reversibility of asphaltene flocculation with chemicals

Reversibility of Asphaltene Flocculation with ChemicalsPriyanka Juyal,* Vickie Ho, Andrew Yen, and Stephan J. Allenson

Nalco Energy Services, 7705 Highway 90-A, Sugar Land, Texas 77478, United States

ABSTRACT: Restabilization of asphaltenes after they precipitate out in the production systems can be of great industrialsignificance with respect to remediation. Here, we discuss the redispersion of asphaltenes in the hydrocarbon phase aided bytreatment chemicals in the laboratory using the asphaltene dispersancy test (ADT), Turbiscan, and modified ADT method.Through extensive laboratory experiments on a suite of dead crude oils and treatment chemicals, redispersion and restabilizationof asphaltenes was observed upon treatment with asphaltene treatment chemicals, even when the destabilized system comprisingof untreated crude oil in heptane was allowed to age for extended periods. We infer that precipitated asphaltenes can beeffectively restabilized by the treatment chemicals. This could have potential applicability in mitigating asphaltene challenges,specifically in topside crude oil production facilities.

■ INTRODUCTIONPhase transitions that lead to precipitation of asphaltenes dur-ing crude oil production are a critical concern from operationaland economic standpoints. Phase separation of asphaltenes canpresent severe problems with the production and recovery ofcrude oils and also during refining and upgrading operations.Near the well-bore, destabilized asphaltenes are known tocause formation damage by clogging the pores of the crude oilformation. Flocculated asphaltenes deposit onto the pumps andthe tubings, thereby restricting flow and decreasing crude oilproduction. Precipitated asphaltenic material may also collect atthe oil−water interface in surface equipment in the produc-tion handling facilities, such as free-water knockouts and heattreaters, causing the formation of stable emulsions that makeoil−water separation extremely difficult. When asphaltenessettle at the bottom of the treaters and contaminate the dis-charge water, they create a unique disposal challenge. Asphaltenesoften deposit in stock tanks, lowering the volume capacity andjeopardizing the discharge line.Because of polydispersity associated with asphaltenes, they

are defined in terms of their solubility as the fraction of crudeoil that is soluble in aromatic solvents, such as toluene, andinsoluble in aliphatic solvents, such as heptane.1−6 Precipitationtakes place when the crude oil loses its ability to keep thoseparticles dispersed, which is influenced by several factors, suchas variations in temperature, depletion of reservoir pressure,and/or changes in the bulk solution composition, and alsoduring enhanced oil recovery operations.7−10 For example, witha reduction in the pressure, the relative volume fraction of thelighter components in the crude oil increases, thereby destabi-lizing the asphaltenes.11 Similarly, compositional changes in-duced by commingling crude oil streams from different wellscan cause asphaltenes to precipitate out.The issue whether asphaltene precipitation is reversible or

not has been variously approached.11,12 The issue has been ex-tensively researched, with well-supported evidence in the pub-lished literature in favor of and also arguments against thereversibility of asphaltene precipitation. Various thermodyna-mic models derive from asphaltene flocculation to be completelyreversible, whereas colloidal models predict otherwise.13−16

Nevertheless, a better understanding of the reversibility of as-phaltene precipitation may contribute to the development ofimproved predictive models of asphaltene precipitation.17,18

Many researchers support partial to complete reversibility ofasphaltene precipitation, but some researchers suggested that itis not possible for the precipitated asphaltenes to go back intosolution. Andersen et al. presented experimental data confirm-ing partial reversibility of asphaltene precipitation.19,20 Uponthe addition of toluene to a mixture of crude oil and heptanewith precipitated asphaltenes, Buckley et al. observed both theappearance and disappearance of asphaltenes over the samenarrow range of mixture refractive index (RI) values, indicatingcomplete reversibility of asphaltene precipitation for the crudeoil examined. Buckley et al. ascribe the difficulty in the determi-nation of the reversibility of the asphaltene precipitation pro-cess to the slow kinetics of dispersion and flocculation.21,22

On the basis of titration experiments, it has been shown thatprecipitated asphaltenes can be redissolved by the additionof a good solvent.23 However, the addition of a solvent is alsoreasonably considered as no indication of complete reversibilitybecause it is not the reverse process of the addition of theprecipitant.11,24 Hirschberg et al. assumed asphaltene aggrega-tion to be reversible but a very slow process.13 Hammami et al.through a series of isothermal pressure depletion experimentsconducted under live oil conditions, provided evidence of as-phaltene precipitation above and redissolution below thesaturation pressure, and concluded that pressure depletion in-ducing asphaltene precipitation is a highly reversible process.25

