00018892[2] asphaltenes

SPE 18892

Asphaltene Deposition: A Comprehensive Description of ProblemManifestations and Modeling Approachesby K.J. Leontaritis,● U. of Illinois

SPE Member“Now with ARCO Oil & Gas Co.

Copyright 1989, Society of Petroleum Engineers, Inc.

This paper wae prepared for presentation at the SPE Production Operetiona Sympoeium held in Oklehoma City, Oklahoma, March 1S-14, 1989.

Thie paper wae selected for preeerdation by an SPE Program Committee following review of information contained in en abetracl aubmiffeu by the author(a). Contents of the pqw,as preeented, hava not been reviewed by the society of Petroleum Engineare and are eubjeot to Sorrestion by the euthor(e). The material, es presented, dose not neaasaarily reflaofeny position of the SocIefy of Patroleum Engineers, its officsre, or mambars. Papera presented at SPE meetings are eubjact to publication review by Editorial Committees of the societyof Petroleum Engineers, Permieekmto copy is resfriited to an abstract of not more than 300 words. Illuetratmnamay not be copied. The abafresf ehould contain amapiouous ackncMadgmsntof where and by whom the paper is presented. Write Publicefions Manager, SPE, P.O. Sox S33SSS,Ffiiherdeon, TX 750SS-SSSS.Telex, 73tfSS9 SPEDAL.

~BSTRAC T asphaltenes different engineers and researchers have.For instance, for the reservoir engineei it may mean

The so-called “asphaltene problem” manifests itself in formation damage and in-situ pluggin~; for thealmost all the facets of production, processing, and production engineer, near well formation damage andtransportation of petroleum, Due to the multiplidty of subsurface +ind surface equipment plugging andthe engineering discipi.ines involved in the above malfunctions; for the refining engineer, distillationprocesses an interdisciplinary approach is beat suited column ind equipment pluggage and tankage capacityfor seeking fundamental understanding of and loss, as well as catalyst deactivation; for thesolutions to the problem. transportation engineer, pipeline pluggage and capacity

loss; and finally, for the asphalt production spechdist,This paper gives a detailed description of how the blending and asphalt quality headaches. Whateverasphaltene problem manifests itself in the field. It one’s vantage point the bottom line of the “asphaltenediscusses in-situ formation damage deep in the problem” is exactly what it says, problems! Thereservoir matrix and near the wellbore, subsurface and literature on the asphaltene flocculation fieldsurface production facilities problems, and refining, experiences and research is extensiveblending, and transportation problems due to [1,2,3~,5,6,7J3,9,10,11,12,13,14,15,16,17,18)].By no meansasphaltene flocculation and deposition. The molecular all the existing literature is cited here due to spaceand colloidal asphaltene models are described and limitations,recommendations are made as to the preferredphysical model, Existing, published analytical One of the most intriguing questions related toasphaltene models based on either molecular or the asphaltene problem is, what the asphaltenescolloidal nature of asphaltenes are presented and themselves are! Not long ago, some experimentalanalyzed, A preferred modeling approach based on the work using field ionization mass spectrometry (FIMS)colloidal physical model is recommended. The was reported [19]where the authors tasted somedoubtcolloidal model permits and supports calculations as to the existence of asphaltenea in crude oil as itsbased on both reversible equilibrium thermodynamics heaviest component, as most researchers feel, hence,and irreversible colloidal flocculation as well. they used the title for their papex “Asphaltenea Where

Are You?”. Since FIMS may be destructing theINTRODUCTION asphaltene micelles and, thus, measuring their

The expression “Asphaltene Deposition Problem”properties in a completely non-aggregated state, whichwo.:ld not reflect their natural state in the oil, a

means different things to different people,. primarilybecause of the variation in vantage points related to

participant of the symposium where the above paperwas Ix&g presented suggested to the authors that amore appropriate title for their paper might have been——

References and illustration at end of paper. “Asphaltenes, What Have You Done with Them?”.This paper refers to work done by the author prior to The above transaction. highlights the elusiveness ofhis affiliation with Arco Oil and Gas Company. asphaltenes; because the one group of adentista could

not find them where they were supposed to be whaeas deposition is omurring during primary production of athe other was asking them what they did with them. reservoir, the asphaltene problems should be expected

to diminish as production progresses with time, whileThe goal of this paper is two fold first, present the keeping everything else constant.

scope of the asphaltene problem and hopefullydemonstrate that asphaltenes are for real, and second, It appears then that the only feasible way a non-give an overview of the industrial and academic asphaltene prone reservoir could get into asphalteneefforts underway for providing solutions to the flocculation and deposition problems deep in theproblem. It is frustrating as well as tantalizing to a reservoir matrix by the route of composition change issaentist or engineer to be invplved in the quest to find by secondary or tertiary oil recovery (EOR). Ina solution to a problem in which the main culprit is secondary recovery a fluid is inja%ed in the reservoirnot even clearly defined and understood. with the purpose of primarily maintaining the

pressure during production. Depending on the natureASPHALTENE PROBLEM MANIFESTATIONS (i.e., composition) of the injected fluid, asphaltene may

start flocculating and depositing in the pores andFormation DamaPe passages of the reservoir matrix (see Fig. 1), thus

resulting in severe and possibly irreversible formationFormation damage due to asphaltene flocculation damage. A similar situation may occur in tertiary oil

and deposition can take place either. deep in the recovery, where chemicals and/or misdble solventsreservoir matrix or near the wellbore. The two cases are added to the formation thus altering theare discussed separately next, composition of the reservoir fluid (for that matter, any

composition changing process employed has the1, DeeD in the Reservoir Matrix potential of resulting in asphaltene flocculation).

