Energy Fuels 2010, 24, 4387–4395 Published on Web 07/19/2010
: DOI:10.1021/ef100574m
Sensitivity Study of Naphthenic Acids from Flow Assurance Deposits Characterized by Low-Resolution Mass Spectrometry Andrew G. Shepherd,† Ken S. Sorbie, Gillian B. Thomson,* and Robin E. Westacott Heriot Watt University, Riccarton, Edinburgh, EH14 4AS, United Kingdom. Current address: Nederlandse Aardolie Maatschappij B.V., Schepersmaat 2, 9405 TA, Assen, The Netherlands.
†
Received May 7, 2010. Revised Manuscript Received June 29, 2010
Soaps constitute one of the more recent challenges in exploration and production flow assurance. In this work, the naphthenic acids from a field soap deposit were extracted and analyzed using low-resolution mass spectrometry, with the aim of examining the effects of ionization sources, operational settings, and the impact of solvents. Results suggest that fast atom bombardment (FAB), atmospheric pressure chemical ionization (APCI), and electrospray (ES) ionization sources have different effects on the samples and lead to different spectra. FAB led to the highest signal for the so-called ARN acids, as well as a range of signals for monocarboxylic acids. The signals for ARN acids obtained using both the ES and APCI sources were similar to each other in terms of intensity. Nevertheless, because of the differences in monocarboxylic acid signals between these sources, the spectrum obtained with APCI, most likely, suggests favorable ionization of ARN acids. This was supported by the analysis of the spectra of a commercial naphthenic acid mixture using the three ionization sources. Mass spectrometry data also showed that the APCI source led to the formation of multimers of higher molecular-weight naphthenic acid for the concentrations studied. More detailed investigations of the operation of the ES source demonstrated that a decrease in source voltage resulted in favored ionization and detection of the ARN acids over the monocarboxylic acids. Moreover, the use of more polar solvents in combination with the ES source also resulted in favored ionization of ARN acids. Use of selected conditions with liquid chromatography followed by tandem mass spectrometry allowed for the identification of four carboxylic acid functionalities for a selected ARN acid ion. Overall, this information points to the importance of various mass spectrometry variables in naphthenic acid detection, particularly with different acid families, and will aid in the development of a combination of quantitative methods for use with field samples. On the basis of our work and the sources examined, the ES source would be the preferred setting for an overview of the general fingerprinting of the naphthenic acids in deposits. The APCI source may be the preferred setting for work focusing on ARN identification and characterization. The ES source was also used to investigate soaps formed in the laboratory conditions using aqueous phases of varying ionic composition. The presence of ARN and non-ARN species in the soap generated in the laboratory suggests competing effects during the soap formation process, as well as the availability of mono- or divalent ions in the parent aqueous phase for the precipitation mechanism.
soap deposits.4-10 Sodium carboxylate soap emulsions have been shown to contain a predominance of fatty acids.4,9 Baugh et al. showed that calcium naphthenate soap scales contained a family of high-molecular-weight naphthenic species, which they called ARN and suggested were tetra-acids.5,6 More recently, Sjoblom and co-workers have shown a range of molecular structures that fit this description by analyzing calcium naphthenate deposits.7,10-12 However, ARN acids
Introduction The precipitation of soaps in the oilfield environment represents a challenge in exploration and production flow assurance.1,2 Soaps may be divided into three major categories: calcium naphthenate soap scales, sodium carboxylate soap emulsions, and bound soap scales.3,13 Recently, a number of research groups have examined the composition of field *To whom correspondence should be addressed. E-mail: g.b.thomson@ hw.ac.uk. (1) Vindstad, J. E.; Bye, A. S.; Grande, K. V.; Hustad, B. M.; Hustvedt, E.; Nergard, B. Presented at the 5th International Symposium on Oilfield Scale, Aberdeen, U.K., 2003; SPE 80375. (2) Hurtevent, C.; Ubbels, S. Presented at the 8th International Symposium on Oilfield Scale, Aberdeen, U.K., 2006; SPE 100430. (3) Turner, M.; Smith, C. Presented at the 7th International Symposium on Oilfield Scale, Aberdeen, U.K., 2005; SPE 94339. (4) Shepherd, A. G.; Thomson, G. B.; Westacott, R.; Sorbie, K.; Turner, M.; Smith, P. C. Presented at the 8th International Symposium on Oilfield Scale, Aberdeen, U.K., 2006; SPE 100517. (5) Baugh, T.; Grande, K. V.; Mediaas, H.; Vindstad, J. E.; Wolf, N. Presented at the 7th International Symposium on Oilfield Scale, Aberdeen, U.K., 2005; SPE 93011. (6) Baugh, T. D.; Wolf, N. O.; Mediaas, H.; Vindstad, J. E.; Grande, K. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 2004, 49 (3), 274–276. r 2010 American Chemical Society
