtotal metals content in the asphaltene. The figure showsthat the asphaltene precipitation rate dependence onn-heptane (i.e., light paraffin hydrocarbon) added to theoil (n) decreases as both the asphaltene molecularweight and the total metals content increase. Othercharacteristics evaluated were the carbonyl abundancesindex (CdO, as defined in our previous work26) and thepropensity for aggregation of asphaltenes through hy-drogen bonding (measured in terms of the relativeabundance of FTIR peaks at 3435 cm-1 (representing

OH and NH groups) and 3050 cm-1 (representingaromatic CH stretching). Their relationships with n aregiven in Figure 5, which shows that n decreases as CdO increases but increases as I3435/I3050 increases. Thisis in contrast to our results for CO2-induced asphalteneprecipitation.25 The results for n versus I3435/I3050 andCdO show that a higher propensity for aggregation isnot beneficial, in terms of the kinetics of light-paraffin-hydrocarbon-induced asphaltene precipitation, whereasa higher CdO content is.

On the other hand, n versus the average number ofcarbons per alkyl side chain (measured as nNMR andnGASPE

26) in Figure 6 shows that an increase in thenumber of carbons in the alkyl side chain decreases n.This is beneficial to the kinetics of light-paraffin-hydrocarbon-induced asphaltene precipitation. Because

Figure 3. Variation of asphaltene precipitation rate depen-dence on asphaltene content (m) with various asphaltenecharacteristics ((O) fa, aromatic carbon fraction; (×) NR, averagenumber of rings per molecule; (4) Cb/Cbn, degree of condensa-tion; and (0) NB, degree of branching of the alkyl side chains).

Figure 4. Variation of asphaltene precipitation rate depen-dence on n-heptane added (n) with (O) asphaltene molecularweight and (4) total metals content.

Figure 5. Variation of asphaltene precipitation rate depen-dence on n-heptane added (n) with (O) the propensity of theasphaltene molecules for aggregation (I3435/I3050) and (4) thecarbonyl abundances index (CdO).

Figure 6. Variation of asphaltene precipitation rate depen-dence on n-heptane added (n) with the average number ofcarbons per alkyl side chain of the asphaltene molecule ((4)nNMR and (O) nGASPE).


a larger number of carbons in the alkyl side chainimplies higher paraffinicity, the result shows that thehigher the paraffinic content of the asphaltene, thelower the contribution of light paraffin hydrocarbonadded to the oil to the rate of asphaltene precipitation.Figure 7 gives the variation of n with the number ofsubstituent rings in the asphaltene molecule. The figureshows that n decreases as r increases. Because r is anindication of how bulky the asphaltene molecule is (i.e.,how large the molecular weight is), the results confirmour earlier result (Figure 4), which showed that nincreases as the molecular weight increases.

3.1.3. Pre-exponential Constant (Frequency Factor) ofAsphaltene Precipitation. The variation of the frequencyfactor (evaluated as ln k0) with various asphaltenecharacteristics are presented in Figures 8-11. Thecharacteristics are molecular weight and total metals

content (Figure 8), propensity for aggregation (I3435/I3050)and carbonyl abundances index (CdO) in Figure 9,number of carbons per alkyl side chain (in terms of nNMRand nGASPE) in Figure 10, and the number of substituentrings (r) in Figure 11. The frequency factor representsthe frequency of collision between asphaltene moleculesand n-heptane (i.e., light paraffin hydrocarbon) mol-ecules to induce asphaltene precipitation. Figure 8shows that the value of ln k0 increases as both themolecular weight and total metals content of asphalt-enes increase. Also, Figure 9 shows that the value of lnk0 increases as the value of I3435/I3050 increases butdecreases sharply with an increase in CdO. In the caseof Figure 10, the results show that the value of ln k0decreases very sharply as the number of carbons per

Figure 7. Variation of asphaltene precipitation rate depen-dence on n-heptane added (n) with the number of substituentrings in the asphaltene molecule (r).

Figure 8. Variation of the frequency factor for asphalteneprecipitation (ln k0) with (O) the molecular weight and (4) thetotal metals content of the asphaltenes.

Figure 9. Variation of the frequency factor for asphalteneprecipitation (ln k0) with (O) the propensity of asphaltenemolecules for aggregation (I3435/I3050) and (4) the carbonylabundances index (CdO).

