Copyright 2000, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 2000 SPE Permian Basin Oil and GasRecovery Conference held in Midland, Texas, 21–23 March 2000.

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Abstract

The work described in this paper has been conducted overseveral years under both laboratory and field conditions.Several field case histories and supporting laboratoryobservations are presented to substantiate the conclusionsexpressed.

Asphaltenes are often invoked to describe the nature oforganic damage and obstructions found when atypical formsof organic deposits appear. In fact a majority of problemsstudied by this laboratory have been misidentified asasphaltene when the true problem was due to paraffin waxdeposition. Asphaltenes are poorly understood, consequentlymethods aimed at dealing with deposit mechanisms andchemical prevention present a doubly difficult challenge.Asphaltene deposition mechanisms and the chemicalstructures responsible are less well characterized than those ofemulsion breaking, corrosion protection, and paraffin depositcontrol. Since most asphaltene deposits are associated withparaffin and there exists many proven methods of paraffincontrol, in many cases it makes better sense to attack organicdeposit problems with paraffin control products than someunproven “asphaltene inhibitor”.

This paper suggests that adequate methods exist today for theremoval and control of paraffin wax damage, and that methodsfor the control of asphaltenes are still in a relatively embryonicstate of development. Until the day comes that asphaltenes arewell understood chemically, as are waxes, emulsions,corrosion, and scale, the hope of economically treating themwill remain a problem for research.

IntroductionAsphaltenes are basically defined as comprising an organicfraction of crude oil that is insoluble when that oil is placed infifty times its volume in pentane.1 This definition is extremelybroad and non-informative, and very little information aboutthe chemical structure of these pentane insoluble fractions isgained by this procedure. In fact if the crude oil is notproperly pretreated to remove water and solids, confusion canarise about the true quantity of “asphaltenes” that are actuallypresent. This operational definition of asphaltenes resultsfrom historical observations made by oil industry personnel,and was never intended to offer a rigorous description of thesesubstances. One outstanding feature of this method ofisolating asphaltenes is its clear tendency to exclude ionicand/or polar species possessing strong intermolecularinteractions from solvation by pentane. Hidden within thisfeature is the fact that higher molecular weight normalparaffins tend to aggregate by inductive charge associationthrough London dispersion forces (Fig.1). Unless the pentanesolution is heated to disrupt these forces of aggregation thehigher normal paraffins will also be included as part of theprecipitated “asphaltenes”.

Many organic molecules can be classified as ionic and/orpolar. Carboxylic acids and their salts are examples ofnaturally occurring organic molecules that can be and areoften found in crude oil (Fig. 2). Additionally, many aminesand anionic amine salts are known to be present in crude oils(Fig. 3). Although the concentration of these molecules isusually low in the majority of crude oils produced, they areoften associated with problems in its production.“Asphaltenes”, as they are operationally defined, can thus bethought of as including these molecules. Further, asmentioned above, normal paraffins can also be included aspart of the isolated “asphaltenes” when no heat has beenapplied to the pentane oil mixture. Although researchersattempting to elucidate structural features of “asphaltenes”have expended considerable efforts, the basic lack of a cleardefinition of these materials hinders progress.2 Vaguedescriptions of “resins and maltenes” are given to describe thecomponents thought to be responsible for the suspension of“asphaltenes” (Fig. 4). Thus, confusion on confusion resultsand speculation replaces fact when dealing with the ill-definednature of asphaltenes.

SPE 59703

Asphaltene: To Treat or NotHarold L. Becker “JR”, Unichem A Division of BJ Services Company

2 HAROLD L. BECKER "JR" SPE 59703

What Do We Really know about asphaltenes?If we constrain our speculation and concede the operationaldefinition, we can only say that asphaltenes are those speciespresent in crude oil that possess a tendency to aggregatedespite the solvation effects of pentane. This definition doesnot exclude the carboxylic acids, amines, and their respectivesalts. Further, it does not exclude those molecules thataggregate through London forces of dispersion such as highermolecular weight paraffins. Thus one or several of the acids,amines, their respective salts, and paraffins could beconsidered asphaltenes. Some additional qualifiers have beenadded to the definition of asphaltenes over the years such asfollowing:

1) Those fractions of crude oil that are insoluble in fiftytimes their volume in pentane are intensely coloredspecies (e.g., black).

2) Those fractions of crude oil that are insoluble in fiftytimes their volume in pentane possess polar character.

3) Those fractions of crude oil that are insoluble in fiftytimes their volume in pentane migrate in an electricfield.

