Energy Fuels 2010, 24, 2320–2326 Published on Web 11/30/2009
: DOI:10.1021/ef900959r
Compositional Variations between Precipitated and Organic Solid Deposition Control (OSDC) Asphaltenes and the Effect of Inhibitors on Deposition by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry† )
Priyanka Juyal,‡ Andrew T. Yen,*,‡ Ryan P. Rodgers,§, Stephan Allenson,‡ Jianxin Wang,^ and Jefferson Creek^ )
‡ Nalco Energy Service, 7705 Highway 90-A, Sugar Land, Texas 77478, §Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, and ^ Chevron Energy Technology Company, 1400 Smith Street, Houston, Texas 77002
Received August 31, 2009. Revised Manuscript Received November 3, 2009
Organic solid deposition control (OSDC) is a live oil test capable of simulating the production conditions of oil streams and can generate asphaltene deposits under production system conditions. OSDC can also simulate gas lift conditions because the producers are looking for artificial lift methods to produce oil from low-energy reservoirs. In this paper, we present the first compositional study on the OSDC deposits under gas lift conditions and compare it to C7 asphaltenes from the same crude oil precipitated in the laboratory, by use of ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. Furthermore, deposits collected from chemically treated fluids were also studied. The negative-ion mass spectra of OSDC deposits from untreated crude oil and asphaltene inhibitor (AI)-treated crude oil are richer in acidic species, such as the Ox and SxOy polar classes, relative to the parent crude oil. The molecular-weight differences for treated deposits relative to the untreated sample may help explain the deposition tendency in the tests. We infer that a correlation of field asphaltene deposition tendency with laboratory screening tests is essential for the advancement of asphaltene research.
depressurization has been previously highlighted with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) by Klein et al.2,3 Recently, a new device was introduced to study asphaltene deposition under live conditions. The organic solid deposition control (OSDC) unit has been used to study both wax and asphaltenes since its inception.4 OSDC is a tool capable of simulating the production conditions of oil streams and can generate asphaltene deposits under production and transportation system conditions. OSDC can also simulate gas lift conditions. In this respect, identification of compositional variations between field deposits, OSDC solids, and precipitated asphaltenes is important because a correlation of field asphaltene deposition tendency with laboratory screening tests is essential for the advancement of asphaltene research. The ability to predict the occurrence and magnitude of asphaltene deposition in wellbores is the key to the feasible solution of the flow assurance challenge. An advanced understanding of the asphaltene deposition mechanism under different conditions is critical to the oil producers at the design stage. The mechanism of asphaltene flocculation and deposition can be understood with an improved understanding of
Introduction As the oil industry moves toward deep and ultra-deep offshore production, achieving greater oilfield efficiency and productivity in such challenging environments is a critical issue. The probability of encountering organic solids and the costs associated with their remediation is expected to increase in deep water/ultra-deep water environments. Flow assurance is a critical concern in subsea developments in ultra-deep water because of the flow distances involved and the temperature and pressure regimes that apply. One of the key flow assurance issues is asphaltene deposition. Well and pipeline clogging can lead to significant economic losses and operational delays. Asphaltenes are defined by their solubility in toluene and insolubility in a saturated hydrocarbon, such as n-pentane or n-heptane (IP143). The amount, chemical composition, and morphology of precipitated asphaltenes vary with the precipitant type, pressure, and temperature.1 In the field, asphaltene precipitation can occur during mixing of incompatible hydrocarbon fluids, miscible flooding, CO2 injections, gas lift operations, and acidizing jobs. The difference in chemical composition of precipitated asphaltenes versus asphaltenes induced by † Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed. E-mail: atyen@ nalco.com. (1) Hammami, A.; Ratulowski, J. In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; pp 617-660.
r 2009 American Chemical Society
(2) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G.; Yen, A.; Asomaning, S. Energy Fuels 2006, 20, 1965–1972. (3) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G.; Yen, A. Energy Fuels 2006, 20, 1973–1979. (4) Zougari, M. J.; Jacobs, S.; Ratulowski, J.; Hammami, A.; Broze, G.; Flannery, M.; Stankiewicz, A.; Karan, K. Energy Fuels 2006, 20, 1656–1663.
