Reversibility of Asphaltene Flocculation with Chemicals - American

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Reversibility of Asphaltene Flocculation with Chemicals Priyanka 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 industrial significance with respect to remediation. Here, we discuss the redispersion of asphaltenes in the hydrocarbon phase aided by treatment 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 restabilization of asphaltenes was observed upon treatment with asphaltene treatment chemicals, even when the destabilized system comprising of untreated crude oil in heptane was allowed to age for extended periods. We infer that precipitated asphaltenes can be effectively restabilized by the treatment chemicals. This could have potential applicability in mitigating asphaltene challenges, specifically in topside crude oil production facilities.

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INTRODUCTION Phase transitions that lead to precipitation of asphaltenes during crude oil production are a critical concern from operational and economic standpoints. Phase separation of asphaltenes can present severe problems with the production and recovery of crude oils and also during refining and upgrading operations. Near the well-bore, destabilized asphaltenes are known to cause formation damage by clogging the pores of the crude oil formation. Flocculated asphaltenes deposit onto the pumps and the tubings, thereby restricting flow and decreasing crude oil production. Precipitated asphaltenic material may also collect at the oil−water interface in surface equipment in the production handling facilities, such as free-water knockouts and heat treaters, causing the formation of stable emulsions that make oil−water separation extremely difficult. When asphaltenes settle at the bottom of the treaters and contaminate the discharge water, they create a unique disposal challenge. Asphaltenes often deposit in stock tanks, lowering the volume capacity and jeopardizing the discharge line. Because of polydispersity associated with asphaltenes, they are defined in terms of their solubility as the fraction of crude oil that is soluble in aromatic solvents, such as toluene, and insoluble in aliphatic solvents, such as heptane.1−6 Precipitation takes place when the crude oil loses its ability to keep those particles dispersed, which is influenced by several factors, such as variations in temperature, depletion of reservoir pressure, and/or changes in the bulk solution composition, and also during enhanced oil recovery operations.7−10 For example, with a reduction in the pressure, the relative volume fraction of the lighter components in the crude oil increases, thereby destabilizing the asphaltenes.11 Similarly, compositional changes induced by commingling crude oil streams from different wells can cause asphaltenes to precipitate out. The issue whether asphaltene precipitation is reversible or not has been variously approached.11,12 The issue has been extensively researched, with well-supported evidence in the published literature in favor of and also arguments against the reversibility of asphaltene precipitation. Various thermodynamic models derive from asphaltene flocculation to be completely reversible, whereas colloidal models predict otherwise.13−16 © 2012 American Chemical Society

Nevertheless, a better understanding of the reversibility of asphaltene precipitation may contribute to the development of improved predictive models of asphaltene precipitation.17,18 Many researchers support partial to complete reversibility of asphaltene precipitation, but some researchers suggested that it is not possible for the precipitated asphaltenes to go back into solution. Andersen et al. presented experimental data confirming partial reversibility of asphaltene precipitation.19,20 Upon the addition of toluene to a mixture of crude oil and heptane with precipitated asphaltenes, Buckley et al. observed both the appearance and disappearance of asphaltenes over the same narrow range of mixture refractive index (RI) values, indicating complete reversibility of asphaltene precipitation for the crude oil examined. Buckley et al. ascribe the difficulty in the determination of the reversibility of the asphaltene precipitation process to the slow kinetics of dispersion and flocculation.21,22 On the basis of titration experiments, it has been shown that precipitated asphaltenes can be redissolved by the addition of a good solvent.23 However, the addition of a solvent is also reasonably considered as no indication of complete reversibility because it is not the reverse process of the addition of the precipitant.11,24 Hirschberg et al. assumed asphaltene aggregation to be reversible but a very slow process.13 Hammami et al. through a series of isothermal pressure depletion experiments conducted under live oil conditions, provided evidence of asphaltene precipitation above and redissolution below the saturation pressure, and concluded that pressure depletion inducing asphaltene precipitation is a highly reversible process.25 Joshi et al. determined the reversibility of the asphaltene flocculation process for a selected crude oil by alternately collecting near-infrared (NIR) spectra at high and low pressures.26 They observed that, if the pressure on the sample is reduced from 13 000 to 6000 psi and then increased back to 13 000 psi within minutes of the pressure drop, the spectral scattering Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 14, 2011 Revised: December 30, 2011 Published: January 8, 2012 2631

