Waxy Crude Oil Emulsion Gel: Chemical Characterization of

Nov 17, 2014 - Higher O/O2 class ratios were associated with crude oils that formed gel emulsions, indicating that O class compounds play an important...
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Waxy Crude Oil Emulsion Gel: Chemical Characterization of Emulsified Phase Extract Components Rosana C. L. Pereira,*,† Rogério M. Carvalho,† Bruno C. Couto,† Márcia Cristina K. de Oliveira,† Marcos N. Eberlin,‡ and Boniek G. Vaz§ †

Cenpes, Petrobras, Rio de Janeiro, Rio de Janeiro 21941-915, Brazil Thomson Mass Spectrometry Laboratory, Institute of Chemistry, University of Campinas (UNICAMP), Campinas, São Paulo 13083-970, Brazil § Instituto de Química, Universidade Federal de Goiás, Goiânia, Goiás 74001-970, Brazil ‡

S Supporting Information *

ABSTRACT: Emulsions of several waxy crude oils exhibiting similar properties were prepared and characterized to better understand the nature of their polar components. Both crude oils and their corresponding emulsions were analyzed by negativeion electrospray Fourier transform ion cyclotron resonance mass spectrometry [ESI (−) FT-ICR MS]. The most abundant classes found in these crude oils were neutral N, acidic O, and O2 heteroatomic compounds. The N class was present in higher relative abundance in all samples, followed by the O class (phenolic-like compounds). The O2 class was minor in all crude oils, corroborating their low total acid number (TAN) values. The O/O2 class ratios were directly correlated with their rheological properties. Higher O/O2 class ratios were associated with crude oils that formed gel emulsions, indicating that O class compounds play an important role in gel emulsion stabilization. The identified phenolic compounds possess a structure that allows for (i) intermolecular hydrogen-bonding interactions that facilitate the formation of large aggregates and (ii) van der Waals interactions (e.g., π stacking and solvophobic effects) that promote attraction of the large aggregates. We propose that the alignment of paraffins and the alkyl side chains of phenolic-like compounds contribute to the emulsion stability and affect the rheological behavior of the highly paraffinic crude oils. The analysis of the emulsion phase confirmed the presence of the O class (phenolic compounds) as well as the acidic O2, O4, O4S, and nitrogen-containing classes (NO and NO2), indicating the surface activity of compounds bearing at least one oxygen atom. These results suggest an enhanced activity of O-containing compounds in gel formation and the stabilization of emulsions of highly paraffinic crude oils.

1. INTRODUCTION

misrepresentations of the emulsion stability models in waxy gel systems. Visintin et al.8 proposed that waxy gel emulsions are stabilized by paraffin particles at temperatures below the wax appearance temperature (WAT) that adsorb or coat water droplets. Once the flocks of solid paraffin amass on or between water droplets, the dispersed water becomes entrapped by a waxy crystal network. The stability of this system can be attributed to two factors: (1) the n-paraffin content and composition [carbon number distribution and molecular weight (MW), for instance] or (2) the polar components present in oil that influence the water−oil interface, facilitating a possible adsorption process. The composition of emulsion films ultimately governs their stability and rheological properties.9 Adsorbed multi-layered colloidal paraffin particles stabilize rigid emulsion films, whereas the adsorbed water and polar compounds from crude oil yield stable emulsion interfacial material.10 In this work, to realize their crucial role in crude oil emulsion stability and, therefore, to characterize the polar components present at the emulsion phase better, a set of similar waxy crude