Figure 10. Variation of the frequency factor for asphalteneprecipitation (ln k0) with the average number of carbons peralkyl side chain of the asphaltene molecule ((4) nNMR and (O)nGASPE).


alkyl side chain increases. This is favorable, in termsof the kinetics of n-heptane-induced asphaltene precipi-tation. Because a larger number of carbons in the alkylside chain implies a higher paraffin fraction, the resultshows that the higher the paraffin content of theasphaltenes, the smaller the contribution of the fre-quency factor to the rate of light-paraffin-hydrocarbon-induced asphaltene precipitation. Finally, Figure 11shows that the value of ln k0 decreases as the numberof substituent rings in the asphaltene molecules in-creases, similar to the trend for n versus this parameter(r).

3.1.4. Activation Energy. Figure 12 illustrates typicalvariations of the activation energy (Ea) with molecularweight and the total metals content of the asphaltenes.The activation energy is an indication of the tempera-ture sensitivity of the asphaltene precipitation process.The result shows that Ea is inversely related to boththe molecular weight and the total metals content. Thisimplies that the lower the asphaltene molecular weight,the more temperature-sensitive the oil is to precipita-

tion. In other words, asphaltene precipitation from lightoils is more temperature-sensitive, as compared toheavier oils. Results from this figure also imply that thepresence of large amounts of metals in the asphaltenesis beneficial, in that it decreases the temperaturesensitivity of the oil, in regard to asphaltene precipita-tion.

Figure 13 shows the variation of Ea with the propen-sity for aggregation of asphaltene molecules throughhydrogen bond formation (I3435/I3050) and the index ofcarbonyl abundances (CdO). The results show aninverse relationship between Ea and I3435/I3050 but adirect relationship between Ea and CdO. The figuresuggests that the temperature sensitivity decreases asthe propensity for asphaltene aggregation increases. Incontrast, the figure illustrates that temperature sensi-tivity of the precipitation process increases with CdO.Figure 14 shows the variation of the activation energywith the average number of carbon per alkyl side chainobtained using GASPE technique (nGASPE), as well asthat obtained using NMR spectroscopy (nNMR).26 Theresults show that the activation energy is directlyproportional to the average number of carbons per alkylside chain. Because a larger number of carbons per alkylside chain indicates increased paraffinicity, the resultsimply that asphaltene precipitation becomes more sen-sitive to temperature as the asphaltene molecule be-comes more paraffinic.

Thus far, we have only looked at how various as-phaltene and oil characteristics influence the kineticparameters in light-paraffin-hydrocarbon-induced as-phaltene precipitation. This has made it possible toidentify the oil/asphaltenes characteristics that affectthe kinetic parameters (m, n, ln k0, Ea) positively, inwhich an increase in the value of the characteristiccontributes to a reduction of the rate of light-paraffin-hydrocarbon-induced asphaltene precipitation, accord-ing to eq 1. We have also identified those factors thatinfluence the kinetic parameters negatively.

Figure 11. Variation of the frequency factor for asphalteneprecipitation (ln k0) with the number of substituent rings inthe asphaltene molecule (r).

Figure 12. Variation of the activation energy for asphalteneprecipitation (Ea) with (O) the molecular weight and (4) totalmetals content of the asphaltenes.

Figure 13. Variation of the activation energy for asphalteneprecipitation (Ea) with (O) the propensity of asphaltenemolecules for aggregation (I3435/I3050) and (4) the carbonylabundances index (CdO).


3.1.5. Onset Point. The only discernible relationshipsbetween the onset point of n-heptane-induced asphalt-ene precipitation and oil or asphaltene characteristicswere with the number of carbons per alkyl side chainof the asphaltene molecule (nNMR and nIR) and the ironcontent in the oil. These are illustrated in Figure 15.The figure shows that the onset point increases as thenumber of carbons per alkyl side chain of the asphaltenemolecule increases. Because an increase in the numberof carbons per alkyl side chain is a reflection of anincrease in the paraffin content of the asphaltenemolecule, this result implies that the stability of crude

oil toward asphaltene precipitation is improved with anincrease in the paraffin content of the asphaltenemolecule. Figure 15 also shows that the onset pointdecreases as the iron content of the oil increases.Because the presence of iron confers some degree ofpolarity on the oil, this result implies that the oilbecomes more unstable, with respect to asphalteneprecipitation, as the iron content or polarity of the oilincreases.