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

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Several chemicals were evaluated for their performance in reversibility of asphaltene precipitation. Here, we report results from chemicals A and B, with more focus on chemical A. Chemical A, a highly successful asphaltene treatment chemical, is nonylphenol formaldehyde resin type chemistry. Chemical B is a polyolefin ester type chemistry in an aromatic solvent. Saturates, Aromatics, Resins, and Asphaltenes (SARA) Analysis. Crude oil samples were submitted to Weatherford Laboratories (Shenandoah, TX) for SARA analysis. Asphaltenes were heptane (C7)-precipitated from the fluids following the IP 143 procedure. ADT. ADT evaluates the presence of asphaltenes and their tendency to precipitate in a crude oil. The test also enables determination of the efficacy of treatment chemicals. An aliquot of untreated and treated crude oil is added to ADT test tubes that contain 10 mL of heptane. Treatment was performed with 100, 250, and 500 ppm by volume (on the basis of crude oil) of the chemical. The percent sedimentation because of gravity after 10, 30, and 60 min is observed and recorded. The sedimentation or precipitation is recorded in milliliters from the graduated ADT tubes. Where no precipitation is observed, the data are recorded as “clear”, and where precipitation is observed but is not measurable, the data are recorded as “trace”. Subsequently, all of the samples are centrifuged for 3 min at 2500 rpm. The transmission of the blank oil sample and treated samples are measured with a colorimeter (PC 910 colorimeter, Brinkman Instruments, Westbury, NY) and recorded as %Tblank and %Ttreated. The blank sample is shaken again on the vortexer such that all of the asphaltenes are dispersed in the solution and the transmission is recorded as 100% dispersion (100%D). Percent inhibition (%I) is calculated from transmission measurements as per the equation

characteristic of destabilization is entirely eliminated, thus suggesting deflocculation or resuspension of asphaltenes. Wang et al. speculated that asphaltene precipitation is less likely to be reversible for crude oils well beyond the onset conditions.27 Partial reversibility or a hysteresis effect indicating slower kinetics of asphaltene redispersion compared to the kinetics of flocculation has been demonstrated by several researchers.16−20,28 However, an improved understanding of not just the asphaltene precipitation mechanism but also the reversibility of precipitation is warranted for further improvements in predictive models and mitigation strategies and remediation chemicals. Chemicals provide cost-effective and invaluable solutions to resolve deposition problems that plague the oil and gas production systems from production, through fluid transportation to topside operations.29−32 Because asphaltene deposition does not occur until after flocculation, polymeric dispersants were developed that stabilize the asphaltenes and prevent flocculation. It is widely believed that these chemicals act in the same manner as the resins and maltenes, interacting with asphaltenes and stabilizing the asphaltene in the crude oil. These asphaltene treatment chemicals have a stronger association with the asphaltenes than the natural resins and maltenes and are able to stabilize the asphaltenes through greater changes in pressure, temperature, shear, and chemical environment. Chang and Fogler studied the stabilization of crude oil asphaltenes in apolar alkane solvents using a series of model alkylbenzenederived amphiphiles as the asphaltene stabilizers and concluded that asphaltenes from crude oil can be effectively dispersed in alkane solutions by the oil-soluble amphiphiles that adsorb strongly to the asphaltene surfaces and establish a stable steric alkyl layer around asphaltene molecules that impedes asphaltene molecular association and precipitation.33 In the field application, it is deemed important that these chemicals are administered to the crude oil before the asphaltenes become destabilized and flocculation occurs.29,30 Reversibility of asphaltene flocculation is crucial from a mitigation point of view. Application of oilfield chemicals in reverting asphaltene flocculation can be of tremendous value, especially in the off-shore oil production, where precipitation can have unfortunate consequences. However, to the best of our knowledge, there are no studies on the reversibility or redispersion of flocculated asphaltene in crude oil aided by chemicals. In this paper, we describe the laboratory analyses on the destabilization and redispersion of flocculated asphaltenes in crude oils aided by commercial asphaltene treatment chemical. Stability analyses were performed on several unstable crude oils using the asphaltene dispersancy test (ADT)34 and Turbiscan.35 Performance of the chemical inhibitors was monitored on crude oils both before and after asphaltene flocculation has occurred. ADT and Turbiscan methodologies were suitably modified to assess the effectiveness of the inhibitors in restabilizing flocculated asphaltenes, when the treatment was administered following destabilization and sedimentation. The performance of the treatment chemicals in resuspending flocculated asphaltenes in the presence of static water was also evaluated to simulate the topside separator condition for a Gulf of Mexico platform.