The formation of crude oil emulsions is very common in the petroleum industry, leading to significant flow assurance challenges during oil production. Such emulsions may be very stable because of the presence of polar compounds, such as asphaltenes and resins, at the crude oil/water interface. Fine solids may also be present in the emulsion systems, forming resistant films at the drop interface.1−4 Although the effect of dispersed water on crude oil rheology has been wellcharacterized, little is known about the effect of water on waxy crude oil emulsions, in which an unusual rheological behavior has been observed.5 These emulsions can be highly stable and exhibit high viscosities, and basic oil properties have failed to sufficiently explain this unusual behavior. Sjoblom and co-workers6 have studied waxy crude gel emulsions and observed a relationship between higher viscosities and lower resin/asphaltene ratios. The content and characteristics of n-paraffin in petroleum systems were also found to directly impact their rheological behavior.7 Composition of polar components in crude oils is very diverse, suggesting a highly complex interaction of such components at the emulsion interfaces. Considering only the total asphaltene content or the resin/asphaltene ratio and neglecting the detailed composition of these fractions may result in © XXXX American Chemical Society

Received: April 28, 2014 Revised: October 24, 2014

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to ensure the validity of the MW distribution from the FT-ICR analyzer. In addition to an external calibration, an internal recalibration was applied to the peak list using Composer software (Sierra Analytics, Modesto, CA) prior to the final peak assignment. A set of theoretical homologues for a specific heteroatom class (most abundant class for each ion mode) was selected as an internal calibrant because of its presence in all samples, low errors, and high average peak intensities. For each spectrum, automated analysis was used to assign formulas to ion peaks with a signal-to-noise ratio (S/N) > 3. The allowed elements were 12C, 1H, 16O, 14N, 32S, and 13C. The maximum allowed formula error was 1 ppm, and the mass limit for empirically assigning elemental formulas was 500 Da. Formulas above 500 Da were assigned through the detection of a homologous series. For each elemental composition, CcHhNnOoSs, the heteroatom class, type [DBE (number of rings plus double bonds)], and carbon number (Cn) were tabulated for the subsequent generation of heteroatom class relative abundance distributions and graphical DBE versus Cn plots.

oils were selected and their resulting emulsions were submitted to phase separation. Comprehensive chemical characterization (class, MW, carbon number, and insaturation level) as measured by double bond equivalent (DBE) distributions of both the crude oils and their corresponding emulsions was performed by ultrahigh-resolution and accuracy negative-ion electrospray Fourier transform ion cyclotron resonance mass spectrometry [ESI (−) FT-ICR MS].

2. EXPERIMENTAL SECTION 2.1. Samples. Six Brazilian waxy crude oils, identified as A−F, were selected for this study. The main properties of the crude oils are reported, and the values of their physical−chemical properties can be considered typical of many waxy crude oils. 2.2. Basic Characterization of Crude Oils. Saturate, aromatic, resin, and asphaltene (SARA) analysis was performed in a thin-layer chromatography−flame ionization detector (TLC−FID) system as reported.11 The asphaltene content was determined by the IP143 method. The WAT was measured by differential scanning calorimetry (DSC)12 in a temperature range from 80 to −40 °C. The total acid number (TAN) was determined according to the ASTM D664 method. The density value was obtained by the ISO12185 method. The water content of the crude oils was measured by Karl Fischer titration. The n-paraffin content was determined as described in our previous work.7 2.3. Emulsion Preparation and Stability. Water-in-oil (w/o) emulsions were prepared using synthetic brine consisting of 5.0 wt % NaCl in Milli-Q water at aqueous volume fractions of 10, 30, 50, and 70%. The crude oils were thermally pre-conditioned in an oven at 60 °C for 1 h to redissolve any precipitated wax prior to the addition of the aqueous phase. The crude oil and water emulsification (250 mL) was performed using an Ultra Turrax homogenizer at 8000 rpm for 3 min. Stability tests with 50 mL of emulsion (duplicate experiment) were performed by centrifugation at 15 000 rpm [19 872 relative centrifugal force (RCF)] and 60 °C to determine the emulsion stability as a function of the water separation volume. 2.4. Rheological Analysis. Rheometric measurements were performed using a Haake controlled-stress rheometer (Thermo Fisher Scientific). The dynamic viscosity was measured using coaxial cylinder geometry (Z20 sensor), while the emulsion was cooled at a programmed cooling rate (1 °C/min) from the starting temperature (60 °C) to the hold temperature (4 °C). The range of the applied shear rate was 20−250 s−1. 2.5. Phase Separation. After centrifugation, the oil, water, and emulsion phases were separated and analyzed. The liquid phases (water and oil) were removed, and the emulsion layer (solid-like) was separated. This emulsion phase (2 g) was submitted to a Dean−Stark distillation procedure with 80 mL of xylene. The resulting extract was analyzed by ESI (−) FT-ICR MS. It is expected the polar components that stabilize emulsion are more concentrated in this emulsion phase extract. 2.6. ESI (−) FT-ICR MS. Crude oil and emulsion phase extract samples (approximately 4 mg) were dissolved in 10 mL of toluene, and 0.5 mL of the resulting solution was transferred to a 1 mL vial and diluted with 0.5 mL of methanol containing 0.2% ammonium hydroxide. The solvents and additives were high-performance liquid chromatography (HPLC)-grade, purchased from Sigma-Aldrich, and used as received. The general ESI conditions were as follows: capillary voltage of 3.10 kV, tube lens of −100 V, and flow rate of 3 μL min−1. Ultrahigh-resolution and accuracy FTMS was performed using a Thermo Scientific 7.2 T Fourier transform ion cyclotron resonance mass spectrometer (LTQ FT Ultra, Thermo Scientific, Bremen, Germany). A scan range of m/z 200−1000 was used, and 100 microscans were accumulated for each spectrum. The average resolving power (Rp) was 400 000 at m/z 400, where Rp was calculated as M/ΔM50%, that is, by the m/z value divided by the peak width at 50% peak height. The time-domain data (ICR signal or transient signal) were acquired at 700 ms. The MW distribution for each sample was first verified by linear trap quadropole (LTQ) analysis

