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Relationship between the Content of Asphaltenes and Bases in Some Crude Oils Tanja Barth,*,† Sylvi Høiland,† Per Fotland,‡ Kjell Magne Askvik,‡ Reidun Myklebust,† and Kristin Erstad† Department of Chemistry, University of Bergen, Allegaten 41, N-5007 Bergen, Norway, and Norsk Hydro Research Centre, Reservoir Technology Group, P.O. Box 7190, N-5020 Bergen, Norway Received October 1, 2004. Revised Manuscript Received April 4, 2005
A strong correlation between the asphaltene content and the total base number (TBN) has been found for a set of 20 crude oils. The correlation coefficient, R2, for a polynomial fit is 0.990, over a range of TBN from 0.15 to 4.14 mg KOH/g oil and asphaltene contents of 0.4-25 mg/g oil. The higher values are for biodegraded oils. Titration of asphaltene fractions and the deasphalted oil shows that the asphaltene fractions are not in themselves bases. Extraction of bases with a formic acid/water constant boiling mixture gives recoveries of 25-37% of the original TBN, and the extracted oils still contain the remaining bases. This makes it difficult to evaluate the relevance of further characterization of the extract, which consists of molecules with molecular weights of 200-600 g/mol, with UV absorbance maxima corresponding to nitrogen heterocyclic compounds. The results suggest that the petroleum bases stabilize asphaltenes in the bulk crude oil and that acid-base interaction could be an important mechanism for asphaltene stability. In-reservoir biodegradation of the oil increases the asphaltene content, TBN, and TAN, but the specific compounds may be different from the original, geologically stable crude oil components.
Introduction Though crude oils are often spoken of as “hydrocarbons”, the components with a molecular composition that includes other elements than C and H are more important for many physical and chemical properties of the oils than the bulk hydrocarbons. Polar compounds, which contain one or more of the heteroelements nitrogen, oxygen, and sulfur, are critical factors for parameters such as viscosity, interfacial activity and chemical stability. The high-molecular-weight asphaltene compounds are the most chemically complex oil constituents and contain a higher proportion of the heteroatoms than the rest of the oil. This results in an inherent instability of the asphaltene solution in the hydrocarbon bulk phase, and interaction with other components of the oil phase is needed for stabilization. If the bulk oil composition changes, the asphaltenes can become unstable in the oil and precipitation can occur.1,2 In production and processing of crude oil, precipitation or sedimentation of a solid asphaltene phase often has negative consequences. The classic asphaltene problem is often related to deposition of solids in the well during production. If the deposition leads to choking of the production, then well intervention is necessary. Asphaltene problems may also occur in a reservoir during * Author to whom correspondence should be addressed. E-mail:
[email protected]. † University of Bergen. ‡ Norsk Hydro Research Centre. (1) Andersen, S. I.; Speight, J. G. Pet. Sci. Technol. 2001, 19, 1-34. (2) Speight, J. G. The chemistry and technology of petroleum, 3rd ed.; Marcel Dekker: New York, 1999.
miscible gas injection or by mixing streams with ‘light’ and ‘heavy’ oils during transport and storage. Once asphaltenes precipitate, the particles may deposit or act as emulsion stabilizers and cause water/oil separation problems. If asphaltene problems occur, a dramatic increase in operational costs may be the result. It is therefore of great practical value to understand the mechanisms that stabilize the asphaltene fraction, and the subject is receiving considerable attention.1-3 The asphaltenes are not a well-defined compound class but rather the extreme end of a continuum of molecular compositions found in crude oils, ranging from the simplest hydrocarbons to the high-molecular-weight compounds containing a high proportion of heterocompounds.2,5 The most widely used definition of asphaltenes relates to solubility. The amount of solids that precipitates in a 40:1 mixture of pentane/oil at 1 bar and ambient temperature is usually given as the asphaltene content, see, e.g., Speight.2 Some workers prefer to use heptane instead of pentane. Asphaltenes are thus defined as a solubility class in the crude oil and are not chemically defined by structure or functional groups. In a further refinement, carbenes are the fraction of asphaltenes that are insoluble in benzene and the carboid fraction is again insoluble in carbon disulfide. The residual oil after the asphaltenes have been removed is termed the maltenes. Further fractionation of the maltenes typically gives a resin fraction, which (3) Wang, J.; Buckley, J. S. Energy Fuels 2003, 17, 1445-1451. (4) Buckley, J. S.; Wang, J. J. Pet. Sci. Eng. 2002, 33, 195-202. (5) Altgelt, K.H.; Boduszynski, M. M. Composition and analysis of heavy petroleum fractions; Marcel Dekker: New York, 1994.