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⎡ (%T ⎤ treated − 100%D) × 100 %I = 100 − ⎢ ⎥ ⎣ (%Tblank − 100%D) ⎦ The results for ADT were also photographically recorded. Modified ADT. For evaluating the redispersion ability of the chemical product, the untreated aliquot of the homogenized crude oil sample was added to the ADT tubes containing 10 mL of heptane. Treatment with the chemical at various dosages followed either immediately or at various intervals (after aging for 30 min, overnight, and for 1 week in the dark), following the destabilization and flocculation/ separation of asphaltenes. In the case of overnight and 1 week of aging of the destabilized oil−heptane solution, the effect of additional stress induced by centrifuging sample sets before aging on the performance of the chemical with respect to reversing the flocculation was also studied. In this way, the ADT methodology was modified and the addition of the chemical followed asphaltene destabilization in the crude oil to determine if the asphaltenes would be redispersed and restabilized into the bulk by the treatment. Percent inhibition (%I) was calculated as with the ADT, and results were also visually followed and recorded with photographs. The scheme of the ADT and modified ADT experiments followed in this work is described with the flowchart reported in Figure 1. Turbiscan. The samples were analyzed on Turbiscan for evaluating the effect of asphaltene inhibitors and aging on the crude oil, using a Turbiscan MA2000 from Formulaction (Davie, FL) that uses a pulsed NIR light source (850 nm). Turbiscan performs multiple light scattering to measure the average transmittance of a sample. It measures both the stability of the asphaltenes in a particular crude oil and the relative performance of treatment chemical in that crude oil. A computer automates this technique, thereby diminishing the possibility of operator error. In a Turbiscan test, an efficient mitigation chemical will lower the percent transmission compared to the blank and remain relatively unchanged during the full length of the test.

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RESULTS AND DISCUSSION Several crude oils were used in the study to examine the efficiency of chemicals in redispersing asphaltenes in the oil phase, following destabilization by a flocculant. Table 1 reports

MATERIALS AND METHODS

Crude Oils and Chemicals. Two crude oil samples, one from the Gulf of Mexico (reported as crude oil A) and another crude oil from a land-based well (crude oil B), were considered in this study. 2632


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Figure 1. Scheme of the experiments followed to evaluate the performance of the chemical in stabilizing asphaltenes (ADT) and in reversing the flocculation 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 crude oil B

Gulf of Mexico Wyoming

27.5 35.0

40.41 59.56

42.06 32.76

12.21 6.95

5.32 0.73

American Petroleum Institute (API) gravities and SARA compositional profile based on the topped oil for two of the crude oils used in this work. Crude oil A is a medium oil, and crude oil 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, as described in the Materials and Methods. For the treated samples, chemical A was added via a micropipet to the crude oil sample and the sample was shaken on the vortexer, followed by heating for an additional hour at 60 °C before injection into the ADT tube. A total of 100 μL of the untreated and treated crude oil A was injected into 10 mL of heptane. Table 2 reports Table 2. Standard ADT Results for Crude Oil A sample crude oil A chemical A a