3. RESULTS AND DISCUSSION Analysis were performed using FT-ICR MS because this technique is known to provide ultrahigh resolution and accuracy as measured by a mass resolving power, m/mΔ50% > 200 000 (mΔ50% is the mass spectral full peak width at half height), from 225 < m/z < 1000, which enables the differentiation of isobaric compounds with an accuracy within 300).15d Because high-MW polar compounds may interact synergistically with paraffins, asphaltenes, and resins to stabilize the emulsions,17 we plotted the number-average (Mn) and weightaverage (Mw) molecular weights calculated from the spectra of

Table 2. Viscosity and Stability Data for the Crude Oil Emulsions Studied sample crude oil emulsion 50% water cut emulsion 70% water cut emulsion 50%

A 20.1 243.3 1156.4

43.2

B

C

D

E

Viscosity (mPa s) at 60 °C and 50 s−1 15.6 11.2 9.7 8.3 101.7 126.0 101.1 66.3 1782.8

1141.5

Stabilitya at 60 °C 47.2 2.0

F 8.0 80.6

235.5

181.7

283.3

62.0

40.4

13.2

a

The water volume resolved after the centrifugation process in 100 mL of sample (mL).

Kilpatrick and co-workers.16 With respect to the waxy systems, emulsion sample D has a low tendency to form wax gel based on the n-paraffin content and WAT values (Table 1). Emulsions A, B, and E exhibited a high water content separation, probably because of the high n-paraffin content (the stability test was performed at 60 °C). Emulsions C and F were the most stable, despite their dissimilar characteristics: emulsion C is a waxy gel, whereas emulsion F is not. It remains unclear whether a possible correlation between the n-paraffin content and the low ratio of resin/asphaltene contributed to the stability/gelification results observed for these emulsions. Emulsions prepared with crude oils that have a low ratio between resin and asphaltene are considered stable as reported by Kilpatrick.16b Emulsions A and B were less stable than emulsion C. In relation to the asphaltene content, A−C exhibited similar characteristics. These results reinforce our previous finding7 that the behavior and stability of waxy gel emulsions are considerably complex and should not be explained on the basis of only the interactions between nparaffins and asphaltenes.