(7) Brandal, O. Interfacial (O/W) properties of naphthenic acids and metal naphthenates, naphthenic acid characterization and metal naphthenate inhibition. Ph.D. Thesis, Norwegian University of Science and Technology, Trondheim, Norway, 2005. (8) Yen, A.; Debord, J.; Asomaning, S.; Craven, M.; Rodgers, R. P.; Marshall, A. G. Presented at the 7th International Conference on Petroleum Phase Behaviour and Fouling, Asheville, NC, 2006. (9) Gallup, D. L.; Star, J. Presented at the 6th International Symposium on Oilfield Scale, Aberdeen, U.K., 2004; SPE 87471. (10) Simon, S.; Nordgard, E.; Bruheim, P.; Sjoblom, J. J. Chromatogr., A 2008, 1200, 136–143. (11) Lutnaes, B. J.; Brandal, O.; Sjoblom, J.; Krane, J. Org. Biomol. Chem. 2006, 4, 616–620. (12) Smith, B. E.; Sutton, P. A.; Lewis, A.; Dunsmore, B.; Fowler, G.; Krane, J.; Lutnaes, B. F.; Brandal, O.; Sjoblom, J.; Rowland, S. J. J. Sep. Sci. 2007, 30, 375–380.
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are a very small fraction of the total naphthenic acids present in soap-forming crude oils.13,14 Thus, detection of these species in crude oils plays a key role in prediction of soap deposition issues.26-28 Difficulties in separating ARN from other crude oil naphthenic acids has motivated work on developing model tetra-acid species for research purposes.33 There are many possible techniques to be used for the analysis of naphthenic acid composition of soaps or crude oil samples. The first step usually comprises extraction of the naphthenic acid species (using liquid or ion-exchange methods). Different extraction methods can lead to significantly different naphthenic acid yields and care must be taken in using this information for prediction purposes. Extraction of naphthenic acids may be followed by chemical derivatization. However, it is also known that the effectiveness of different derivatization agents is related to the size and structure of the naphthenic acids.15 By far the most common and effective method of identifying particular naphthenic acid species (with or without derivatization) is using mass spectrometry. The literature contains many interesting reports of various ionization methods employed for crude oil analysis but not for the purpose of screening of soap-forming systems (e.g., soapforming crude oils or solid deposits). Soft ionization source varieties are preferred for naphthenic acid studies because these lead to less fragmentation and, therefore, a less complex assignment of the mass-to-charge ratios generated. Among the most popular sources used with low-resolution setups are fast atom bombardment (FAB)16-18 and electrospray (ES).19-21 There have been limited detailed comparative studies for these sources.30 Atmospheric pressure chemical ionization (APCI), which is another type of soft ionization source, has been found to be at least 1 order of magnitude more sensitive than ES when tested with commercial naphthenic acid samples, and the formation of unwanted clusters using the FAB source has also been reported.22 APCI and ES sources were used with model naphthenic acid solutions and Maya crude oil, where it was shown that ES consistently produced molecular ion peaks with no fragmentation and greater signal intensities and APCI resulted in the formation of molecular peaks but also unwanted cluster ions.18 High-resolution instruments have also been successfully adapted for the use of specific ionization sources, which has allowed for a wealth of detailed heteroatom information