Joshi et al. determined the reversibility of the asphaltene floc-culation process for a selected crude oil by alternately collectingnear-infrared (NIR) spectra at high and low pressures.26 Theyobserved that, if the pressure on the sample is reduced from13 000 to 6000 psi and then increased back to 13 000 psiwithin minutes of the pressure drop, the spectral scattering

Special Issue: 12th International Conference on Petroleum PhaseBehavior and Fouling

Received: September 14, 2011Revised: December 30, 2011Published: January 8, 2012

Article

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© 2012 American Chemical Society 2631 dx.doi.org/10.1021/ef201389e | Energy Fuels 2012, 26, 2631−2640

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characteristic of destabilization is entirely eliminated, thus sug-gesting deflocculation or resuspension of asphaltenes. Wang et al.speculated that asphaltene precipitation is less likely to be rever-sible for crude oils well beyond the onset conditions.27 Partialreversibility or a hysteresis effect indicating slower kinetics ofasphaltene redispersion compared to the kinetics of flocculationhas been demonstrated by several researchers.16−20,28 However,an improved understanding of not just the asphaltene preci-pitation mechanism but also the reversibility of precipitation iswarranted for further improvements in predictive models andmitigation strategies and remediation chemicals.Chemicals provide cost-effective and invaluable solutions to

resolve deposition problems that plague the oil and gas produc-tion systems from production, through fluid transportationto topside operations.29−32 Because asphaltene deposition doesnot occur until after flocculation, polymeric dispersants weredeveloped that stabilize the asphaltenes and prevent floccu-lation. It is widely believed that these chemicals act in the samemanner as the resins and maltenes, interacting with asphaltenesand stabilizing the asphaltene in the crude oil. These asphal-tene treatment chemicals have a stronger association with theasphaltenes than the natural resins and maltenes and are ableto stabilize the asphaltenes through greater changes in pres-sure, temperature, shear, and chemical environment. Changand Fogler studied the stabilization of crude oil asphaltenes inapolar alkane solvents using a series of model alkylbenzene-derived amphiphiles as the asphaltene stabilizers and concludedthat asphaltenes from crude oil can be effectively dispersed inalkane solutions by the oil-soluble amphiphiles that adsorbstrongly to the asphaltene surfaces and establish a stable stericalkyl layer around asphaltene molecules that impedes asphal-tene molecular association and precipitation.33 In the fieldapplication, it is deemed important that these chemicals areadministered to the crude oil before the asphaltenes becomedestabilized and flocculation occurs.29,30 Reversibility of asphal-tene flocculation is crucial from a mitigation point of view.Application of oilfield chemicals in reverting asphaltene floccu-lation can be of tremendous value, especially in the off-shore oilproduction, where precipitation can have unfortunate con-sequences. However, to the best of our knowledge, there are nostudies on the reversibility or redispersion of flocculated asphal-tene in crude oil aided by chemicals.In this paper, we describe the laboratory analyses on the

destabilization and redispersion of flocculated asphaltenes incrude oils aided by commercial asphaltene treatment chemical.Stability analyses were performed on several unstable crude oilsusing the asphaltene dispersancy test (ADT)34 and Turbiscan.35

Performance of the chemical inhibitors was monitored on crudeoils both before and after asphaltene flocculation has occurred.ADT and Turbiscan methodologies were suitably modified toassess the effectiveness of the inhibitors in restabilizing floc-culated asphaltenes, when the treatment was administered fol-lowing destabilization and sedimentation. The performanceof the treatment chemicals in resuspending flocculatedasphaltenes in the presence of static water was also evaluatedto simulate the topside separator condition for a Gulf of Mexicoplatform.

■ MATERIALS AND METHODSCrude Oils and Chemicals. Two crude oil samples, one from the

Gulf of Mexico (reported as crude oil A) and another crude oil from aland-based well (crude oil B), were considered in this study.

Several chemicals were evaluated for their performance in reversi-bility of asphaltene precipitation. Here, we report results from chem-icals A and B, with more focus on chemical A. Chemical A, a highlysuccessful asphaltene treatment chemical, is nonylphenol formalde-hyde resin type chemistry. Chemical B is a polyolefin ester type chemistryin an aromatic solvent.

