- Email: [email protected]
An analysis of crude oil–acid reaction products by size-exclusion chromatography
Fuel 80 (2001) 33±40
www.elsevier.com/locate/fuel
An analysis of crude oil±acid reaction products by size-exclusion chromatography M. Rietjens a,*, M. Nieuwpoort b a
Halliburton B.V., European Research Centre, Weversbaan 1-3, 2352 BZ, Leiderdorp, Netherlands b School for Laboratory Education, Leidsedreef 5, 2352 BA, Leiderdorp, Netherlands Received 25 November 1999; revised 12 April 2000; accepted 18 April 2000
Abstract The contact of acid with crude oil generates a reaction product known as acid-sludge which has been interpreted as polymerised asphaltenes due to Friedel±Crafts reactions. A size-exclusion chromatographic analysis of crude oils and fractions showed, however, that acid-sludge is composed of small molecules. The greatest increase in molecular weight was observed for the nonpolar fractions. The results could be explained by assuming surfactant-like behaviour of protonated basic crude oil components. Aggregate formation, therefore, explains the higher molecular weights observed. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Crude oil±acid complexes; Acid-sludge; Aggregation; Size-exclusion chromatography
1. Introduction Acid formulations are frequently injected to stimulate oil wells. This improves the permeability of the rock matrix, enhancing oil production. A typical acid formulation is based on 15% HCl, but higher acid strengths are also used. Contact of acid with the crude oil, however, generates a precipitate known as acid-sludge [1,2]. PrecipitatesÐ organic or inorganicÐare undesirable because of their potential for plugging pores and impairing formations. It is commonly believed that acid-sludge is polymeric, with an asphaltenic nature, and is induced by Friedel±Crafts-type reactions. Recent experiments have cast doubt on this view. For example, in one experiment, a crude oil was diluted with toluene and acid was added. After 15 min, the ¯uid was checked for sludge. A critical toluene concentration was identi®ed, below which acid-sludge was formed. Above this concentration, no acid-sludge was formed [3]. Even asphaltene-free crude oils and condensates can generate acid-sludge when exposed to acid [4]. Clearly, these ®ndings do not support the asphaltene polymerisation theory; however, they do support a recently published hypothesis based on aggregation [3]. The results of size-exclusion chromatography (SEC) on acid-sludge samples generated from various crude oils and several types of acids have helped * Corresponding author. Fax: 131-71-581-3400. E-mail address: [email protected] (M. Rietjens).
determine the nature of acid-sludge. This paper provides details of the SEC results that support the aggregation theory. SEC is an analytical tool for determining the molecular weight of polymers or compounds with relatively high molecular weight. For example, SEC has been successfully applied in ageing studies of road asphalt [5], molecular weight distribution of asphaltenes in crude oils [5], and aggregation behaviour of asphaltenes [6]. Aggregation of asphaltenes has been observed even in highly diluted and excellent solvents such as tetrahydrofuran (THF). As a result, a shift in the chromatograms toward higher molecular weights can be seen. Because asphaltenes are known to be a constituent of most crude oils, aggregation of asphaltenes can be anticipated in the analysis of acid-sludge by SEC. The term acid-sludge is used to classify crude oil±acid precipitates for distinguishing these sludges from other sludges related to crude oils. 2. Procedures 2.1. Sample preparation Five crude-oil samples were analysed: N'Kossa (Angola), Monti Enoc (Italy), Monti Alpi Nord (Italy), Skjold (Denmark), and Tengiz (Russia). Tengiz is a transparent crude oil containing virtually no asphaltenes. Before the samples were treated and analysed, they were centrifuged
0016-2361/01/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(00)00073-9
34
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
for 15 min at 4500 rpm to remove the insoluble residues. The crude-oil samples were separated into polar/nonpolar fractions and asphaltene/de-asphaltenated fractions. All samples were analysed before and after acid treatment. 2.2. Separation of the polar and nonpolar constituents This technique was adapted from SjoÈblom et al. [7]. In our work, the treatment of the crude oils with SiO2 did not remove the black colour of the oil, as opposed to SjoÈblom et al. Therefore, the nonpolar fractions still had components that contained heteroatoms which can be basic in character. The purpose of this investigation was to examine the sludging behaviour of the crude oil with most of the polar compounds removed. A 6.0-ml aliquot of the crude oil was transferred to a test tube, and 1.00 g of SiO2 was added. The SiO2 had been conditioned for several days in a oven at 508C. Under these conditions, the silica was activated, but not to the extent that irreversible adsorption occurred [8]. The test tubes were regularly shaken during a 15-min period, then they were centrifuged. The oil was transferred to a new test tube and treated once more with 1.00 g of SiO2. This latter oil layer is referred to as the nonpolar fraction. Both SiO2 fractions were combined, and the polar components were desorbed through two washings: ®rst with CH2Cl2 1 5% methanol and then with THF. The extracts were collected, and the solvents evaporated in a vacuum oven. Before further testing, these samples were dissolved in 2.0 ml of toluene. They are referred to as polar fractions. From the results in Figs. 2±6 it is observed that both the polar and nonpolar fractions have lower molecular weights. This ®nding indicates that irreversible adsorption of polar components probably occurred to some extent. 2.3. Asphaltenes and de-asphaltenated fractions Asphaltenes and de-asphaltenated fractions were prepared by mixing each crude oil sample with n-hexane in a ratio of 1:20 and ®ltering off the precipitated asphaltenes [9]. 2.4. Acid-sludge sample preparation Four types of acids were used to generate acid-sludge: (a) 15% HCl; (b) 15% HCl 1 1000 mg/l of Fe 31 (as FeCl3); (c) a 25% w/w trichloroacetic acid (TCA) solution in toluene; and (d) a solution of approximately 3000 ppm Fe 31, added as FeCl3, in dioxane. The ®rst two acids are water-based and form two-phase systems. The other two acids, a Brùnsted and a Lewis acid, respectively, are miscible with oil and form one-phase systems. In all the tests, both the acid-sludge and the nonsludging oil phases were analysed. The tests with 15% HCl formulations were carried out by placing 2.0 ml of the acid in a test tube and adding 2.0 ml of the oil sample. The tubes were allowed to stand statically at 1008C for 1.5 h.