to be obtained from crude oil samples. More recently, this has been applied to the study of soap deposit samples.23,24 The main advantage here is that there are no additional requirements to separate the naphthenic acids prior to analysis in crude oil samples. However, extraction steps may be required when focusing on ARN detection. The present work had three major objectives. The first objective was to perform a sensitivity analysis of the naphthenic acids extracted from a field deposit of calcium naphthenate soap scale using low-resolution mass spectrometry. The different ionization sources selected for this work consisted of FAB, ES, and APCI, all operated in the negative mode. Additional sensitivity analyses were conducted using the ES source coupled to a single quadrupole, where the effects of a range of solvents as well as different operational variables were investigated. A second objective was to use liquid chromatography mass spectrometry (LCMS) to focus on particular acid structures. To our knowledge, there has been limited use of LCMS for the study of naphthenic acids from field deposits.5 However, LCMS has not been combined with tandem mass spectrometry (MS/MS) for this purpose. Previous work in our laboratories has indicated that LC separation of naphthenic acids from field deposits is necessary for the study of so-called ARN acids through MS/MS, because the first ionization of the ARN species cannot be fragmented without prior LC separation.13 Thus, further experiments were conducted using LC and MS/MS, with equipment conditions chosen to focus on the ARN acids within the naphthenic acid fraction of the field deposits. The third objective of this work was to use mass spectrometry to investigate the outcome of phase behavior tests during the formation of soap systems in the laboratory. Earlier investigations on model naphthenic acid systems have shown that the phase behavior (e.g., formation of a laboratory soap scale or soap emulsion), when carried out in laboratory conditions, was a function of the naphthenic acid species and concentration in the oil phase, presence of mono- and/or divalent cations, bicarbonate anions, and pH in the water phase. Our results also showed that these variables affected the final crystalline composition of the solids formed, as given by energy-dispersive X-ray and X-ray diffraction.25 Obtaining information on the final composition of laboratory soaps may also provide a better understanding of the mechanism of soap formation and the effects of particular acid structures applicable to field conditions. For instance, claims have been made in other publications that (a) the field soap scale samples consisted almost exclusively of so-called ARN naphthenic acid species and (b) the presence of ARN in parent crude oils
(13) Shepherd, A. G. A mechanistic analysis of naphthenate and carboxylate soap-forming systems in oilfield exploration and production. Ph.D. Thesis, Heriot Watt University, Edinburgh, U.K., 2008. (14) Smith, P. C.; Turner, M.; Smith, D.; Roberts, S. Crude oil screening process. European Patent 20070251409, 2007. (15) Jones, D. M.; Watson, J. S.; Meredith, W.; Chen, M.; Bennett, B. Anal. Chem. 2001, 73 (3), 703–707. (16) Fan, T. P. Energy Fuels 1991, 5, 371–375. (17) Wong, D. C.; van Compernolle, R.; Nowlin, J. G.; O’Neal, D. L.; Johnson, G. M. Chemosphere 1996, 32, 1669–1679. (18) Laredo, G. C.; Lopez, C. R.; Alvarez, R. E.; Cano, J. L. Fuel 2004, 83 (11), 1689–1695. (19) Rudzinski, W. E.; Oehlers, L.; Zhang, Y.; Najera, B. Energy Fuels 2002, 16, 1178–1185. (20) Rogers, V. V.; Liber, K.; MacKinnon, M. D. Chemosphere 2002, 48, 519–527. (21) Lo, C., C.; Lee, B. G.; Bunce, N. J. Anal. Chem. 2003, 75, 6394– 6400. (22) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217–233. (23) Mapolelo, M. M.; Stanford, L. A.; Rodgers, R. P.; Yen, A. T.; Debord, J. D.; Asomaning, S.; Marshall, A. G. Energy Fuels 2009, 23 (1), 349–355. (24) Brandal, O.; Hanneseth, A.-M.; Hemmingsen, P. V.; Sjoblom, J.; Kim, S.; Rodgers, R. P.; Marshall, A. G. J. Dispersion Sci. Technol. 2006, 27 (3), 295–305.
(25) Shepherd, A. G.; Thomson, G. B.; Westacott, R. E.; Neville, A.; Sorbie, K. Presented at the International Symposium on Oilfield Chemistry, The Woodlands, TX, 2005; SPE 93407. (26) Mediaas, H.; Grande, K.; Hustad, B.-M.; Hovik, K. R.; Kummernes, H.; Nergard, B.; Vinstad, J. E Presented at the Tekna Oilfield Chemistry Symposium, Geilo, Norway, 2005. (27) Brocart, B.; Hurtevent, C.; Volle, J. L. Presented at the 7th International Conference on Petroleum Phase Behaviour and Fouling, Amsterdam, The Netherlands, 2005. (28) Hanneseth, A.-M. D. An experimental study of tetrameric acids at w/o interfaces. Reactivity, inhibition, mechanism and emulsion formation. Ph.D. Thesis, Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway, 2009. (29) Shepherd, A. G.; Thomson, G. B.; Westacott, R. E.; Sorbie, K Presented at the Chemistry in the Oil Industry X, Manchester, U.K., 2007. (30) Mohammed, M. A.; Sorbie, K. S. Colloids Surf., A 2009, 349, 1–18.