10.1021/ef049750a CCC: $30.25 © 2005 American Chemical Society Published on Web 05/21/2005
Content of Asphaltenes and Bases in Crude Oils
can be defined as the fraction of the crude oil which is precipitated by propane after asphaltene precipitation. The remaining propane-soluble material is then termed gas-oil. Alternatively, the resins are defined as the compounds retained on a silica or alumina chromatography column when a hydrocarbon solvent is used for elution. All of these fractions can be described by their solubility parameters.4 Theory predicts that compounds with similar solubility parameters are mutually soluble. The solubility parameters of the resins are intermediate between the asphaltenes and the hydrocarbons and are suggested to act as modifiers of the bulk oil solvent properties, keeping the asphaltenes in solution, as discussed by Wang and Buckley.3 However, the precise mechanism of the interactions between the resins and asphaltenes are not clearly understood. Acidic and basic compounds are part of the resin fraction of oil.5 The acidic and basic functional groups contain oxygen, nitrogen, and sulfur, have surfactant properties, and are interfacially active. Since the asphaltenes also contain heteroelements and polar functional groups, including acidic and basic sites, interactions between different groups of heterocompounds are central when asphaltene stabilization is considered. It is well established that oils with a high density (“heavy oils”) have a high content of heterocompounds, especially asphaltenes, and often a high content of acidic compounds as measured by titration (TAN, total acid number).6,7 Oils that have been biodegraded in the reservoir are typically both heavy and acidic. The content of bases has to a smaller degree been investigated and evaluated relative to their source and effects on oil properties. Acid-base interactions can potentially contribute to the stabilization of asphaltenes, but no clear correlation between the content of asphaltenes and petroleum acids or bases has been shown, though some data has been evaluated in this context.8 Acid and base amphiphilic additives have been shown to have good effects for stabilizing asphaltenes from specific oils.9,10 However, when sample sets of many crude oils are to be evaluated, many variations in the procedure for determining the total base number (TBN) and asphaltene content are possible, so the results are not easily comparable from laboratory to laboratory or even from operator to operator. Possible correlations may therefore be obscured by procedural variations. Determination of the total titrable bases in crude oil is usually made using a standard procedure.11,12 In this procedure, the TBN is given as the amount of potassium hydroxide (KOH) that would require the same amount of acid titrant as 1 g of oil. A comparable scale is used in TAN determinations,13 though here it is the amount (6) Meredith, W.; Kelland, S.-J.; Jones, D. M. Org. Geochem. 2000, 31, 1059-1073. (7) Barth, T.; Høiland, S.; Fotland, P.; Askvik, K. M.; Pedersen, B. S.; Borgund, A. E. Org. Geochem. 2004, in press. (8) Yang, S.-Y.; Hirasaki, G. J.; Basu, S.; Vaidya, R. J. Pet. Sci. Eng. 2002, 33, 203-215. (9) Auflem, I. H.; Havre, T. E.; Sjoblom, J. Colloid Polym. Sci. 2002, 280, 695-700. (10) Østlund, J.-A.; Nyde´n, M.; Fogler, S. H.; Holmberg, K. Colloids Surf., A 2004, 234, 95-102. (11) ASTM2896-88, Annual Book of ASTM Standards; American Society of Testing Materials: Philadelphia, 1988; Section 5. (12) Dubey, S. T.; Doe, P. H. SPE Res. Eng. 1993, Aug., 195-200. (13) ASTM664-89, Annual Book of ASTM Standards, American Society of Testing Materials: Philadelphia, 1989; Section 5.