Table 3. Standard ADT for Crude Oil B with Chemicals A and Ba sample crude oil B chemical A

chemical B

10 min (mL)

30 min (mL)

1h (mL)

%T

%I

0 100 250 500

2 clear clear clear

1.5 clear clear clear


62.2 41.4 30.8 29.7

0 63 95 99

10 min (mL)

30 min (mL)

1h (mL)

%T

%I

0 100 250 500 100 250 500

0.1 trace trace trace trace trace trace



71.1 41.4 30.8 29.7 59.9 56.4 54.9

0 56 72 82 57 75 83

“Trace” refers to visual observation of asphaltene particles but not measurable.

a

a

dosage (ppm)

dosage (ppm)

is applied to the crude oil prior to injection into heptane. To determine if the chemistries used in asphaltene control could also be effective in reversing the flocculation of asphaltenes and, thus, redispersing and restabilizing the crude oil, we modified the ADT procedure as described in the previous section. The crude oil was destabilized by the addition to a large excess of heptane, followed by the addition of the chemical to the solution. The treatment was administered immediately, after aging for 30 min, overnight aging, and also after 1 week of aging in the 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 heptane. The asphaltenes were allowed to precipitate with gravity, followed by injection of various dosages (100, 250, and 500 ppm) of the chemical, such that the treatment with chemical A followed asphaltene destabilization in the crude oil. The chemical injection followed immediately after the addition of the crude oil sample to heptane, and the dosage was based on the quantity of the crude oil plus the heptane solution. Thus, for the modified ADT experiments, the chemicals were likewise diluted in toluene to make the effective concentration of the chemical the same as for the conventional ADT. Thus, 1% dilution of the chemicals in toluene was prepared, and crude oil + heptane solution was treated with 100, 250, and 500 ppm

“Clear” refers to no sedimentation observed.

sedimentation because of gravity recorded at 10, 30, and 60 min of the ADT duration for the blank crude oil A and the crude oil treated 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 increased with an increasing dosage of chemical A. Thus, it is evident that chemical A is effective in stabilizing asphaltenes against precipitation for crude oil A at all of the treatment rates tested. Standard ADT results for crude oil B treated with both chemicals A and B is displayed in Table 3. It is evident that both of the chemicals are effective in preventing asphaltene precipitation 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 2633


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of the diluted chemical, to match the total concentration with that for conventional ADT. To check that the reversibility of flocculation was not a result of extra toluene added with the chemical in terms of the solvency that it would impart to the solution for asphaltenes (although only a few parts per million of toluene), we conducted modified ADT experiments with treatment with 1000 ppm of toluene and xylene. Treatment with these solvents was performed 2.5 h after destabilization of the crude oil with heptane. The percent inhibition for the blank sample and the samples treated with 1000 ppm of toluene and xylene remained unchanged; that is, these aromatic solvents provided no inhibition against flocculation (data not reported here). Thus, it was evident that the reversibility of asphaltene flocculation was because of the dispersant action of the inhibitor. ADT results for this approach when the treatment was rendered immediately upon destabilization of crude oil A are listed in Table 4. It is apparent from Table 4 that chemical