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would be a prerequisite for the formation of soap scales in the field.5,26 However, other claims have reported crude oil samples with ARN that did not lead to soap scale precipitation problems.27 Our experience is that the presence and concentration of ARN in soap deposits is highly dependent upon the sample history. Moreover, the presence of production chemicals in field samples can alter the distribution of naphthenic acids detected, compared to a nontreated sample.4
Table 1. ES Operational Variable Sensitivity Test Experimental Matrix experiment
1
2
3
4
5
ion spray voltage (ISV) (kV) nebulizer gas setting (NEB) curtain gas setting (CUR)
-4.50 10 12
-3.75 10 12
-3.00 10 12
-4.50 10 6
-4.50 5 12
Table 2. ES Organic Solvent Phase Properties for Sensitivity Analysis
Experimental Section A calcium naphthenate soap scale field deposit was used for all of the sensitivity studies. Further characterization of this sample has been described elsewhere.13 Naphthenic acids were obtained from this sample according to the following procedure: the deposit (300 g) was ground using a mortar and pestle and stirred with acetone (500 mL) for 1 h. The sample was filtered, reground, and treated with a fresh volume of acetone. The process was repeated using three additional solvents: methylene chloride, toluene, and isooctane. At the end of the wash treatment, the solvent was removed under vacuum, giving approximately 200 g of material. Some of this material (100 g) was added to 3 M hydrochloric acid (400 mL) and toluene (300 mL). This mixture was magnetically stirred and heated at 40 °C under reflux for 2 h, after which the organic portion of the deposit had become dissolved in the organic phase. The whole extraction process was checked to ensure that the majority of the naphthenic acids in the deposits were captured. Filtering removed approximately 7 g of solids, which included sand and other insoluble materials. The remaining liquid was rinsed with water to wash out most of the hydrochloric acid and placed in a rotary evaporator to remove the toluene. A representative aliquot of this final sample (0.1 g) was dissolved in a 50:50 (vol/vol) glacial acetic acid/toluene mixture (5 mL). The resulting solution was washed with ultra-high-quality water (1 mL, HPLC grade). The organic solvent phase was then used in the mass spectrometry experiments. To this phase, 35 vol % aqueous ammonia solution (10 μL) was added to aid ionization. This facilitates charge transfer within the solvent phase. Because the field soap sample used was not thought to be contaminated with production chemicals, mass spectra (up to m/z 1500) were obtained for the naphthenic acids without derivatization. Each instrument was calibrated prior to the experiments with a commercial naphthenic acid internal standard; however, detection is not quantitative because of the difference in ionization efficiencies. A commercial naphthenic acid mixture from Acros (Geel, Belgium) was also studied to aid in the interpretation of the soap naphthenic acid spectra. The soft ionization sources used for both soap naphthenic acids and commercial naphthenic acids were FAB, APCI, and ES. For the FAB analysis, a VG ZAB-SE instrument was used with cesium iodide and a resolution of 1200. The voltage of the atom gun was 30 keV; the discharge was approximately 3 mA; the orientation of the ion beam gun was close to 110°; and the pressure in the source housing was between 5.5 and 10 mbar. The matrix used was a 50:50 (vol/vol) mixture of nitrobenzylalcohol (NBA) and triethanolamine (TEA). The APCI was carried out using a PE-Sciex API 150 EX mass spectrometer with a heated nebulizer source. The mass analyzer consisted of a single quadrupole. Nitrogen was used as both a nebulizer (10 psi) and curtain gas (12 psi). The nebulizer current voltage and temperature were -6 kV and 400 °C, respectively. The focusing and extraction potentials used were -50 and -5 V, respectively. The analysis parameters were ion energy of -1.0 V, deflector voltage of 300 V, and electron multiplier of 2000 V. The ES instrument used was also the PE-Sciex API 150EX single quadrupole mass spectrometer. Samples were infused using a Harvard Syringe pump with flows between 1 and 10 μL/min. The source temperature used was 350 °C, and the needle voltage for ionspray was -4.5 kV. Nitrogen was used as both a nebulizer gas (6 psi) and curtain gas (60 psi).