After being centrifuged at 3000 rpm for 5 min, the upper layer, or the nonsludging phase, was removed and prepared for analysis. To collect the acid-sludge residing at the interface, a 4-ml aliquot of toluene was added to the tube and gently mixed with the oil layer. Then, the solution was centrifuged and the upper layer was largely drawn off and discarded except for the last 0.5 ml. This process was repeated until the acid-sludge was visible as solid particles at the interface, with clean toluene above it. These particles were collected and prepared for analysis. The acid-sludge tests with TCA and FeCl3 were conducted differently. A 2.0-ml aliquot of oil was placed in test tubes. Forty microlitres of the TCA solution was added to each tube. In the tests with FeCl3, a volume of 50 ml of the FeCl3 solution was added to each tube. The tubes were mixed and allowed to stand statically at ambient temperature for 15 min. After the test tubes were centrifuged, the sludge was removed from each tube and prepared for analysis. 2.5. Size-exclusion chromatography All the crude-oil samples and fractions were analysed before and after acid treatment. After acid treatment, both the acid-sludge and the nonsludging phases were analysed. Dilutions in THF were prepared which contained approximately 1000 mg/l of the sample. For the acid-sludge samples, dilutions were prepared with a colour intensity similar to the other samples. To avoid problems associated with air oxidation, these dilutions were analysed within 1 h after preparation [5]. The molecular weight (M) of the asphaltenes ranges from several hundred Daltons up to tens of thousands Daltons. Because it was expected that sludge is not polymeric in nature, a column was selected that separated optimally in this range. For SEC, a Waters 625 LC system equipped with a 486 tuneable absorbance detector was used. The system runs under the Millennium 2.00 software package. A Styragel HT3 column (4.6 mm ID £ 300 mm) was used. It is suitable for a molecular weight between 300 and 30,000 g/mol. As a guard column, a Styragel 4.6 mm ID £ 30 mm column was used. The test conditions included ambient temperature, a ¯ow rate of 0.35 ml/min, eluent THF, and UV/Vis detection at a wavelength of 275 nm. 3. Results 3.1. SEC analysis 3.1.1. Calibration The basic theory of SEC will be discussed as far as necessary for an understanding of the results presented [10,11]. SEC separates compounds based on the hydrodynamic radius of the molecules. Because polymers used for calibration and asphaltenes have different hydrodynamic radii, different retention volumes or times can result with the
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40 Table 1 Correction of molecular weights according to Reerink and Lijzenga Molecular weight calibration curve
Molecular weight corrected according to Reerink and Lijzenga
250 500 1000 2000 5000
201 545 1479 4013 15,015
same molecular weight. Dif®culties with calibration have been documented by several authors [5,12,13]. Application of the universal calibration curve, based on the Mark± Houwink relation, a plot of [h ]M versus Ve, where [h ] is the intrinsic viscosity and Ve the elution volume, does not hold for asphaltenes. Asphaltenes have a lower hydrodynamic volume because they are more compact, and elute at the same volume as lower-molecular-weight polystyrenes. Maltenes and polystyrenes have similar compactness [5]. According to Reerink and Lijzenga [14], a correction can be made by describing the hydrodynamic volume according to the disc model. A list of molecular-weight conversions is shown in Table 1. Generally, higher molecular weights are calculated for asphaltenes with a molecular weight above 500 g/mol, using a calibration curve based on polystyrene and toluene. In Figs. 2±6 the Reerink±Lijzinga corrections were not used. Another complication with asphaltenes, or aromatic compounds in general, is that the elution behaviour is different for peri-condensed aromatics such as pyrene and coronene, and cata-condensed aromatics, such as naphthalene and anthracene. Peri-condensed aromatics require higher elution volumes compared with cata-condensed aromatics of the same molecular weight [12]. Because no satisfactory explanation has been determined for this [12,15], the retention volume is not uniquely linked to molecular weight. Based on polystyrene and toluene as the calibration compounds, an estimation of the molecular weight is determined by the calibration curve, Eq. (1). Both calibration samples were run in triplicate log Mps 21:157Ve 1 7:182
r 2 0:9999
1
where Mps is the molecular weight relative to polystyrene. Based on these considerations, the results can be compared on a relative basis, but the absolute values should be interpreted with caution. Based on duplicate runs, an error of 0.2% was determined for the retention volumes. This error translates to 1% after application of Eq. (1), and to 7% after conversion of log(M) values to molecular weight. This applies to molecular weights calculated for peak values. For the upper values of the molecular weight, an error of 13% was estimated. 3.1.2. Effects of aggregation In asphaltene applications, THF is often used as an eluent
35