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of KOH that is needed to neutralize the oil (mg KOH/g oil) that is reported. From a general understanding of acid-base equilibria, the TBN and TAN values should be seen as potentially related, since the chemical function as acid or base depends on the pH of the system rather than on inherent properties of the molecules. A protonated base is an acid, and a deprotonated acid is a base (using Bronsted acid/base definitions). In this work, we report a correlation between the TBN and the asphaltene content observed in a set of 20 crude oils taken mainly from the Norwegian continental shelf. Systematic trends in other physical and chemical characteristics of the oils are discussed, including the effect of biodegradation. Some preliminary work on analysis of the bases by extraction and characterization of the extracts is included. Experimental Section The experimental procedures have been described in Barth et al.7 in more detail, and are only briefly summarized here. Samples. Eighteen oil samples, distributed over nine fields, were supplied by Norsk Hydro, and two oils were supplied by Statoil. The primary sample set consisted of crude oils that had been stored in closed, refrigerated sample containers. Oil (0.5-1 L) was transferred to aluminum flasks, and void volumes filled with nitrogen gas. The different measurements were performed as soon as possible after the oil was received in the laboratory. Aliquots of the oil were removed from the stock bottle after heating to 60 °C and homogenization to ensure a representative composition of each sample. Additional samples were in part received in larger volumes, and in several cases, the sample collection and storage had been less rigorous, so addition of production chemicals cannot be excluded. Determination of Asphaltene Content. The asphaltenes were precipitated by refluxing a weighted portion of the oil with a 40 times excess (per volume) of hexane for 6 h. The precipitated asphaltenes were immediately removed by filtration of the hot solution through a Whatman GF/C glass fiber filter and dissolved in dichloromethane (DCM). The concentration of the asphaltenes in the DCM solution was determined gravimetrically, by deposition of a 10 µL solution on the weighing pan of a Cahn electrobalance (range 0.0001-2 mg), letting the solvent evaporate, and noting the weight of the nonvolatile residue after 20 min. TBN and TAN Titration. A Metrohm autotitrator (model 798 MPT Titrino) connected to a Metrohm Solvotrode combined LL pH glass electrode (model 6.0229.100) was used for the determination of acid and base numbers. The base numbers, TBN, were determined by the standard procedure ASTM2896-8811 with modifications according to Dubey and Doe.12 The procedure uses perchloric acid dissolved in acetic acid as titrant and methylene isobutylketone as solvent for the crude oil samples. Acid numbers were determined according to ASTM664-89,13 which is a standardized potentiometric titration with KOH in 2-propanol. For both TAN and TBN determination, at least three parallel titrations of the crude oil samples were made, using 5-20 g of oil in each titration. The TBNs were determined within an uncertainty of approximately 5%. For TAN values, an uncertainty of less than 4% is normal for values of TAN exceeding 1 mg KOH/g oil, but errors as large as 20% could be observed for very low TANs. For some oils, a test for the presence of already-neutralized bases was performed by adding an excess of the strong base tetramethylammonium hydroxide (TMAH) dissolved in 2-propanol and repeating the titration. The content of the weaker
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petroleum bases was calculated as the difference between the total bases and the excess strong base, which gave a separate inflection point in the titration curve. Asphaltene and maltene fractions were titrated to determine the distribution of the titrable bases after the fractionation. The extracted bases (see below) and the residual oil after extraction were also titrated to determine the efficiency of the extraction procedure. Density Determination and Determination of Degree of Biodegradation. The density of the crude oil was determined using an Anton Paar DMA60 densitometer connected to an Anton Paar DMA602HT measuring cell at 20 °C. The degree of biodegradation was estimated as described by Peters and Moldowan14 on the basis of gas chromatograms of the hydrocarbon fraction of the oils, combined with mass spectrometric detection of specific biomarker structures. Extractable Base Fraction. An extractable base fraction was produced from the crude oil by liquid-liquid extraction with a constant-boiling formic acid/water mixture (75.5:24.5 by volume, bp ) 107 °C). Twenty grams of oil was diluted with 25 mL of hexane and extracted with 5 × 10 mL of the formic acid/water mixture in a separatory funnel under nitrogen. For the oils with high TBN values, the separation of the phases could take up to 24 h in the first extraction step but the following aliquots of the formic acid/water mixture could then be removed after 30-60 min separation time. The pooled extracts were re-extracted with 10 mL portions of pentane until colorless to remove oil hydrocarbons, evaporated to dryness, and redissolved in a dichloromethane/methanol solvent (97:3 by volume). The yield was quantified gravimetrically as described for the asphaltenes. The efficiency of the extraction with regards to basic compounds was determined by titration of extract and residual oil to determine the TBN of each fraction. The equivalent weight of the bases was calculated from the extract TBN and yield in mg/g oil. UV spectra of extracts diluted with dichloromethane to a concentration of 0.4 mg/mL were recorded in quartz cuvettes over a 200-600 nm range. Gel Permeation Chromatography of the Extractable Bases. Gel permeation chromatography was performed on a PL-gel 3 µL MIXED-E column using tetrahydrofuran as the mobile phase at a rate of 0.5 mL/minute. The detector used was a Sedex 55 light scattering detector (S.E.D.E.R.E, France), which is linearly sensitive to all compounds with a boiling point above 100 °C. The molecular weight calibration was based on representative standards covering a range of molecular weights from 122 (benzoic acid) to 599 g/mol (vanadyloctaethyl porphyrin), with the addition of polystyrene standards at 30 000 and 70 000 g/mol for the higher molecular weights. A linear relationship between the logarithm of the molecular weight of seven standard compounds, and the retention time (RT) was observed with a correlation coefficient of 0.98. Measurement of Nitrogen-to-Carbon Atomic Ratio. The ratio of C and N was determined on a Carlo Erba elemental analyzer, using aniline for calibration.