solvent
molar mass (g/mol)
viscosity (cP at 20 °C)
dipole moment (D)
toluene methanol acetonitrile
92.14 32.04 41.05
0.59 0.59 0.35
0.36 1.69 3.90
Table 3. MS/MS Settings Used for Naphthenic Acid Analysis mode collision gas energy (V) collision gas thickness (molecules/mL) ionization source scan time (ms) ionization gas declustering potential (V) focus potential (V) entrance potential (V) collision energy TOF (eV) air pressure in cell (torr) exit potential (V)
CID argon 50-60 2.19 1012 electrospray 500 nitrogen -30 -130 8 -19 2 10-3 4
A sensitivity analysis using toluene as the organic solvent phase was also performed using ES with the following operational variables: ion spray voltage (ISV), nebulizer gas (NEB), and curtain gas flow rates (CUR). The experimental matrix for this sensitivity study is shown in Table 1. The gas flow parameters (NEB and CUR) are digitally set and work off an array of internal valves. Exact flow rates can be calculated, but the digital values are most commonly used for comparative purposes. The gas flow supplied to the instrument was 60 psi. The minimum value for CUR was 6 psi because a lower setting would result in the collapse of the vacuum needed to operate the system. The ES source coupled to the single quadrupole analyzer was further used in sensitivity studies where different organic solvent phases were examined for injection. Sample preparation and ES settings followed the procedures described earlier. The organic solvents investigated were toluene, toluene/methanol (50:50, vol/vol), and toluene/methanol/acetonitrile (25:25:50, vol/vol/vol). Selected organic solvent properties are shown in Table 2. Liquid chromatography was conducted with an Agilent 1100 ion trap SL. HPLC mode was used with a Zorbax 300 SB-C3 (20 40 mm) column, followed by flow into a Purospher RP 18 column (250 4 mm) with a 5 μm coating. The Purospher column was kept at 35 °C, and the eluents used were acetone/acetonitrile (25:75, vol/vol) at a flow rate of 0.5 mL/min. For the MS/MS experiments, the instrument used was a hybrid quadrupole time of flight (TOF), in which ions are selected using a quadrupole analyzer and fragmented in a quadrupole collision cell. Detection and separation of the ions are carried out in the TOF sector. The full equipment settings are presented in Table 3. The organic solvent phase used in these tests was toluene. The phase behavior procedure to generate laboratory soaps used 50 mL of the naphthenic acid extracts separated from the soap sample (according to the method mentioned previously) and 50 mL of an aqueous phase. Deionized water was used as a reference to which calcium ions (as CaCl2 3 6H2O), sodium ions (as NaCl), barium ions (as BaCl2), and bicarbonate ions (as NaHCO3) were added in different concentrations. The naphthenic acid extracts and the different aqueous phases were placed in contact and shaken vigorously for 1 min. Solids formed were 4389
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Figure 1. Comparison of ES, APCI, and FAB (NBA/TEA matrix) spectra for naphthenic acids from the field deposit.
Figure 2. Comparison of ES, APCI, and FAB (NBA/TEA matrix) spectra for the commercial naphthenic acid mixture.
In Figure 1, it can be observed that the FAB spectrum shows a high signal observed around m/z 1230, which has been assigned to the first ionization of the so-called ARN acids.5 Other m/z signals in the spectra represent other naphthenic acid species. The FAB source resulted in a good signal for the ARN acids. This is probably due to the enhanced charge transfer between the naphthenic acids and the hydroxyl groups and carboxyl group in the matrix. In FAB, the molecular ions are produced as a consequence of deprotonation of naphthenic acids in the matrix. For homogeneous ionization, the matrix and solute need to be well-mixed. Charge transfer may be facilitated between the FAB matrix and the ARN acids because of the polarity of the species. It is possible that, with the current FAB setup, preferential ionization of the ARN acids and lower molecular-weight monocarboxylic acids prevent detection of the intermediate molecular-weight acids. In Figure 1, it can also be noticed that the overall signal of ARN acids is comparable for both APCI and ES sources. Only the ES spectrum showed evidence of monocarboxylic acids with m/z between 100 and 500. In APCI, ionization is a function of charge transfer between the solvent molecules and the sample molecules conducted in a
filtered with a Schleicher and Schuell 597HY hydrophobic silicone-coated paper, with a retention diameter of 4.7 μm. The naphthenic acid distribution of these laboratory solids was obtained by redissolving the samples following the procedure described earlier for the field soap sample. Similar low-resolution mass spectrometry conditions for the ES ionization source were used.
