Direct assessment of inhibitor and solvent effects on the deposition

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Direct assessment of inhibitor and solvent effects on the deposition mechanism of asphaltenes in a Brazilian crude oil Lia Beraldo da Silveira Balestrin, Renata Dias Francisco, Celso Aparecido Bertran, Mateus Borba Cardoso, and Watson Loh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00043 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Figure 1. (a) QCM results shown as change in frequency as a function of time for crude oil with different nheptane contents. The percentage displayed in the chart legend indicates the concentration of n-heptane relative to the asphaltene precipitation onset (1.8 mL C7/g oil). The data were obtained in duplicate, with the uncertainty indicated by the width of the lines. (b) QCM results for the oil with n-heptane at 95% of the onset plotted in log scale at y-axis. The intersection of the dotted line indicates the transition point. 156x55mm (300 x 300 DPI)

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Figure 2. (a) QCM results shown as change in frequency as a function of time indicating asphaltene deposition onto gold surface. Data collected for systems with different inhibitors (DBSA – dodecylbenzene sulfonic acid, NF – nonylphenol and CA 1 and CA 2 – commercial additives) at 95% of the onset. Replicates were presented separately. (b) is a zoom of (a). 522x304mm (300 x 300 DPI)


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Figure 3. QCM results shown as change in frequency as a function of time for crude oil with n-heptane concentration at 95% of the onset at varying amounts of nonylphenol: from 0 to 1 wt %. The data were obtained in duplicate and the width of the lines indicates the uncertainty. (b) is a zoom of (a). 137x55mm (300 x 300 DPI)


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Figure 4. Images obtained by confocal microscopy with laser scanning combining laser intensity with optical view of oil-immersed samples at 95% of the onset for (a) sample without additives and samples with (b) 1 wt % of NF, (c) 1 wt % of DBSA, (d) 0.5 wt % of DBSA + 0.5 wt % of NF, (e) 1 wt % of CA 1. Scale bar of 500 μm. 93x82mm (300 x 300 DPI)


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Figure 5. Combined images of laser intensity with optical images obtained by confocal microscopy with laser scanning of gold surfaces immersed for 24 h in oil with n-heptane concentration of 95% of the onset. The bottom images are magnifications. 419x593mm (300 x 300 DPI)


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Figure 6. Atomic force microscopy images from gold surfaces immersed for 1 h in oil with n-heptane at 95% onset in the absence of additives, after washing with 0.5 mL of n-heptane. The image shows two regions of the sample. On the left, topography; on the right, phase contrast maps. 72x62mm (300 x 300 DPI)


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Figure 7. Instability index for asphaltene precipitation as a function of time. Measurements performed in duplicate for samples without additives; and in the presence of 1 wt% of NF; 1 wt% of DBSA; 1 wt% of CA 1; 1 wt% of CA 2; or in the mixture of 0.5 wt% of DBSA and 0.5 wt% of NF. (b) is a zoom of (a) 549x209mm (300 x 300 DPI)


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Figure 8. Size distributions and cumulative curves for pure oil and with 1 wt% of CA 1 samples. The oil + 1 wt % of DBSA, oil + 1 wt % of NF and oil + 0.5 wt % of DBSA + 0.5 wt % systems were suppressed to facilitate the visualization of the results, since they overlapped the pure oil system data. 296x209mm (300 x 300 DPI)


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Figure 9. Representation of asphaltene aggregation behavior, adapted from reference 26. 127x86mm (300 x 300 DPI)


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Figure 10. Scheme indicating the process of asphaltenes deposition in the absence and presence of aggregation inhibitors. 31x27mm (300 x 300 DPI)


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Direct assessment of inhibitor and solvent effects on the deposition mechanism of asphaltenes in a Brazilian crude oil Lia Beraldo da Silveira Balestrina, Renata Dias Franciscoa, Celso Aparecido Bertrana, Mateus Borba Cardosob, Watson Loha a

b

Institute of Chemistry – University of Campinas (UNICAMP) - Campinas, SP, Brazil.