Results Bulk Oil Properties. Table 1 gives the contents of asphaltenes, bases, and acids of the oils, together with their densities and an estimate of the degree of biodegradation on the Peters and Moldowan scale.14 Sample names starting with B indicate that the oils are biodegraded, while the S oil samples have not been microbially altered to any significant degree. Oils from a specific (14) Peters, K. E.; Moldowan, J. M. The Biomarker Guide; Prentice Hall: Englewood Cliffs, NJ, 1993; pp 252-265.
Barth et al. Table 1. Crude Oil Propertiesa asph TBN TAN biodegr density sample mg/g oil mg KOH/g mg KOH/g P and M scale g/mL B1a B1b B2a B2b B2c B3a B4a B4b B4c S1a S2a S2b S3a S3b S4a S4b S5a S5b S7a S7b
14.14 17.20 3.08 5.81 2.97 25.37 3.79 1.94 0.73 2.73 0.42 0.26 2.17 2.80 0.69 0.14 1.43 0.20 1.39 0.43
2.79 3.43 1.06 0.89 1.44 4.14 1.21 1.29 1.1 0.94 0.35 0.31 0.72 1.16 0.15 0.39 0.47 0.45 0.65 0.74
2.4 2.22 3.2 2.66 3.2 2.11 1.1 1.51 1.4 0.13 0.04 0.04 0.03 0.16 0.026 0.04 0.12 0.10 0.12 0.02
2 2 6 6 5 8 3 3 2 0 0 0 0 0 0 0 0 0 0 0
0.942 0.941 0.914 0.918 0.920 0.895 0.900 0.897 0.858 0.841 0.837 0.856 0.833 0.791 0.841 0.845 0.831 0.853 0.846
a Original sample set is bold. Asp, asphaltene content; biodegr, level of biodegradation.
Figure 1. Plot of asphaltene content as a function TBN. Initially measured values, squares; supplemental data, circles; solid line, curve for initial data set; stippled line, curve for all data points. Table 2. Distribution of Bases in Asphaltenes and Maltenes TBN TBN, maltenes/ asph./ eq. w. aspha sample TBN, oil g oil g oil % recovery base funct. B1b B2c B4c S3b S4b S5b a
3.4 1.44 1.11 1.16 0.39 0.45
3.3 1.12 1.03 1.17 0.3 0.35
0.2 0.03 0.005 0.023 0.05 0.02
103 80 93 103 90 78
7516 3066 3365 6828 157 561
Eq. w. asph, weight per mole functional groups, g/mol.
