The chemical and physical structure of petroleum: effects on recovery operations

Ž .Journal of Petroleum Science and Engineering 22 1999 3–15

The chemical and physical structure of petroleum: effects onrecovery operations

James G. Speight )

Western Research Institute, 365 North 9th Street, Laramie, WY 82070-3380, USA

Accepted 21 January 1997

Abstract

In a mixture as complex as petroleum, recovery chemistry can only be generalized because of the intricate and complexnature of the molecular species that make up the crude oil. It is this complexity that leads not only to difficulties in analyzingthe recovered material but also in analyzing the original oil in place. Moreover, the incompatibility of crude oil constituents

Ž .with each other is a continuing issue and the occurrence of suspended organic solids during recovery especially thermalreduces the efficiency of a variety of processes. More detailed knowledge of the composition and reactivity of petroleum willhelp in understanding the means by which models can be applied to understanding recovery processes. The models that are

Ž .proposed as a means of being applicable to the prediction of sediment i.e., asphaltene, resin, wax formation and depositionfrom petroleum due to changes in pressure, temperature and composition fall somewhat short in their structure. Furthermodeling needs involve an understanding of the chemistry of these materials and reflect the more modern approach to thephysico-chemical structure of petroleum in order to more correctly predict the onset of precipitation as well as the locationand amount of the sediment deposition in the producing wells and in oil-transport pipelines. q 1999 Elsevier Science B.V.All rights reserved.

Keywords: Complexity; Physico-chemical structure; Sediment

1. Introduction

It is the general consensus that it is the polar, i.e.,heteroatom, constituents of petroleum which are re-sponsible for the formation of suspended organic

Žsolids during a variety of recovery processes Islam,.1994; Park et al., 1994 . However, an area that

remains largely undefined, insofar as the chemistryand physics are still speculative, is the phenomenonof the incompatibility of the crude oil constituents, asmight occur during these operations. The formation

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Žof a suspended solid phase during recovery as well.as during refining operations is related to the chemi-

cal and physical structure of petroleum; the latter isgreatly influenced by the former.

By way of a brief series of definition, the failureof petroleum fractions to mix and the separation of aseparate phase is usually referred to as incompatibil-ity. When incompatibility occurs, the constituentsŽ .usually the asphaltenes and the resins that form aseparate phase are variously referred to as a precipi-tate, sediment, andror sludge formation dependingupon the nature of the material and the causes of theseparation.

0920-4105r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0920-4105 98 00051-5

( )J.G. SpeightrJournal of Petroleum Science and Engineering 22 1999 3–154

Fig. 1. The laboratory fractionation of petroleum.

Petroleum can be considered to be a delicatelybalanced system insofar as the different fractions

Žwhich contain hydrocarbons saturates and aromat-. Ž .ics as well as heteroatom constituents Fig. 1 .

Although the heteroatom constituents tend to concen-Žtrate in the higher molecular weight fractions the

. Ž .asphaltenes and resins Fig. 2 , nitrogen, oxygen,and sulfur species that are in near-neutral molecular

Fig. 2. The relative concentration of the heteroatoms after fractionation.

( )J.G. SpeightrJournal of Petroleum Science and Engineering 22 1999 3–15 5

locales will also occur in the saturates and aromatics,remembering that the nomenclature is not necessarilyprecise and that the composition of each fraction is a

Ž .function of the separation process Speight, 1991 .Asphaltenes are recognized as being a complex

mixture of species of varying molecular weight andŽ .polarity Fig. 3 . Thus, carbenes and carboids are

lower molecular weight highly polar species that arepredominantly products of thermal processes andmight not occur in the typical recovery process.However, the application of thermal techniques, suchas fire flooding, to petroleum recovery can producesuch species and they will either deposit on thereservoir rock or appear as suspended solids in theoil. In fact, it is recognized that it is the polar speciesin the crude oil that govern the oil–rock interactions

Ž .in the reservoir Bruning, 1991 from which manysediments can arise.

With the exception of the carbenes and the car-boids, the fractions are compatible proÕided thereare no significant disturbances or changes made to

Ž .the system. Such changes are: 1 the alteration ofŽ .the natural abundance of the different fractions; 2

the chemical or physical alteration of the constituentsas might occur during recovery, especially changesthat might be brought on by thermal processes; andŽ .3 alteration of the polar group distribution as mightoccur during oxidation by exposure to aerial oxygenduring the recovery process. In the reservoir, asphal-tene incompatibility can cause blockages of the pores

Fig. 3. Representation of asphaltenes and carbenesrcarboids onthe basis of molecular weight and polarity.

and channels through which the oil must move dur-Žing recovery operations Park et al., 1994; Islam,

.1994; Leontaritis, 1996 .All of these incidents cause disturbances to the

petroleum system. However, when such disturbancesoccur, it is the higher molecular weight constituentsthat are most seriously affected eventually leading to

Žincompatibility precipitation, sediment formation,.sludge formation depending upon the circumstances.