Results and Discussion Figure 1 contains a comparison of the spectra of the naphthenic acids extracted from the calcium naphthenate soap scale field deposit obtained using the soft ionization sources, FAB, APCI, and ES. The overall detection of the naphthenic acids by mass spectrometry is a function of many factors, including the energy delivered by the ionization sources. In this sensitivity study, it was not possible to operate the sources using the exact same energy ranges because of hardware limitations. However, the results were reproducible. The atom gun of the FAB source was operated at 30 keV. However, the nebulizer current voltage for the APCI was -6 kV, which was close to the ion spray voltage for the ES, which was -4.5 kV, and this allows for a better comparison. 4390
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Figure 3. Comparison of ES spectra for sensitivity experimental cases (refer to Table 1).
Figure 4. Comparison of ES spectra for organic solvent phase spectra: (a) toluene/methanol/acetonitrile, (b) toluene/methanol, and (c) toluene.
plasma by partial discharge around a conductor placed at a high potential. This phenomenon is known as the “corona discharge”.31 Charge transfer may therefore be facilitated in the presence of polar molecules, such as the ARN acids. If ARN acids contain four carboxylic groups, they should be more polar than monocarboxylic naphthenic acids. This may explain the absence of monocarboxylic acids with low m/z values in the APCI spectra. Figure 2 shows the spectrum of a commercial naphthenic acid analyzed with the same sources discussed previously. The average molecular weight for commercial naphthenic acid mixtures have a certain degree of statistical variation, reported to be between 220 and 239 Da, with bimodal-type distribution mostly a result of monocarboxylic acids.19,22 Such commercial mixtures contain a very small amount of species with m/z above 400. Rudzinski et al. showed that observed higher m/z values for the commercial naphthenic acid mixtures occurred because of multimer formation.19 The FAB source shows a
high selectivity for monocarboxylic acids with low molecular weight. The ES shows low signal intensity; however, the spectrum is consistent with the trends reported in the literature.19 The APCI spectrum does not show many species below m/z 250, which is rather surprising because the average molecular weight for this sample is 230 Da. In addition, the spectrum for this source shows m/z species above 400, which are most likely multimers generated during the ionization process of the naphthenic acids in the parent sample and a function of the concentration used.32 Given the effects discussed here, FAB and ES could potentially be used for the purpose of identification for naphthenic acids from flow assurance samples, where an overall fingerprint would be preferred. The APCI source could be potentially used for studies where the focus would be detection of ARN acids, as a result of the favored ionization effects, but not for the detection of the lower molecular-weight acids. Analysis of soap-forming crude oils using the sources mentioned in this paper have also been carried out. The FAB source resulted in poor spectra when applied directly to the analysis of crude oils (results not shown). Thus, it is likely that additional procedures, such as ion-exchange separation of the naphthenic acids from the crude oil, would be necessary, prior
(31) de Hoffmann, E.; Stroobant, V. Mass Spectrometry: Principles and Applications, 2nd ed.; Wiley: New York, 2001. (32) Rodgers, R. P.; Rahimi, O.; Messer, B.; Phillips, T.; Marshall, A. G. Presented at the 61st Annual Corrosion Nace Conference and Exposition, San Diego, CA, 2006.