Brazilian Nanotechnology Laboratory (LNNano) and Brazilian Synchrotron Light Laboratory (LNLS), CNPEM – Campinas, SP, Brazil * corresponding author: [email protected]

ABSTRACT

The study of asphaltene deposition mechanism is critical to understand and solve important problems in the petroleum industry. The same is valid for the selection of inhibitors to control or prevent asphaltene flocculation and/or deposition. However, most of the current information on these processes is obtained by experiments performed using model solvent systems. In the present study, we used quartz crystal microbalance (QCM) measurements as well as laser scanning confocal microscopy to characterize the asphaltene deposition directly measured in a Brazilian crude oil at different conditions of flocculant concentration and using inhibitors with different chemical features. Measurements under accelerated sedimentation (LUMiSizer) were also employed to evaluate inhibitor capacity in crude oil systems, in this case using a


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large excess of n-heptane. Overall, QCM results suggest that diffusion-limited aggregation (DLA) model can be used to describe systems close to or above the concentration of the onset of asphaltene precipitation. And the transition to a behavior that follow the reaction-limited aggregation (RLA) model occurs when an inhibitor is added or the flocculant concentration is reduced farther from the onset. Moreover, accelerated sedimentation shows that the inhibitors tested act by preventing aggregate growth. Therefore, these results highlight the importance of performing time-dependent experiments directly in crude oils and support the use of these methodologies to optimize inhibitor selection for different crude oils.

INTRODUCTION

It is well known that asphaltenes can flocculate due to composition changes during the injection of carbon dioxide and natural gas, or with temperature or pressure variations. From these instability conditions, the deposition of asphaltenes in the rock pores can occur, reducing their permeability. Moreover, asphaltene particles can also be bubble-charged to the pipe walls, reducing the effective diameter available for oil transport and requiring more frequent cleanings, as well as the addition of chemicals to remove deposited asphaltene. Asphaltene aggregation can lead to more severe consequences, such as the abandonment of the extraction well, when sludge formation blocks the oil flow. Damage can also occur in pumps, separators, filters and other production equipment, due to the flocculation of asphaltene particles, increasing costs with maintenance and cleaning.1 A series of studies have shown that these problems are associated to asphaltene colloidal aspects and the formation of hierarchical aggregates1-3, in some cases detected


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directly in crude oils.4 Different techniques can be used to detect and monitor asphaltene aggregates, including direct observation in crude oil systems by using optical microscopy or more sophisticated techniques such as atomic force miscroscopy5 and electron microscopy6 or scattering techniques such as SAXS or SANS.4,7-10 In addition to their colloidal structure, another facet that was recently brought to attention is the importance to characterize the asphaltenes aggregation kinetics. These studies were neglected for a long time, limiting themselves to short time scales. Kinetic studies11 of asphaltenes in crude oils with the addition of alkanes have shown that the precipitation time can range from a few minutes to several months, depending on the alkane concentration employed. The authors suggest that the addition of a precipitant leads to the destabilization of the aggregates dispersed in the oil by changing the properties of the latter. Thus, asphaltenes can aggregate into larger particles, detectable by an optical microscope. The aggregation rate becomes faster as the flocculant concentration increases due to the destabilization of more asphaltene nanoparticles, as well as the reduction of medium viscosity. As a consequence, the collision frequency is increased.11 In addition, that study proposed that aggregation can be described by a diffusion-limited aggregation (DLA) model leading to the growth of Brownian treeshaped structures.12 Data obtained by Goual and coworkers showed for a Tensleep crude oil that the asphaltene molar mass distribution became narrower as the carbon number of the alkane used to precipitate the asphaltene increases. In addition, they observed that adsorbed asphaltene films become more viscoelastic as the system approaches the alkane concentration of the precipitation onset. On the other hand, oil samples analyzed only with the addition of toluene resulted in rigid films with a thickness of 3.5 nm, regardless