4) Those fractions of crude oil that are insoluble in fiftytimes their volume in pentane are comprised ofmolecules that contain heteroatoms (e.g., Nitrogen,Oxygen, and Sulfur).

Although these qualifiers help in the characterization ofasphaltenes they do little to negate the possibility thatcarboxylic acids, amines, their salts, and higher molecularweight paraffins are included as part of the pentaneprecipitated “asphaltenes”. Intensely colored species oftenresult from very small quantities of highly conjugatedmolecules (repeating units of doubly bonded carbonsseparated by two saturated carbons), or metallic complexes(Fig. 5). Carboxylic acids, amines, and their respective saltspossess polarity and as such they can also migrate in electricfields (Fig.6). Carboxylic acids, amines, and their respectivesalts also include heteroatoms (Fig. 7). Therefore, a case canbe made that what is classified as asphaltene might just aseasily be called heteroatom molecular species inclusion in acrude oil .

Laboratory TestingMuch of the laboratory work conducted on asphaltenes hasbeen directed at methods for the solublization or suspension ofthese materials. One such method involves the precipitationof the “asphaltenes” and resolvation or resuspension of themin large volumes of incompatible solvent (i.e., pentane). Inthis test, a small sample of the crude oil (e.g., 1 c.c.) is placedin a large volume of pentane (e.g., 50 c.c.) and allowed toprecipitate, whereupon a small quantity (e.g., 1000 PPM) ofsuspending agent or “test compound” is added and the resultsof its addition observed. If the “test compound” is effective,the precipitated “asphaltene” will be dispersed and remaindispersed throughout the pentane. Although this method doessuggest that certain chemical compounds are useful in the

dispersion of materials that are incompatible with lighterhydrocarbons, it does little to indicate whether it will besuccessful in the actual crude oil system.

Another test procedure involves the employment of coretesting apparatus to simulate some of the reservoir conditionsunder which the crude is produced. The test apparatusconsists of high-pressure pump, Hastler core holder, Bareasandstone core, forward/backward valving, pressuretransducer, U.V./Visible spectrometer, and two-channelrecorder (Fig. 8). Samples of asphaltene collected from thefield or precipitated from the crude are added to xylene andpumped onto a brine-saturated core while the pressure andabsorbency are measured. Xylene pumping is continued until abase-line pressure and absorbency is achieved. Once a highbase-line pressure and absorbency are established, chemicaladditives are added to the xylene flush and pumped onto thecore. If the additives are effective, they will cause a drop inpressure and an increase in absorbency greater than the xylenealone. Testing indicated that various additives were moreeffective than xylene alone, and that fatty bi-polar moleculeswere among the most effective additives for the removal ofdamage.3

Infrared and H1NMR analysis of various field-collected andlaboratory prepared samples of asphaltenes have yieldedspectra that are mostly consistent with those of hydrocarbons.In fact the absence of functional group adsorption bands in theinfrared indicate that these structures contain less than fewpercent heteroatom functionality. Thus, instrumentalanalytical methods have not yet clearly defined or structurallyelucidated many of the attributes of asphaltenes.

Field Problems related to “Asphaltenes”What circumstances promote the formation of “asphaltene”deposits in the oil field? During the last decade the use of CO2

floods have gained increasing acceptance as a method ofenhancing the recovery of petroleum. CO2 is injected into theproducing formation at pressures above its critical pressure(e.g., pressures that reduce its gas volume to a volume where itexists in a liquid state). CO2 is polar but it does not possess adipole moment and it cannot act to solvate dipolar species ornon-polar components within crude oil, it can only act as adiluent. Thus CO2 at critical pressure behaves as pentane doeswhen mixed with crude oil; it acts as a non-interactive matrixwith charged species and a marginal solvent for lowhydrocarbons. As CO2 breakthrough (the appearance of fluidsat the well bore) occurs, organic deposits begin to obstruct theflow of reservoir fluids. It is the character of these depositsthat often contributes to the confusion about the nature ofasphaltenes. These deposits are regularly obtained fromsucker rods, and well tubing and saved for future analysis.However, they are not generally saved in sealed containers.The gases have been allowed to escape from the sample andwhat is left is a black hard substance very often referred to asasphaltene.

SPE 59703 ASPHALTENE: TO TREAT OR NOT 3

Acid treatments to remove inorganic scale obstructions arecommon occurrences in the oil field. These acid treatmentsare frequently accompanied by the appearance of organicsludge’s that, if not controlled, plug perforations and reduceproduction. It is commonly accepted that this organic sludgeresults from the incompatibility of “asphaltenes” with acid.However, from the discussion above, it should be clear thatcarboxylic acids or amine salts could also be responsible forthe obstructions resulting from acidification of the formation.