It is generally accepted [lA,6,11aOZl,22,23,24,25&] In miscible EOR a displadng fluid (e.g., propane,that asphaltene flocculation may take place because of C02, NGL, etc.) is injected into the formation with theeither reservoir fluid temperature, pressure and purpose of mixing completely with the reservoir oilcomposition changes or electrokinetic effects due to and carry it to the surface. Ideally, in miscible recoverystreaming potential generation during reservoir fluid all of the accessible oil is recoverable due to completeflow. Usually, in the case of normal reservoir miscibility between the miscible solvent and theproduction, no significant changes in temperature are r~oir oil. It .$sthis property of the miscible solvent,encountered. Furthermore, although a gradual which makes it a favorable candidate for EOR that alsopressure drop during the production life of- the gives it the potential of causing asphaltene flocculationreservoir in most cases doea occur, a drop in the in the reservoir and suhequent in-situ deposition andpressure is generally accepted to act in a way that pore plugging.inhibits asphaltene flocculation [1,4,6,11,15,20,21,22,23].Hence, as the reservoir becomes older asphaltene It has been amply demonstrated that most (if notproblems are expected to diminish (indeed, in all cases all) of the familiar miscible solvents can causeknown to the author they did). Consequently, only two asphaltene flocculation. The cpestion that has not beendominant mechanisms of asphaltene flocculation may convincingly answered yet ,is how much of” thebe considered likely to occur in the reservoir, by flocculated material will actually deposit in-situ andcomposition change and/or by electrokinetic effect% what are the consequences. This is basicly a dynamic

problem (flow related) and depends on the specifici. Flocculation Due To Composition Changes flowing system (fluid and media). There is an inherent

difficuhy in trying to answer the above questionsTwo common ways that the composition of a because, first, these answers cannot be easily

reservoir fluid (deep in the reservoir “matrix) changes generalized and should be tailored obtained for aare by normal depletion during which the lighter particular system and, second, simulating actualcdtnponents of the reservoir fluid are produced in reservoir conditions in the laboratory is very difficult.higher proportion (i.e., during primary production by The mechanism of the oil displacement process and adissolved gas drive) and injection of fluids for number of other factors (e.g., related to the asphalteneenhanced oil recovery (EOR). flocculation mechanism, the geometry of the mass

media, etc.) will dictate the magnitude of the in-situThe reservoir fluid composition change during deposition.

primary oil production is nearly always towards thedirection that results in a drop of both the GOR and the It is important to note that flocculation and in-API gravity, Both of these composition trends have as situ deposition are two separate phenomena. It isa consequence a drop in the asphaltene flocculation possible to have flocculation without substantial in-tendency of the reservoir fluid. The above are, of situ deposition, provided that the flow dynamics andcourse, general trends and may not occur with every the nature of the flocculated material are such thatoil. Hence, generally speaking, even if asphaltene permit. erosion and/or carrying of the deposit by the

ear The Well fireflowing fluid (the deposits may accumulate, however,in the production wells and separators). Needless is As discussed above, asphaltene flocculation due tosay that the flocculated materials contain sizable the streaming potential generated by the flowingamount of entrapped oil which, if deposited in the reservoir fluid is possible, However, there seems to bereservoir with the asphaltenes, will not be recovered, a certain threshold in fluid velocity beyond whichIronically, the entrapment of oil by the flocculating asphaltene flocculation may occur. This threshold forasphaltenes gives them fluidity and acts against different fluids, conduit mass medium, and geometrydeposition, Because the flocculation is essentially has not been adequately studied to date. However, weirreversible, dropping the pressure of the flocculated do know that the higher the velocity of the fluid thesystem may result in initiation of the deposition higher the generated streaming potential and as aprocess (this is possible in well tubings). result the more severe the asphaltene flocculation

[4,20,22,23,24,27], In a producing reservoir matrixNettability reversal of the reservoir rock caused higher velocities occur near the wellbore, where all the

by asphaltene flocculation and deposition is also a produced fluids converge, Consequently, reservoirdistinct possibility. As Figure 2 shows, the highly polar asphaltene problems caused by electrokinetic effects areand heavier than water flocculating asphaltene expected to concentrate near the well bore, providedmicelles may diffuse through the water and be that the volubility of the micellea in oil is favorable to

adsorbed onto the rock thus making it oil wet. This (or near) flocculation. In these cases properly designedshould affect oil recovery adversely, because the matrix acidization jobs can restore full well productionpermeability to oil is expected to diminish and rate by cleaning up the deposited asphaltenes and,residual saturations to increase. thus, eliminating the formation damage[12,17,1821,271.

This appears to be a fairly routine process, althoughcostly, as the author can confirm from personal

ii. Flocculation Due To Electrokinetic Effects experience.