Thus, the dispersibility of the higher molecularweight constituents becomes an issue that needsattention. And one of the ways by which this issuecan be understood is to be aware of the chemical andphysical character of the higher molecular weightconstituents. By such means, the issue of dispersibil-ity, and the attending issue of incompatibility can beunderstood and even predicted.

2. Discussion

2.1. Asphaltenes

Asphaltenes are, by definition, a solubility classthat is precipitated from petroleum, heavy oil, andbitumen by the addition of an excess of a liquid

Žparrafinic hydrocarbon Girdler, 1965; Andersen andBirdi, 1990; Speight, 1991, 1994; Speight and Long,

.1996 . In addition, the composition of the as-phaltenes fraction is dependent upon the nature of

Ž .the hydrocarbon precipitant Fig. 4 , the ratio of thevolume of the precipitant to the volume of feedstockŽ . Ž .Fig. 5 , to the contact time Fig. 6 , and to thetemperature at which the precipitation occursŽMitchell and Speight, 1973; Speight et al., 1984;

.Speight, 1991; Andersen and Stenby, 1996 .By virtue of the means of separation, the asphal-

tene fraction is chemically complex but it can beconveniently represented on the basis of molecular

Ž . Ž .weight and polarity Fig. 3 Long, 1979, 1981 andfor different crude oils the slope of the line repre-senting the distribution of molecular weight and the

Ž .variation in polarity will vary Speight, 1994 . As-phaltenes can not be crystallized in the usual senseof the word. However, and by way of emphasis ofthe complex nature of this fraction, asphaltenes can


Fig. 4. Asphaltene yield as a function of the carbon number of theparaffinic precipitant.

be sub-fractionated by the use of a variety of tech-niques to produce fractions that vary considerably inmolecular weight and which also vary considerablyin terms of the functional group types and functional

Žgroup content Francisco and Speight, 1984; Speight,.1991 . These data lend support to, and reinforce, the

Fig. 5. Asphaltene yield as a function of precipitant:oil ratio.

Fig. 6. Asphaltene yield as a function of time.

concept that asphaltenes are complex mixtures ofmolecular sizes and various functional types.

In addition, any variation of the major parametersŽprecipitant, precipitant:oil ratio, time, and tempera-

.ture can cause substantial variations in the natureand amount of the separated asphaltenic material. Itcan vary from a dark brown amorphous solid to ablack tacky deposit, either of which under the preva-lent conditions could be termed an asphaltene.

The elemental compositions of asphaltenes varyover only a narrow range corresponding to HrCratios of 1.15"0.05%, although values outside of

Ž .this range are often found Speight, 1991 . Notablevariations do occur in the proportions of the het-eroelements, in particular in the proportions of oxy-gen and sulfur. On the other hand, the nitrogencontent of the asphaltenes has a somewhat lesserdegree of variation. This is, perhaps, not surprisingsince exposing asphaltenes to atmospheric oxygencan substantially alter the oxygen content, and expos-ing a crude oil to elemental sulfur, or even tosulfur-containing minerals, can result in sulfur up-take.

The data from the various studies intimate thatasphaltenes, viewed structurally, contain condensedpolynuclear aromatic ring systems bearing alkyl side


chains. The formulae derived from the studies in-voked the concept of large polynuclear aromaticsystems and efforts were made to describe the totalstructures of asphaltenes in accordance with mag-netic resonance data and results of other spectro-scopic and analytical techniques. The heteroatomsŽ .nitrogen and sulfur have also been identified asoccurring in a variety of locations, including cyclic

Ž .and non-cyclic systems Speight, 1991, 1994 . Theoxygen functions are considerably different to thenitrogen and sulfur function there being a lesseroccurrence of oxygen in being systems and a greater

Žtendency for oxygen to occur as hydrogenated e.g.,. Ž .phenolic functions Speight, 1991, 1994 .Ž .Metals i.e., nickel and vanadium occur predomi-

nantly in the asphaltene fraction with lesser amountsŽ .occurring in the resins fraction Reynolds, 1994 .