Saturates, Aromatics, Resins, and Asphaltenes (SARA)Analysis. Crude oil samples were submitted to Weatherford Labo-ratories (Shenandoah, TX) for SARA analysis. Asphaltenes were hep-tane (C7)-precipitated from the fluids following the IP 143 procedure.

ADT. ADT evaluates the presence of asphaltenes and their tendencyto precipitate in a crude oil. The test also enables determination of theefficacy of treatment chemicals. An aliquot of untreated and treatedcrude oil is added to ADT test tubes that contain 10 mL of heptane.Treatment was performed with 100, 250, and 500 ppm by volume (onthe basis of crude oil) of the chemical. The percent sedimentationbecause of gravity after 10, 30, and 60 min is observed and recorded.The sedimentation or precipitation is recorded in milliliters fromthe graduated ADT tubes. Where no precipitation is observed, the dataare recorded as “clear”, and where precipitation is observed but is notmeasurable, the data are recorded as “trace”. Subsequently, all of thesamples are centrifuged for 3 min at 2500 rpm. The transmission ofthe blank oil sample and treated samples are measured with a colori-meter (PC 910 colorimeter, Brinkman Instruments, Westbury, NY)and recorded as %Tblank and %Ttreated. The blank sample is shakenagain on the vortexer such that all of the asphaltenes are dispersed inthe solution and the transmission is recorded as 100% dispersion(100%D). Percent inhibition (%I) is calculated from transmissionmeasurements as per the equation

= −−−

×⎡⎣⎢

⎤⎦⎥I

T DT D

% 100(% 100% )(% 100% )

100treated

blank

The results for ADT were also photographically recorded.Modified ADT. For evaluating the redispersion ability of the chem-

ical product, the untreated aliquot of the homogenized crude oilsample was added to the ADT tubes containing 10 mL of heptane.Treatment with the chemical at various dosages followed either im-mediately or at various intervals (after aging for 30 min, overnight, andfor 1 week in the dark), following the destabilization and flocculation/separation of asphaltenes. In the case of overnight and 1 week of agingof the destabilized oil−heptane solution, the effect of additional stressinduced by centrifuging sample sets before aging on the performanceof the chemical with respect to reversing the flocculation was alsostudied.

In this way, the ADT methodology was modified and the additionof the chemical followed asphaltene destabilization in the crude oil todetermine if the asphaltenes would be redispersed and restabilized intothe bulk by the treatment. Percent inhibition (%I) was calculated aswith the ADT, and results were also visually followed and recordedwith photographs.

The scheme of the ADT and modified ADT experiments followedin this work is described with the flowchart reported in Figure 1.

Turbiscan. The samples were analyzed on Turbiscan for evaluatingthe effect of asphaltene inhibitors and aging on the crude oil, using aTurbiscan MA2000 from Formulaction (Davie, FL) that uses a pulsedNIR light source (850 nm). Turbiscan performs multiple light scat-tering to measure the average transmittance of a sample. It measuresboth the stability of the asphaltenes in a particular crude oil andthe relative performance of treatment chemical in that crude oil.A computer automates this technique, thereby diminishing the possi-bility of operator error. In a Turbiscan test, an efficient mitigationchemical will lower the percent transmission compared to the blankand remain relatively unchanged during the full length of the test.

■ RESULTS AND DISCUSSIONSeveral crude oils were used in the study to examine the effi-ciency of chemicals in redispersing asphaltenes in the oil phase,following destabilization by a flocculant. Table 1 reports

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American Petroleum Institute (API) gravities and SARA com-positional profile based on the topped oil for two of the crudeoils used in this work. Crude oil A is a medium oil, and crudeoil B is a light oil.ADT. ADT was performed on a Gulf of Mexico crude oil

(crude oil A) and chemical A as the treatment chemical, asdescribed in the Materials and Methods. For the treated sam-ples, chemical A was added via a micropipet to the crude oilsample and the sample was shaken on the vortexer, followedby heating for an additional hour at 60 °C before injection intothe ADT tube. A total of 100 μL of the untreated and treatedcrude oil A was injected into 10 mL of heptane. Table 2 reports

sedimentation because of gravity recorded at 10, 30, and 60 minof the ADT duration for the blank crude oil A and the crude oiltreated with chemical A.From Table 2, it can be observed that no precipitation was

observed at all treatment rates and significant percent inhibition(%I) is achieved relative to the untreated crude oil. %I in-creased with an increasing dosage of chemical A. Thus, it isevident that chemical A is effective in stabilizing asphaltenesagainst precipitation for crude oil A at all of the treatment ratestested.Standard ADT results for crude oil B treated with both