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of the asphaltene concentration employed. Data obtained from crude oil diluted in toluene also shows that the kinetics is initially governed by adsorption.13 Zahabi et al.14 reported kinetic data also in model systems of about 16 hours of data acquisition, showing that a steady state is not reached during this period. In addition, the deposition extends to multilayer structures, with a possible structural change of the material surface or depletion of the nanoaggregates from the liquid. QCM (quartz crystal microbalance) measurements conducted on gold surface and silica particles showed that the adsorption is more effective on gold surfaces. 14 Cagna et al.15 showed that asphaltene adsorption takes longer than the predicted by a mechanism involving diffusive motion of the species towards the interface. However, the experimental results of asphaltene adsorption in the interfacial region are quite similar when considering asphaltene samples containing wider molecular mass distributions. Carrier et al.16 employed a Quartz Crystal Resonator (QCR) to determine the asphaltene onset of flocculation. Data showed that the frequency and dissipation are extremely sensitive to asphaltene flocculation. However, the resonant frequency changes due to variation in solution viscosity and adsorption increase over time. As adsorption phenomenon appears earlier than asphaltene flocculation, this property cannot be used to reliably detect the flocculation onset. On the other hand, the dissipation change, related only to viscosity changes, can be used to detect the beginning of asphaltene flocculation. The authors also propose the QCR as an attractive tool to probe the flocculation of asphaltene that can be used under high pressure to study flocculation on live oil decompression. Modeling studies performed by Chapman and coworkers of systems employing crude oil in heptol present a liquid loading effect (liquid gets trapped in the solid


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structure of asphaltene deposits).17 On the other hand, this effect is negligible in a model system. Thus, direct mass analysis in oil systems is hampered. Their results showed that the kinetics of asphaltene deposition is initially governed by adsorption, in which the asphaltene-surface interaction is dominant. Over time, the asphaltene-asphaltene interaction becomes more important, necessitating the use of kinetic precipitation and deposition constants to adjust the data.17 Based on results from dynamic light scattering measurements (photoncorrelation spectroscopy) adapted to opaque fluids, Yudin et al. proposed that asphaltene aggregation in alkane solutions follows two different kinetic laws: diffusionlimited aggregation (DLA) and reaction-limited aggregation (RLA), depending on the initial particles size. The authors also associated the kinetics of asphaltene aggregation with vanadium content in the asphaltene fraction, describing in the highest vanadium content asphaltene sedimentation even in toluene solutions.18 Ashoori et al. also proposed the use of DLA and RLA mechanisms when studying colloidal particles of asphaltenes in toluene-heptane mixtures.19 In a microfluidic platform, Qi et al.20 studied in situ pore-scale (50 and 100 μm) asphaltene deposition using different solvents combining microfluidic experiments with high-resolution optical imaging. The authors worked with bitumen samples and showed that solvents with more aliphatic character produces more asphaltenes and pore-space damage when compared with solvents with a fraction of aromatic/naphthenic components. Therefore, even though the literature discusses the mechanism of asphaltene aggregation, there is still a gap in the understanding of how aggregation proceeds and changes by the inhibitor addition to a crude oil. In this sense, the present study was conducted aiming to monitor asphaltene adsorption onto surfaces directly from crude


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oils under different conditions such as flocculant content and the presence of model and commercial inhibitors. Besides, stability tests were performed using an accelerated sedimentation methodology in order to assess inhibitors performance. Asphaltene deposits from crude oil were also investigated using laser scanning microscopy allowing a qualitative investigation of surface coverage caused by asphaltene deposition. This work uses a unique set of experimental techniques allowing the investigation of the asphaltenes deposition and precipitation from a more detailed perspective, correlating effect of inhibitors with stability and size of the aggregates.

EXPERIMENTAL

Chemicals A Brazilian crude oil, provided by PETROBRAS, was employed for this study. A summary of its composition data is listed in Table 1. °API and its saturated, aromatic, resins and asphaltene contents were determined by Petrobras using the SARA methodology. Asphaltene content precipitated with n-pentane (C5I) was determined following the IP143/84 methodology.21 The onset of asphaltene precipitation was determined for this oil using a beaker containing 1 g of the oil. At room temperature and under constant stirring, 100 µL of n-heptane was added every 10 min22 (equilibration time). After each addition, a small droplet was analyzed at an optical microscope (Nikon ECLIPSE 50i) to check for the presence of asphaltene particles. The concentration in which objects appear for the first time (onset) for this oil is 1.8 mL nheptane/ g oil.


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Table 1. Properties of the Brazilian crude oil used in this study. Saturated

Aromatics

Resins

Asphaltenes

Asphaltenes C5I (wt

(wt %)

(wt %)

(wt %)

(wt %)

%) / IP143/84

53

24

21

2.2

3.6

°API 27.9

Three pure solvents, n-pentane, n-heptane and toluene (Sigma-Aldrich, ≥ 99%); two model inhibitors, 4-dodecylbenzenesulfonic acid – DBSA (Sigma-Aldrich, ≥ 95%) and 4-nonylphenol – NF (provided by Oxiteno); and two commercial additives (CA 1 and CA 2), were employed in asphaltene adsorption and precipitation experiments. In the case of CA 1 inhibitor, it is a polymer solution in an aromatic medium containing an amino compound as main active component, while inhibitor CA 2 contains a phenolic polymer as main active compound.