However, from a structural viewpoint, metals aremuch more difficult to integrate into the asphaltenesystem. It is known that the nickel and vanadiumoccur as porphyrins but whether or not these are anintegral part of the asphaltene structure is not known.Some of the porphyrins can be isolated as a separatestream from petroleum.

Asphaltene molecular weights are variableŽ .Speight, 1991, 1994 there being the tendency toassociate even in dilute solution in non-polar sol-vents. However, data produced using highly polarsolvents indicate that the molecular weights, in sol-vents that prevent association, usually fall into the

Ž .range 2000"500 Speight et al., 1985 .As a result of these studies, it has been possible to

derive various models for petroleum asphaltenes,some of which can be used to explain the chemicaland physical properties of the asphaltenes con-

Ž .stituents Speight, 1991, 1994 and offers some de-gree of predictability as a function of the molecularmodel. These models, that might be more in keepingwith behavioral characteristics, have smaller polynu-clear aromatic systems and also span the range of

Žfunctional types as well as molecular sizes Speight,.1991, 1994 . In addition, such models can have the

large size dimensions that have been proposed forasphaltenes since mobility effects can increase the‘effective diameter’ of the asphaltenes moleculesthereby presenting larger dimensions that is the casefor the stationary molecule. In fact, asphaltenesmolecules can be considered to be ‘molecular

chameleons’ insofar as they can vary in dimensionsdepending upon the degree of mobility and the angleof rotation about an axis andror the freedom ofrotation about one, or more, of the bonds.

2.2. Structure of petroleum

An early hypothesis of the physical structure ofŽ .petroleum Pfeiffer and Saal, 1940 indicated that

asphaltenes are the centers of micelles formed byadsorption, or even by absorption of part of themaltenes, that is, resin material, on to the surfaces orinto the interiors of the asphaltene particles. Thus,most of those substances with greater molecularweight and with the most pronounced aromatic na-ture are situated closest to the nucleus and are sur-rounded by lighter constituents of less aromatic na-

Žture. The transition of the intermicellar dispersed or.oil phase is gradual and almost continuous. Since

Žasphaltenes are incompatible with the oil saturates.and aromatics fraction, asphaltene dispersion is

Ž .mainly attributable to the resins polar aromaticsindicating that the resins are, under ambient condi-tions, a necessary constituent and that by their pres-

Žence they prevent incompatibility Swanson, 1942;Witherspoon and Munir, 1960; Koots and Speight,

.1975; Mushrush and Speight, 1995 .Furthermore, resins from one crude oil might

disperse asphaltenes from a different crude oil withŽ .some difficulty, if at all Koots and Speight, 1975 .

The instability of many such ‘heterogeneous’ blendsindicate that there may be significant structural in-compatibility among the asphaltenes from one crudeoil and the resins of another crude oil.

The means by which asphaltenes and resins inter-act to exist in petroleum remains the subject of

Žspeculation but hydrogen bonding Moschopedis and.Speight, 1976a; Acevedo et al., 1985 and the forma-

Ž .tion of charge-transfer complexes Yen, 1974 havebeen cited as the causative mechanisms. There isevidence that asphaltenes participate in charge-trans-

Žfer complexes Penzes and Speight, 1974; Speight.and Penzes, 1978 but the exact chemical or physical

manner in which they would form in petroleum isstill open to discussion.

The original concept of the asphaltene–resin mi-celle invoked the concept of asphaltene–asphaltene

Ž .association to form a graphite-like stack Fig. 7


Fig. 7. The stack-type concept of the asphaltene micelle.

which acted as the micelle core which, in turn, wasstabilized by the resins. However, the concept ofhydrogen-bonding interactions being one of themeans of association between the asphaltenes andresins has, however, led to a reconsideration of theassumed cluster as part of the micelle. When resinsand asphaltenes are present together, hydrogen-bond-ing studies show resin–asphaltene interactions arepreferred over asphaltene–asphaltene interactionsŽ .Moschopedis and Speight, 1976a . If the same inter-molecular forces are projected to petroleum, as-phaltenes in petroleum as single entities that arepeptized, and effectively dispersed, by the resinsŽ .Fig. 8 . However, whatever the means by which theindividual molecular species are included in the mi-

celle, the structure is recognized as being complexŽ .Bardon et al., 1996 .

2.3. Asphaltene deposition during recoÕery opera-tions

Asphaltenes and asphaltene-related materials areknown to deposit as sediments during recovery oper-ations in the vicinity of production wells duringmiscible floods, after acid stimulation, or during

Žpressure changes Burke et al., 1990; Islam, 1994;.Park et al., 1994 . Many reservoirs produce without

any such problems until the oil stability is perturbedduring later stages of oil production.