3.2. Inhibitor Effectiveness. The effectiveness ofeach chemical additive to inhibit n-heptane (light-paraffin-hydrocarbon)-induced asphaltene precipitationwas evaluated from Tables 3A and 3B, in terms of thefractional difference ∆i/i, as defined in eq 2:

where i represents m, n, k, or w.In this definition, a positive value for the kinetic

parameters k (k ) k0 exp[-Ea/(RT)]), m, and n indicatethat the chemical additive is an effective inhibitor andthe magnitude represents the degree of effectiveness.The converse is also true. On the other hand, a negativevalue for the equilibrium parameter (i.e., the onset pointof precipitation, w) shows that the inhibitor is effectiveand the magnitude represents the degree of effective-ness. The converse is also true.

3.2.1. Dodecylbenzenesulfonic Acid (DDBSA). Table 4shows the effectiveness of DDBSA on precipitationbehavior of the three crude oil samples. Results showthat the addition of DDBSA enhances the asphaltenecontent dependence of the precipitation rate (m) for theL-O and M1-O oils and reduces it for the M2-O oil. Thisresult shows that, in regard to suppressing the effect ofm, DDBSA was effective only for the least-aromatic,least-polycondensed oil (M2-O).26 On the other hand,Table 4 shows that the n-heptane dependence of theprecipitation rate (n) was enhanced by 109% and 88%for the L-O and M1-O oils, respectively, but was sup-pressed by 102% for the M2-O oil. The other kineticparameter considered was the rate constant (k). DDBSAwas able to reduce the asphaltene precipitation rateconstant by 100% for the L-O and M1-O oils, butenhanced k for the M2-O oil. In the case of the onsetpoint (equilibrium parameter), Table 4 shows thatDDBSA produced beneficial effects by increasing theonset points by 6.7%, 0.5%, and 0.2% for the L-O, M1-O, and M2-O oils, respectively. One can conclude fromTable 4 that DDBSA was more effective with the least-aromatic medium oil (M2-O),26 in terms of all the kineticparameters, whereas it was more effective with themore-aromatic oil, L-O,26 in terms of the equilibriumparameter.

3.2.2. Nonyl Phenol (NP). Table 5 shows the effective-ness of nonyl phenol (NP) on asphaltene precipitationfrom three crude oil samples. The results show that the

Figure 14. Variation of the activation energy for asphalteneprecipitation (Ea) with the average number of carbons per alkylside chain of the asphaltene molecule ((4) nNMR and (O) nGASPE).

Figure 15. Variation of the onset point for asphalteneprecipitation (w) with (×) the iron content in the oil and theaverage number of carbons per alkyl side chain ((O) nNMR and(4) nIR) of the asphaltene molecule.

Table 4. Effectiveness of Dodecylbenzenesulfonic Acid(DDBSA) during n-Heptane Flooding

L-O M1-O M2-O

∆m/m -0.825 -0.453 0.999∆n/n -1.094 -0.883 1.025∆k/k 1 1 -8449008467∆w/w -0.0675 -0.005 -0.002

∆ii

)inoninhibited - iinhibited

inoninhibited(2)


addition of NP produces a detrimental effect as itenhanced the asphaltene dependence of the precipita-tion rate (m) for all three oils. Table 5 also shows nosigns of suppressing the n-heptane content dependenceof the precipitation rate (n). Instead, this parameter wasenhanced by 19%, 58%, and 200% for the L-O, M1-O,and M2-O oils, respectively. In contrast, the rateconstant of asphaltene precipitation was reduced by100% for the L-O, M1-O, and M2-O oils. Thus, NP hadits best kinetic effect only in terms of reducing the rateconstant and, as such, the overall asphaltenes precipita-tion rate. In terms of the onset point (the equilibriumparameter, w), NP was effective for all three oils.

3.2.3. Toluene. Table 6 shows that the effectivenessto inhibit asphaltene precipitation from the three crudeoil samples with toluene was similar to those with NP.Results show that the addition of toluene enhanced theasphaltene content dependence of the precipitation rate(m) for all three oils. Behavior that was the same asthat previously reported was observed for the n-heptanedependence of precipitation (n), with an enhancementof ∼49%, ∼18%, and ∼132% for the L-O, M1-O, andM2-O oils, respectively. In contrast, other results inTable 6 indicate that the rate constant of asphalteneprecipitation was reduced by 100% for the L-O, M1-O,and M2-O oils, just as observed in the case with NP.Table 6 also shows that a beneficial effect of tolueneaddition is also observed in terms of the onset points.These were improved by 7%, 1%, and 3% for the L-O,M1-O, and M2-O oils, respectively. On the basis of theseresults, it would appear that, for n-heptane-inducedasphaltene precipitation, one improvement was ob-served in the form of delaying the onset point. The othersignificant benefit was obtained in terms of a drasticreduction of the rate constant, and, as such, the overallrate of asphaltene precipitation.