Methods Quartz crystal microbalance (QCM) The sample preparation for measurements using the QCM200 (Stanford Research Systems) quartz microbalance (QCM25 oscillator of 5 MHz) involved the use of the crude oil over a wide n-heptane concentration range. To prepare the samples, n-heptane was added at a rate of 0.125 mL min-1 per oil gram until the desired concentration was reached. This will be expressed with respect to the precipitation onset determined previously by optical microscopy as 1.8 mL nheptane/g oil. In addition, the effect of inhibitors such as the model compounds – DBSA and NF, and commercial additives – CA 1 and CA 2 at asphaltene precipitation was analyzed in the system where the heptane concentration was at 95% of the oil onset. In this case, the inhibitor was added in the desired amount previously dissolved in the


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volume of n-heptane that would be added to achieve the concentration of 95% of the onset. The additive concentration was expressed at wt% related to the oil mass. The experiment was conducted by closing the beaker where the quartz crystal was inserted with a polypropylene cap, and maintaining it in a water bath at 25 °C. The QCM sensor was kept upright throughout the measurement. Some experiments with a heavy crude oil sample with a lower °API ( = 11,0) were also performed. It was not possible to obtain a stable signal due to the high viscosity of the medium, even increasing the temperature to 50 °C. The operating principle of the balance is associated with the piezoelectric reverse effect. It is sensitive to the adsorption and adhesion of materials onto the sensor, detected by the reduction of the fundamental oscillation frequency of piezoelectric crystal. The deposited mass can be calculated by the relationship between the frequency reduction and the adsorbed mass, as described by the Sauerbrey equation.23 This relationship considers the formation of a rigid deposited film.

∆𝑓0 =

―2𝑓20

∆𝑚 𝐴 𝜇𝑞𝜌𝑞

where ∆𝑓0 is the fundamental frequency variation, 𝑓0 is the fundamental frequency, 𝐴 is the crystal effective area, 𝜌𝑞 is the quartz density, 𝜇𝑞 is the quartz shear modulus and ∆ 𝑚 represents the mass change.23

In order to verify the consistency of measurements over long times, experiments with water, n-heptane and pure oil were carried out, confirming the stability of the baseline and the correct sealing of the system. An example is shown in Figure S1 of the Supplementary Material.


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Laser scanning confocal microscopy The Keyence 3D laser scanning confocal microscope VK-X200 (laser wavelength: 408 nm, laser spot radius: 200 nm, magnification: 24000x) was employed to analyze the deposits formed on the gold QCM sensor. Samples with n-heptane concentration of 95% of the onset were prepared as described above, and the crystal was removed from the oil medium at: 1 h, 4 h, 8 h and 24 h. After this step, the substrates were left upright for at least 24 h in order to drain any excess of oil before measurements under the microscope. The microscope operation is based on the amount of light reflected by the sample. The lenses scan the z-axis at each x-y point, and the software reconstructs the sample in three-dimensional way. Thus, this microscope allows one to analyze samples that present significant height differences, because all the points are placed in focus.24 This represents a gain in resolution and image definition that would not be achieved in a conventional optical microscope. Furthermore, it is not necessary to perform sample preparation steps, such as washing and drying steps, which would be necessary, for example, with atomic force microscopy.5

Accelerated sedimentation The LUMiSizer equipment is designed to assess the stability of colloidal dispersions. In addition, it allows one to estimate particle sizes, even in concentrated samples employing the STEP (Space and Time resolved) Technology.25 The analyses were carried out with a mixture of oil, n-heptane and toluene in the volume proportion of 1:8.3:2.7. This ratio of flocculant and diluent was optimized in order to dilute as little