The parameters that govern sediment formationand deposition of asphaltenic materials frompetroleum are related to the composition of the crude

Ž .oil Table 1 as well as the parameters used for theŽ .recovery process Table 2 . It must also be recog-

nized that the material that deposits from the crudeoil as a separate phase is more aromatic and richer inheteroatom compounds than the original crude oil. Infact, in some cases, especially when oxidation hasoccurred, the deposited material is more aromaticand richer in heteroatoms than in asphaltenes.

In terms of the crude oil parameters that influencesediment formation, there has been considerable fo-cus on the asphaltene content as well as the chem-istry and physics of the asphaltenes relationship tothe remainder of the oil. For example, while theasphaltene content of petroleum oils varies over a

Ž .wide range Koots and Speight, 1975; Speight, 1991 ,asphaltene content is not the single determining in-fluence on sediment formation. It has been noted that

Fig. 8. The asphaltene–resin concept of the micelle.


Table 1Crude oil properties and their general relationship to sediment formation

Parameter Comments

Asphaltene content affects light oil recovery and heavy oil recoveryinfluences oil–rock interactionsforms sediment when gases are dissolved in the oilthermal methods can change relationship to oil

Ž .Asphaltene:resin ratio )1.0 oil is stableŽ .-1.0 oil is unstable

Heteroatom content provides polarity, responsible for incompatibilitypreferential reaction with oxygen

Aromaticity can cause incompatibility in paraffinic environmentsAPI gravity low gravity, high asphaltene contentViscosity high viscosity, high asphaltene contentOil medium can be changed by thermal recovery processesOxidation changes functional group composition

asphaltene:resin ratios are usually below unity in oilsthat are stable and higher than unity in oils thatexhibit ready precipitation of asphaltenic materialŽ .Sachanen, 1945 .

Even though it is generally understood that as-phaltene content of a crude oil increases with de-creasing API gravity, asphaltene precipitation hasalso been reported in light oils as well. Such an

Ž . Ž .effect gas deasphalting Speight, 1991 arises fromthe increased solubility of hydrocarbon gases in thepetroleum as reservoir pressure increases during mat-uration. Incompatibility will also occur when as-phaltenes interact with reservoir rock, especiallyacidic functions of rocks, through the functional

Ž .groups e.g., the basic nitrogen species just as theyinteract with adsorbents. And, there is the possibilityfor interaction of the asphaltene with the reservoirrock through the agency of a single functional groupin which the remainder of the asphaltene molecule

Žremains in the liquid phase vertical association rela-. Ž .tive to the rock surface Fig. 9 . On the other hand,

the asphaltene constituents can react with the rock at

Žseveral points of contact horizontal association rela-. Ž .tive to the rock surface Fig. 10 thereby enhancing

the bonding to the rock and, in some cases, effectingrecovery operations to an even greater extent. Both

Žmodes of reaction can entrap other species such as.resins and aromatics within the space between the

rock and the asphaltene.Another area where incompatibility might play a

detrimental role during recovery operations occurs asa result of aerial oxidation. In general, it is the morepolar species which oxidize first with or without thepresence of catalysts leaving an oil that is relatively

Žfree of heteroatom species Moschopedis and Speight,.1978 . Thus, oxidation is a means for the production

of highly polar species in a hydrocarbon oil leadingto the deposition of polar sediments and being analo-gous to the deasphalting procedure. Thus, after in-corporation of oxygen to an oil-dependent limit,significant changes occur to asphaltenes and resins.These changes are not so much due to oxidativedegradation but to the incorporation of oxygen func-tions that interfere with the natural order of intra-

Table 2Recovery process parameters and their relationship to sediment formation

Parameter Comments

Carbon dioxide injection lowers pH by changing oil compositionMiscible flooding rich gas lowers solvent power of oilOrganic chemicals cause incompatibility through low solvent powerAcidizing interaction with constituentsPressure decrease changes oil compositionTemperature decrease similar effect to pressure decrease


Fig. 9. Vertically oriented asphaltene–mineral interactions.

molecular and intermolecular structuring leading toŽthe separation of asphaltenic material Moschopedis

.and Speight, 1976b, 1977 .Thus, with alterations in these parameters, the

formation of a sediment can occur although thenature of the sediment will vary and be dependentupon the process of formation. There are also severalprocess-related destabilizing forces that can causeprecipitation of asphaltenic material and each in-volves disturbance to the equilibrium that existswithin petroleum.