3.3. Interrelationship between Additive Effec-tiveness and Oil/Asphaltene Characteristics. Itshould be mentioned that not all of the inhibitoreffectiveness parameters exhibited discernible relation-ships with oil/asphaltene characteristics. Also, not allof the inhibitors exhibited discernible patterns withprecipitation behavior and oil/asphaltene characteris-tics. All of the chemical additives were effective, in termsof ∆w/w and ∆k/k for almost all the oils; therefore, wedecided to use these parameters to evaluate the inter-relationships between additive effectiveness, precipita-tion behavior, and oil/asphaltene characteristics. Notethat the ∆w/w values, as calculated by eq 2, are negativevalues, as shown in Tables 4-6. However, absolute

∆w/w values are used in this section, based on ourdefinition of effectiveness for the onset point.

3.3.1. Dodecylbenzenesulfonic Acid (DDBSA). Thereis no discernible pattern in the variation of ∆w/w withw and the oil/asphaltene characteristics. This is at-tributed to the limited number of data points availableto us in making the evaluation. It is expected that, witha larger number of data points, it will be possible todraw conclusions in regard to the presence or absenceof definite trends. In the case of ∆k/k, DDBSA wasclearly not effective for the M2-O oil, whereas itproduced 100% reductions for the L-O and M1-O oils.The 100% reduction in k for the L-O and M1-O oils canbe attributed to using a higher-than-necessary concen-tration of DDBSA. It is expected that a smaller concen-tration of DDBSA should enable the ranking of L-O andM1-O oils, in regard to which oil yields the best ∆k/keffect with DDBSA. It would then be possible todetermine if interrelationships exist between inhibitoreffectiveness, precipitation behavior, and oil/asphaltenecharacteristics.

3.3.2. Nonyl Phenol (NP). Figure 16 shows the inter-relationship between nonyl phenol (NP) effectiveness(measured as an improvement in the onset point (∆w/w), precipitation behavior (w), and oil/asphaltene char-acteristics (the number of carbons per alkyl side chainof asphaltene molecules (measured as nNMR and nIR) andiron content in the oil). Figure 16 shows that theefficiency criterion (∆w/w) increases as the oil stability(onset point) and the paraffin fraction of the asphaltenemolecule each increase. However, it decreases as theiron content of the oil increases. This suggests that NPis most effective with the most-stable oil of low polarityand a low aromatic fraction of the asphaltene molecule.

In the case of ∆k/k, NP produced 100% reductions forall three oils. As in the case of DDBSA, the 100%reduction in k for the L-O, M1-O, and M2-O oils can be

Table 5. Effectiveness of Nonyl Phenol (NP) duringn-Heptane Flooding

L-O M1-O M2-O

∆m/m -0.183 -0.273 -0.691∆n/n -0.191 -0.576 -2.0∆k/k 1 1 1∆w/w -0.051 -0.027 -0.104

Table 6. Effectiveness of Toluene during n-HeptaneFlooding

L-O M1-O M2-O

∆m/m -0.167 -0.22 -0.636∆n/n -0.493 -0.179 -1.321∆k/k 1 1 1∆w/w -0.072 -0.0133 -0.026

Figure 16. Interrelationships between (4) the onset pointefficiency (∆w/w), the onset point (w), and various oil/asphalt-ene characteristics for nonyl phenol (NP). ((O) denotes nNMR

data (the number of carbons per alkyl side chain of theasphaltene molecule, as determined by NMR methods) and (+)denotes nIR data (the number of carbons per alkyl side chainof the asphaltene molecule, as determined by FTIR methods),whereas (]) denotes the iron content).


attributed to the use of higher-than-necessary concen-tration of NP. Thus, it is expected that a smallerconcentration of NP would enable the ranking of L-O,M1-O, and M2-O, in regard to which oil yields the best∆k/k effect with NP. It would then be possible todetermine if interrelationships exist between inhibitoreffectiveness, precipitation behavior, and oil/asphaltenecharacteristics.