field is indicated by a number, and a, b, and c indicate separate samples or wells within each field. The asphaltene contents of the oils vary from 25.4 mg/g oil for sample B3a to 0.14 mg/g oil for sample S4b. The TBN varies from 4.14 mg KOH/g oil (0.07 mol/kg) for sample B3a to 0.15 mg KOH/g oil (0.003 mol/kg) for oil S4a. A plot of the asphaltene content as a function of TBN is shown in Figure 1, where the initially measured values are given as squares and supplemental data measured over a 2-year period is given as circles. A clear relationship of increasing asphaltene content with increasing TBN is observed. For the original sample set, the linear correlation coefficient, R2, is 0.952. However, a second-order polynomial function fits even
Content of Asphaltenes and Bases in Crude Oils
Energy & Fuels, Vol. 19, No. 4, 2005 1627
Table 3. Properties of the Extractable Bases sample
ext. bases mg/g oil
ext. BN mg KOH/g
recovery (% of TBN)
ext. eq.w. (g/eq)
mol wt (GPC)
N/C(base) (atomic)
B1a B2a B4b S1a S3a S4a S5a
5.2-6.5 2.84 2.54-2.32 3.06 2.09 0.74 1.89
0.716 0.243 0.353
26% 23% 27%
400-500 660 370-400
48-178
0.0124
200-746 135-554
0.267
37%
440
0.0130 0.0102 0.0075 0.0098
0.175
37%
610
better and is shown in the figure as a solid line with the following equation:
Asphaltenes ) 0.16 + 1.42TBN + 1.15TBN2 R2 ) 0.990 Data from analysis of additional oil samples that have been received from different sources, where the quality and handling of the oils is more uncertain, have been added over a 2-year period of time. The additional data shows more scatter, but in general fits with the trend. The best-fit line including all samples still has a correlation coefficient R2 ) 0.965 (shown as a stippled line). The values given in Table 1 show that the biodegraded oils generally have higher TBN values and asphaltene contents than the nonbiodegraded oils. The TAN and density are systematically highest in the biodegraded oils. Qualitatively, it was observed that, while the nondegraded oil S4b could be retitrated after addition of TMAH base solution to give the same TBN values as the initial titration, this was not possible for the B1b biodegraded oil. The titration curves became unstable, and the TBN values were much lower than for the initial determination, with very large deviations between parallels. Base Content of Asphaltenes and Bulk Oil. The base content of the asphaltene fraction and the maltenes (bulk or residual deasphalted oil) has been determined by TBN titration of the two fractions of six of the additional oils. The oils used for this procedure were acquired in larger volumes, as there were not sufficient amounts of the original samples for all measurements to be performed. The results are given in Table 2 and show that the bases remain in the bulk oil phase after deasphalting. The TBN of the asphaltene fractions are negligible, and the maltene fractions contain nearly as much base as the original oil. Composition of Petroleum Bases. Further investigation of the petroleum base composition has been attempted by extraction of bases and chemical characterization. An extraction procedure using a formic acid/ water constant-boiling mixture for liquid-liquid extraction was developed. However, the recovery of bases was in the range of 26-37% of the original TBN, which is not very satisfactory (Table 3). As a control, the basestripped residue of oil B4b was titrated after deasphalting, and the TBN of the residual oil was found to correspond to the nonextracted bases. Again, the asphaltenes contained very little basic functionalities: TBNbase extract ) 0.35 mg KOH/g oil, TBNasphaltenes ) 0.09 mg KOH/g oil, TBNresidual oil ) 0.88 mg KOH/g oil, sum 1.29 mg/g KOH/g oil, which sums up to the same value as initial measurement on the crude oil at 1.293 mg KOH/g oil. The low recovery values are therefore
150-608 46-467
considered to be caused by incomplete extraction and not by loss of basic components during the procedure. The equivalent weight per basic functionality for the extracts is also given in Table 3, and gives values of 400-600 g/equiv, which is in the lower end of the expected range for petroleum resins. The range of molecular weights determined by GPC is also given in Table 3 and gives even lower values, but overall in an overlapping range. The nitrogen content (Table 3) of the extract is significantly higher than that for the crude oils as a whole (measured range of selected oils, N/C ) 0.0010.01), as would be expected if nitrogen bases contribute to the base fraction. However, the values stay in a range of atomic N/C of 0.01 and so are not high, so the simple pyridine or quinoline type structures with N/C values of around 0.1 do not seem to be quantitatively important. UV spectra of the extractable bases showed three distinct shoulders at 211, 236, and 263 nm wavelengths and a long tail continuing into the visible part of the spectrum (>400 nm), as shown in Figure 2. The maximum at 211 nm is general for aromatic compounds. At 236 nm, a number of nitrogen aromatic compounds have an absorption maximum. A typical example is pyrrole (C4H4N). At 262 nm, indole (pyrrole with a benzene ring attached) has an absorbance maximum. This compound has a second maximum at 280-288 nm, which we do not see in these spectra, but the relative intensities of the maxima can be strongly influenced by substituents. The lack of maxima at higher wavelengths than 350 nm show that three- or four-ring systems are not present in any considerable amount in the base fraction. Attempts were made at molecular analysis by gas chromatography-mass spectrometry (GC-MS) on a column suitable for basic compounds, but no separation into specific components was achieved. The components appeared as an unresolved mixture (UCM) both for biodegraded and nonbiodegraded oils. Discussion The data present an intriguing relationship between the titrable bases in a crude oil, as measured by TBN, and the asphaltene content, as illustrated in Figure 1. Though similar correlations have been considered previously by other authors,15,8 no clear trends have been observed and acid-base interactions have been discarded as stabilization mechanisms. The lack of observable correlations could be caused by the well-known problems in obtaining consistent data, especially for the asphaltene content determination. Asphaltene precipitation procedures are very difficult to standardize, as demonstrated by a round-robin test performed at the establishment of the Norwegian Industry Guide to
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Barth et al.