It has been shown that ‘asphaltenes carry an In the production of reservoir fluids containingintrinsic charge. This charge has been considered partly sizable amount of asphaltenes excessively largeresponsible for the stability of the asphaltene-resin drawdowns must be avoided, because large drawdownsmicelles [4,14,20zl~# 24]. Hence, as the rnicelles are result in higher fluid velocities deeper in theaggregating and ready to flocculate from a volubility in formation, which in turn may cause formationoil standpoint, the similar charges on the micelle damage that is very difficult to remove. Whenkernels prevent them from doing so (refer also to a opening a well into production for the first time orrelated discussion h later sections). At this point two after a workover, it is customary to go through anthings may happen to cause flocculation of the initial cleanup period at which time the well ismicelles; first, a further composition change (i.e., directed to the cleanup facilities. Any debiis, mud, andaddition of a flocculant) that would make the micelles cleaning or workover fluids that have entered theeven less soluble in the oil and, as a result, cause the formation or sit in the sump or tubing of the wellopposing electrical charges be overcome by the van der must be cleaned out before the well is placed onWaal-London attractive (dispersion type) forces production. For asphaltenic oils this step is veryand/or, second, generation of a streaming potential crucial, because the flow rate during cleanup must bedue to flow (or application of an external potential) large enough to remove all undesirables from thelarge enough to neutralize the electrical charges and vicinity of the well while at the same time being smalldisturb the force balance between the micelles thus enough to avoid asphaltene flocculation and resultingcausing them to flocculate. formation damage, Excessive pressure reduction (or

drawdown) should also be avoided because it promotesThe first route of flocculation is possible to occur deposition of flocculated asphaltenes.

deep in the reservoir matrix and was discussed in theprevious section. The second mechanism of Well Problemsflocculation is also possible to occur in the reservoir.Two very important factors in this case are the velocity In the well asphaltene deposition may manifestof the flowing reservoir fluid and its composition itself in many ways, but the most obvious one is 10ssof(which dictates the volubility of the asphaltene micelles production. Higher and higher choke settings arein it). Other factors that influence the electrokinetic required to maintain the same production level untilprocess are the zeta-potential of asphaltenes, their size, the choke is fully open and the production lossestemperature, transport properties of the oil (i.e., cannot be compensated by oping the choke further.ViSUWity),and Other [4,14,20~1~,23,2427,28,29~]. ThiS Eventually, the well may “bridge” and stop flowingtopic has been discussed in more detail in other completely or continue flowing at an extremely smallpublications of the author [20,2~ and is briefly dealt flow rate in a rather “burping” way where the flowwith later in this paper, starts and stops at different time intervals on its own

volition,

4 Asphaltene Deposition: A Comprehensive Description ofProblem Manifestations and Modeling Approaches SPE 18892

Complete plugging of the well is very undesirable Surface facilitimbecause the cost of asphaltene cleanup to restoreproduction is very large. A customary method of In the production surface facilities asphaltenecleanup in this case is “bailing” by wireline, This flocculation and deposition causes tremendouscleanup method is slow and costly, especially if the problems. What actually happens is that flocculatedasphaltene plug in the well tubing is very hard and asphaltenes deposit on just about everything the crudelong. However, other techniques such as drilling the oil comes in contact with. The biggest impact of thisasphaltene plug inside the tubing or hydroblasting problem is on safety and process control andthrough it (e.g., by using a coiled tubing unit) have monitoring equipment. For instance, level control andbeen used with, of course, a much higher cleanup cost, or indication equipment as well as sight glasses ondue to the normally high mobilization cost of these production separators and other vessels plug up and asunits. These type of cleanup operations create also a result the equipment overflow. The consequences ofother kind of Iegistical problems. For instance, this may vary in importance and intensity, Highdepending on how soft or hard the asphaltene plug is a pressure safety devices may not actuate and if safetyvery large amount of washings may need to be relief valves are also plugged by asphaltene depositsdisposed of. In the case of offshore operations this can and fail to open, when needed, the results may verybe a very big problem, especially near beaches or tourist well be catastrophic. Thus failure of safety and processareas. In this case a mobile barge or standby small monitoring and control devices is a major problemtanker would be of big help, as the author confirms caused by asphaltene deposition.from personal experience.

In general, it may be said that all vessels and pipesThe most desirable and recommended alternative or even more broadly all production wells and

is to undertake well asphaltene cleanup operations equipment internals are susceptible to asphaltenebefore the well plugs up completely, In general the plugging. As a result, identification and prediction ofwells must be monitored daily to detect production the asphaltene flocculation potential of a crude prior tolosses. In this case wireline scraping can be used design and construction of the production andsuccessfully while the well effluent is flowing to the processing facilities is absolutely essential. There wecleanup facilities, and as a result carrying the various equipment design techniques that may be usedasphaltene scrapings with it. This method may be very to alleviate or minimize the effects of asphalteneeffective in onshore operations. However, in offshore deposition, if it is expected a priori that asphalteneoperations the logistical problem of disposing of the flocculation will occur. This emphasizes thecrude oil-asphaltene scrapings needs to be considered importance of modeling efforts that have as a purposecarefully. Routing the well to the production separator prediction of the phase behavior of asphaltenea inwhile scraping asphaltene deposits could result in crude oil.serious production problems. Alternatively, a solvent(e.g., xylene) bath and wash of the tubing followed by The design techniques referred to above fors~aping while the well is flowing may bean acwptable processing asph altenic crude oils dqmtd on the extendreinedy. of operations in the production facilities and other

factors. Detailed description of these design techniquesPluggage of downhole safety valves (DHSV) and is beyond the scope of this paper. However, an example