[5,6,12], but CHCl3 and benzene containing 5% methanol have been applied as well. The molecular weight of asphaltenes is complicated by the association of asphaltenes in solution. Because asphaltene aggregation does not suddenly begin at the critical micelle concentration (CMC), but probably proceeds through a stepwise mechanism, the formation of small assemblies can be expected even at low asphaltene concentrations [6]. For dissociating acid-sludge most completely, THF was selected as a solvent and eluent because of its excellent solvency. However, the effects of asphaltene aggregation can be anticipated in the analysis of acid-sludge. 3.1.3. Air oxidation In the study of road asphalt, an increase in viscosity was observed as a result of dissolving the asphalt and evaporating the solvent [5]. Other experiments showed that asphalt and asphaltenes are very sensitive to air oxidation when diluted [5,15]. A similar case is the analysis of crude oils and acid-sludge samples by SEC. The samples were strongly diluted with THF, and were thus exposed to relatively large amounts of oxygen. For this reason, diluted samples were analysed as quickly as possible, within 1 h. It is assumed that this time was suf®cient to dissociate the acid-sludge aggregates. 3.2. Analysis before acid treatment A plot of a typical chromatogram for the Tengiz sample is shown in Fig. 1. The results obtained with SEC are plotted in Figs. 2±6. In each chromatogram, two peaks were generally observed, but only the results of the peaks representing the highest molecular weights are shown. The peaks representing lower-molecular-weight compounds are of interest, but because no detailed analysis of the components in these peaks is available, they were not considered further. Furthermore, focusing on the highest-molecular-weight components demonstrates more convincingly that acidsludge is not polymeric in nature. A direct analysis of the pure crude-oil samples and their fractions before acid treatment showed an interesting feature. Compared to the pure crude-oil samples, equal or reduced molecular weights were observed for the polar and nonpolar fractions, whereas the asphaltene fractions from N'Kossa and Monti Alpi have a two- to three-fold higher molecular weight (Figs. 2 and 5, respectively). The deasphaltenated fractions generally had equal molecular weights except for Monti Alpi, which had half the molecular weight. This increase in molecular weight of the asphaltene fractions could not be attributed to chemical reactions of any kind. Because asphaltenes have a strong tendency to aggregate, it is reasonable to attribute the observed increases in molecular weight to aggregation [5,6,15]. 3.3. Analysis after acid treatment In a few cases, the molecular weight of the nonsludging
36
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
Fig. 1. Size-exclusion chromatogram obtained on the Tengiz sample.
phase was higher than the same untreated crude oil or fraction. This was true for the polar fraction of Skold (Fig. 3) and Monti Enoc crude oil (Fig. 4). For the other cases, equal values for the molecular weights were observed. Conversely, higher molecular weights were generally observed for the acid-sludge samples. The biggest increase in molecular weight was among the nonpolar fractions. The increase was up to ®ve-fold, when based on peak values, or
10-fold when based on upper values. Taking the Reerink± Lijzenga corrections into account, the increases in molecular weights were even larger, but this did not alter the conclusions. Most polar fractions have higher molecular weights, but the increase is small, 1.5- to 2-fold. Results of the asphaltene fractions were similar to the polar fractions. In two cases, N'Kossa and Monti Enoc, equal molecular weights were obtained. Increases in molecular weight of up to two-fold were observed for Skjold and Monti Alpi.
Fig. 2. Results for N'Kossa crude-oil sample. The white bar indicates the peak value of the molecular weight, and the grey bars show the upper value of the molecular weight.
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
37
Fig. 3. Results for Skjold crude-oil sample.
Differences between acid-sludges generated by various acids were also considered. The upper values of the molecular weights were similar for all acid-sludges, but the molecular weights calculated on peak values differed. Acid-sludge induced from FeCl3/dioxane showed a lower peak molecular weight compared with acid-sludge induced from 15% HCl formulations. This was also generally true for acid-sludge generated from TCA.
4. Discussion 4.1. Polymerisation theory Oil®eld researchers have generally accepted that acidsludge is formed as a result of Friedel±Crafts-type reactions. This view is based on the following: (1) Lewis and Brùnsted acids are known to catalyse Friedel±Crafts and
Fig. 4. Results for Monti Enoc crude-oil sample.
38
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
Fig. 5. Results for Monti Alpi crude-oil sample.
related reactions [16]; (2) a number of heterocyclic compounds that are present in oil are prone to polymerise under acidic conditions [17]; and (3) acid-sludge is formed when a crude oil is exposed to acid. The stronger the acid concentration, the more acid-sludge is formed. The presence of small concentrations of Fe 31 considerably increases the amount of acid-sludge [1,2,18,19]. These observations suggest that the polymerisation theory based on Friedel± Crafts reactions is accurate. However, an analysis that reveals polymeric products has never been performed.