Figure 2. UV traces of selected base extracts (absorbance vs wavelength, 200-600 nm): samples B1a, B4b, S1a, and S4a.
Organic Geochemical Analysis (NIGOGA).16 Here, the acceptable range of values for the Norwegian standard oil NSO-1 is quite wide, 1-4% with a most probable (15) Buckley, J. S.; Lord, D. L. J. Pet. Sci. Eng. 2003, 39, 261-273. (16) Norwegian Industry Guide to Organic Geochemical analysis edition 4.0, http://www.npd.no/engelsk/nigoga/nigoga4.pdf.
value of 1.9%, and this is the case where the participating laboratories all used the same procedure. The data presented in this paper have been collected over a 3-year period of time, with different operators but with very careful reproduction of the experimental procedures, and even so the values seems to diverge more as time
Content of Asphaltenes and Bases in Crude Oils
Energy & Fuels, Vol. 19, No. 4, 2005 1629 Table 4. Linear Correlation Coefficients (R) between Crude Oil Properties in This Data Set asp. TBN TAN biodeg. density
Figure 3. Comparison of data from this paper and Yang et al.8 for TBN and asphaltene content. Diamonds, this paper; triangles and squares, series from Yang et al.; circles, unsystematic values from Yang et al. No sample with TBN above 5 mg KOH/g oil or asphaltene content above 100 mg/g confirm the trends, and these samples have been excluded from the plot.
passes. Thus, the consistency of the data is a critical factor for observing the relationship presented here. If the data published by Yang et al.8 is divided into subsets where the naming seems to indicate a common origin, comparable trends can be extracted if outliers are removed from the plot, as shown in Figure 3. The simplest explanation for the observed correlation would be if the bases were incorporated into the asphaltene molecules, but this is easily disproved by the separate determination of TBN of the asphaltenes and maltenes. Titration of each fraction shows that the asphaltenes contain a very low proportion of the bases. A corresponding TAN titration also gives low values for titrable acids in the asphaltenes (B1b asphaltenes, TAN ) 0.09 mg KOH/g oil; B4c asphaltenes, TAN ) 0.01 mg KOH/g oil; other oils below detection level), so a simple acid-base relationship is not immediately indicated. However, further work is needed on this, especially as the standard TAN titration only registers the acids with strengths at least comparable to carboxylic acids, while more weakly acidic groups such as phenols (pKa > 9) are not registered (unpublished data). Also, during both the TBN titration and the liquid-liquid extraction of the bases, a black precipitate has been observed in some of the high-asphaltene oils as the bases are neutralized or removed. This supports the idea that the bases are stabilizing the asphaltenes in the crude oil solution and lose their ability to do this when neutralized. When trying to extract the bases by an organic/ aqueous acid phase, the procedure used in this work only attains an efficiency of one-fourth to one-third of the total base content and the larger part of the bases remain in the stripped oil, see Table 3. This suggests that the major part of the bases is present in a strongly hydrophobic state. This may be due to a high molecular weight of the molecule or protection of basic groups by interactions with other components in the oil. The structural information obtained on the portion of the bases that is extracted thus cannot be assumed to represent all basic compounds. However, if the chemical structures found in the extract are relevant, polyaromatic systems containing nitrogen bases make an important contribution to the petroleum base fraction. It is difficult to evaluate if published data on petroleum
asph.