Steam floods have been employed by production companies inattempt to recover some of the heavier crude oils for manyyears. The concept of these efforts is based on the idea thatheavier crude oils are recoverable by viscosity reductionaffected by heating so that they can migrate through restrictivepore throats to the well bore. However, if “asphaltenes” aresalts of acids or base, they usually comprise speciespossessing melt points that are usually in excess of thoseobtainable by steam under pressure. Thus, the nature of thecrude oils obtained by steam flood methods must include thosefractions of crude oil that are rendered mobile by elevatedtemperatures. Furthermore, those temperatures must beobtainable by steam under pressure. So what is the nature ofcrude production under steam flood? It should be noted thatpolar or charged organic species are among the least likely tobe rendered movable by increased temperature. However, thepolar nature of the water used in the steam infusion can beconsidered as a motive media. Thus, it can be seen thatanalysis of fluids produced in this way can include both polarand non-polar species.

Case History I; the Consequences of MisinterpretationA major oil producer in Mexico had determined that theirproduction problems were due to an accumulation ofasphaltenes in their production string. Their initial productionwas determined to be forty six hundred barrels per day, butafter two days the well would plug off and production wouldgo to zero. The well had a depth of fifty seven hundredmeters, and a bottom hole temperature of one hundred fiftydegrees centigrade. The production companies’ technical staffwas certain that with such high well temperatures thelikelihood of paraffin deposits was small. Thus, the depositsthat plugged the well were described as “asphaltenes”. Thisdescription led to the use of aromatic solvent clean-outsutilizing expensive coiled tubing units. After a two-day coiledtubing round trip, the well again began producing at the fortysix hundred barrel per day rate and promptly fell again to zeroafter two additional days. Interestingly, laboratory analysis ofdeposits removed from the well tubing showed them to consistof a fifty-fifty mixture of paraffins and asphaltenes. Afterseveral of these coiled tubing procedures, the ProductionCompany began looking for other methods of controllingorganic deposition.

Several specialty chemical companies were invited to test theirvarious chemistries for the control of asphaltene deposits. The

Production Company established a criterion for successfultreatment. This criterion required that the wellhead pressuredrop no lower than twenty seven hundred pounds per squareinch from the initial pressure of thirty two hundred pounds persquare inch over a two-month period. All but one of thesespecialty chemical companies met with failure and coiledtubing treatments were resumed after each failure. Solvents,dispersents, and combinations of solvent and dispersents wereincluded among the many classifications of treatmentsunsuccessfully performed. The only successful treatmentachieved was one that employed the use of a paraffin crystalmodifier treatment. Crystal modifier chemical was added on acontinuous basis down the backside of the well. After twenty-seven months of this treatment, the well pressure was found tobe twenty nine hundred pounds per square inch, and theproduction had fallen from forty eight hundred barrels per dayto thirty two hundred barrels per day.

Case History II; Organic Deposit RemovalA large production company located in Wyoming had beenconducting CO2 floods for several years and had implementeda solvent wash program that utilized xylene to remove thedeposits obstructing production. In fact, this company had amonthly allotment of funds for the purchase of xylene, since itwas found to be so effective in removing organic deposits.High-pressure pump trucks were used to pump xylene into thewell, and the wells were shut in for twenty-four hours to allowsufficient contact time of the xylene to break-up the organicdeposits. Some wells were on a monthly schedule, whileothers were on weekly or bi-weekly solvent clean-upschedules. Analysis of solids retrieved from the tubing, andsucker rods showed that the deposit constituted approximatelya fifty-fifty weight ratio of paraffin to asphaltene just the sameas those reported in Case History I above. Although severallaboratory tests including cold finger, pour point, and viscositydata indicated that a crystal modifier could be appliedsuccessfully to this field, no field trials were authorized.

Several reasons were given why there was a reluctance toallow the application of a crystal modifier product in this field.Perhaps, the most convincing reason involved the poorphysical characteristics of the crystal modifier products (e.g.,they were known to freeze in the winter, thus rendering themun-pumpable). This situation meant that they had to switchtreating programs back and forth between the winter andsummer seasons. This also meant that they had to keep onhand the pressure treating trucks for winter clean-upprocedures. Although this laboratory is strongly disposed tobelieve that a crystal modifier application to this field problemwould have met with success, we cannot disapprove of theproducing companies’ choice to remain committed to remedialsolvent treatment methods. However since the time thisdecision was made, substantial improvements have been madeto many of these crystal modifier products. The reliablewinterization of these products has finally been achieved, andthe limitation of seasonal application has been removed.