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as possible while still having measurable precipitation kinetics. At lower concentrations of n-heptane, precipitation was too slow and/or the amount of asphaltene precipitated did not lead to detectable transmission patterns. Increasing beyond the concentration of n-heptane used, the kinetics of precipitation was too fast, and it was not possible to obtain transmission profiles with precision and adequate sampling. Experiments were performed with different inhibitors: model compounds – DBSA and NF, and commercial additives – CA 1 and CA 2. The additive concentration was expressed at wt% with respect to the oil mass. The oil:inhibitor ratio used was the same as in the QCM experiments. However, due to the requirement for dilution, the final concentrations (oil and inhibitor) were seven times lower. The samples were conditioned in polyamide cuvette (optical path of 2 mm) and rotated at 400 rpm for 22 h. This equipment operates at the wavelength of 865 nm and the analyses were performed at 25 °C in duplicate. Problems were also encountered when using the LUMiSizer with heavy oils (°API = 11,0) due to the high opacity of the mixture, which prevented monitoring of asphaltene deposition. Use of these techniques is possible at higher oil dilutions, but this would deviate from the objectives of the present study of using samples as close as possible to crude oils.

RESULTS

Figure 1 (a) shows the results obtained with QCM technique on asphaltene adsorption onto gold as a function of time and as an effect of increasing contents of flocculant (n-heptane). The increase in -Δf suggests a mass gain, agreeing with the expected asphaltene deposition. However, calculation of the actual mass deposited is not possible with this type of system, using the Sauerbrey equation, because the


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formation of a rigid film of the asphaltene deposit does not occur. 26 For this reason, we have chosen to present the data in terms of frequency variation, which represents an estimate of the change in mass over the sensor. (a)

(b)

Figure 1. (a) QCM results shown as change in frequency as a function of time for crude oil with different n-heptane contents. The percentage displayed in the chart legend indicates the concentration of n-heptane relative to the asphaltene precipitation onset (1.8 mL C7/g oil). The data were obtained in duplicate, with the uncertainty indicated by the width of the lines. (b) QCM results for the oil with n-heptane at 95% of the onset plotted in log scale at y-axis. The intersection of the dotted line indicates the transition point.

The measurements represented in Figure 1 indicate a greater mass gain as the flocculant content increases. This mass gain increases more significantly in time at the beginning of the measurements (a zoom of the first ~2 h is shown in Fig. S2) and


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decreases its slope with time, suggesting a saturation behavior. Another interesting observation from the data in Figure 1 is that the amount of asphaltene deposited over a certain interval does not increase linearly with the flocculant content, because a more prominent increase is observed as the onset of precipitation is approached. When plotted in log scale at y-axis (as in Fig. 1b), these data seem to conform to two distinct behavior, with the break indicating a transition. For the other plots (Fig. S3), the time at which this transition takes place is the same. The effect on inhibitors on QCM deposition curves at n-heptane content of 95% of onset is shown in Figure 2. This figure also contains replicates of the QCM measurements, which confirm that these measurements are very reproducible at the beginning (around the first 6 h), becoming less reproducible for longer measurements possibly as a consequence of the thickening and complexity of the deposited layer. (a)

(b)

Figure 2. (a) QCM results shown as change in frequency as a function of time indicating asphaltene deposition onto gold surface. Data collected for systems with different inhibitors (DBSA – dodecylbenzene sulfonic acid, NF – nonylphenol and CA 1


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and CA 2 – commercial additives) at 95% of the onset. Replicates were presented separately. (b) is a zoom of (a).

In terms of additive effects, the results on Figure 2 show that DBSA does not significantly affect the mass gain when compared to the system without additive. The other inhibitors (NF, CA 1 and CA 2), on the other hand, significantly reduce the mass gain. Furthermore, a 1:1 mixture of DBSA and NF causes an intermediate effect which is not far from pure NF, suggesting that the effect is controlled by the latter. For this reason, we have performed studies varying NF content below 1 wt %, as shown in Figure 3. These results clearly show that NF displays an already significant inhibition capacity even at concentrations as low as 0.1 wt %.

(a)

(b)


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Figure 3. QCM results shown as change in frequency as a function of time for crude oil with n-heptane concentration at 95% of the onset at varying amounts of nonylphenol: from 0 to 1 wt %. The data were obtained in duplicate and the width of the lines indicates the uncertainty. (b) is a zoom of (a).

To further investigate the asphaltene deposits caused by n-heptane addition to pure oil or in the presence of additives, the gold plate surfaces from the QCM equipment were analyzed after 1–24 h of experiment with a laser scanning confocal microscope. Figure 4 shows the results observed for all systems with the inhibitors previously studied by QCM. These results show the presence of black spots over the gold surface, which confirms the deposition measurements obtained with the QCM instrument. Moreover, these images clearly reveal the presence of more black spots at the system without the addition of any inhibitor after 24 h of experiment.