chemicals A and B is displayed in Table 3. It is evident thatboth of the chemicals are effective in preventing asphalteneprecipitation in crude oil B at all of the treatment rates applied.Modified ADT. To evaluate the effectiveness of a chemical

in controlling asphaltene precipitation with ADT, the chemical

is applied to the crude oil prior to injection into heptane. Todetermine if the chemistries used in asphaltene control couldalso be effective in reversing the flocculation of asphaltenes and,thus, redispersing and restabilizing the crude oil, we modifiedthe ADT procedure as described in the previous section. Thecrude oil was destabilized by the addition to a large excess ofheptane, followed by the addition of the chemical to the solu-tion. The treatment was administered immediately, after agingfor 30 min, overnight aging, and also after 1 week of aging inthe dark after the crude oil was injected into heptane (Figure 1).A total of 100 μL of homogenized crude oil A sample was

added to the ADT centrifuge tubes containing 10 mL of hep-tane. The asphaltenes were allowed to precipitate with gravity,followed by injection of various dosages (100, 250, and500 ppm) of the chemical, such that the treatment with chem-ical A followed asphaltene destabilization in the crude oil. Thechemical injection followed immediately after the addition ofthe crude oil sample to heptane, and the dosage was basedon the quantity of the crude oil plus the heptane solution.Thus, for the modified ADT experiments, the chemicals werelikewise diluted in toluene to make the effective concentrationof the chemical the same as for the conventional ADT. Thus,1% dilution of the chemicals in toluene was prepared, and crudeoil + heptane solution was treated with 100, 250, and 500 ppm

Figure 1. Scheme of the experiments followed to evaluate the performance of the chemical in stabilizing asphaltenes (ADT) and in reversing theflocculation of the asphaltenes (modified ADT) when treatment was performed immediately or at various intervals after destabilization.

Table 1. API Gravities and Relative Percentages (by Weight) of SARA-Fractionated Components of the Topped Crude Oils

sample source API saturates aromatics resins asphaltenes

crude oil A Gulf of Mexico 27.5 40.41 42.06 12.21 5.32crude oil B Wyoming 35.0 59.56 32.76 6.95 0.73

Table 2. Standard ADT Results for Crude Oil Aa

sampledosage(ppm)

10 min(mL)

30 min(mL)

1 h(mL) %T %I

crude oil A 0 2 1.5 1.3 62.2 0

chemical A100 clear clear clear 41.4 63250 clear clear clear 30.8 95500 clear clear clear 29.7 99

a“Clear” refers to no sedimentation observed.

Table 3. Standard ADT for Crude Oil B with Chemicals Aand Ba

sampledosage(ppm)

10 min(mL)

30 min(mL)

1 h(mL) %T %I

crude oil B 0 0.1 0.5 0.4 71.1 0

chemical A100 trace trace trace 41.4 56250 trace trace trace 30.8 72500 trace trace trace 29.7 82

chemical B100 trace trace trace 59.9 57250 trace trace trace 56.4 75500 trace trace trace 54.9 83

a“Trace” refers to visual observation of asphaltene particles but notmeasurable.



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of the diluted chemical, to match the total concentration withthat for conventional ADT. To check that the reversibility offlocculation was not a result of extra toluene added with thechemical in terms of the solvency that it would impart to thesolution for asphaltenes (although only a few parts per millionof toluene), we conducted modified ADT experiments withtreatment with 1000 ppm of toluene and xylene. Treatmentwith these solvents was performed 2.5 h after destabilization ofthe crude oil with heptane. The percent inhibition for the blanksample and the samples treated with 1000 ppm of toluene andxylene remained unchanged; that is, these aromatic solventsprovided no inhibition against flocculation (data not reportedhere). Thus, it was evident that the reversibility of asphal-tene flocculation was because of the dispersant action of theinhibitor. ADT results for this approach when the treatmentwas rendered immediately upon destabilization of crude oil Aare listed in Table 4. It is apparent from Table 4 that chemical

A is highly effective in restabilizing asphaltenes in the crudeoil A sample, even when the treatment followed after thedestabilization step in the modified ADT.Modified ADT results for crude oil B sample with chemicals