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

rates of 100, 250, and 500 ppm was added to the test tubes as labeled. The ADT tubes were vortexed for 10−15 s and visually observed for asphaltene instability as reflected from sedimentation at 10, 30, and 60 min from the addition of the chemical. Figure 3a shows ADT photographs for the test tubes at the end of 0 (treated and shaken), 10, 30, and 60 min after the addition of the chemical. Figure 2b is the starting reference for this step. It can be clearly deduced from the visual observation in Figure 3a that chemical A at all dosages is effective in redispersing and stabilizing the already precipitated asphaltenes for crude oil A, as reflected from no sedimentation in the sample tubes at the end of 60 min for the treated samples. In the next modification of the ADT procedure, the severity of the sedimentation was increased by centrifugation, followed by storing the test tubes overnight. Two sets of four ADT test tubes labeled blank, 100, 250, and 500 ppm (from left to right in Figure 3) were prepared. A 100 μL aliquot of crude oil was added to all eight test tubes containing 10 mL of heptane. The tubes were shaken thoroughly, and sedimentation was observed after 10, 30, and 60 min from the injection of the crude oil into heptane. Asphaltene precipitation can be observed from the ADT test tubes as time progresses, and there is significant precipitation at the end of 60 min. At the end of 60 min, one set was centrifuged for 30 min at 2000 rpm. The visual observations for both of the sets of test tubes before the addition of the chemical are depicted in panels b and c of Figure 3, labeled as “blank” and “blank + centrifuge”, respectively. The precipitated asphaltene layer becomes significantly compact following centrifugation (blank + centrifuge). The kinetics of the precipitation was further increased by storing the destabilized samples for 1 week in the dark before the treatment was deployed (Figure 3d). This test was also performed on two sets of test tubes, and the second set was subjected to additional stress by centrifugation for 30 min before storage for 1 week in the dark (Figure 3e). This way, the asphaltene sediment was allowed to age, and the efficacy of chemical A treatment on the aged deposit and under increased stress was determined. Visual observation of the photographs in panels a−e of Figure 3 asserts the effectiveness of chemical A in redispersing and restabilizing the flocculated asphaltenes. However, some precipitate is observed at the end of 60 min for both sets of test tubes that were centrifuged before overnight and 1 week of extended aging in the dark. It seems that centrifugation followed by aging causes congealing of the precipitate, such that the physical state of the centrifuged asphaltene is different relative to the non-centrifuged asphaltene. This diminishes the effectiveness of the inhibitor, probably because some fraction of the congealed asphaltene is not readily available to interact with the inhibitor. Additional stress might cause the formation of larger asphaltene aggregates that may require increased

Table 4. Modified ADT for Crude Oil A with Chemical A sample crude oil A chemical A

dosage (ppm)

10 min (mL)

30 min (mL)

1h (mL)

%T

%I

NA 100 250 500

2 clear clear clear



62.2 33.4 31.2 31

0 88 94 95

A is highly effective in restabilizing asphaltenes in the crude oil A sample, even when the treatment followed after the destabilization 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 Table 5. Modified ADT for Crude Oil B with Chemicals A and B sample crude oil B chemical A

chemical B

dosage (ppm)

10 min (mL)

30 min (mL)

1h (mL)

%T

%I

NA 100 250 500 100 250 500

2 trace trace trace trace trace trace



71.1 53.7 53.6 52.7 54.1 53.9 53.2

0 89 89 94 87 88 91

homogenized crude oil B sample was added to the ADT centrifuge tubes containing 10 mL of heptane. The chemical injection followed immediately after the addition of the crude oil sample to heptane. Thus, modified ADT results for crude oil B from Table 5 also confirm the effectiveness of the chemicals in redispersing the asphaltenes, when treatment was deployed post-destabilization. Chemical A gave consistently better performance compared to the other chemicals used in this study; thus, we have focused more 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 four ADT test tubes containing 10 mL of heptane, and the mixture was shaken vigorously and allowed to stand for 30 min to facilitate asphaltene flocculation and precipitation. The test tubes were 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 2634


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Figure 3. Modified ADT results for crude oil A when the chemical was injected following destabilization: (a) 30 min after destabilization, (b) after overnight 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 each set 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.

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

performed before aging, it appears that the precipitated asphaltenes are only partially restored to the solution upon treatment with the chemical. 2635


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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.