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when the more polar acetonitrile is added to the organic solvent phase is ARN ionization preferred. Again, changes in the signal intensity were not observed for the second and third ionizations of the ARN acids during these experiments. Figure 5 presents an ion chromatogram for the naphthenic acids extracted from the field deposit using the settings described earlier. The fraction at 17.5 min retention time was concentrated and used in the MS/MS experiments. Figure 6 presents the fragmentation spectrum of the parent ARN acid ion (m/z 1230). At least four major daughter ions were obtained from this fragmentation procedure, each of which was further fragmented, thus leading to a sequence of MS4 experimental data. These were m/z 1212, 1150, 1106, and 1088, all shown in Figure 6. A possible assignment of these fragmentation patterns is presented in Table 4. Interpretation of the data suggests the loss of three carbon dioxide groups, two water groups, and one carbon monoxide group for the sequence of fragmentation experiments. These losses may be associated to fragmentation of four carboxylic acid functionalities in the parent ion, m/z 1230 (the first ionization of the ARN species), which provides additional evidence for the presence of four carboxylic acid groups in the ARN structure. However, these groups appear to be affected differently under the influence of collision dissociation, which could be a result
to the use of this source for crude oil naphthenic acid analysis. Further work should be directed at the precise effects of the ARN concentration on spectra response. The next discussion is related to the experimental matrix described in Table 1, and Figure 3 contains a comparison of the different spectra generated. All spectra show the presence of m/z species around 1230, which can be attributed to the first ionization of the so-called ARN acids.6 The second and third ionizations of the ARN species were also observed, although at very low intensity (e.g., less than 4% of the total signal). As the ISV is reduced from -4.5 to -3.0 kV, the first ionization ARN acid signal is increased relative to the signals of the monocarboxylic acids. Note that, for experiment 3, there was a very high signal-to-noise ratio, yet the ARN acids were still predominantly detected in the mass spectrum. This trend could be explained by the preferential ionization of the ARN acids discussed previously, even at low voltages. Changes in the signal intensity for the second and third ionizations of the ARN acids were not observed during these experiments. Figure 4 shows the spectra for the three different organic solvent phase combinations used with the ES source. The ARN acid signal is reduced with the presence of methanol. The ARN acid signal is at its maximum in the presence of acetonitrile. Methanol and acetonitrile have higher dipole moments compared to toluene (see Table 2), which aids charge transfer during ionization. These solvents also have lower molecular weight than toluene, which also aids the formation of the aerosol spray (although this effect is less pronounced with methanol because it has very close viscosity to toluene). The use of methanol leads to the preferential ionization of naphthenic acids other than ARN acids, and this can be observed in the spectrum in Figure 4. Only
Table 4. MS/MS Parent and Daughter Ions and Mass Assignments m/z of product ion
1230
1212
1150 1106
Figure 5. Ion chromatogram for the naphthenic acids extracted from the field deposit.
1088
m/z of daughter ion
cumulative mass loss
assignment
1212 1168 1150 1132 1106 1088 1168 1150 1132 1106 1088 1132 1106 1088 1090 1064 1072
18 44 18 26 44 44 44 18 18 26 18 18 26 18 16 26 16
loss of H2O loss of CO2 loss of H2O loss of CO loss of CO2 loss of CO2 loss of CO2 loss of H2O loss of H2O loss of CO loss of H2O loss of H2O loss of CO loss of H2O loss of O loss of CO loss of O
Figure 6. MS/MS spectra of m/z 1230 parent ion (diamond). Annotated peaks represent main daughter ions generated from the ion at m/z 1230.
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Figure 7. Mass spectrometry of laboratory solids formed from single cationic aqueous phases (in captions). m/z = mass-to-charge ratio.
aqueous phases were used with the naphthenic acid extracts in the static bottle tests, a solid formed that contained ARN acids (m/z 1230) and a broad distribution of other species between m/z 300 and 600. The use of a barium-rich aqueous phase leads a reduction in the relative percentage of ARN species present in the solid formed. When sodium ions are used, no naphthenic acid species are observed in the laboratory solids with m/z values above 300 and no ARN is detected in the sample. Figure 8 presents the composition of the laboratory solids formed with the presence of bicarbonate ions in the aqueous phase. No ARN species were detected in the solid formed from an aqueous phase that did not contain calcium, even in the presence of bicarbonate ions. Cation hydration effects have been used to explain the naphthenic
of the different alkyl groups attached to each carboxylic group. This however would need further investigation for a more definite conclusion. The mass spectrometry results of naphthenic acid distribution of the solids formed in the laboratory during the phase behavior tests are now discussed. The effect of aqueous phases containing single cationic species (calcium, sodium, and barium) without bicarbonates is presented in Figure 7. Clear differences can be observed in the spectra. When calcium-rich (33) Nordgard, E. L. Model compounds for heavy crude oil components and tetrameric acids. Characterization and interfacial behaviour. Ph.D. Thesis, Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway, 2009.