their actuation mechanism is another serious problem of such a.techniq’s is designing vessel bridles in a waycaused by asphaltene flocculation and deposition. that permits frequent on stream flushing or cleaningMalfunction of the DHSV’S has the potential of by the operator without requiring maintenanceresulting in very serious consequences. DHSV’Sleft in personnel to get involved, Also, parallel vesselservice for long periods without checking their operation or provision of spare equipment whereoperation in wells where asphaltene deposition is economically feasible is another design remedy. This,occurring are bound to malfunction. The same holds of course, just scratches the surface of the processfor other valves of the x-mass tree, e.g., the hydraulic design concerns that are to be addressed if the crude oilmaster the wing valve and other. Needle’ valves on has been shown to be prone to asphaltene flocculation.the top of x-mass trees for isolation ai pressure gaugesthat measure tubing head pressures are particularly Evidently, it is of utmost importance for thesusceptible to asphaltene plugging (although one prospective developer and producer of a new oilwould thing that flocculating micelles would not reach discovery to know whether the oil would undergothat dead space), There have been cases, known to the asphaltene flocculation prior to being produced or laterauthor, where the operator removed the pressure during processing. This kind of information wouldgauge believing that the needle valve was closed with spark a number of design changes starting from thesubsequent blowing of the asphaltene plug resulting in reservoir, weils, production, and processing facilities.the well shootrng 150 atmospheres asphalvmic sour Designing the above facilities for the case of nocrude oil high into the air, asphaltene flocculation and finding out later that

asphaltenes flocculate is a very &atly problem that mayeven threaten the economic recovery of the field. It is

SPE 18892 K. J. Leontaritis 5

sounder and more economical to design theproduction facilities for asphaltene flocculation (if thathappens to be the case) the first time around ratherthan trying to retrofit later. On the other hand,overdesigning the facilities to handle an asphalteneproblem that would never materialize is also aneconomic no-no, Hence, it is obvious that the bestalternative is to have a way to reliably predictasphaltene flocculation in order that proper planningand design is done at the beginning,

In offshore production operations the “skimpile”and /or the oily water clean-up system are veryimportant and at the same time troublesome tooperate systems. Oily water coming from the vesseldrains and/or from cleaning the platform decks mustbe carefully de-oiled and cleaned before it is disposed ofinto the sea or re-injected. Flocculated asphaltenes,because they are surface active and their density isgreater than water’s, cause serious problems in theoperation of these equipment (usually the mainprinciple of operation in these equipment is based onthe gravity difference of the fluids being separated,with water, of course, being heavier than oil).

Refininp and Trans~ort ation Fac ilitiq

A uarallel may be drawn between the asphalteneproblems during production and refining, because ofthe similarity of some of the processing steps andequipment. All problems mentioned aboutinstrumentation for the production side apply to therefining side as well, provided that the crude oil isasphaltenic and asphaltene flocculation is possible atthe processing conditions of the oil and its derivatives,

For instance, asphaltene flocculation that wouldcause plugging in tower and vessel internals is a verydistinct possibility. A crude oil stabilizer once in awhile (e.g., every two to four months, depending onthe asphaltene deposition severity) needs to be takenout of service for cleaning. The clean-up method maybe hot water washing, steaming, solvent circulationand wash, or whatever means have been deemedappropriate for cleaning the stabilizer. This may be avery costly operation, especially if the tower isnormally run at full capacity and it is not possible tomake up the production losses with the availableequipment. If it is known a priori (i.e., during theprocess design step) that asphaltene flocculation wouldbe a very distinct possibility, a number of provisionscould be made in the original design to either preventthe asphaltene problem or else permit stabilizercleaning without production losses (e.g., provide over-capacity in the stabilkr and storage facilities to permitmaking up for the lost throughput or provide a secondstabilizer running in parallel). Whatever the decision,in general, it is much cheaper to provide these extrafacilities during the original design and constructionthan any time after corninissioning and startup.

Similar reasoning applies to other processingequipment, i.e., heat exchangers, pumps, heaters,filters, etc... A frequent problem with pumps isplugging of the cyclone separator that supplies fluidfrom the discharge to the pump seals, hence, resultingin premature pump failure. Also suction strainers plugup very often and, as a result, onstream redundancy inthis case may be justifkd. There is virtually no limit tothe number of design changes and provisions thatcould be made to alleviate the asphaltene problem, if itis known a priori that asphaltenes will be flocculatingduring the production and processing of the crude oil.It was suggested recently [31] that asphalteneflocculation may be the main culprit of crude unit heatexchanger train fouling. Thus, eliminating orminimizing asphaltene flocculation in this heatexchanger train could result in hundreds of thousandsof dollars annual savings for the petroleum refiners.

ASPHALTENEPHYSICALMODEL

General Considerations

As it was demonstrated in the previous sectionthe asphaltene problem is very serious and real and,with the trend of the oil industry toward heavier moreasphaltenic oils, bound to get worse. The mostimportant and nagging question in the minds of mostresearchers of the asphaltene problem (and certainly tothe author) is what is the true state of asphaltenes inthe original oil? or how do asphaltenes exist in the oilbefore any attempt is made to separate them?

The answer to the above question is very crudalin our quest to understanding and predicting the phasebehavior of asphaltenes. It would be the ultimate ofgood luck if one were to develop a model that predictsasphaltene phase behavior without having accurateknowledge of how asphaltenes exist in the oil and themechanism by which they flocculate. Hence, oneshould start by asking what appears to be a pertinentquestion at this point, what is known to date about thenature of asphaltenes in the original oil?