The dosage response chart shown in Figure 4 reports the percent inhibition from all of the ADT and modified ADT conducted for crude oil A. The chart highlights the response to the chemical when the treatment was administered before destabilization as per standard ADT and at varying intervals after destabilization. Chemical A, at all treatment rates, successfully redisperses and stabilizes the precipitated asphaltenes for crude oil A but, interestingly, not to the same extent. Observed trends for percent inhibition for samples treated with 250 and 500 ppm of the inhibitor suggest time dependence of asphaltene reversibility or redissolution. It is observed that percent inhibition reduces with increased storage before the treatment was administered. Percent inhibition for sample sets that were centrifuged is also lower than their counterparts that were not centrifuged before storage and treatment. The trend also indicates partial reversibility of asphaltene flocculation for when the treatment was administered after the destabilization under modified ADT conditions relative to conventional ADT. Thus, the dosage response chart reinforces the visual observations from Figure 3. To further verify the results from ADT, we repeated a few tests with Turbiscan analysis. The Turbiscan is used as an automated ADT and uses multiple light scattering to measure the percent transmission of a sample. It measures both the stability of the asphaltenes in a particular crude oil and the relative performance of asphaltene inhibitors in that crude oil.

As the oil is destabilized, the transmission through the sample increases, most likely because of particle flocculation in the oil and, consequently, flocculation and sedimentation of asphaltenes. The rate by which the transmission changes gives a measure of how quickly the oil is destabilized; i.e., the faster the transmission increases, the less stable is the oil. For treated samples, if the chemical is effective, it will keep the asphaltenes well-solubilized in the solution and prevent precipitation, which will keep the average transmittance unchanged. However, if the chemical is ineffective, the asphaltene particles will flocculate and precipitate, resulting in a rapid increase in the average transmission. Turbiscan data from Figure 5 show the stability trends for untreated crude oil A and the effect of treatment with chemical A. It can be easily inferred that chemical A effectively renders stability to crude oil A at all of the treatment rates employed here. 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 modified ADT, in the case of Turbiscan analysis, prior to treating with the chemical, 100 μL of crude oil was added to the Turbiscan tube containing heptane to destabilize the asphaltenes. With this method, unstable asphaltenes would precipitate out over 30 min. The chemical was subsequently added at the various treatment rates, and tubes were shaken before Turbiscan analysis. It is evident from Figure 6 that the chemical, at all 2636


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

treatment rates, effectively redisperses flocculated asphaltenes into the crude oil solution. The results from Turbiscan are thus in qualitative agreement with ADT results. Effect of Hand-Shaking versus Vortexing after Chemical Treatment. Experiments were carried out to examine the impact of hand-shaking or vortexing, after the chemical was injected to the destabilized and aged crude oil−heptane mixture. This was performed to assess the influence of the method of mixing on the performance of the chemical in redispersing the flocculated asphaltenes. The method of mixing is likely to enable an increased interaction between the chemical and the asphaltene particles and facilitate effective restabilization of the flocculated asphaltenes. With this objective, two sets of two ADT test tubes, labeled blank and treated, were prepared. ADT with a 100 μL aliquot of untreated crude oil injected into 10 mL of heptane was conducted. The tubes were shaken 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 the dark, followed by injected of 250 ppm of the chemical. One set of test tube was given moderate hand-shaking, 10 times, and the other set of the test tubes was vortexed at high speed for 15 s. The test tubes were then set for observation for the next 60 min. The visual observations for both sets of test tubes before the addition of the chemical and after treatment are reported in Figure 7. Visual observation of the modified ADT in Figure 7 indicates that hand-shaking shows less sedimentation in the untreated ADT tube for the hand-shaken sample. The tubes treated with 250 ppm of the inhibitor reflect a similar performance of the chemical in terms of sedimentation at the end of 10, 30, and 60 min duration of the test. The formation of relatively more stable emulsion with hand-shaking as compared to a fast shaker is reported by Allenson et al. and Lang et al. while evaluating the effect of shear on the emulsion droplet size.36,37 They report more than 3 times higher viscosity values and 1/3 smaller water droplet size for the hand-shaken emulsions relative to the fast-shaker-generated emulsions. From our modified ADT experiments, we infer that hand-shaking provides as good of mixing as vortexing and redisperses the flocculated asphaltenes to the same ability as with the fast vortexing.