Figure 1. ESI (−) FT-ICR MS spectra of methanol/toluene solutions of the six crude oils. C

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Figure 2. Correlation between Mn and Mw and m/z ratios of the polar compounds detected by ESI (−) MS of crude oils.

Figure 3. Relative abundance of the acidic NSO classes in crude oil samples.

compositions of the crude oils. The group 2 crude oils exhibited O2 class compounds in higher abundance than the crude oils from group 1, although it was not the case for TAN values of all samples. To verify the differences in the class profiles of the crude oils, we compared the O/O2 class abundance ratio (Figure 4). Two groups with distinct O/O2 class ratios were identified. Group 1

the crude oils (Figure 2). We anticipated that group 1 (samples A−C with higher viscosities and gel behaviors) would exhibit higher Mn/Mw values compared to group 2 (samples D−F with lower viscosity emulsions). In comparison of samples A and B of group 1 to samples D and E of group 2, it is possible that indeed high-MW polar compounds contributed to the higher viscosity and gel behavior of these samples. For example, highMW polar compounds possessing long carbon chains may form a link between water and wax, contributing to the gel network and emulsion stabilization. Samples C and F, however, exhibited a distinct profile in their respective groups, preventing the correlation of the MW of their polar compounds to their rheological behavior. For these samples, other factors can explain their rheological behavior. To investigate the overall changes in the chemical compositions of the crude oils, we plotted the distributions of the heteroatom classes (Figure 3). The abundance of the dominant homologous series identified in each spectrum was divided by the total observed ion abundance in each spectrum. All six crude oils contained the same molecular classes, namely, the pyrrolic benzologue species containing a single nitrogen atom. Only subtle differences were observed in the

Figure 4. O/O2 class ratios for crude oils A−F. D

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Figure 5. Acidic NSO class relative abundance for emulsion phase extracts (EE) and their corresponding parent crude oils A−F.

Figure 6. Plots of DBE versus Cn for O and O2 classes for crude oil samples A−F.

E

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the emulsion interface and the differences in the n-paraffin content support the theory of a multi-component, multi-layered interface adsorption. The crude oils from group 1 exhibit a high abundance of O2 compounds with DBE of 1 as well as a high content of O-containing compounds and n-paraffins, providing favorable conditions for multi-component, multi-layered interface adsorption and contributing to the high stability of their emulsions and gel networks. In contrast, the crude oils from group 2 contain O2 compounds with DBEs ranging from 1 to 10 as well as a low content of O-containing compounds and nparaffins compared to the crude oils from group 1. Sample E exhibits a high content of n-paraffin but a lower abundance of O-containing compounds compared to the crude oils from group 1.