For example, carbon dioxide causes the destabi-lization of the petroleum equilibrium by loweringpH, by changing oil composition, and by creatingturbulence. Usually, asphaltene precipitation in-creases as the volume of carbon dioxide available tothe crude oil increases during the later stages ofcarbon dioxide injection or stimulation. The most

noticeable primary locations of asphaltene depositionare the wellbore and the pump regions. In addition,

Ž .flooding of a rich gas miscible flooding destabi-lizes the asphaltene–crude oil mixture by loweringthe solvent power of the solution. The hydrocarbongases used in such applications effectively cause

Ž .deasphalting gas deasphalting, solvent deasphaltingof the crude oil. The negative effect of rich gas is ata maximum near the bubble point; this effect isalleviated after the bubble point is reached. Simi-larly, organic chemicals such as isopropyl alcohol,methyl alcohol, acetone, and even some glycol, alco-hol, or surfactant based solvents, that do not have anaromatic component, may selectively precipitate as-phaltenes and resins.

Asphaltene precipitation may be caused by wellstimulation, such as acidizing which involves a dras-tic shift in local chemical equilibria, pH and libera-

Fig. 10. Horizontally oriented asphaltene–mineral interactions.


tion of carbon dioxide. It may also increase theconcentration of some ions, such as iron, which willpromote the formation of asphaltenic sediment.

A decrease in the pressure is another importantfactor that influences the onset of solids depositionfrom petroleum. In fact, the effect of pressure partic-ularly noticeable just above the bubble point forcrude oils that are rich in light ends. Depending onthe location of the pressure decrease, deposition mayoccur in different parts of the reservoir as well as inthe wellbore and in the production stream. Further-more, a decrease in pressure is usually accompaniedby a decrease in temperature which can also causephysico-chemical instability leading to the separationof asphaltenic from the oil. Pressure change alonecan also invoke similar asphaltene precipitation.

The asphaltenic material in crude oil is electri-cally charged through the existence of zwitterions or

Žpolarization within the molecular species Preckshotet al., 1943; Katz and Beu, 1945; Penzes and Speight,1974; Speight and Penzes, 1978; Fotland and Alfind-

. Žsen, 1996 . Therefore any process such as the flow.through reservoir channels or through a pipe which

can induce a potential across, or within, the oil willalso result in the electrodeposition of asphaltenicmaterial through the disturbance of the stabilizingelectrical forces. In addition, neutralization of themolecular charge will also result in the formation ofa sediment.

2.4. Models of organic deposition

The mechanism of asphaltene precipitation is veryŽcomplex Speight et al., 1984; Andersen and Speight,

.1993; Buckley, 1996 and controversy as to thenature of asphaltene solutions persists. However,petroleum consists of a mixture of oil, aromatics,

Ž .resins, and asphaltenes Fig. 1 and it is necessary toconsider each of the constituents of this system as acontinuous or discrete mixture interacting with eachother. The omission of any one fraction could lead to

Ž .errors in the outcome Andersen and Speight, 1993 .There are two general approaches to the consider-

Žation of the nature of asphaltenes in oil Hirschberget al., 1984; Mansoori et al., 1987; Park et al., 1994;

.Islam, 1994 . The first approach considers as-phaltenes to be dissolved in oil in a true liquid state.In this case, asphaltene precipitation is considered to

depend on thermodynamic conditions of temperature,pressure and composition. This particular approachrecognizes asphaltene precipitation as a thermody-namically reversible process. The second approachconsiders asphaltenes to be solid particles which aresuspended colloidally in the crude oil and are stabi-lized by resin molecules; consequently, the deposi-tion process is considered to be irreversible. If thematerial precipitated is a reacted derivative of the

Ž .asphaltenes i.e., a sediment , this may be true but ifthe precipitated material is unreacted asphaltene thenthis assumption may need a correction.

One question of major interest during recoveryoperations is the timing of sediment formationrde-position and the amount of the organic material willseparate from the oil under specific conditions.

Two different models have been proposed to ex-plain the behavior of petroleum and the potentialfor solids deposition during recovery operationsŽKawanaka et al., 1989; Islam, 1994; Park et al.,

.1994 .The continuous thermodynamic model utilizes the

theory of heterogeneous polymer solutions is utilizedfor the predictions of the onset point and amount oforganic deposits from petroleum crude. A stericcolloidal model which is capable of predicting theonset of organic deposition has also been developedand a combination of these two models results in afractal aggregation model. These efforts have gener-ally been adequate to predict the asphaltene–oil in-

Žteraction problems phase behavior andror floccula-.tion wherever it may occur during oil production

and processing.In the continuous thermodynamic model the de-

gree of dispersion of the high molecular weightorganic constituents in petroleum depends upon thechemical composition of the petroleum. Precipitationof the high molecular weight material can be ex-plained by a change in the molecular equilibria thatexist in petroleum through a change in the balance ofoil composition. Moreover, the precipitation processis considered to be reversible. Indeed, the reconstitu-tion of petroleum after fractionation has been

Ž .demonstrated Koots and Speight, 1975 and lendssupport to this model.