Table 6 reports modified ADT results for the hand-shaking versus vortexing. It can be inferred that both of the methods Table 6. Modified ADT for Destabilized Crude Oil A: HandShaking versus Vortexing sample crude oil A crude oil A

mixing handshaking handshaking vortexing vortexing

dosage (ppm)

10 min (mL)

30 min (mL)

1h (mL)

%T

%I

NA

trace

0.1

1.6

61.6

0

250

clear

trace

trace

33.2

61

NA 250

trace clear

0.5 trace

1.6 trace

62.8 30.7

0 68

provide comparable restabilization of the precipitated asphaltenes as deduced from reasonably close percent inhibition data. Crude Oil B. We repeated the ADT and modified ADT experiments reported in the preceding section on another highly unstable crude oil, crude oil B, sourced from a landbased well in Wyoming. The performance evaluation of chemical A and other chemicals with respect to asphaltene stabilization was tested with ADT and modified ADT. Visual observations from ADT and modified ADT for this crude oil sample are reported in Figure 8. The photographs show that chemical A shows a similar performance as for crude oil A in redispersing the precipitated asphaltenes and restabilizing the crude oil B. Redispersibility in the Presence of Static Water. The platform associated with crude oil A in the Gulf of Mexico does not produce much water. However, the separator holds static water, and over time, asphaltenes from the crude oil migrate to the oil−water interface, leading to the formation of an asphaltenic pad layer that eventually settles down in the separator. Figure 9 shows the picture of extensive fouling because of settling of asphaltenes in the separator. To simulate separator conditions and the formation of the interfacial layer, Turbiscan analyses were performed under static water conditions. The tests were run for an extended duration of 30 h with 8 mL (80%) of either whole crude oil or destabilized crude oil in heptane and 2 mL of water (20%). For static conditions, 2 mL of water was injected into the Turbiscan tube, followed by the addition of 8 mL of crude oil or destabilized crude oil in 2637


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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 are labeled blank, 100, 250, and 500 ppm from left to right.

oil−water interface in the separators. In a small system, such as the Turbiscan tube, the short duration of the test might be insufficient for the asphaltenes to adsorb at the interface. To induce asphaltene destabilization and study their interfacial behavior, 100 μL of crude oil was injected into 10 mL of heptane and allowed to stand for 2 days to precipitate the asphaltenes. Before initiating Turbiscan, the crude oil−heptane solution was hand-shaken, and 8 mL were added to a Turbiscan tube containing 2 mL of water. To evaluate the effect of the stabilizing chemical, the destabilized crude oil−heptane mixture was hand-shaken, treated with 100 ppm of the chemical, and subjected to Turbiscan in a Turbiscan tube containing 2 mL of water. The transmission through the glass vial was measured every 30 min, and the test was run for 30 h. Figure 10 reports the Turbiscan analyses conducted on destabilized crude oil, untreated and treated with the stabilizing chemical. The destabilization and flocculation of asphaltenes for untreated crude oil and the formation of the rag layer over time are evidenced from the increase in transmission through the sample (top panel of Figure 10). Interestingly, a clean interface and stable transmission are observed throughout the length of the test for the destabilized sample when treated with the chemical (bottom panel of Figure 10), indicating the efficacy of the chemical in controlling the size of the asphaltene particles, preventing flocculation and eventual migration to the oil−water interface. Photographs of the Turbiscan tubes at the end of the 30 h experiment reported in Figure 11 show that, for the untreated sample, the asphaltenes start to flocculate and move to the oil− water interface forming a rag layer. However, for the chemicaltreated sample, no rag layer is formed and a clean oil−water interface is observed. It seems that the chemical keeps the size of the asphaltene particles in control, keeping them welldispersed in the oil phase and, thus, preventing their adsorption

Figure 9. Asphaltene settling in a separator on a Gulf of Mexico platform.

heptane. In another variation of the test condition, water and crude oil were hand-shaken before Turbiscan. It was observed that, under static conditions, no asphaltenes precipitated out and migrate to the interface, as evidenced by stable transmission in the Turbiscan graph and also from visual observation of the clean oil−water interface (no rag layer) in the Turbiscan tube at the end of the 30 h test (results not shown here). Upon hand-shaking before Turbiscan, the oil and water form a very stable emulsion, which does not break out even at the end of 30 h. Transmission remained stable for this emulsion through the 30 h duration of the test, indicating no asphaltene destabilization (results not shown). The stability of asphaltenes in these tests could be attributed to the slow diffusion and kinetics of 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 significantly greater length of time and frequent charge of fresh fluids, when destabilized asphaltenes from the field could migrate to the 2638