There has been a number of experimente.~ sincethe 1930’swho tried to provide an answer to the abovequestion [4,6,7,8,9,10,11,12,13, 14,24,25,32, 33s4351.Unfortunately, no one has been able to demonstrateconvincingly and beyond any reasonable doubt what isthe state of asphaltenes as they originally exist in oil.Nevertheless, the information compiled so far by theplethora of researchers on the topic of asphaltenes doespoint in a certain direction that suggests the “mostprobable” state of asphaltenes in the original oil.

Some of the most important facts on asphaltenesgenerated over the years by various experimentersreferen~d above are

i. Asphaltenes have an intrinsic charge that maybe positive or negative depending on their oilsource. If asphaltenes are placed under the

6. Asphaltene Deposition: A Comprehensive Description of. . . ..- . SPE 1-l’romem Manifestations and Modeliruz Ammoaches

*’–-––

influence of an external ekwtrical field they aspha!tenea in their natural state are polydisperse.migrate to the oppositely charged electrode. Pfeiffer and %al depicted asphaltenea in their original

ii. Resins act as peptizing agents for the state as monodisperae, although they did raise theasphaltenes, They are highly polar and, hence, question of polydispersity. Many recent investigatorsattracted by the charged asphaltene kernels (i.e., believe that asphaltenes in the original oil arernioellecenters). They concentrate around the pdydisperse [8,9,19a#2647#34].surface of the asphaltenes forming a protectivelayer, Resins and asphaltenes together, as The short range steric intermolecular repulsivedescribed above, may be trolledmicelles. These forces between resin molecules adsorbed on differentmicelles are separate molecukwentities of the asphaltene kernels keeps them from flocculating.crude oil and subject to all thermodynamic There is also another weaker and of longer rangechanges the rest of the components undergo. repulsive force present between the asphalteneHowever, the addition of an adequate amount particles because of their similar electrical charge, Thisof a flocculant, e.g., a paraffin like n-pentane, repulsive force can be overcome or neutralized bycauses partial or total destruction of the rrdcelle mechanical (large pressure drop, agitation) or electricaland may result in irreversible flocculation of the (opposing streaming potential) means.asphaltenes,

iii, Substantially high streaming potentials have Since the asphaltene-resin-oil system appears tobeen measured between the inlet and outlet of be a colloidal suspension, as Figure 3 indicates, it istubes and corepluga through which asphaltene behooving to investigate whether colloidal sciencecontaining oils or solutions have flowed. In one techniques can be applied hereto predict the stability ofcase, the generated streaming potential was the system. To date, there exist two theories thatconsidered responsible for flocculationof the account for the stability of colloidal dispersiona, thesuspended asphaltenea [241. “DLVO Theory” and the “5teric Stabilization Theory”

iv. Both asphaltenes and resins exist in the oi! in a [28,291.The DLVO theory is applicable to electrolyticpolydisperse state and, as a result, molecular systems, where stabilization is due to the electricalweight distributions are used to describe them. double-layer. The steric stabilization theory is

v. Asphaltenes and resins have a tendency to applicable to systems where the dispersed phaseaggregate with each other. The degree of (particulate phase) is stabilized by repulsiveaggregation depends on the oil composition (i.e., interactions between molecules adsorbed on thesolvent power of oil). surface of the suspended particles, Based on what is

Although a number of other experimental known to date about the mture of the asphaltenes andobservations and conclusions have been made the other constituents of the petroleum fluids it is believedabove five appear to be accepted by the overwhelming that the colloidal asphaltene dispersions in oil belongmajority of asphaltene reaeiychers. Hence, a physical in the latter (steric) category.model describing the state of asphaltenes in oil mustincorporate and explain the above five observations. However, the young steric stabilization theory

does not offer as of now any means to produceQnerallv Ameed As~halten e Phvsical Model quantitative results. As a result, either the steric

stabWzation theory would have to be extended or aIn this section a physical model of the asphaltene new theory permitting full quantitative treatment of

resin oil system is described. This model is a natural the asphaltene-resin-oil system would have to becultivation of the five observations discussed in the

$gproposed.

previous section, The basic concept embodied in themodel is that asphalt~es exist in the oil in colloidal AJYT C L MODELS OF ASPHALTENEsuspension, stabilized by resins adsorbed on their %surface (see Figure 3). Pfeiffer and S@ [36], in 1940,were the first researchers to propose an asphaltene To date, there have been two differentphysical model for bitumens similar to the one shown fundamental approaches to the development ofin Figure 3 (although Nellesteyn (37) was first to analytical models predicting asphaltene flocculation.propose the colloidal nature of bitumens). Later, One is the “Molecular-Thermodynamic” approachWitherspoon et al. [13] contended that baaed on their [25,26,381 and the other is the “Thermodynamic-uhracentrifuge data of Illinois basin oils the Pfeiffer Colloidal” approach [20x71,The underlying principlesand Seal model for bitumens was applicable to oils as “ of each of these approaches are discussed next.well. More experimental work followed thatpinpointed to the applicability of the Pffeifer and Saal ]jolecular-1’hermodvnarnic A~r.woachmodel for asphaltenes in oil. Recently (20,27) themodel was revived and utilized to formulate a One of the first molecular-thermodynamicthermodynamic-colloidd model for predicting the modeling efforts for predicting the asphaltene phasephase behavior of asphaltenes. One important point, behavior, was published by Fussel [38]. ?usselnot fully explained by Pfeiffer and Saal, is whether attempted to describe the phase behavior of asphaltic