2320
pubs.acs.org/EF
Energy Fuels 2010, 24, 2320–2326
: DOI:10.1021/ef900959r
Juyal et al.
their molecular nature and morphology. This knowledge also translates into the design of next-generation chemistries suitable for ultra-deep water flow assurance and improved predictive models for asphaltene instability. Here, we present the first compositional characterization study on the OSDC deposits under gas lift conditions and examine compositional variations with C7 asphaltenes from the same crude oil generated in the laboratory by ultra-high-resolution FT-ICR MS. Positive- and negative-ion ESI facilitates characterization of basic and acidic components found in the asphaltenes, parent crude oil, and deposits.5 Elemental compositions are presented as heteroatom class distribution graphs (i.e., NnOoSs), rings plus double bonds [double bond equivalents (DBEs)] to reveal the degree of aromaticity, and carbon distribution to reveal the degree of alkylation of aromatic cores. The OSDC deposits are found to be richer in acidic species, such as the Ox and SxOy polar classes, relative to the parent crude oil. The same trend was observed with field asphaltene deposits and asphaltenes induced from depressurization,2 emulsion rag layer6,7, and steam-assisted gravity drainage (SAGD) deposits.8
Table 1. Relative Percentages (by Weight) of SARA-Fractionated Components of the Crude Oil saturates
aromatics
resins
asphaltenes
52
30
12
6
Table 2. Deposit from the OSDC Test from Blank Crude Oil and after Treatment with Two Different Chemistries
test duration (h) wall deposits (g)
oil þ lift gas (untreated)
oil þ lift gas chemical A
oil þ lift gas chemical B
2 0.287
2 0.335
2 0.571
(ASphaltene InStability Trend) modeling9,10 during design stage and confirmed by SDS (Solids Detection System) measurement. Therefore precipitation occurred during OSDC tests. For the chemical treatment tests, the same oil/lift gas ratio was applied as in the blank test, except that 500 ppm by weight of selected asphaltene inhibitor was added to the oil before charging into the OSDC cell. After deposition test, the deposits from the wall of cell outer cylinder was extracted with dichloromethane, and the solvent was evaporated to retrieve the deposits. Sample Preparation for ESI FT-ICR MS. The parent crude oil, C7 asphaltenes, and deposits generated from OSDC (untreated and treated crude oils) were dissolved in toluene to a concentration of 2 mg/mL. Samples were further diluted to 1 mg/mL in a standard electrospray solvent (60:40 toluene/ methanol, vol/vol) for mass spectrometric analysis. The volume of methanol was kept at a minimum to avoid precipitation of asphaltenes. A representative aliquot (1 mL) of each sample was spiked with 10 μL of 20% ammonium hydroxide (NH4OH) in methanol to facilitate deprotonation of the acidic species to yield [M - H]- ions, whereas 10 μL of 2% formic acid (HCOOH) in methanol was used to protonate the basic species to generate [M þ H]þ ions. Each sample was delivered to the mass spectrometer ionization source via a syringe pump at a rate of 500 nL/min through a 50 μm inner diameter fused silica micro-ESI needle under typical ESI conditions (2.0 kV; tube lens, 350 V; heated capillary current, 4.20 A). Mass Analysis. Each sample was analyzed with a custom-built 9.4 T 22 cm horizontal room-temperature bore diameter (Oxford Corp., Oxney Mead, U.K.) FT-ICR mass spectrometer at the National High Magnetic Field Laboratory.11 A modular ICR data acquisition system (MIDAS) was used to collect and process ICR data.12,13 Ions were accumulated externally in a linear octopole ion trap for 5-15 s and transferred through radio-frequency (rf)-only multipoles to a 10 cm diameter, 30 cm long open cylindrical Penning ion trap.14 Octopole ion guides were operated at 2.0 MHz with a 100 Vp-p rf amplitude and a 900 μs transfer period.15 Broadband frequency-sweep (chirp) dipolar excitation (∼70-641 kHz, at a sweep rate of 50 Hz/μs and peak-peak amplitude of 190 Vp-p) was followed by direct
Materials and Methods High-performance liquid chromatography (HPLC)-grade toluene, n-heptane, methanol, ammonium hydroxide, and formic acid in methanol (Fisher Scientific) were used as supplied. The crude oil is from a new development in the Gulf of Mexico that is not yet under production, with an American Petroleum Institute (API) gravity of 26, gas/oil ratio (GOR) of ∼300 standard cubic feet/stock tank barrel (SCF/STB), and asphaltene onset pressure of 1% are shown.