exhibits higher O/O2 values, whereas group 2 exhibits lower values. These contrasting values suggest that O-containing molecules play an important role in the rheological behavior of crude oils. These compounds possess a structure that allows for (i) intermolecular hydrogen-bonding interactions that facilitate the formation of large aggregates and (ii) van der Waals interactions (e.g., π-stacking and solvophobic effects) that bring the large aggregates together.17 We believe that the alignment of paraffins and the alkyl side chains of phenolic-like molecules contribute to the formation of stable emulsion gels and greatly affect the rheological behavior of these crude oils. Further considerations can be made about the effects of Ocontaining compounds on emulsion stability. According to Visitin et al.,4 wax particles can be strongly adsorbed at liquid− liquid interfaces, forming pickering emulsions below the WAT. This phenomenon increases the stability of the emulsions by increasing the interfacial film viscosity and reducing the coalescence of the drops. It can be suggested that, because of the strong interactions between wax crystals and the drop surface, the growth of the gel network involves the droplets themselves. The O-containing compounds strengthen these interactions, trapping water inside the gel network. To further investigate these O compounds, the emulsion phase extracts were also analyzed. Figure 5 shows the percent relative abundances for the negative-ion heteroatomic classes in the w/o emulsion interface material of crude oils. Similar to the parent oils, the N class was predominant in the emulsion phase extract samples. However, these samples also presented the O class (phenolic compounds) as well as acidic components, such as O2, O4, and O4S, and NOx classes (NO and NO2), demonstrating surface activity for compounds with at least one oxygen atom. Although the O class was present in lower relative abundance in these materials compared to the parent oils, we believe that molecules from this class play an important role in gel formation and emulsion stabilization. To understand the structural impact of the O and O2 classes on the rheological behavior of the two groups, we also compared the DBE and Cn range (Figure 6). The most abundant O class ions in all of the oils exhibited DBE values between 4 and 12, with Cn from C10 to C52, and this pattern was similar across all samples. Alkylated monoaromatic phenols (DBE of 4) were predominant, followed by mono- and dicyclic monoaromatic phenols (DBE of 5 and 6, respectively). Di- and triaromatic phenols (DBE of 7 and 10, respectively) were also present in all of the crude oils. Such polyaromatic phenols can exhibit a sheet-like three-dimensional structure that stacks as thin, rigid, multi-layered emulsion films.18 For the O2 class, a distinct profile was observed in the two oil groups. The most abundant ions in group 1 exhibited a DBE of 1 characteristic of fatty acids, with Cn from C10 to C30. In contrast, group 2 exhibited DBEs ranging from 1 to 6 and Cn ranging from C10 to C50. The trends in the profiles of the Ox classes suggest a multilayered w/o interface behavior, as observed previously.1 We suspect that the low MW O2 molecules (samples A−C have lower MW O2 molecules than samples D−F) first adsorb in a monolayer at the interface, the O-containing compounds adsorb to the fatty acids, and the paraffins align with the alkyl chains of the O compounds in a third layer through van der Waals interactions. Previously, Poteau et al.18c and Wu et al.18d described results that might support our theory that small surface active compounds adsorb at the oil-in-water (o/w) interface. The high abundance of Ox compounds adsorbed at

4. CONCLUSION Detailed composition of six crude oils and their emulsion phase extracts provided by ESI (−) FT-ICR MS indicates that O- and O2-containing molecules contribute to the formation of highly stable and viscous emulsions. The oil samples that produced waxy gel emulsions exhibited higher O/O2 ratios, whereas the oil samples that failed to produce gelled emulsions exhibited low O/O2 ratios. The superior viscosities observed in the waxy emulsions A− C result from their greater abundance of O-containing compounds (phenols), linear fatty acids (low mass compounds), and n-paraffins. In contrast, the inferior viscosities observed in the waxy emulsions D−F are due to the low abundance of O-containing compounds, the low abundance of O2 compounds, and the low abundance of n-paraffins. Crude oil E represents the only deviation from this pattern, with a high abundance of n-paraffins. Despite its high n-paraffin content, crude oil E exhibits a low abundance of O-containing compounds, which degrades the synergism that is responsible for the rheological behavior observed in crude oils A−C. As in our previous work, herein, we demonstrate that the presence of water increases the magnitude of the rheological properties of waxy crude oil gels. This water effect is attributed to the network produced by the aggregation of wax and water, which is mediated by polar compounds. In this study, a comparison of the polar compositions of crude oils and their emulsions suggested the presence of complex interactions at the emulsion interfaces and that gel network formation and further emulsion stabilization are due to the presence of multilayered films composed of low mass fatty acids, O-containing compounds, and paraffins. The extraction of O-containing compounds for subsequent incorporation into crude oils that fail to form waxy crude oil gels could provide additional evidence for the involvement of these compounds in gel network formation.

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ASSOCIATED CONTENT

S Supporting Information *

Acidic NSO class relative abundance for emulsion interface materials and their corresponding parent crude oils (Table S1), high-viscosity emulsion (solid-like gel) (Figure S1), and phase separation after the centrifugation procedure (Figure S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Petrobras for permission to publish this work. REFERENCES

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