M

SPE 18892 K. J. Leontaritis 7

systems by using the equation of state approach. Her rather than being of a single size. The polydispersemodel, which was baaed on the Redlick-Kwong EOS, polymer theory ef Scott and Magat [42,43]was used inused a simultaneous calculation of vapor-liquid and place of the Flory-Huggins theory to calculate theliquid-liquid equilibria to predict the phase behavior of asphaltene-oil solid-liquid phase equilibria. One subtleasph.altic crudea. Fussel considered the flocculating difference between the two approaches is that in theasphaltenes as a separate “heavy liquid phase in Hirschberg et al. model the flocculation of asphaltenesthermodynamic equilibrium with the light liquid and is determined by an LLE calculation, whereas in thevapor phases. An inherent weakness of Fussel’s Kawanaka and Mansoori model the flocculation ofmethod is that the extended corresponding states asphaltenes is determined by a solid-liquidtheory (which is embodied in the R-K EOS) is known equilibrium, Hence, implicitly, Hirschberg et al.not to address accurately highly polar (i.e., asphaltene considered the asphaltene pkwe as a liquid whereasphase) and dissimilar in constituent size systems. Kawanaka and Mansoori considered it a solid,

Nevertheless, both in Hirschberg et al,’s and KawanakaFussel’s work was followed by Hirschberg et al.’s and Mansoori’s model the asphaltene flocculation

(25) proposal which considered the asphaltene entities process is necessarily taken as reversible,as monodisperse polymeric molecules, as a resultpolymer theories (i.e., the Flory-Huggins model) were The molecular thermodynamic approach toused to describe the “molecular” behavior of asphaltene flocculation is a well rounded techniqueasphaltenes. Hirschberg and his co-workers proposed a that utilizes the conventional thermodynamicmolecular-thermodynamic liquid model for describing methods for phase equilibria, The main handicap ofasphaltene phase behavior in reservoir crudes upon the method seem to lie in that it doea not take intochanges in pressure, temperature, and composition. account, sufficiently, the physical model that wasThe main concept which separates this model from the described in the previous section (2a) which has beencolloidal model is that asphaltenes are molecules derived over the years and ia based on actualdissolved in the oil like any ‘other molecule (see Figure experimental observations. Namely, this method doea4). The amount dissolved in the oil is a function of the not t*e into account the effect of resins on asphaltenethermodynamic conditions of the system. phase behavior. Furthermore, it treats asphalteneFurthermore, the process of precipitation and flocculation as a molecular reversible phenomenonredissolution is considered to be complete reversible which is opposite to the majority of availabledepending on the thermodynamic state of the system. experimental and field data. This model, it maybe said,As a result, conventional thermodynamic phase is more suitable to be app!!.d when predhking asphaltequilibrium methods may be utilized (possibly using phase separation th?.3 a tbaltene flocculation and.an EOS) to predict the phase behavior of asphaltenes in deposition.the liquid oil. This is similar to Fussel’s method exceptthat the Flory-Huggins theory [39,40] is utilized to Asphalt separating from crude oil is a heavyperform the liquid-liquid equilibrium calculation liquid phase, e.g., the asphalt phase obtained from thebetween the oil and asphaltene phases. propane crude deasphalting process. It has been

Hirschberg et al. instead of using a full three phaseproposed [l,ll&,44] that in asphalt (and bitumens ingeneral) the asphaltene-resin micelles are still in tact,

model (as Fussel had previously done) chose a i.e., not destroyed. Hence, hi crude oil deasphalting thecombination of a vapor-liquid and liquid-liquid model, asphaltene-resin micelles simply precipitate from theby assuming that VLEand LLEare independent of each original oil mixture essentially unaltered together withother. The liquid phase composition was calculated the intermicellar liquid (which contains oil andfirst by performing VLE calculations, using the Soave propane among other) as another “liquid phase. SinceEOS [41], and assuming no asphaltene precipitation. the micelles are still in tact and as a result act l% theThen, the Flory-Huggins model was used, based on a rest of the molecules and are soluble in oil, addition ofnumber of simplifying assumptions, to calculate the oil to this asphalt liquid phase (i.e., changing itsamount of asphaltenes precipitated from the liquid oil composition, or even its temperature and pressure) isphase, provided (i.e., assuming) that the newly formed expected to result in complete mixing (44). Hence, theasphaltene phase does not affect the vapor-liquid process of asphalt precipitation as a separate phase mayequilibrium phase split. This assumption may be too be considered as being very close to reversible,severe, especially for highly uspludticsystems in which especially near the onset of flocculation. This may bethe liquid oil composition changes drastically after the case when a heavy second liquid phase reportedlyflocculation of asphaltenes. separates reversibly in high pressure cells when oil is

mixed with light paraffins, C02. etc.An apparent improvement to Hirschberg et al.’s

approach was proposed by Kawanaka and Mansoori As it will be made evident in the next section the[341. Their contribution was to extend Hirschberg’s reversibility or irreversibility of asphaltenemethod to the so called “polydisperse” case, where flocculation depends on the state of the asphalteneasphaltenes are considered to be a component of the resin micelles during flocculation. If the micellescrude oil that has a molecular weight distribution separate from the oil essentially as they are in their

-

8 Asphaltene Deposition A Comprehensive Description of SPE 1SS92Problem Manifestations and Modelimz Arwroaches

original state then the flocculation process may bereversed by simply changing the thermodynamic stateof the system (i.e., by changing the temperature,pressure, and composition). As mentioned before, hithis case, it should rather be more suitable to talk ofasphalt phase separation than asphaltene flocculation.