The ratio of polar to non-polar molecules and theratio of high- to low-molecular-weight molecules ina complex mixture such as petroleum are the two


factors primarily responsible for maintaining mutualsolubility. The stability of the system is altered bythe addition of miscible solvents causing the highmolecular weight andror the polar molecules toseparate from the mixture either in the form ofanother liquid phase, or as a solid precipitate. Hydro-gen bonding and the sulfur- andror nitrogen-contain-ing segments of the separated molecules could startto aggregate for polymerization and, as a result,produce a solid phase which separates from the oil.

In the steric colloidal model, the high molecularweight materials in petroleum are considered to besolid particles of different sizes suspended col-loidally in the oil and stabilized by other petroleum

Ž .constituents i.e., resins adsorbed on their surface.The original hypothesis of the physical structure

Ž .of petroleum Pfeiffer and Saal, 1940 which resinsplayed a role in the stabilization of the asphaltenesand later confirmation of the role of the resins in

Žpetroleum Swanson, 1942; Witherspoon and Munir,.1960; Koots and Speight, 1975 is supportive of this

model.In the fractal aggregation model, it is assumed

that pi–pi interactions are the principal means bywhich asphaltenes associate. This assumption maynot be completely valid because of the evidence thatfavors hydrogen bonding between the molecularspecies and the observation that asphaltene–resininteractions may predominate over asphaltene–asphaltene interactions in petroleum. The conceptthat asphaltene–asphaltene interactions may be thepredominant interactions is true for solutions of as-phaltenes, and this is reflected in the molecular

Ž .weight data Speight et al., 1985 but there is noguarantee that these interactions are predominant in

Žpetroleum especially with evidence that indicates.the high potential for other interactions

Ž .Moschopedis and Speight, 1976a; Speight, 1994 .In as much as the high molecular weight con-

stituents of petroleum have a wide range of polarityŽ .and molecular weight distribution Fig. 3 , such

compounds have been considered to act as heteroge-neous polydisperse polymers. However, knowingwith a new understanding of the nature of as-

Ž .phaltenes Speight, 1994 , there may be some diffi-culties with this assumption. Nevertheless, in orderto predict the phase behavior of the these con-stituents, it has also been assumed that the properties

of these constituents depend on their molecularweights. The model also assumes that the as-phaltenes are partly dissolved and partly in the col-loidal state thereby accounting for both the solubilityand colloidal effect of high molecular weight organicconstituents in the lower molecular weight con-stituents. The proposed models can provide the toolfor making satisfactory prediction of the phase be-havior of the deposition of high molecular weightmaterials.

Because the issue of the deposition of asphaltenicmaterials problem is complex, it is necessary toattempt an understanding of the deposition mecha-nism before an accurate and representative modelcan be formulated. Utilization of kinetic theory offractal aggregation has enabled the development ofthe fractal aggregation model. This model allows todescribe properly several situations, such as phasebehavior of heavy organic deposition, the mechanismof the association of the high molecular weightconstituents, the geometrical aspects of aggregates,the size distribution of the sediments, and the solubil-ity of the high molecular weight constituents in thesolution under the influence of a miscible solvent.

Another potential model involves use of the solu-bility parameter of the asphaltenes and the surround-ing medium. It is known that the solubility of as-phaltenes varies with the solubility parameter of the

Žsurrounding liquid medium Mitchell and Speight,.1973 and calculation or estimation of the solubility

parameters of various liquids is known. From thesedata, it is possible to estimate point at which asphal-tenic material is precipitated when the compositionof the oil is changed by the addition of a hydrocar-

Ž .bon liquid Mitchell and Speight, 1973 . In fact, thesolubility parameter has been used successfully todetermine the character of heavy oils and investigate

Ž .the separation of asphaltenic material Wiehe, 1996 .The solubility parameter concept also recognized thegradation of polarity of the asphaltenes as selectiveprecipitation occurs during the addition of a non-solvent when the most polar constituents are precipi-

Ž .tated first Andersen and Speight, 1992 .The models which apply the solubility parameter

concept calculate the interaction through an assump-tion of the total crude oil as formed by asphaltenesand deasphaltened oil; hence the system is regardedas a two component system. The changes in phase