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Figure 4. Images obtained by confocal microscopy with laser scanning combining laser intensity with optical view of oil-immersed samples at 95% of the onset for (a) sample without additives and samples with (b) 1 wt % of NF, (c) 1 wt % of DBSA, (d) 0.5 wt % of DBSA + 0.5 wt % of NF, (e) 1 wt % of CA 1. Scale bar of 500 μm.

For the systems with DBSA, some black spots related to asphaltene deposits appear earlier than for the other systems (see supplementary material, Figures S4 and S5), agreeing with the lower onset for heptane induced asphaltene precipitation determined with this additive (see Table S1). The decrease in the onset using DBSA as an additive, although unusual, has already been reported in the literature, describing that this value could increase or decrease by changing the concentration of the inhibitor.27


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In general, a higher density of black spots is observed in the systems with DBSA, DBSA-NF mixture and no addition of inhibitor. Moreover, it is clear the higher concentration of aggregates in the system without additives as the immersion time increases. Besides that, we can divide the inhibitor capacity in two groups: the first one: with the crystal almost completely clean (CA 1 and NF); and the other with a clear tendency of asphaltene deposition (DBSA, DBSA-NF mixture and control, with not additives). In order to confirm the black spots as asphaltenes, we analyzed some oil samples subject to different treatments. Figure 5 shows micrographs obtained for the gold surface after QCM measurements in oil with n-heptane at 95% of the onset after being washed with n-heptane (in order to remove the excess of oil) and toluene (as an attempt to remove also asphaltene deposits). For this, the system without inhibitor, which remained in contact with the oil-flocculating medium for 24 h, was washed with 0.5 mL of n-heptane. This removed the oil residue (maltenes, but not asphaltenes) that was forming the flow paths and recovered the particle surfaces. One can clearly see dark spots at the gold surface after oil removal, becoming darker and smaller after washing with n-heptane. After magnification, these deposits resemble asphaltene particles observed upon microscopy after flocculation with alkanes. Toluene washing, on the other hand, seems to remove them, as expected for being a good solvent. These results confirm the attribution of these black spots as asphaltene deposits.


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Figure 5. Combined images of laser intensity with optical images obtained by confocal microscopy with laser scanning of gold surfaces immersed for 24 h in oil with nheptane concentration of 95% of the onset. The bottom images are magnifications. Figure S6 (available in Supplementary Material), in turn, shows that the aggregates, prior to washing, actually are roughly spherical due to the presence of residual oil film. These aggregates present a width of ca. 15 µm and height around 7 µm. Moreover, Figure S7 (available in Supplementary Material), shows that the aggregates continue to grow, reaching extensions of up to ca. 0.5 mm after 196 h of exposure (for the system without additive). This result agrees with the QCM data, which shows a clear increase in the material deposited even after 24 h. It is important to point out that the residual oil observed in some images may be masking smaller particles of asphaltenes. An indication of this effect is the way the oil film presents itself, forming interference paths that are affected by single points, apparently small aggregates of asphaltenes (Figure S4 of the Supplementary Material). To verify this hypothesis, the surface of the sample without additive that was in contact with the oil for 1 h was washed with 0.5 mL of n-heptane. Following, the surface was


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analyzed in a Park Systems NX-10 atomic force microscope, which revealed aggregates with heights up to 0.1 μm (Figure 6). The presence of these objects might indicate that they already existed on the surface and were masked by the presence of a residual oil film, washed by n-heptane, though one cannot rule out heptane induced precipitation.

Figure 6. Atomic force microscopy images from gold surfaces immersed for 1 h in oil with n-heptane at 95% onset in the absence of additives, after washing with 0.5 mL of nheptane. The image shows two regions of the sample. On the left, topography; on the right, phase contrast maps.

To further investigate the deposition behavior, stability tests were also performed by another technique (accelerated sedimentation) in order to assess inhibitor capacity. The equipment used, LUMiSizer, also allows an estimate of aggregate sizes. Differently from the previous experiments, these were conducted at large excess of nheptane (mixture of oil, n-heptane and toluene in the volume proportion of 1:8.3:2.7). These conditions were optimized to produce a convenient amount of asphaltene and for samples with enough transmission to allow size measurements.