Thermodvnamic<ol16idal Amx’each

Recently, based on the colloidal nature ofasphaltenes, a thermodynamic-colloidal model (T-CModel) of asphaltene flocculation was proposed, asdepicted in Figure 5, for predicting the phase I#taviorof asphaltenes in crude oil and its derivatives [20,271.The basic principle behind the formulation of the T-CModel is that the transfer of peptizing agents (i.e.,resins) from the asphaltene phase to the oil phase andvice versa is responsible for the aggregation ofasphaltene micelles and their flocculation into largerentities which causes them to drop out of suspension.The highly polar centers (i.e., kernels) of theasphaltene micelles have a natural tendency to attracteach other (i.e., aggregate), grow in 8* (i.e.~flocculate)~and, as a result, drop out of soiution or suspension.

However, the outer layer of an asphaltene micelle(i.e., the peptizkg layer or resins) serves as a stabilizerwhich due to steric repulsion with the outer layer ofdifferent rnicelles keeps it from flocculating (see Figure6). As Pfeiffer and Saal imagined 50 years ago, there is asmooth transition in terms of physical and chemicalproperties from the inner side of the peptizing layer tothe outer edges. For example, the highly polar resinmolecules tend to concentrate at the inner side of thepeptizing layer, whereas the less polar outside.

The size of the asphaltene micelle and thethickness of the peptizing layer at the onset depends ona number of factors, e.g., nature of asphaltenes andresins, composition of oil (or solvent power of oil),temperature, pressure, and other, Whether theasphaltene micelles remain peptized or flocculatedepends on the solvent properties of the oil at theprevailing thermodynamic conditions. A pertinentquestion at this point is, at the existing equilibriumdistribution of peptizhtg agents between the asphalteneand oil phases is the solvent power of the liquid oilphase large enough to keep the asphaltene micellesfrom flocculating? or, in different terms, are the stericrepulsion forces between the asphaltene micelle outerlayers large enough to oppose the attractive forcesbetween the inner and highly polar micelle centers(i.e., he kerneh)? The pr-s of flocculation may bedescribed as follows (see Figure ~

a. As a certain crude oil or bitumen sample istitrated against a flocculant (i.e.,n-heptane) twoimportant changes with respect to asphaltenemicelle flocculation are occuming

i. the solvent power of the oil changes in away that the outer layers of peptizingagents are transferred to the oil phase.

I

ii. the asphaltene mioelle overall polarity (orsromatiaty) or tendency to flocculateincreases because the oil phase isbecoming less polar (or less aromatic)whereas the asphaltene phase is becomingmore polar (or more aromatic).

b. At the onset of asphaltene flocculation the outerlayer of the micelle is stripped deep enough andthe solvent power of the oil has been Iowerwl tothe point that the altered micelles may nowattach to each other grow in size and drop out ofsuspension.

It is evident from the above description that themicelle size at the onset of flocculation is a function ofthe flocculant used during titration. The original T-CModel, in order to keep the mathematics simple,treated the nature and size of the asphaltene micelle atthe onset as a function of temperature only. Thisassumption may be acceptable for flocculant likenormal heptane and heavier paraffins with which themaximum amount of flocculated asphaltenes isessentially constant. For flocculant equivalent tolighter paraffins, however, the nature and size offlocculated asphaltene micelles is a function of bothtemperature and type of flocculant and as a result theassumption needs to be relaxed, In the important caseof predicting asphaltene flocculation during gas EORthe miscible injectants often used are equivalent tonormal paraffins lighter than heptane. As a result, -inthese cases the T-C Model would require as inputexperimental onsets of the oil with lighter paraffins aswell (i.e., n-C3, n-C4, n-CS), in order to relax theassumption that the micelle size is independent of theflocculant.

The T-C Model has been described in more detailpreviously in the referenced publications of theauthor. In brief, the model utilizes conventionalmacroscopic thermodynamb and the equation of stateapproach to perform VLE equilibrium calculations forestablishing the liquid phase from which asphaltenesmay flocculate. Next the model uses thethermodynamic equality of the chemical potentialsbetween phasea at equilibrium to determine the split ofpeptizing agents (i.e., resins) between the oil andasphaltene phases, Speafically, at equilibrium, !~~chemical potential of the resin in the oil phase is equalto its chemical potential in the asphaltene phase, i.e.:

AaphakonaPhaw OHPh-

~R@n = kaaifl 1

In the T-C Model the chemical potential of theresin in the 011phase is calculated by considering theresins as large polymeric molecules, hence, invokingthe Flory-Huggins statistical thermodynamic polymertheory [25#9#1. The following equation is baaed onthe above theory

I.-

. 7

~MR Mn- (PR)r,fon the asphaltenes and cause them to flocculate[4,1440,Z23,24XI. In those cases where flocculation of=ln(@)n+l-~+Xn

m= RT 2 asphalt~zweoccurs without significant thermodynamicchange in the produced fluids, such as in early stages of

Where @mvolume fraction of resin in the liquid primary production with small drawdown, asphaltenedeposition caused by electrokinetic eff- is possible.Vn,VM,molar’volume of resin and liquid

mixture respectively

~ = (vfi~(&&)2, Hory-Hu@Minteraction parameter CONCLUSIONS AND RE~ED FUT~& volubility parameter RESEARC~