Fig. 11. Estimation of the solubility parameter from the HrC atomic ratio.

equilibrium is caused by the changes in the solubilityparameter of either of the two pseudo-components,which may happen either by dissolution of gas oralkane in the deasphaltened oil phase or by changesin temperatures. The amount of precipitated as-phaltenes is calculated as the differences in as-phaltenes present in the oil and the solubility ofasphaltenes at the saturation point. In the differentmodels the change in the composition of the deas-phaltened oil is taken as significant for the phaseequilibrium, and various methods, i.e., cubic equa-tions of state, are applied to determine the propertiesof this fraction.

ŽFurther development of this concept Speight,.1994 has led to the graphical representation of the

Fig. 12. Potential changes in the solubility parameter due tointeractions that lead to sediment deposition.

solubility parameters of polynuclear aromatic sys-tems and estimation of the solubility parameter ofthe asphaltenes based on hydrogen:carbon ratiosŽ .Fig. 11 . Further development of this knowledgecan allow the progress of asphaltene deposition to be

Ž .followed Fig. 12 and the region of sediment forma-Ž .tion or the region of instability and incompatibility

to be estimated using a simplified phase diagramŽ .Fig. 13 . Obviously more work is need to apply the

Fig. 13. Representation of petroleum as a three-phase systemshowing a region of instability.


mathematical procedures to this concept but it doesinvoke the principle of knowledge of the chemicaland physical properties of the asphaltenes and theoil.

In general, these proposed models are, to a de-gree, applicable to the prediction of heavy organic

Ž .deposition asphaltene, paraffinrwax, resin frompetroleum due to changes in pressure, temperatureand composition. However, the use of assumptionsthat do not reflect, or recognize, what might be theactual chemical and physical structure of petroleumcan lead to errors in the data. Further modeling mustinvolve recognition of the more modern concepts ofthe structure of petroleum as well as application ofthe models to the predictability of the location andamount of the deposition of the sediments inside theproducing wells and oil-transport pipelines.

3. Conclusions

Petroleum is a complex system that depends uponthe relationship of the constituents fractions to eachother and the relationships are dictated by molecularinteractions. Thus, recovery chemistry can only begeneralized because of the intricate and complexnature of the molecular species that make up thecrude oil leading to difficulties in analyzing not onlythe recovered material but also the original oil inplace. Moreover, the incompatibility of crude oilswith each other is a continuing issue and the occur-rence of suspended organic solids during recoveryŽ .especially thermal reduces the efficiency of a vari-ety of processes.

Asphaltene precipitation or the mere presence ofasphaltenes may invoke many implications in recov-ering asphaltic crude oils. Asphaltene precipitationmay occur under various thermally or non-thermallyenhanced oil recovery schemes or even primary pro-duction conditions.

These models that are proposed models as beingŽapplicable to the prediction of sediment i.e., asphal-

.tene, resin, wax formation and deposition frompetroleum due to changes in pressure, temperatureand composition. Further modeling must involve anunderstanding of the chemistry of these materialsand reflect the more modern approach to thephysico-chemical structure of petroleum in order to

more correctly predict the onset of precipitation aswell as the location and amount of the sedimentdeposition in the producing wells and in oil-transportpipelines.

References

Acevedo, S., Mends, B., Rajas, A., Lairs, I., Rives, H., 1985. Fuel64, 1741.

Andersen, S.I., Birdi, K.S., 1990. Fuel Science and TechnologyInternational 8, 593.

Andersen, S.I., Speight, J.G., 1992. Div. Fuel. Chem. Am. Chem.Ž .Soc. 37 3 , 1335, Preprints.

Andersen, S.I., Speight, J.G., 1993. Fuel 72, 1343.Andersen, S.I., Stenby, E.H., 1996. Fuel Science and Technology

International 14, 261.Bardon, C., Barre, L., Espinat, D., Guille, V., Li, M.H., Lambard,

J., Ravey, J.C., Rosenberg, E., Zemb, T., 1996. Fuel Scienceand Technology International 14, 203.

Ž .Bruning, I.M.R.deA., 1991. Oil and Gas Journal 89 3 , 38.Buckley, J., 1996. Fuel Science and Technology International 14,

55.Burke, N.E., Hobbs, R.E., Kashou, S.F., 1990. Journal of

Petroleum Technology 42, 1440.Fotland, P., Alfindsen, H., 1996. Fuel Science and Technology

International 14, 101.Francisco, M.A., Speight, J.G., 1984. Div. Fuel Chem. Am.