4 HAROLD L. BECKER "JR" SPE 59703

Organically soluble crystal modifier products have beensuccessfully suspended in a highly polar solvent matrix thatallows them to be pumped at temperatures well below zerodegrees F. Given the advances made in crystal modifiertechnology, the laboratory results reported in this case history,and the success reported in Case History I above, it wouldappear that application of crystal modifier technology wouldhave been successful.

Case History III: Steam FloodsA major producing company in California who has beenconducting steam floods for over twenty years recentlynoticed (ca., 1999) that their water cuts were going up, andthat their LACT sales were diminishing. For many years thiscompany and its predecessor had accepted the theory that theywere removing high asphaltene crude from a formation thathad, long ago, spent its natural gas drive. The crude oilrecovered from this formation was found to be extremelyviscous (e.g., 8-10 API Gravity), and intensely colored(black). The combination of these observations suggested toboth producers that the nature of the crude oil they weredisplacing with steam fit with the reported characteristics ofasphaltene. Concurrently, analysis conducted by specialtychemical companies reported that asphaltenes were present inthe oil samples obtained from the field. Consequently theysettled on asphaltene as the cause of problem depositsresponsible for treater interface problems and were deterredfrom examining alternative causes of the diminishing crude oilsales. During the last several years a number of specialtychemical companies have been contracted by the producingcompany to solve the interface problems. In the majority ofcases, those specialty companies were sidetracked by theasphaltene issue and failed to satisfy the requirements of thecustomer. It was not until very recently (ca., August 1999)that this laboratory had an opportunity to examine the natureof the problem.

The viscosity of the crude oils produced from these areas isvery high (e.g., twenty thousand to forty thousand centipoiseat sixty degrees F.). This high crude oil viscosity requires theprovision of special physical treating equipment, such as tubeheat exchangers and high temperature dehydration vesselscapable of maintaining fluid transport. Many steam floodsmaintain a down-hole temperature in excess of four hundreddegrees F. in order to assure the mobility of crude oils withinthe reservoir. Unique problems abound in such a system, non-the least important of which is the tightness of the emulsionformed during the processes of crude oil recovery and itssubsequent treatment. One such problem involved theplugging of one of the post well heat exchangers by what wasdescribed as asphaltene. Laboratory analysis of the depositplugging the heat exchanger revealed that it was mainly finesand covered by heavy black crude oil. This finding did notrule out the possibility that the “black crude oil” wasasphaltene, however, it also did not rule out the possibility thatthe deposit was a high boiling paraffin wax. Thus, a sample ofthe deposit was obtained from the heat exchanger and diluted

fifty-fifty volume to volume with kerosene, divided intofractions and treated at one thousand parts per million withvarious crystal modifier products. Several of these dilutetreated and untreated samples were allowed to reach roomtemperature and examined to see if they would flow whentilted at an angle. All but one of the dilute deposit samplesremained solid (resisted flow when displaced from the uprightposition). Once this crystal modifier was indicated as havingpotential for inclusion in the field treatment program,continued testing was performed to examine if it would alsohave a beneficial effect on the resolution of interfaceproblems. This testing revealed a marked improvement in thewater oil interface, and provided additional emphasis for theincorporation of crystal modifier products in this steam floodoperation.

ConclusionSpecialty chemical companies, indeed, know and employ afew effective chemical methods for dealing with corrosion,scale, emulsions, and paraffin problems. However, thesemethods are extremely limited and suggest that a highspecificity of product activity is required for an effectivesolution to the problem. If the nature of the problem is poorlydefined, as is that of asphaltene and no clear chemical handlecan be used to alter their behavior, then a redefinition of theproblem and its possible solution should be offered.“Asphaltenes, resins, and maltenes” offer convenient butobscure descriptions of problem species.

References: 1 ASTM D-2006, D-2, “ASTM STANDARDS PETROLEUMPRODUCTS, LUBRICANTS, AND FOSSIL FUELS”2 Becker, H.L. (J.R.), “Crude Oil Emulsions, Waxes, andAsphaltenes”, PennWell, (1997).3 Becker, H.L. (J.R.), Thomas, D.C., Raul de Real Ruis,“Asphaltene Formation Damage Remediation”, SPE ProductionOperations Symposium, Oklahoma City, Oklahoma, 21-23March 1993.

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Figure 1: London Dispersion forces producing Wax Crystals

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Figure 3: Organic amine salts commonly present in crude oil

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Figure 4: Proposed asphaltene structure surrounded by resins and/or maltenes

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Figure 7: Proposed de-peptized asphaltene di-mer unit

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Figure 8: Core testing device used for studying asphaltenes