Equation 2, which is derived from the F1ory- It should be evident that predicting asphalteneHuggins theory, is based on a number of assumptions, phase behavior is a very difficult undertaking.such as: However, as often emphasized in this paper and in

previous literature, the importance of the task ofi. a monodisperse polymer solution behaves identifying the state of asphaltenes in the original oil

like a chain, i.e., having mobile segments of and the mechanism of flocculation as they relate toequal size to and behaving as a solvent asphaltene phase behavior prediction cannot be over-

emphasized. It is clear that selecting the molecular ormoleculeii. the lattice theory of liquids is invoked colloidal physical model of asphaltene flocculationiii. the polymer is amorphous (i.e., non- leads to entirely different scientific methods or

qtalline) approaches for pre@cdng asphaltene phase behavior. Ifiv, the system is athermal (i.e., mixing of solvent Pffeifer and Saal’s original physical model depicts

and polymer is without energy effects) correctly the state of asphaltenes in petroleum then theT-C Model is bound to give more accurate and realistic

It should be noted here that the main reason for asphaltene phase behavior predictions. If asphaltenesusing the Flory-Huggins model to do the resin exist in oil in molecular state then molecularchemical potential calculations is simplicity. In fact, thermodynamic models would make more accurateutilizing a polydisperse polymer theory, e.g., the Scott predictions.and Magat theory, may prove to be more accurate,assuming that the required input data of the model Hence, the accuracy of the mathematical modelscould be obtained accurately. Another possible choice is described in this paper d~ds lirgely not only onthe use of a cubic equation of state. This, however, is their intricate details but on how accurately thenot recommended because cubic equations of state are physical models they are baaed on reflect the real statenormally based on the extended corresponding states of asphaltenes in petroleum as well, That is why at thistheory which is known not to be accurate for systems time it is more important to work diligently towardwith heavy, highly dissimilar, and polar establishing the real state of asphaltenes than do moremolecules.l%e choice of the method for calculating the mathematical modeling that is based on uncertainresin chemical potential is left to the individual foundations, This type of research appears to be moreengineer. Not enough work haa been done in this area of the basic type rather than applied and should beto tilt the balance one way or another, from the tuned toward generating high quality experimentalauthor’s viewpoint. For a more detailed description of data on asphaltene structure and phase behavior usingthe recommended procedure for calculating the onset contemporary analytical techniques,of asphaltene flocculation the reader is directed to theoriginal publication of this work [20Z71. As it has been hopefully demonstrated in this

paper the asphaltene deposition problem is real andSince asphaltenes are known to carry an intrinsic has affected seriotily the economics of petroleum

similar charge [4,14,20,21,22Z3,24,27],their colloidal recovery, production, and processing worldwide. Withsuspension stability should be influenced by the the expected trend of the petroleum industry towardstrength of this charge. Figure 8 depicts the process heavier and more asphaltic crudes the economics ofthrough which the charge of asphaltenes could petroleum recovery, production, and processing isinfluence the onset of asphaltene flocculation. Since bound .to get worse, The economic recovery of oil fromthe opposing electrical force is considered to& weak some reservoim, even during primary pn?ductiow hasaqd long range, it is not expected to play a role in the been seriously challenged by the asphaltene depositionflocculation process if the system is far from the onset. problem. The petroleum industry is being forced toHowever, if the system is close to the onset it is direct more attention and capital to the research ofpossible to have flocculation caused exclusively by asphaltene deposition because of the threat it poses toelectrokinetic effects. That is, generation of streaming the economic recovery of oil. Those companies thatpotential due to flow large enough to cancel the charge take the lead in this process will be affected the least by

the adversities of the problem. The impact of the

10 Asphaltene Deposition: A Comprehensive Description ofProblem Manifestations and Modeling Approaches. .

solution(s) to asphaltene deposition wil! be industry- Engineering, p. 229, Aug. 1988,Elsevier Publisherswide affecting the economics of oil from the reservoir B. V., Amsterdam, The Netherlands.all the way to its final destination,

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SPE 18892 K. J. Leontaritis 11

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of The Society of Petroleum Engineers, Dallas,Texas,Sept. 27-30,1987.

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r

flg.1 . in.Siiu Aaplwllorn IM@llon

s= 18$92

Q,.:,‘/,

;.j ‘.,,.,. 0,~,.,. ‘/

, ‘..,. ‘.,

h~’4

,,‘/’, ,

,,, ‘ .’. ,

/ ‘,:,,

,’,,\

;:;::.

Fig.3- Pep!izatlon of Asphaltenes

W&f

gmln

-011

Fig. 2. AaplUIIOIWSsdsettwd on tha rock cuiaa wtbbllify chm?gos

~- .

o-b.,,..:..

With Resins

.

I‘/ql\, a.;’

0–0 --0 lo\

0 -!-Q I -*

Fig. 4- Molaoular-lhorrnodyMmlc Model

%/

.

WE 18892

---- ---- -.

LegmkO rninmoleculeg

● uomtk molecules

— p~c, naphthcnic, and olefinic molecules

&@akllepallicl.orKemd9

Fig. 5- Thermodynamic-cdioiddModel

Fig.6 - Asphaltmra-l?esin Micello

SPE18892 “

p=-!?!?

La4~%FA- \

Fig. 7-The addition of ● ffoooufant cauae8 thinning oftha paptlzlng Iay.r wound the aapfraltanawaain mlcalle.

A

B c

4D E

F24