A and B are summarized in Table 5. A total of 200 μL of

homogenized crude oil B sample was added to the ADT cen-trifuge tubes containing 10 mL of heptane. The chemical injec-tion followed immediately after the addition of the crude oilsample to heptane.Thus, modified ADT results for crude oil B from Table 5 also

confirm the effectiveness of the chemicals in redispersing theasphaltenes, when treatment was deployed post-destabilization.Chemical A gave consistently better performance compared tothe other chemicals used in this study; thus, we have focusedmore on chemical A in the rest of the paper.Next, we delayed the treatment with the chemical to 30 min

after destabilization. A blank crude oil sample was added to fourADT test tubes containing 10 mL of heptane, and the mixturewas shaken vigorously and allowed to stand for 30 min to faci-litate asphaltene flocculation and precipitation. The test tubeswere observed at 0 and 30 min (panels a and b of Figure 2).Subsequently, the test tubes were labeled blank, 100, 250,

and 500 ppm (from left to right). Chemical A at the dosage

rates of 100, 250, and 500 ppm was added to the test tubes aslabeled. The ADT tubes were vortexed for 10−15 s andvisually observed for asphaltene instability as reflected fromsedimentation at 10, 30, and 60 min from the addition of thechemical. Figure 3a shows ADT photographs for the testtubes at the end of 0 (treated and shaken), 10, 30, and 60min after the addition of the chemical. Figure 2b is thestarting reference for this step. It can be clearly deducedfrom the visual observation in Figure 3a that chemical A atall dosages is effective in redispersing and stabilizing thealready precipitated asphaltenes for crude oil A, as reflectedfrom no sedimentation in the sample tubes at the end of60 min for the treated samples.In the next modification of the ADT procedure, the severity

of the sedimentation was increased by centrifugation, followedby storing the test tubes overnight. Two sets of four ADT testtubes labeled blank, 100, 250, and 500 ppm (from left to rightin Figure 3) were prepared. A 100 μL aliquot of crude oil wasadded to all eight test tubes containing 10 mL of heptane. Thetubes were shaken thoroughly, and sedimentation was observedafter 10, 30, and 60 min from the injection of the crude oil intoheptane. Asphaltene precipitation can be observed from theADT test tubes as time progresses, and there is significant pre-cipitation at the end of 60 min. At the end of 60 min, one setwas centrifuged for 30 min at 2000 rpm. The visual observa-tions for both of the sets of test tubes before the addition of thechemical are depicted in panels b and c of Figure 3, labeledas “blank” and “blank + centrifuge”, respectively. The precipita-ted asphaltene layer becomes significantly compact followingcentrifugation (blank + centrifuge).The kinetics of the precipitation was further increased by

storing the destabilized samples for 1 week in the dark beforethe treatment was deployed (Figure 3d). This test was alsoperformed on two sets of test tubes, and the second set wassubjected to additional stress by centrifugation for 30 min be-fore storage for 1 week in the dark (Figure 3e). This way, theasphaltene sediment was allowed to age, and the efficacy ofchemical A treatment on the aged deposit and under increasedstress was determined.Visual observation of the photographs in panels a−e of

Figure 3 asserts the effectiveness of chemical A in redispersingand restabilizing the flocculated asphaltenes. However, someprecipitate is observed at the end of 60 min for both sets of testtubes that were centrifuged before overnight and 1 week ofextended aging in the dark. It seems that centrifugation fol-lowed by aging causes congealing of the precipitate, such thatthe physical state of the centrifuged asphaltene is differentrelative to the non-centrifuged asphaltene. This diminishes theeffectiveness of the inhibitor, probably because some fractionof the congealed asphaltene is not readily available to interactwith the inhibitor. Additional stress might cause the formationof larger asphaltene aggregates that may require increased

Table 4. Modified ADT for Crude Oil A with Chemical A

sampledosage(ppm)

10 min(mL)

30 min(mL)

1 h(mL) %T %I

crude oil A NA 2 1.5 1.3 62.2 0

chemical A100 clear clear clear 33.4 88250 clear clear clear 31.2 94500 clear clear clear 31 95

Table 5. Modified ADT for Crude Oil B with Chemicals Aand B

sampledosage(ppm)

10 min(mL)

30 min(mL)

1 h(mL) %T %I

crude oil B NA 2 1.5 1.3 71.1 0

chemical A100 trace trace trace 53.7 89250 trace trace trace 53.6 89500 trace trace trace 52.7 94

chemical B100 trace trace trace 54.1 87250 trace trace trace 53.9 88500 trace trace trace 53.2 91

Figure 2. ADT for crude oil A (untreated) at (a) 0 min and (b)30 min.




agitation to break to be available to interact with the treatmentchemical, restabilize, and restore to the original state by thechemicals. For both of the cases where centrifugation was

performed before aging, it appears that the precipitatedasphaltenes are only partially restored to the solution upontreatment with the chemical.