3.3.3. Toluene. As in the case of DDBSA, there is nodiscernible pattern in the variation of ∆w/w with w andthe oil/asphaltene characteristics. Again, this is at-tributed to the limited number of data points availableto us in making the evaluation. It is expected that, witha larger number of data points, it will be possible todraw conclusions in regard to the presence or absenceof definite trends. In the case of ∆k/k, toluene produced100% reductions for all three oils, as in the case of NP.Also, as in case of DDBSA and NP, the 100% reductionin k for the L-O, M1-O, and M2-O oils can be attributedto the use of a higher-than-necessary concentration oftoluene. Thus, it is expected that a smaller concentra-tion of toluene would enable the ranking of L-O, M1-Oand M2-O, in regard to which oil yields the best ∆k/keffect with toluene. It would then be possible to deter-mine if interrelationships exist between inhibitor ef-fectiveness, precipitation behavior, and oil/asphaltenecharacteristics.

3.3.4. Industrial Applications. The results from thiswork should enable the correct choice of chemicalinhibitor for prevention of light-hydrocarbon-inducedasphaltene precipitation by inhibition during lighthydrocarbon flooding. The information needed to makethis choice includes the nature of the oil (whetherparaffinic or aromatic) and whether precipitation shouldbe controlled by the kinetic parameter or the equilibri-um parameter. In terms of using the kinetic parameters(m, n, and k), one must make the choice of the overridingparameter, k, because this has the controlling influenceon the rate. In this case, DDBSA drops out, because itis not effective for all the oils, in terms of k. In terms ofthe equilibrium parameter, all three inhibitors areeffective; however, NP is the choice, because its ef-fectiveness is better and it is more environmentallybenign than toluene.

Inhibitor effectiveness studies were performed at onepressure (17.2 MPa) and temperature (338 K). However,it is worthwhile to perform studies at other tempera-tures and pressures. Previous studies have shown thatm, n, k, and w were affected by temperature,24 whereasw was affected by pressure.29 Therefore, it is expectedthat inhibition effectiveness of the chemicals, in terms

of m, n, k, and w, will also be affected by both temper-ature and pressure.

4. Conclusions

(1) The asphaltene precipitation rate dependence onasphaltene content of the oil (m) decreased as theheteroatoms content of both the crude oil and asphalt-enes increased, but increased as the aromatic carbonfraction and the degree of branching of the asphaltenemolecules each increased. On the other hand, asphalt-ene precipitation rate dependence on the amount ofn-heptane (i.e., light paraffin hydrocarbon) added (n)decreased as the paraffin fraction of the asphaltenesincreased, but increased as the propensity of the as-phaltene molecules for aggregation increased. Thetrends for the frequency factor for asphaltene precipita-tion (k0) were similar to those for n.

(2) In contrast, the activation energy for asphalteneprecipitation decreased as the propensity for aggrega-tion increased, but increased as the paraffin fraction ofthe asphaltene molecules increased. Furthermore, theequilibrium parameter (the onset point) increased as theparaffin fraction of the asphaltene molecules increased,but decreased as the iron content of the oil increased.

(3) Dodecylbenzenesulfonic acid (DDBSA) was moreeffective with the least-aromatic medium oil (M2-O), interms of all the kinetic parameters, whereas it was moreeffective with the more-aromatic oil (L-O), in terms ofthe equilibrium parameter. A significant benefit ob-tained with the three chemical additives was the drasticreduction of the rate constant, and, as such, the overallrate of asphaltene precipitation.

(4) Nonyl phenol (NP) exhibited its highest inhibitionefficiency (∼10%), in terms of the onset point on the oilwith the highest onset point (i.e., the most-stable oil)with the lowest iron content, and which also had thehighest average number of carbon per alkyl side chain(nIR) in the asphaltene molecules. DDBSA and toluenedid not exhibit any discernible relationships.

Acknowledgment. The authors thank the Petro-leum Technology Research Center (PTRC), Regina forfinancial support, and Dr. Sam Huang and Mr. BartSchnell of the Saskatchewan Research Council (SRC),Petroleum Branch, Regina for their technical support.

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(29) Ramachandran, S.; Breen, P.; Ray, R. Chemical ProgramsEnsure Flow and Prevent Corrosion in Deepwater Facilities and FlowLines. Baker Hughes Inc., 2000, http://www.bakerhughes.com/baker-hughes/inDepth/72k/Petrolite.pdf.


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