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

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CONCLUSION The ADT procedure was modified wherein treatment with the chemical was administered after destabilization of the crude oil by injecting into a large excess of heptane. On the basis of our modified ADT experiments in the laboratory with a suite of dead crude oils, we infer that asphaltene treatment chemicals are highly effective in redispersion and restabilization of asphaltenes in the hydrocarbon phase, after destabilization with heptane. The chemicals were effective even after the destabilized mixture of crude oil and heptane was allowed to age for an extended period. This is contrary to the belief that treatment with asphaltene stabilizers should necessarily be applied before destabilization and flocculation of asphaltenes have occurred. The chemical was effective in controlling the migration of destabilized asphaltene to the oil−water interface, as evidenced by Turbiscan measurements. On the basis of the observations from the stock tank oils studied, we infer that precipitated asphaltenes can be effectively restabilized by the treatment chemicals. These results suggest that the asphaltene control chemicals could be applied to reverse asphaltene flocculation and could have potential applicability in mitigating asphaltene challenges in topside crude oil production facilities.

Figure 11. Visual observation of Turbiscan tubes at the end of the 30 h experiment.

at the oil−water interface, even when the treatment was performed post-destabilization and aging. Thus, visual observations of the Turbiscan tubes containing untreated and treated samples 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 asphaltenes even after they have flocculated out of the crude oil. These observations could help in examining the potential for topside applications of asphaltene treatment chemicals and could be an option in mitigating topside asphaltene problems even after settling/precipitation has already occurred.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS The authors thank the Nalco Company for permission to publish the results in this study. Helpful discussions with Alberto Montesi 2639


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(30) Allenson, S. J.; Walsh, M. Proceedings of the Society of Petroleum Engineers (SPE) International Symposium on Oilfield Chemistry; Houston, TX, Feb 18−21, 1997; SPE 37286. (31) Juyal, P.; Le, V.; Yen, A.; Allenson, S. J. J. Dispersion Sci. Technol. 2011, 32, 1096−1104. (32) Asomaning, S.; Yen, A. Proceedings of the Chemistry in the Oil Industry VII: Performance in a Challenging Environment; Royal Society of Chemistry (RSC): London, U.K., 2002; pp 277−286. (33) Chang, C.; Fogler, H. S. Langmuir 1994, 10, 1749−1757. (34) Stankiewicz, B. A.; Flannery, M. D.; Fuex, N. A.; Broze, J. G.; Couch, J. L.; Dubey, S. T.; Leitko, A. D.; Nimmons, J. F.; Iyer, S. D.; Ratulowski, J.; Westrich, J. T. Prediction of asphaltene deposition risk in E&P operations. Proceedings of the Spring National Meeting of the American Institute of Chemical Engineers; New Orleans, LA, March 11− 14, 2002; pp 1738−1744. (35) Ostlund, J. A.; Russel, T. J.; Chia, L. The application of a novel method to the study of heavy fuel oil stability and the performance of additives. SAE [Tech. Pap.] 2004, DOI: 10.4271/2004-28-0092. (36) Lang, F.; Weather, T. Use of emulsion breaker chemical to reduce the apparent viscosity of production fluids. Proceedings of the Rio Oil and Gas Expo and Conference; Rio de Janeiro, Brazil, Sept 13− 16, 2010. (37) Allenson, S. J.; Yen, A. T.; Lang, F. Application of emulsion viscosity reducers to lower produced fluid viscosity. Proceedings of the Offshore Technology Conference (OTC) Brazil; Rio de Janeiro, Brazil, Oct 4−6, 2011; OTC-22443-PP.

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

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Reversibility of Asphaltene Flocculation with Chemicals - American

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