Figure 3. Modified ADT results for crude oil A when the chemical was injected following destabilization: (a) 30 min after destabilization, (b) afterovernight aging, (c) after centrifuge and overnight aging, (d) after 1 week of aging, and (e) after centrifuge and 1 week of aging. Test tubes in eachset are labeled blank, 100, 250, and 500 ppm from left to right.

Figure 4. Dosage response chart relative to conventional ADT for crude oil A when treatment with chemical A was administered post-destabilization.





The dosage response chart shown in Figure 4 reports thepercent inhibition from all of the ADT and modified ADTconducted for crude oil A. The chart highlights the response tothe chemical when the treatment was administered before de-stabilization as per standard ADT and at varying intervals afterdestabilization. Chemical A, at all treatment rates, success-fully redisperses and stabilizes the precipitated asphaltenes forcrude oil A but, interestingly, not to the same extent. Observedtrends for percent inhibition for samples treated with 250 and500 ppm of the inhibitor suggest time dependence of asphal-tene reversibility or redissolution. It is observed that percentinhibition reduces with increased storage before the treatmentwas administered. Percent inhibition for sample sets that werecentrifuged is also lower than their counterparts that were notcentrifuged before storage and treatment. The trend alsoindicates partial reversibility of asphaltene flocculation for whenthe treatment was administered after the destabilization undermodified ADT conditions relative to conventional ADT. Thus,the dosage response chart reinforces the visual observationsfrom Figure 3.To further verify the results from ADT, we repeated a few

tests with Turbiscan analysis. The Turbiscan is used as anautomated ADT and uses multiple light scattering to measurethe percent transmission of a sample. It measures both thestability of the asphaltenes in a particular crude oil and therelative performance of asphaltene inhibitors in that crude oil.

As the oil is destabilized, the transmission through the sampleincreases, most likely because of particle flocculation in theoil and, consequently, flocculation and sedimentation of asphal-tenes. The rate by which the transmission changes gives ameasure of how quickly the oil is destabilized; i.e., the fasterthe transmission increases, the less stable is the oil. For treatedsamples, if the chemical is effective, it will keep the asphalteneswell-solubilized in the solution and prevent precipitation, whichwill keep the average transmittance unchanged. However, ifthe chemical is ineffective, the asphaltene particles will floccu-late and precipitate, resulting in a rapid increase in the averagetransmission.Turbiscan data from Figure 5 show the stability trends for

untreated crude oil A and the effect of treatment with chemicalA. It can be easily inferred that chemical A effectively rendersstability to crude oil A at all of the treatment rates employedhere.Results shown in Figure 6 are for the crude oil A sample in

heptane that was allowed to sit for 30 min. As with the modi-fied ADT, in the case of Turbiscan analysis, prior to treat-ing with the chemical, 100 μL of crude oil was added to theTurbiscan tube containing heptane to destabilize the asphal-tenes. With this method, unstable asphaltenes would precipitateout over 30 min. The chemical was subsequently added at thevarious treatment rates, and tubes were shaken before Turbiscananalysis. It is evident from Figure 6 that the chemical, at all

Figure 5. Turbiscan graphs of untreated and treated crude oil A.

Figure 6. Turbiscan graph for destabilized crude oil A in heptane. Chemical injection followed 30 min after the addition of crude oil into heptane.





treatment rates, effectively redisperses flocculated asphaltenesinto the crude oil solution. The results from Turbiscan are thusin qualitative agreement with ADT results.Effect of Hand-Shaking versus Vortexing after