In the early-1960s, Lewis acids such as FeCl3 were shown to be capable of polymerising benzene to polyphenyl in the presence of small amounts of water [20]. However, a high concentration of FeCl3 was required in a ratio of 1:1 to benzene. FeCl3 was presumed to function both as a catalyst and as an oxidant. In contrast to benzene, the sterically hindered 1,3,5-trimethylbenzene could be dimerised and chlorinated, but not polymerised [21]. The more reactive compound, naphtalene, reacted at ambient temperature, but formed only 1,1 0 -bi-naphtyl. It was theorised that the
Fig. 6. Results for Tengiz crude-oil sample.
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
highest degree of polymerisation is associated with the most active propagating species, i.e. the carbonium ion that possesses the least delocalisation as in benzene [22]. The work by Kovacic et al. shows that few aromatic compounds can polymerise under ideal conditions. Later work by the group of Speight on reactions of asphaltenes with metal chlorides showed the elements Cl and O to be incorporated into the structure of asphaltenes, but no evidence of highermolecular-weight asphaltenes was provided [9]. In a later publication, the oxidation of bitumen in the presence of various metal salts was described. The results clearly indicated no increase in molecular weights of the asphaltenes [23]. On the contrary, lower-molecular-weight asphaltenes were identi®ed. This observation was attributed to enhanced intramolecular association of the oxidised, more polar, asphaltenes [23]. The results presented here also do not support the polymerisation theory. In addition, several theoretical arguments and arguments based on observations have been brought forward against this theory [3]. 4.2. Acid-sludge aggregation A parallel with asphaltene aggregation can be drawn. For this reason, the current theory on asphaltene aggregation is explained ®rst. A distinction must be made between aggregation of crude-oil components in the crude oil itself and aggregation of these components in THF. 4.2.1. Crude oil The presence of several permanent dipoles per molecule is the primary cause of asphaltene aggregation [24]. Yen et al. have postulated an aggregation theory for asphaltenes in which aggregates of various sizes exist, as shown in Scheme (2) [25,26] Free asphaltenes $ assembly $ cluster $ flocs
2
The free asphaltenes are the smallest, with a diameter of approximately 1.8 nm. Flocs have a diameter of approximately 5 mm, which is large enough to settle out. Although the shape of asphaltene molecules is far from idealÐlong alkyl chains with a hydrophilic headgroupÐthe initially formed assemblies could resemble a reversed micelle. A description in reversed micellar terminology has, in fact, been proposed by Yen et al. [27] for asphaltenes. Stack formation as occurs in asphaltene assemblies is another important mechanism [28]. Acid-sludge consists of particles that are similar in size to ¯occulated asphaltenes [4]. It is assumed that similar stages exist in acid-sludge formation as shown in Scheme (3). The term ªpresludgeº is used to identify components that are prone to form acid-sludge H1
Free presludge $ protonated presludge $ assembly $ cluster $ flocs
3
This theory is being explored with success. It states that the
39
®nal stage, acid-sludge, is basically a ¯occulated material that results from aggregation of protonated crude-oil components. Whether assemblies and clusters truly exist is under investigation. This work shows that the basic constituents have molecular weights in the same order as the original crude-oil components. The onset of acid-sludge, as detected by microscopy, is characterised by particles .300 nm. This is the same value as calculated for centrifugation at 3000 rpm. Thus, particles ,300 nm remain in the nonsludging phase, whereas particles .300 nm settle out. Upon contact with a water-based acid such as 15% HCl, crude oil can form acid-sludge, but the transport of HCl to the oil phase is required [3,4]. As a result, acid-sludge formation requires time. When crude oils are mixed with acids dissolved in organic solvents (TCA/toluene and FeCl3/ dioxane), acid-sludge forms almost instantaneously. The interaction of an acid (Brùnsted or Lewis) with some crude-oil components results in protonated species or metal complexes which are more polar. These complexes can be caused by simple acid±base interactions. However, electron donor/acceptor complexes can also form [28,29]. It has been demonstrated that HCl is incorporated into the structure of acid-sludge, but the nonsludging phase also contains a substantial amount of HCl acid [3]. This HCl acid can be simply dissolved in the oil phase or incorporated into assembles and clusters. Large aggregates, at least up to the ¯occulation stage, are evidently formed in the acidsludge itself. We propose that the large ¯occulates are formed from the smaller clusters which, in turn, are formed from assemblies. We hypothesise that the smallest assemblies are formed from protonated crude oil±acid complexes. Protonation of basic sites of, for example, nitrogen-based crude-oil components, renders surfactant-like species that have a natural tendency to aggregate. This process could resemble reversed micelle formation of ordinary surfactants in organic solvents [30]. Stack formation, as occurs in asphaltenes, is another important mechanism. In addition, the number of polar sites or hydrophilic groups is relevant, as well. Compounds with fewÐor just oneÐhydrophilic group and a bulky nonpolar group could aggregate as classical surfactants aggregate in organic media [30]. If a component contains several polar sites, such as asphaltenes, the solubility of these components in the crude oil will be small and the components will be prone to aggregate even when unprotonated. 4.2.2. Tetrahydrofuran When various samples are dissolved in THF, the aggregation behaviour is altered. THF is a semipolar solvent, in contrast to a crude oil, which is basically nonpolar. The fact that acid-sludge dissolves very well in THF shows that the ¯occulated material dissociates. In fact, a new equilibrium is established that is quanti®able through SEC. Thus, aggregates at the assembly or cluster level could still be present in THF. Because the basicity of THF is low, crude oil±HCl complexes probably remain largely undissociated. This may