Results and Discussion OSDC Results. The amount of deposits in mass collected after the OSDC tests are summarized in Table 2. The mass of deposited solids from Table 2 shows that the amount/extent of deposition increased with the addition of asphaltene inhibitor, which can also be observed from the photos taken from the outer cylinder after deposition tests (Figure 1). MS. ESI MS identifies polar compound classes from crude oil and its associated fractions by generation of quasi-molecular charged species of the type (M þ H)þ or (M - H)- by protonation or deprotonation affected by an acidic or basic reagent. The acidic molecules in the petroleum samples are deprotonated to form (M - H)- ions, and basic compounds in the sample are protonated to form (M þ H)þ ions, recorded in the negative and positive ESI mass spectra, respectively. The process requires no pre-chromatographic separation and is mild, so that it is well-suited for the analysis of complex materials, such as petroleum and the deposits, without fragmentation. Positive- and negative-ion mass spectra were collected for the parent crude oil, C7 asphaltenes, and OSDC solids (from untreated and treated crude oils) to illustrate compositional variations in these deposits and the effect of treatment with asphaltene inhibitors.
Compositional disparity between C7 asphaltenes and OSDC solids for this crude oil was readily apparent by the difference in the response to ESI for these samples. C7 asphaltenes for this crude oil show a very poor signal under electrospray conditions relative to the OSDC solids. Because ESI is selective to polar species, this suggests the low polarity of C7 asphaltenes in comparison to OSDC solids that show an abundant signal. Further, the spectral data for C7 asphaltenes could not be processed because of the poor signal quality. Negative-ion electrospray selectively ionizes acidic species, typically carboxylic acids, phenols, and near-neutral nitrogencontaining heteroaromatic species, such as carbazoles. Figure 2 reports the relative abundance for different heteroatom classes highlighted from the negative-ion mass spectra for parent crude oil and OSDC deposits from untreated and treated (chemical A and chemical B) crude oil. As discussed earlier, treatment with inhibitors did not control deposition but led to an overall increase in the amount of solids generated from OSDC (Table 2). Our result is different from a recent paper, where a modified flow-through system was used.21 This is apparent from Figure 2, where it is clear that the negative-ion mass spectra for the deposits have a noticeably higher abundance of SxOy classes, several of which do not show up in the mass spectra for the parent crude oil. This
(16) Ledford, E. B. J.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984, 56, 2744–2748. (17) Shi, S. D.-H.; Drader, J. J.; Freitas, M. A.; Hendrickson, C. L.; Marshall, A. G. Int. J. Mass Spectrom. 2000, 195-196, 591–598. (18) Kendrick, E. Anal. Chem. 1963, 35 (13), 2146–2154. (19) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2001, 73 (19), 4676–4681. (20) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1186–1193.
(21) Akbarzadeh, K.; Ratulowski, J.; Lindvig, T.; Davies, T.; Huo, Z.; Broze, G.; Howe, R.; Lagers, K. SPE Annual Technical Conference and Exhibition, New Orleans, LA, Oct 4-7, 2009; SPE 124956.
2322
Energy Fuels 2010, 24, 2320–2326
: DOI:10.1021/ef900959r
Juyal et al.
Figure 3. Relative iso-abundance color-contoured plots of DBEs versus carbon number for the acidic SO4 class from the parent crude oil and OSDC deposits from untreated and treated crude oils.