Chemical Treatment. Experiments were carried out to exam-ine the impact of hand-shaking or vortexing, after the chemicalwas injected to the destabilized and aged crude oil−heptanemixture. This was performed to assess the influence of themethod of mixing on the performance of the chemical in re-dispersing the flocculated asphaltenes. The method of mixing islikely to enable an increased interaction between the chemicaland the asphaltene particles and facilitate effective restabiliza-tion of the flocculated asphaltenes. With this objective, two setsof two ADT test tubes, labeled blank and treated, wereprepared. ADT with a 100 μL aliquot of untreated crude oilinjected into 10 mL of heptane was conducted. The tubes wereshaken thoroughly, and sedimentation was observed after 10,30, and 60 min from the injection of the crude oil into heptane.The destabilized mixture was allowed to age for 1 day in thedark, followed by injected of 250 ppm of the chemical. One setof test tube was given moderate hand-shaking, 10 times, and theother set of the test tubes was vortexed at high speed for 15 s.The test tubes were then set for observation for the next60 min. The visual observations for both sets of test tubesbefore the addition of the chemical and after treatment arereported in Figure 7.Visual observation of the modified ADT in Figure 7 indicates

that hand-shaking shows less sedimentation in the untreatedADT tube for the hand-shaken sample. The tubes treated with250 ppm of the inhibitor reflect a similar performance of thechemical in terms of sedimentation at the end of 10, 30, and 60min duration of the test. The formation of relatively morestable emulsion with hand-shaking as compared to a fast shakeris reported by Allenson et al. and Lang et al. while evaluat-ing the effect of shear on the emulsion droplet size.36,37 Theyreport more than 3 times higher viscosity values and 1/3 smallerwater droplet size for the hand-shaken emulsions relative tothe fast-shaker-generated emulsions. From our modified ADTexperiments, we infer that hand-shaking provides as good ofmixing as vortexing and redisperses the flocculated asphaltenesto the same ability as with the fast vortexing.

Table 6 reports modified ADT results for the hand-shakingversus vortexing. It can be inferred that both of the methods

provide comparable restabilization of the precipitated asphal-tenes as deduced from reasonably close percent inhibition data.

Crude Oil B. We repeated the ADT and modified ADTexperiments reported in the preceding section on anotherhighly unstable crude oil, crude oil B, sourced from a land-based well in Wyoming. The performance evaluation of chem-ical A and other chemicals with respect to asphaltene stabi-lization was tested with ADT and modified ADT. Visualobservations from ADT and modified ADT for this crude oilsample are reported in Figure 8. The photographs show thatchemical A shows a similar performance as for crude oil A inredispersing the precipitated asphaltenes and restabilizing thecrude oil B.

Redispersibility in the Presence of Static Water. Theplatform associated with crude oil A in the Gulf of Mexico doesnot produce much water. However, the separator holds staticwater, and over time, asphaltenes from the crude oil migrateto the oil−water interface, leading to the formation of an as-phaltenic pad layer that eventually settles down in the sepa-rator. Figure 9 shows the picture of extensive fouling because ofsettling of asphaltenes in the separator. To simulate separatorconditions and the formation of the interfacial layer, Turbiscananalyses were performed under static water conditions. Thetests were run for an extended duration of 30 h with 8 mL(80%) of either whole crude oil or destabilized crude oil inheptane and 2 mL of water (20%). For static conditions, 2 mLof water was injected into the Turbiscan tube, followed by theaddition of 8 mL of crude oil or destabilized crude oil in

Figure 7. Modified ADT results for crude oil A aged for 1 day. Impact of hand-shaking versus vortexing after chemical injection on the chemicalperformance.

Table 6. Modified ADT for Destabilized Crude Oil A: Hand-Shaking versus Vortexing

sample mixingdosage(ppm)

10 min(mL)

30 min(mL)

1 h(mL) %T %I

crudeoil A

hand-shaking

NA trace 0.1 1.6 61.6 0

hand-shaking

250 clear trace trace 33.2 61

crudeoil A

vortexing NA trace 0.5 1.6 62.8 0vortexing 250 clear trace trace 30.7 68




heptane. In another variation of the test condition, water andcrude oil were hand-shaken before Turbiscan. It was observedthat, under static conditions, no asphaltenes precipitated outand migrate to the interface, as evidenced by stable trans-mission in the Turbiscan graph and also from visual observationof the clean oil−water interface (no rag layer) in the Turbiscantube at the end of the 30 h test (results not shown here). Uponhand-shaking before Turbiscan, the oil and water form a verystable emulsion, which does not break out even at the end of30 h. Transmission remained stable for this emulsion throughthe 30 h duration of the test, indicating no asphaltene destabi-lization (results not shown). The stability of asphaltenes inthese tests could be attributed to the slow diffusion and kineticsof migration of the asphaltenes to the oil−water interface.However, in the real field conditions, the pad layer at the oil−water interface in the separator is formed over a significantlygreater length of time and frequent charge of fresh fluids, whendestabilized asphaltenes from the field could migrate to the