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Figure 8. Mass spectrometry of laboratory solids formed from bicarbonate-containing aqueous phases (in captions). m/z = mass-to-charge ratio.
acid behavior at the oil-water interface, and this can be used to interpret the data in this work.7,27 The presence of ARN in both the calcium and barium solids generated in the laboratory attests to the preference of this naphthenic acid species for divalent cations. Note that if ARN acids contain four carboxylic groups as shown in this work, they could potentially bind with up to four divalent ions. Divalent soaps have lower solubility products than monovalent soaps. Under field conditions, the predominant divalent species in the aqueous phase will be the origin of the predominant cation present in the soap scale. For the soap-forming systems analyzed in a previous publication,29 the predominant divalent species was calcium. Only trace amounts of barium in field soap scales were reported and assigned mostly to barium sulfate.4 However, there is anecdotal evidence of barium naphthenates found in certain field deposits. The presence of other naphthenic acid species in the divalent laboratory soaps supports some variety of competition with the ARN species during the precipitation reaction. Fatty acids, have lower molecular weight and, therefore, would have the potential to diffuse faster to the oil-water interface for reaction. ARN species having higher molecular weight would diffuse slower to the oil-water interface. Nevertheless, ARN would have the potential to bind with up to four calcium ions. Moreover, ARN acids have been shown to be more surface-active then selected monocarboxylic acid species.7,13
The absence of ARN in the soap formed from the sodium-rich aqueous phase is interesting. Monovalent cations, such as sodium, may react with only one carboxylic group at a time. The sticky nature of field soap scale deposits has been attributed to the ability of ARN acids to form a network with the divalent calcium ions.27 For the experiments carried out in this work, the most likely explanation for the absence of ARN in the sodium solid is the enhanced solubility of the lower molecular-weight fatty acids in the bulk of the aqueous phase containing sodium ions. In the soap-scale-forming produced waters analyzed in a previous publication, the calcium/sodium concentration ratio was calculated to be between 0.02 and 0.26. It is speculated that produced waters with higher sodium concentrations do not have the potential for soap scale formation under field conditions. Conclusions Naphthenic acids from a calcium naphthenate soap scale deposit were examined without derivatization using three different ionization sources, FAB, APCI, and ES. The use of these sources produced different spectra. The FAB source showed the highest signal for ARN acid as well as monocarboxylic acids. The APCI and ES showed similar signals for ARN acids. APCI presented a favorable ionization of ARN acids, poor detection of monocarboxylic acids, and increased 4394
Energy Fuels 2010, 24, 4387–4395
: DOI:10.1021/ef100574m
Shepherd et al.
evidence of multimer formation. Thus, the FAB and ES spectra were suggested to be a more realistic representation of the overall naphthenic acid fingerprint present in the field deposit sample, under the conditions studied. Use of different organic solvent phases resulted in different ionization effects, particularly for the ARN acids, mostly because of a combination of polarity and molecular weight. The ion spray voltage of the ES equipment was also shown to lead to high overall variation in the naphthenic acid spectra. As voltage is decreased, the ARN acid was ionized preferentially over the monocarboxylic acids. The results in this work suggest that a combination of polar solvents (e.g., acetonitrile) and the APCI source would most likely result in an improved detection for the ARN acids from field deposit samples. LC and MS/MS focusing on further analysis of the ARN acids were able to show further evidence for the presence of four carboxylic acid groups. This was carried out using fragmentation patterns from parent ions at m/z 1230 (the first ionization of the ARN acid). Further work should attempt to apply this information to crude oil samples. In addition, the effect of the ARN concentration should be investigated.
In phase behavior tests, the final naphthenic acid distribution found in the soaps was a direct consequence of the aqueous phase used. When sodium ions were present in the aqueous phase, soaps were generated but without the presence of ARN. No ARN species were detected in the solid formed from an aqueous phase that did not contain divalent cations, even in the presence of bicarbonate ions. The presence of other naphthenic acid species in the divalent laboratory soaps attests to competition of naphthenic acids during the precipitation reaction: a combination of diffusion effects (from bulk oil phases to the oil-water interface), the presence of specific ions in the aqueous phase, and the positioning of the naphthenic acids at the oil-water interface. ARN acids having four carboxylic groups would be favored at the oil-water interface as opposed to monocarboxylic acids. However, monocarboxylic acids would have the ability to diffuse faster to the interface. In addition, monocarboxylic acids are more watersoluble, which would favor partitioning from the oil phase to the water phase. These competing effects and the impact on deposition and modeling should be further investigated. Nevertheless, the experiments described in this work may provide preliminary insight into the complex phase behavior and field observations from soap-forming producing fluids.
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