observation is consistent with the conclusions from characterization work reported by Klein et al.2,3 Schaub et al. also reported a high abundance of acidic oxygen and SxOy classes in SAGD deposits.8 Apparently, acidic oxygen and SxOy classes are becoming enriched in the deposits relative to the parent crude oil, which could be perhaps ascribed to their increased polarity and, hence, increased molecular interactions, leading to deposition. The parent crude oil has a high relative abundance of N1 class ions, the carbazole and its analogues, compared to the deposits, whereas the SO4 class is most abundant for deposits from untreated and treated crude oils. S2O4, SO5, and S2O5 class ions do not show up in the parent crude oil mass spectrum, indicating enrichment of these species in the deposits. The several-fold enrichment of acidic SxOy species has been reported previously. These small polar species do not match some of the proposed asphaltene structures in the literature. However, they are part of the asphaltene continuum, as defined by the operational definition of asphaltenes. Klein et al.2,3 compared the relative abundance profiles for these polar heteroatom classes to differentiate between pressure-drop asphaltenes and the heptane-insoluble asphaltenes from oil collected from the same well. They reported that the SO4 class is the most abundant for pressure-drop asphaltenes, whereas the N1 class (pyrrole) is most abundant for heptane-insoluble asphaltenes. These polar compounds have also been seen in the interfacial material collected from the oil/water interface. Stanford et al.6,7 reported a high relative abundance of SO4 species in emulsion interfacial material with a higher carbon number and DBE relative to the parent crude. In some instances, the SxOy species are not detected in the parent oil. Schaub et al.8 performed a systematic evaluation of deposit and fluid samples collected from different locations in one inverted SAGD facility and reported that the dominant species that contribute to fouling are the SxOy species. The 3D contour iso-abundance color-coded plots are a convenient way of representing the number of rings plus
double bonds (to reveal the degree of aromaticity) and carbon distribution (to indicate the extent of alkylation of aromatic cores). Figure 3 shows the color-coded DBE versus carbon number plots for the SO4 class in the parent crude oil and the deposits. These are relatively less abundant in the parent crude oil, with a maximum DBE of about 12. The SO4 species for the deposits range from 1 < DBE < 16, with the highest relative abundance at DBE = 1. There is selective enrichment of the SO4 species in all of the runs (i.e., run 1, untreated crude oil; run 2, crude oil treated with chemical A; and run 3, crude oil treated with chemical B), specifically SO4 species with a DBE range of 1-5. There is a shift in carbon numbers from 65 for run 1, 60 for run 2, and 55 for run 3 for this class, whereas the DBE distribution is near constant. The difference in the molecular-weight (MW) distribution of OSDC deposits showed that the treated samples have lower MW distribution, which means that the asphaltene inhibitors are effective in dispersing asphaltenes. The lower MW asphaltenes are more likely to be trapped in the boundary layer during the test, thus increasing the deposition tendency. The test result is consistent with recent modeling results by Vargas et al.22 Similarly, DBE versus carbon number plots for the SO3 class in Figure 4 show the enrichment of this class for run 1 (untreated) and run 2 (treated with chemical A), where a DBE distribution of 1-18 and carbon number distribution of 20-60 are observed. The difference in the mechanism of action of two different chemistries is also apparent by the absence of this class in run 3 (treated with chemical B), whereas it appears in the mass spectrum for run 2 (treatment with chemical A). A similar observation is made from the isoabundance color contour plots for the S2O3 ion class in Figure 5, where this class has a relatively low abundance for parent crude oil but appears over a much higher DBE and carbon number (22) Vargas, F. M. C.; Wang, G.; Creek, J. Development of an asphaltene deposition simulator. Proceedings of the 10th Annual International Conference on Petroleum Phase Behavior and Fouling, Rio de Janeiro, Brazil, June 14-18, 2009.
2323
Energy Fuels 2010, 24, 2320–2326
: DOI:10.1021/ef900959r
Juyal et al.
Figure 4. Relative iso-abundance color-contoured plots of DBEs versus carbon number for SO3 species from the parent crude oil and OSDC deposits from untreated and treated crude oils with chemical A and chemical B.
Figure 5. Relative iso-abundance color-contoured plots of DBEs versus carbon number for S2O3 species from the parent crude oil and OSDC deposits from untreated and treated crude oils. Data are derived from negative-ion FT-ICR mass spectra.