Ž .Chem. Soc. 29 1 , 36, Preprints.Girdler, R.B., 1965. Proc. Assoc. Asphalt Paving Technol. 34, 45.Hirschberg, A., de Jong, L.N.J., Schipper, B.A., Meijer, J.G.,

1984. Soc. Pet. Eng. J. 283.Ž .Islam, M.R., 1994. In: Yen, T.F., Chilingarian, G.V. Eds. ,

Asphaltenes and Asphalts. Developments in Petroleum Sci-ence. Vol. 40, Chap. 11, Elsevier, Amsterdam.

Katz, D.L., Beu, K.E., 1945. Ind. Eng. Chem. 43, 1165.Kawanaka, S., Leontaritis, K.J., Park, S.J., Mansoori, G.A., 1989.

Ž .In: Borchardt, J.K., Yen, T.F. Eds. , Oil Field Chemistry:Enhanced Recovery and Production Simulation. SymposiumSeries No. 396, Chap. 24, American Chemical Society, Wash-ington, DC.

Koots, J.A., Speight, J.G., 1975. Fuel 54, 179.Leontaritis, K.J., 1996. Fuel Science and Technology International

14, 13.Ž .Long, R.B., 1979. Div. Petrol. Chem. Am. Chem. Soc. 24 4 ,

891, Preprints.Ž .Long, R.B., 1981. In: Bunger, J.W., Li, N. Eds. , The Chemistry

of Asphaltenes. Advances in Chemistry Series No. 195, Amer-ican Chemical Society, Washington, DC.

Mansoori, G.A., Jiang, T.S., Kawanaka, S., 1987. Arabian J. Sci.Ž .Eng. 13 1 , 17.

Mitchell, D.L., Speight, J.G., 1973. Fuel 52, 149.Moschopedis, S.E., Speight, J.G., 1976a. Proc. Fuel 55, 187.Moschopedis, S.E., Speight, J.G., 1976b. Proc. Assoc. Paving

Technol. 45, 78.Moschopedis, S.E., Speight, J.G., 1977. J. Mater. Sci. 12, 990.


Moschopedis, S.E., Speight, J.G., 1978. Fuel 57, 25.Mushrush, G.W., Speight, J.G., 1995. Petroleum Products: Insta-

bility and Incompatibility. Taylor and Francis, Washington,DC.

Park, S.J., Escobedo, J., Mansoori, G.A., 1994. In: Yen, T.F.,Ž .Chilingarian, G.V. Eds. , Asphaltenes and Asphalts. Develop-

ments in Petroleum Science. Vol. 40, Chap. 8, Elsevier,Amsterdam.

Penzes, S., Speight, J.G., 1974. Fuel 53, 192.Pfeiffer, J.P., Saal, R.N., 1940. Phys. Chem. 44, 139.Preckshot, G.W., Delisle, N.G., Cottrell, C.E., Katz, D.L., 1943.

AIME Trans. 151, 188.Ž .Reynolds, J.G., 1994. In: Yen, T.F., Chilingarian, G.V. Eds. ,

Asphaltenes and Asphalts. Developments in Petroleum Sci-ence. Vol. 40, Chap. 10, Elsevier, Amsterdam.

Sachanen, A.N., 1945. The Chemical Constituents of Petroleum.Reinhold, New York.

Speight, J.G., 1991. The Chemistry and Technology of Petroleum,2nd edn. Marcel Dekker, New York.

Ž .Speight, J.G., 1994. In: Yen, T.F., Chilingarian, G.V. Eds. ,Asphaltenes and Asphalts. Developments in Petroleum Sci-ence. Vol. 40, Chap. 2, Elsevier, Amsterdam.

Speight, J.G., Long, R.B., 1996. Fuel Science and TechnologyInternational 14, 1.

Speight, J.G., Penzes, S., 1978. Chemistry and Industry 729.Speight, J.G., Long, R.B., Trowbridge, T.D., 1984. Fuel 63, 616.Speight, J.G., Wernick, D.L., Gould, K.A., Overfield, R.E., Rao,

B.M.L., Savage, D.W., 1985. Rev. Inst. Francais du Petrole40, 51.

Swanson, J., 1942. J. Phys. Chem. 146, 141.Wiehe, I.A., 1996. Fuel Science and Technology International 14,

289.Witherspoon, W., Munir, Z.A., 1960. Producers Monthly 24, 20.Yen, T.F., 1974. Energy Sources 1, 447.

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The chemical and physical structure of petroleum: effects on recovery operations