oil−water interface in the separators. In a small system, such asthe Turbiscan tube, the short duration of the test might beinsufficient for the asphaltenes to adsorb at the interface.To induce asphaltene destabilization and study their inter-

facial behavior, 100 μL of crude oil was injected into 10 mL ofheptane and allowed to stand for 2 days to precipitate theasphaltenes. Before initiating Turbiscan, the crude oil−heptanesolution was hand-shaken, and 8 mL were added to a Turbiscantube containing 2 mL of water. To evaluate the effect of thestabilizing chemical, the destabilized crude oil−heptane mixturewas hand-shaken, treated with 100 ppm of the chemical, andsubjected to Turbiscan in a Turbiscan tube containing 2 mL ofwater. The transmission through the glass vial was measuredevery 30 min, and the test was run for 30 h.Figure 10 reports the Turbiscan analyses conducted on de-

stabilized crude oil, untreated and treated with the stabilizingchemical. The destabilization and flocculation of asphaltenes foruntreated crude oil and the formation of the rag layer over timeare evidenced from the increase in transmission through thesample (top panel of Figure 10). Interestingly, a clean interfaceand stable transmission are observed throughout the length ofthe test for the destabilized sample when treated with thechemical (bottom panel of Figure 10), indicating the efficacy ofthe chemical in controlling the size of the asphaltene particles,preventing flocculation and eventual migration to the oil−waterinterface.Photographs of the Turbiscan tubes at the end of the 30 h

experiment reported in Figure 11 show that, for the untreatedsample, the asphaltenes start to flocculate and move to the oil−water interface forming a rag layer. However, for the chemical-treated sample, no rag layer is formed and a clean oil−waterinterface is observed. It seems that the chemical keeps the sizeof the asphaltene particles in control, keeping them well-dispersed in the oil phase and, thus, preventing their adsorption

Figure 8. Modified ADT results for crude oil B when the chemical was injected following destabilization: (a) ADT, (b) after overnight storage, (c)after centrifugation and overnight storage, (d) after 1 week of storage, and (e) after centrifugation and 1 week of storage. Test tubes in each set arelabeled blank, 100, 250, and 500 ppm from left to right.

Figure 9. Asphaltene settling in a separator on a Gulf of Mexicoplatform.





at the oil−water interface, even when the treatment wasperformed post-destabilization and aging. Thus, visual obser-vations of the Turbiscan tubes containing untreated and treatedsamples concur with Turbiscan results.On the basis of a suite of crude oils studied, it is inferred

that chemical treatment helps redisperse and stabilize the as-phaltenes even after they have flocculated out of the crude oil.These observations could help in examining the potential fortopside applications of asphaltene treatment chemicals andcould be an option in mitigating topside asphaltene problemseven after settling/precipitation has already occurred.

■ CONCLUSIONThe ADT procedure was modified wherein treatment with thechemical was administered after destabilization of the crude oilby injecting into a large excess of heptane. On the basis of ourmodified ADT experiments in the laboratory with a suite ofdead crude oils, we infer that asphaltene treatment chemicalsare highly effective in redispersion and restabilization of asphal-tenes in the hydrocarbon phase, after destabilization with hep-tane. The chemicals were effective even after the destabilizedmixture of crude oil and heptane was allowed to age for anextended period. This is contrary to the belief that treatmentwith asphaltene stabilizers should necessarily be applied beforedestabilization and flocculation of asphaltenes have occurred.The chemical was effective in controlling the migration ofdestabilized asphaltene to the oil−water interface, as evidencedby Turbiscan measurements. On the basis of the observationsfrom the stock tank oils studied, we infer that precipitatedasphaltenes can be effectively restabilized by the treatmentchemicals. These results suggest that the asphaltene controlchemicals could be applied to reverse asphaltene flocculationand could have potential applicability in mitigating asphaltenechallenges in topside crude oil production facilities.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

■ ACKNOWLEDGMENTSThe authors thank the Nalco Company for permission to publishthe results in this study. Helpful discussions with Alberto Montesi

Figure 10. Turbiscan for 80:20 destabilized crude oil (in heptane) and water: (top) untreated and (bottom) treated with stabilizing chemical afterdestabilization and storage.

Figure 11. Visual observation of Turbiscan tubes at the end of the30 h experiment.



mailto:[email protected]



and Robert A. Pinnick, Chevron North America Explorationand Production Company, are gratefully acknowledged.

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reversibility of asphaltene flocculation with chemicals

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