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Figure 7 shows the change in the determined instability index versus time (in seconds). The instability index is calculated by observing changes of the transmission profiles along the entire length of the cuvette. An instability index of 1 indicates complete phase separation, and the lower this index, the more stable the dispersion. These results show that, under these conditions of large flocculant excess, only the commercial additive CA 1 delays the asphaltene precipitation significantly. It is important to note that with this technique, it is not possible to replicate the same experimental conditions used in QCM due to the opacity of the medium for the latter. As discussed in the sample preparation section, the inhibitor and oil concentrations for LUMiSizer experiments are about seven times lower than in the QCM experiments. Therefore, this concentration reduction implies in reducing the inhibitor capacity, as shown in Figure 3, which indicates the equivalence of the two results. (a)

(b)

Figure 7. Instability index for asphaltene precipitation as a function of time. Measurements performed in duplicate for samples without additives; and in the presence of 1 wt% of NF; 1 wt% of DBSA; 1 wt% of CA 1; 1 wt% of CA 2; or in the mixture of 0.5 wt% of DBSA and 0.5 wt% of NF. (b) is a zoom of (a)


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It was also possible to estimate the size of the aggregates, taken as diameters of equivalent spherical particles, and providing continuous and dispersed phase information (refractive index and density for both, and viscosity, only for the first). Figure 8 and Table 2 summarize the obtained results.

Figure 8. Size distributions and cumulative curves for pure oil and with 1 wt% of CA 1 samples. The oil + 1 wt % of DBSA, oil + 1 wt % of NF and oil + 0.5 wt % of DBSA + 0.5 wt % systems were suppressed to facilitate the visualization of the results, since they overlapped the pure oil system data.

Table 2. Estimates of the average aggregate size obtained by the LUMiSizer System Crude oil Crude oil + 1 wt % of DBSA Crude oil + 1 wt % of NF Crude oil + 1 wt % of CA 1 Crude oil + 1 wt % of CA 2 Crude oil + 0.5 wt % of DBSA+ 0.5 wt % of NF

Apparent diameter (μm) 3±1 3.5 ± 0.9 3±1 0.2 ± 0.1 3±1 4±1

In order to verify the accuracy of the aggregate size determined by the accelerated sedimentation measurements in LUMiSizer, we analyzed the sample with pure crude oil in an optical microscope (Figure S8 of Supplementary Material). The average size for the observed aggregates was (3.9 ± 0.1) μm, in a good agreement with the LUMiSizer estimates.


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In addition, the results shown in Table S2 and Figure S9 (Supplementary Material) allow an assessment of the evolution of phase separation both in terms of the transmission profiles (Fig. S9) and aggregates size (Table S2). The Table S2 indicates the presence of particles in the range of 1–4 μm in the systems of pure oil and oil + DBSA or NF. On the other hand, when CA 1 is added, the particles are smaller, in the range of 0.1–0.4 μm. With the addition of CA 2, a broader size range around 0.3–5 μm is observed. Accordingly, DBSA and NF, in the concentrations used, do not delay the aggregation and the sedimentation occurs fast (ending in 40 min), consequently these additives are not effective in preventing aggregate growth. On the other hand, CA 1 considerably reduces growth and retards phase separation (over 22 h). As a intermediate behavior, CA 2 displays less significant effect: aggregate growth is prevented, but a few larger ones remain (bigger than 1 μm). As a result, phase separation occurs in around 4 h.

DISCUSSION

The QCM data are a result of asphaltene aggregation and transport from these aggregates to the surface of the crystal involving a multistage process.17 However, the adsorption mechanisms may be different depending on how far the system is from the precipitation onset.17 In general, asphaltene flocculation in organic solvent is propose to follow four steps: molecular self-association in solution, asphaltene particles nucleation and growth of asphaltene particles, followed by particles aggregation resulting in phase separation. 29 The present results show that the QCM technique is capable to detect changes in mass due to deposition of asphaltenes over gold surface, allowing one to monitor its