40
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
not be the case with FeCl3-induced acid-sludge because THF can form complexes with Fe(III) [31]. Furthermore, compared with HCl, TCA is less polar and more soluble in THF, and crude oil±TCA complexes may be dissociated more than their HCl equivalents. These effects are tentatively shown in the data. Although the upper values of the molecular weights are equal for all acid-sludges, the molecular weights based on peak values are smaller for acidsludges derived from TCA and FeCl3. Because the peak values represent the majority of components, this ®nding points to a shift in Scheme (3) toward dissociation. The polar fraction is relatively rich in compounds containing heteroatoms such as nitrogen, which are generally basic in character, whereas the nonpolar fraction, by comparison, is relatively poor in these compounds. The acid-sludge of the nonpolar fractions shows the highest increase in molecular weight. It is not unreasonable to assume that compounds in this fraction are more surfactant-like: nonpolar alkyl and/or aryl groups with very few polar groups. Upon protonation, such compounds are more likely to aggregate as classical surfactants aggregate in organic solvents, whereas compounds carrying several polar groups distributed over the molecule are less likely to aggregate. This explains the higher molecular weight observed. In contrast, the polar fractions and acid-sludge generated from the polar fractions are more soluble in a semi-polar solvent such as THF, and have less tendency to aggregate. The higher molecular weights observed for the untreated asphaltene fractions con®rm the tendency of asphaltenes to aggregate, even in a solvent such as THF. When acid complexes were formed, no further increase in molecular weight was observed. Conversely, an increase in molecular weight was observed for the acid-sludge of the de-asphaltenated fractions. Resins are part of the de-asphaltenated fraction, and their propensity to aggregate is low. As with the nonpolar fractions, their tendency to form aggregates increases greatly when acid complexes are formed. In both cases, the acid complexes generated acquired surfactant-like properties and formed aggregates. Recently, it was found that sludge contains a much higher resin content than untreated crude oils, con®rming the ®ndings reported here [2]. 5. Conclusions The ®ndings reported in this paper do not indicate that acid-sludge is polymeric in nature, but they do indicate that acid-sludge consists of aggregates. Acid-sludge particles can be easily dissolved in THF, and were shown to consist of relatively small-molecular-weight components. Increased molecular weights were identi®ed, but could be explained by the aggregation hypothesis. The most
pronounced increases in molecular weight observed were among the nonpolar and de-asphaltenated fractions. Upon the formation of acid complexes, these components had surfactant-like properties and formed aggregates. The most polar species, asphaltenes, and polar fractions have a natural tendency to aggregate, even when unprotonated. Upon formation of acid complexes, this propensity is slightly enhanced, which explains the minor increases in molecular weights that were observed for these fractions. References [1] Moore EW, Crowe CW, Hendrickson AR. J Petrol Technol 1965:1023. [2] Wong TC, Hwang RJ, Beaty DW, Dolan JD, McCarthy RA, Franzen AL. Soc Petrol Engrs, Prod Fac 1997:51. [3] Rietjens M, Nieuwpoort M. European Formation Damage Conference, 31 May 31±1 June 1999, The Hague. Soc. Petrol. Engrs, Paper 54727. [4] Rietjens M. European Formation Damage Conference, 2±3 June 1997, The Hague. Soc. Petrol. Engrs, Paper 38163. [5] Wu CS. Handbook of size-exclusion chromatograph. New York: Marcel Dekker, 1995. (chap. 8). [6] Andersen SI. J Liq Chromatogr 1994;17(19):4065. [7] Sjoblom J, Urdahl O, Hoil H, Christy HH, Johansen EJ. Progr Coll Polym Sci 1990;82:131. [8] Later DW, Wilson BW, Lee ML. Anal Chem 1985;57:2979. [9] Speight JG. Fuel 1970;50:175. [10] Braam WGM. Scheidingsmethode: chromatogra®e. Groningen: Wolters-Noordhoff, 1992. (chap. 5). [11] Young RJ, Powell PA. Introduction to polymers. London: Chapman & Hall, 1991. (Sections 3.17.1±3.17.6). [12] Bergmann JG, Duffy LJ, Stevenson RB. Anal Chem 1971;43:131. [13] Snyder LR. Anal Chem 1969;41:1223. [14] Reerink H, Lijzenga J. Anal Chem 1975;47:2160. [15] Altgelt KH. In: Altgelt KH, Gouw TH, editors. Chromatographic science, vol. 11. New York: Marcel Dekker, 1979. (chap. 12). [16] Olah GA. Friedel±Crafts chemistry. New York: Wiley, 1973. [17] Badger GM. The chemistry of heterocyclic compounds. New York: Academic Press, 1961. [18] Jacobs IC, Thorne MA. Soc. Petrol. Engrs, Paper 14823. [19] Houchin LR, Dunlap DD, Arnold BD, Domke KM. Soc. Petrol. Engrs, Paper 19410. [20] Kovacic P, Koch FW. J Org Chem 1963;28:1864. [21] Kovacic P, Koch FW. J Org Chem 1965;30:3176. [22] Kovacic P, Wu C. J Org Chem 1961;26:759. [23] Moschopedis S, Speight JG. Fuel 1978;57:235. [24] Maruska HP, Rao BML. Fuel Sci Technol Int 1987;5(2):119. [25] Yen TF. In: Yen TF, Chilingarian GV, editors. Asphaltenes and asphalts I, development in petroleum science 40. Amsterdam: Elsevier, 1994. (chap. 5). [26] Dickie DP, Yen TF. Anal Chem 1967;39:1847. [27] Lian H, Lin JR, Yen TF. Fuel 1993;73:423. [28] Taylor R. Electrophilic aromatic substitution. New York: Wiley, 1990. (chap. 2). [29] March J. Advanced organic chemistry. 3rd ed. New York: Wiley, 1985 (p. 74±7, 485). [30] El-Seoud OA. In: Hinze WL, editor. Organized assemblies in chemical analysis, vol. 1. Greenwich (CT): JAI press, 1994. [31] Cotton FA, Wilkinson G. Advanced inorganic chemistry. 4th ed. New York: Wiley. (p. 759).