range in the deposits from untreated crude oil (run 1) and the deposits from treated crude oil (run 2). The absence of this ion class in the deposits from the crude oil treated with chemical B (run 3) conveys that chemical B may be able to react or neutralize these acidic classes and prevent their appearance in the deposits. The DBE versus carbon number plots for S2O4 class in Figure 6 show that this class is absent for the parent crude oil but appears in all three OSDC deposits. This also confirms that these acidic classes significantly contribute to the deposition tendency of crude oil. The inability of the two chemistries used in runs 2 and 3 in controlling these classes is also evident. Effect of Inhibitors from Negative-Ion ESI FTICR-MS. Differences in negative-ion spectra for OSDC solids for runs 2
and 3 (Figure 2) suggest the difference in performance characteristics of the different inhibitors used for the study. Ion classes, such as NO2, SO2, O3, and S2O5 appear in the mass spectra of untreated deposits and deposits from run 2 treated with chemical A. Apparently, these species that are otherwise absent are becoming concentrated in the deposit where no treatment was administered. Also, chemical A is not able to control the deposition of these species, whereas chemical B appears to inhibit these species, as reflected by their nonappearance in the spectrum of the deposit from run 3 treated with chemical B. This may be due to the inability of chemical A to interact with these species and keep them suspended in the oil matrix. Another point of difference is noticed for acid ion class O1, most likely phenols, which is abundant in parent crude oil, but is not highlighted in the mass spectra of OSDC 2324
Energy Fuels 2010, 24, 2320–2326
: DOI:10.1021/ef900959r
Juyal et al.
Figure 6. Relative iso-abundance color-contoured plots of DBEs versus carbon number for acidic S2O4 species from the parent crude oil and OSDC deposits from untreated and treated crude oils. This class is absent in the negative-ion mass spectra of parent crude oil.
Conclusions In this work, we detail the compositional analysis of a crude oil and deposits generated from the crude oil under OSDC conditions with and without inhibitor treatments by both positive and negative ESI FT-ICR MS. OSDC solids respond to ESI, whereas C7 asphaltenes under study show no signal in the mass spectrum under ESI conditions, thus confirming that the OSDC deposits are more polar and compositionally different from C7 asphaltenes. Positive-ion ESI FT-ICR MS highlights that there are no significant compositional differences in the basic species, in terms of carbon number and number of rings plus double bonds, specifically in the Nx and NxSy species. The enrichment of SyOx-type species in the negative-ion ESI mass spectra for the deposits suggests that the polar nature of such species has a definite role to play in the deposition mechanism. Such acidic species have also been previously implicated in other characterization studies involving field and SAGD deposits. The results from this study and other studies on deposition listed in the references point out that these polar species are asphaltenes but just not what has been historically thought of as asphaltenes. Asphaltene continuum spans a broad range of MW and aromaticity but more importantly, in this case, polarity. Compositional variations between different deposits highlight the difference in the mechanism of action for the different types of chemistries used for inhibition. The study also suggests that, under OSDC conditions, certain types of inhibitors are actually contributing to the deposition tendency of this crude oil. The disparity in the mechanism of action for different chemistries is apparent from the difference in the heteroatom class distribution under positive- and negative-ion ESI conditions. In view of the current and future production scenarios where field deposition is expected to be an increasingly important flow assurance issue with several deep/ultra-deep water projects becoming online in a few years, characterization of these deposits is of immense importance to drive asphaltene research further and to face new challenges. We infer that a correlation of field asphaltene deposition tendency with laboratory screening tests is essential for the advancement of asphaltene research.
Figure 7. Heteroatom class distribution derived from positive-ion FT-ICR mass spectra for parent crude oil and OSDC deposits from untreated and treated crude oils. Only classes with relative abundances >1% are reported.
solids from untreated crude oil or solids from crude oil treated with chemical B (run 3). The O1 class ions, however, appear in the mass spectrum of the deposit from run 2, generated from crude oil treated with chemical A. Figure 7 shows the relative abundance distribution for different heteroatom classes in the positive-ion mass spectra for the parent crude oil and the deposits from untreated and treated crude oils with two different chemistries. Interestingly, there does not appear to be striking differences in the basic compound class speciation for the parent crude oil and the OSDC deposits from untreated and treated crude oils. The most basic nitrogen class, N1, indicative of pyridine and its benzanalogues, is the most abundant ion class for the parent crude oil and the deposits. The N1S1 species are the second most abundant in all of the samples. The samples are abundant in basic nitrogen and nitrogen-sulfur-containing species (NSx) and least abundant in nitrogen-oxygen (NOx) and sulfur-oxygen species. 2325
Energy Fuels 2010, 24, 2320–2326
: DOI:10.1021/ef900959r
Juyal et al.
Acknowledgment. This work was supported by the National Science Foundation (NSF) Division of Material Research through DMR-06-54118 and the State of Florida. Skillful FT-ICR MS
analysis by Mmilili M. Mapolelo is highly appreciated. We gratefully acknowledge the Chevron Corporation for providing the samples for this study and for the permission to publish the results.
2326