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aggregation both in a qualitative and quantitative manner. The resulting kinetic data can be represented by two power law functions, suggesting that the asphaltene adsorption proceeds via at least two distinct mechanisms. Some experimental limitations hinder the direct transformation of the frequency data into direct mass information due to effects of liquid trapped in aggregate structures, for instance, but that would become more important at later stages of deposition. In addition, results reported in the literature17 evidence only negligible loading effects on asphaltene solutions due to changes in oil viscosity, that could also affect these results. Therefore, one can assume that the curves of -Δf values as a function of time represent a good description of the quantity of asphaltene deposited on the gold substrate. In the current literature,28-30 aggregation in colloidal systems can be generally viewed as a process that is controlled either by diffusion that modulate particle collisions, what is known as the DLA model, or assuming that there is an energy barrier for the aggregation according to the RLA model.30 According to the results reported in the literature, for asphaltene aggregation at the beginning of aggregation, the RLA model is dominant, and then the aggregation process becomes controlled by DLA with the increase of aggregate size.28-30 Figure 9, adapted from reference 26, depicts how asphaltene precipitation process can be described by these models: the RLA and DLA regimes followed by the sedimentation of the aggregates.


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Figure 9. Representation of asphaltene aggregation behavior, adapted from reference 28.

From this standpoint and considering the results obtained in this work with QCM measurements, we observe that, at low n-heptane concentrations (0-75% of the onset), the shape of the curve resembles that of the RLA curves. On the other hand, at higher concentrations (95-115% of the onset), the curves display the DLA and sedimentation profiles. This trend agrees with recent attempts to correlate these aggregation models to asphaltenes, though obtained in model systems.28 Besides, this observation emphasizes the importance of considering kinetics when dealing with asphaltene aggregation, an important factor that is sometimes neglected.31 From these QCM results, the asphaltene deposition takes more than 70 h, but it is possible to distinguish the different behavior among different compositions using data recorded up to 1.5 h. Increasing n-heptane content, more unstable asphaltene nuclei are formed leading to more frequent efficient collisions and, hence, faster deposition. Figure 2 suggests that adding different inhibitors to the formulation at 95% of the onset affects the deposition mechanism, consistent with a change from a DLA to an RLA model behavior. This result highlights the role of the inhibitor in the stabilization of asphaltene particles, minimizing their aggregation.


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Besides, decreasing the NF concentration (Figure 3), we observed curves with a DLA-like profile (0 and 0.00125 wt.%) indicating an insufficient concentration to produce more stable particles. DBSA data, however, does not show the effect expected considering its well-known activity as asphaltene dispersant,32,33 indicating that this concentration is below that necessary to the transition from DLA to RLA behavior, i.e., to prevent asphaltene growth in this crude oil. The advantage of using this methodology (QCM combined with laser scanning confocal microscopy) is the identification of the mechanism (DLA and RLA) and the minimum inhibitor concentration to produce deposition curves that follow the RLA model, therefore producing a slower deposition. In another perspective, results derived from accelerated sedimentation measurements allowed the assessment and discrimination of effects of both model and commercial inhibitors at large flocculant excess, confirming earlier reports.34 The instability index is able to indicate the efficiency of an inhibitor in the delay/prevention of phase separation (asphaltene precipitation). In general, it is noted that, in the dilution conditions uded (oil:n-heptane:toluene = 1:8.3:2.7), only commercial inhibitors are capable of retarding asphaltene precipitation. The CA 1 inhibitor is more effective, delaying the process by ~ 22 h; while the CA 2, only in ~ 4 h (Table S2). This technique also allowed an assessment of the size distribution of asphaltene particles that could be compared with results obtained from microscopy investigation (Fig. S8). The size distribution is well correlated to the sample stability. While samples without CA 1 present aggregates with an average size of 3 μm, the system with this inhibitor presents aggregates of 0.2 μm. This result indicates that the inhibitor efficiency is associated with its actuation mechanism, which reduces aggregate growth. Therefore,


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for this oil, the use of the CA 1 inhibitor may indicate greater success in remediation of a well. Figure 10 shows schematically what might be occurring in the system in the absence and presence of inhibitors, respectively. It is important to emphasize that the inhibitors behave differently because their action involves affinity and interaction with distinct groups. The results of QCM combined with LUMiSizer show an efficiency screening allowing the selection of the best inhibitor to specific oil. In this case, their efficiency follows the ensuing order: DBSA < DBSA-NF mixture < NF < CA 2

Direct assessment of inhibitor and solvent effects on the deposition

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