www.elsevier.com/locate/fuel
An analysis of crude oil±acid reaction products by size-exclusion chromatography M. Rietjens a,*, M. Nieuwpoort b a
Halliburton B.V., European Research Centre, Weversbaan 1-3, 2352 BZ, Leiderdorp, Netherlands b School for Laboratory Education, Leidsedreef 5, 2352 BA, Leiderdorp, Netherlands Received 25 November 1999; revised 12 April 2000; accepted 18 April 2000
Abstract The contact of acid with crude oil generates a reaction product known as acid-sludge which has been interpreted as polymerised asphaltenes due to Friedel±Crafts reactions. A size-exclusion chromatographic analysis of crude oils and fractions showed, however, that acid-sludge is composed of small molecules. The greatest increase in molecular weight was observed for the nonpolar fractions. The results could be explained by assuming surfactant-like behaviour of protonated basic crude oil components. Aggregate formation, therefore, explains the higher molecular weights observed. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Crude oil±acid complexes; Acid-sludge; Aggregation; Size-exclusion chromatography
1. Introduction Acid formulations are frequently injected to stimulate oil wells. This improves the permeability of the rock matrix, enhancing oil production. A typical acid formulation is based on 15% HCl, but higher acid strengths are also used. Contact of acid with the crude oil, however, generates a precipitate known as acid-sludge [1,2]. PrecipitatesÐ organic or inorganicÐare undesirable because of their potential for plugging pores and impairing formations. It is commonly believed that acid-sludge is polymeric, with an asphaltenic nature, and is induced by Friedel±Crafts-type reactions. Recent experiments have cast doubt on this view. For example, in one experiment, a crude oil was diluted with toluene and acid was added. After 15 min, the ¯uid was checked for sludge. A critical toluene concentration was identi®ed, below which acid-sludge was formed. Above this concentration, no acid-sludge was formed [3]. Even asphaltene-free crude oils and condensates can generate acid-sludge when exposed to acid [4]. Clearly, these ®ndings do not support the asphaltene polymerisation theory; however, they do support a recently published hypothesis based on aggregation [3]. The results of size-exclusion chromatography (SEC) on acid-sludge samples generated from various crude oils and several types of acids have helped * Corresponding author. Fax: 131-71-581-3400. E-mail address: [email protected] (M. Rietjens).
determine the nature of acid-sludge. This paper provides details of the SEC results that support the aggregation theory. SEC is an analytical tool for determining the molecular weight of polymers or compounds with relatively high molecular weight. For example, SEC has been successfully applied in ageing studies of road asphalt [5], molecular weight distribution of asphaltenes in crude oils [5], and aggregation behaviour of asphaltenes [6]. Aggregation of asphaltenes has been observed even in highly diluted and excellent solvents such as tetrahydrofuran (THF). As a result, a shift in the chromatograms toward higher molecular weights can be seen. Because asphaltenes are known to be a constituent of most crude oils, aggregation of asphaltenes can be anticipated in the analysis of acid-sludge by SEC. The term acid-sludge is used to classify crude oil±acid precipitates for distinguishing these sludges from other sludges related to crude oils. 2. Procedures 2.1. Sample preparation Five crude-oil samples were analysed: N'Kossa (Angola), Monti Enoc (Italy), Monti Alpi Nord (Italy), Skjold (Denmark), and Tengiz (Russia). Tengiz is a transparent crude oil containing virtually no asphaltenes. Before the samples were treated and analysed, they were centrifuged
0016-2361/01/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(00)00073-9
34
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
for 15 min at 4500 rpm to remove the insoluble residues. The crude-oil samples were separated into polar/nonpolar fractions and asphaltene/de-asphaltenated fractions. All samples were analysed before and after acid treatment. 2.2. Separation of the polar and nonpolar constituents This technique was adapted from SjoÈblom et al. [7]. In our work, the treatment of the crude oils with SiO2 did not remove the black colour of the oil, as opposed to SjoÈblom et al. Therefore, the nonpolar fractions still had components that contained heteroatoms which can be basic in character. The purpose of this investigation was to examine the sludging behaviour of the crude oil with most of the polar compounds removed. A 6.0-ml aliquot of the crude oil was transferred to a test tube, and 1.00 g of SiO2 was added. The SiO2 had been conditioned for several days in a oven at 508C. Under these conditions, the silica was activated, but not to the extent that irreversible adsorption occurred [8]. The test tubes were regularly shaken during a 15-min period, then they were centrifuged. The oil was transferred to a new test tube and treated once more with 1.00 g of SiO2. This latter oil layer is referred to as the nonpolar fraction. Both SiO2 fractions were combined, and the polar components were desorbed through two washings: ®rst with CH2Cl2 1 5% methanol and then with THF. The extracts were collected, and the solvents evaporated in a vacuum oven. Before further testing, these samples were dissolved in 2.0 ml of toluene. They are referred to as polar fractions. From the results in Figs. 2±6 it is observed that both the polar and nonpolar fractions have lower molecular weights. This ®nding indicates that irreversible adsorption of polar components probably occurred to some extent. 