TBN
TAN
biodeg.
density
1.000
0.961 1.000
0.598 0.598 1.000
0.572 0.577 0.860 1.000
0.726 0.762 0.899 0.726 1.000
base compositions can be used for comparison, as no information on the recovery relative to TBN is given.17-20 Further work on both the extraction and the characterization of the bases is clearly needed before specific chemical structures can be suggested. The possibility of the TBN measuring conjugate bases, i.e., deprotonated acids, and thus reflecting position of the acid-base balance in the oil rather than the total amount of specifically basic compounds, should be considered. The acids and bases are often regarded as separate compound classes, but the positive correlation observed in this data set (Table 4) supports the idea that an equilibrium state gives a more correct picture than a model based on separate, noninteracting compound classes. In a high-TAN oil, there could easily be increased protonation of basic functionalities. These could both be separate compounds or parts of larger molecules. Previous studies of petroleum acids have shown them to be largely made up of multifunctional compounds, 21,7 and amphotheric compounds are also indicated. This would most probably also be the case for the compounds observed as bases in the oil. However, titration procedures developed to detect neutralized bases by addition of excess acid and repeated titration of the oils only slightly increased the TBN values of the nonbiodegraded oils and considerably reduced the TBN values for the biodegraded oils, which suggests that the acidification gave pH-dependent reactions that remove the base functionalities from their original state. It also illustrates the importance of sample handling for production of reliable data if chemically unstable compounds are critical for the acid/base status of biodegraded oils. Even though the established procedures cannot clearly show that there are acid-base differences between asphaltenes and maltenes, the most reasonable interpretation of the relationship between the TBN and asphaltene content is that the bases stabilize the dissolution or dispersion of asphaltene moieties in the crude oil. As shown in Table 4, the correlation coefficient for TBN and asphaltene content is very clearly higher than between any other pair of properties. This suggests a functional relationship and not just co-variation between groups of chemical and physical properties. Dutta and Holland22 titrated a single asphaltene sample (Cold Lake asphaltenes, from very heavy oil) and found that it contained 0.3 mM/g strong and intermediate (17) McKay, J. F.; Weber, J.; Latham, D. R. Anal. Chem. 1976, 48, 891-898. (18) Green, J. B.; Hoff, R. J.; Woodward, P. W.; Stevens, L. L. Fuel 1984, 63, 1290-1301. (19) Merdrignac, I.; Behar, F.; Albrecht, P.; Briot, P.; Vandenbroucke, M. Energy Fuels 1998, 12, 1342-1355. (20) Oleveira, E. C.; Vaz de Campos, M. C.; Lopes, A. S’A.; Vale, M. G. R.; Caramano, E. B. J. Chromatogr. A 2004, 1027, 171-177. (21) Tomczyk, N. A.; Winans, R. E.; Shinn, J. H.; Robinson, R. C. Energy Fuels 2001, 15, 1498-1504. (22) Dutta, P. K.; Holland, R. J. Fuel 1984, 63, 197-201.
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bases and 0.6 mM/g strong to intermediate acids. Further work on determining functional groups on the Cold Lake crude oil asphaltenes has shown carboxylic functionalities in some of the asphaltene fractions.10 One possible model for acid-base interactions would be to place the acidic functionalities in peripheral positions on the asphaltene molecule, where they would be acsessible for interaction with basic compounds in the bulk oil phase. The nature of the nitrogen base functionalities, which typically are a part of the aromatic compounds that comprise the central units of the asphaltenes, would hinder a corresponding interaction with acidic resins in the bulk oil phase. Models of asphaltene stabilization that include acid-base interactions are thus indicated, and evaluation of the acid and base content of the oils should be included when the risk of asphaltene precipitation is evaluated. Biodegradation is clearly important for the acid-base composition of the crude oils and is perhaps the single most significant factor determining the acid-base chemistry, though the TBN-asphaltene correlation spans both biodegraded and nonbiodegraded oils. Biodegradation is seen to increase the TBN values of the oils, as well as the TAN values, though the difference is not as
Barth et al.
large for the TBN as for the TAN.7 The process seems to introduce new compound types into the oil, changing the acid-base properties, as observed in the re-titration tests, and also increasing the asphaltene content. Multivariate classification shows that the oils in this data set comprise two separate classes on the basis of whether the oil has been exposed to biodegradation in the reservoir, with no overlap between the classes. However, the number of samples at present is small for detailed statistical analysis. Further investigations should focus on the different chemical nature of the nonhydrocarbon fractions of biodegraded and nonbiodegraded oils and search for possible biological sources and precursor structures for these fractions, not limiting the analysis to geologically stable molecules. Acknowledgment. Gro Valle, Bent Skaare Pedersen, and Tadesse Negash are thanked for analytical work. Norsk Hydro is thanked for funding the project. Statoil is thanked for supplying the two additional oil samples. EF049750A