2.3. Asphaltenes and de-asphaltenated fractions Asphaltenes and de-asphaltenated fractions were prepared by mixing each crude oil sample with n-hexane in a ratio of 1:20 and ®ltering off the precipitated asphaltenes [9]. 2.4. Acid-sludge sample preparation Four types of acids were used to generate acid-sludge: (a) 15% HCl; (b) 15% HCl 1 1000 mg/l of Fe 31 (as FeCl3); (c) a 25% w/w trichloroacetic acid (TCA) solution in toluene; and (d) a solution of approximately 3000 ppm Fe 31, added as FeCl3, in dioxane. The ®rst two acids are water-based and form two-phase systems. The other two acids, a Brùnsted and a Lewis acid, respectively, are miscible with oil and form one-phase systems. In all the tests, both the acid-sludge and the nonsludging oil phases were analysed. The tests with 15% HCl formulations were carried out by placing 2.0 ml of the acid in a test tube and adding 2.0 ml of the oil sample. The tubes were allowed to stand statically at 1008C for 1.5 h.
After being centrifuged at 3000 rpm for 5 min, the upper layer, or the nonsludging phase, was removed and prepared for analysis. To collect the acid-sludge residing at the interface, a 4-ml aliquot of toluene was added to the tube and gently mixed with the oil layer. Then, the solution was centrifuged and the upper layer was largely drawn off and discarded except for the last 0.5 ml. This process was repeated until the acid-sludge was visible as solid particles at the interface, with clean toluene above it. These particles were collected and prepared for analysis. The acid-sludge tests with TCA and FeCl3 were conducted differently. A 2.0-ml aliquot of oil was placed in test tubes. Forty microlitres of the TCA solution was added to each tube. In the tests with FeCl3, a volume of 50 ml of the FeCl3 solution was added to each tube. The tubes were mixed and allowed to stand statically at ambient temperature for 15 min. After the test tubes were centrifuged, the sludge was removed from each tube and prepared for analysis. 2.5. Size-exclusion chromatography All the crude-oil samples and fractions were analysed before and after acid treatment. After acid treatment, both the acid-sludge and the nonsludging phases were analysed. Dilutions in THF were prepared which contained approximately 1000 mg/l of the sample. For the acid-sludge samples, dilutions were prepared with a colour intensity similar to the other samples. To avoid problems associated with air oxidation, these dilutions were analysed within 1 h after preparation [5]. The molecular weight (M) of the asphaltenes ranges from several hundred Daltons up to tens of thousands Daltons. Because it was expected that sludge is not polymeric in nature, a column was selected that separated optimally in this range. For SEC, a Waters 625 LC system equipped with a 486 tuneable absorbance detector was used. The system runs under the Millennium 2.00 software package. A Styragel HT3 column (4.6 mm ID £ 300 mm) was used. It is suitable for a molecular weight between 300 and 30,000 g/mol. As a guard column, a Styragel 4.6 mm ID £ 30 mm column was used. The test conditions included ambient temperature, a ¯ow rate of 0.35 ml/min, eluent THF, and UV/Vis detection at a wavelength of 275 nm. 3. Results 3.1. SEC analysis 3.1.1. Calibration The basic theory of SEC will be discussed as far as necessary for an understanding of the results presented [10,11]. SEC separates compounds based on the hydrodynamic radius of the molecules. Because polymers used for calibration and asphaltenes have different hydrodynamic radii, different retention volumes or times can result with the
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40 Table 1 Correction of molecular weights according to Reerink and Lijzenga Molecular weight calibration curve
Molecular weight corrected according to Reerink and Lijzenga
250 500 1000 2000 5000
201 545 1479 4013 15,015
same molecular weight. Dif®culties with calibration have been documented by several authors [5,12,13]. Application of the universal calibration curve, based on the Mark± Houwink relation, a plot of [h ]M versus Ve, where [h ] is the intrinsic viscosity and Ve the elution volume, does not hold for asphaltenes. Asphaltenes have a lower hydrodynamic volume because they are more compact, and elute at the same volume as lower-molecular-weight polystyrenes. Maltenes and polystyrenes have similar compactness [5]. According to Reerink and Lijzenga [14], a correction can be made by describing the hydrodynamic volume according to the disc model. A list of molecular-weight conversions is shown in Table 1. Generally, higher molecular weights are calculated for asphaltenes with a molecular weight above 500 g/mol, using a calibration curve based on polystyrene and toluene. In Figs. 2±6 the Reerink±Lijzinga corrections were not used. Another complication with asphaltenes, or aromatic compounds in general, is that the elution behaviour is different for peri-condensed aromatics such as pyrene and coronene, and cata-condensed aromatics, such as naphthalene and anthracene. Peri-condensed aromatics require higher elution volumes compared with cata-condensed aromatics of the same molecular weight [12]. Because no satisfactory explanation has been determined for this [12,15], the retention volume is not uniquely linked to molecular weight. Based on polystyrene and toluene as the calibration compounds, an estimation of the molecular weight is determined by the calibration curve, Eq. (1). Both calibration samples were run in triplicate log Mps 21:157Ve 1 7:182
r 2 0:9999
1
where Mps is the molecular weight relative to polystyrene. Based on these considerations, the results can be compared on a relative basis, but the absolute values should be interpreted with caution. Based on duplicate runs, an error of 0.2% was determined for the retention volumes. This error translates to 1% after application of Eq. (1), and to 7% after conversion of log(M) values to molecular weight. This applies to molecular weights calculated for peak values. For the upper values of the molecular weight, an error of 13% was estimated. 3.1.2. Effects of aggregation In asphaltene applications, THF is often used as an eluent
35
[5,6,12], but CHCl3 and benzene containing 5% methanol have been applied as well. The molecular weight of asphaltenes is complicated by the association of asphaltenes in solution. Because asphaltene aggregation does not suddenly begin at the critical micelle concentration (CMC), but probably proceeds through a stepwise mechanism, the formation of small assemblies can be expected even at low asphaltene concentrations [6]. For dissociating acid-sludge most completely, THF was selected as a solvent and eluent because of its excellent solvency. However, the effects of asphaltene aggregation can be anticipated in the analysis of acid-sludge. 3.1.3. Air oxidation In the study of road asphalt, an increase in viscosity was observed as a result of dissolving the asphalt and evaporating the solvent [5]. Other experiments showed that asphalt and asphaltenes are very sensitive to air oxidation when diluted [5,15]. A similar case is the analysis of crude oils and acid-sludge samples by SEC. The samples were strongly diluted with THF, and were thus exposed to relatively large amounts of oxygen. For this reason, diluted samples were analysed as quickly as possible, within 1 h. It is assumed that this time was suf®cient to dissociate the acid-sludge aggregates. 3.2. Analysis before acid treatment A plot of a typical chromatogram for the Tengiz sample is shown in Fig. 1. The results obtained with SEC are plotted in Figs. 2±6. In each chromatogram, two peaks were generally observed, but only the results of the peaks representing the highest molecular weights are shown. The peaks representing lower-molecular-weight compounds are of interest, but because no detailed analysis of the components in these peaks is available, they were not considered further. Furthermore, focusing on the highest-molecular-weight components demonstrates more convincingly that acidsludge is not polymeric in nature. A direct analysis of the pure crude-oil samples and their fractions before acid treatment showed an interesting feature. Compared to the pure crude-oil samples, equal or reduced molecular weights were observed for the polar and nonpolar fractions, whereas the asphaltene fractions from N'Kossa and Monti Alpi have a two- to three-fold higher molecular weight (Figs. 2 and 5, respectively). The deasphaltenated fractions generally had equal molecular weights except for Monti Alpi, which had half the molecular weight. This increase in molecular weight of the asphaltene fractions could not be attributed to chemical reactions of any kind. Because asphaltenes have a strong tendency to aggregate, it is reasonable to attribute the observed increases in molecular weight to aggregation [5,6,15]. 3.3. Analysis after acid treatment In a few cases, the molecular weight of the nonsludging
36
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
Fig. 1. Size-exclusion chromatogram obtained on the Tengiz sample.
phase was higher than the same untreated crude oil or fraction. This was true for the polar fraction of Skold (Fig. 3) and Monti Enoc crude oil (Fig. 4). For the other cases, equal values for the molecular weights were observed. Conversely, higher molecular weights were generally observed for the acid-sludge samples. The biggest increase in molecular weight was among the nonpolar fractions. The increase was up to ®ve-fold, when based on peak values, or
10-fold when based on upper values. Taking the Reerink± Lijzenga corrections into account, the increases in molecular weights were even larger, but this did not alter the conclusions. Most polar fractions have higher molecular weights, but the increase is small, 1.5- to 2-fold. Results of the asphaltene fractions were similar to the polar fractions. In two cases, N'Kossa and Monti Enoc, equal molecular weights were obtained. Increases in molecular weight of up to two-fold were observed for Skjold and Monti Alpi.
Fig. 2. Results for N'Kossa crude-oil sample. The white bar indicates the peak value of the molecular weight, and the grey bars show the upper value of the molecular weight.
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
37
Fig. 3. Results for Skjold crude-oil sample.
Differences between acid-sludges generated by various acids were also considered. The upper values of the molecular weights were similar for all acid-sludges, but the molecular weights calculated on peak values differed. Acid-sludge induced from FeCl3/dioxane showed a lower peak molecular weight compared with acid-sludge induced from 15% HCl formulations. This was also generally true for acid-sludge generated from TCA.
4. Discussion 4.1. Polymerisation theory Oil®eld researchers have generally accepted that acidsludge is formed as a result of Friedel±Crafts-type reactions. This view is based on the following: (1) Lewis and Brùnsted acids are known to catalyse Friedel±Crafts and
Fig. 4. Results for Monti Enoc crude-oil sample.
38
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
Fig. 5. Results for Monti Alpi crude-oil sample.
related reactions [16]; (2) a number of heterocyclic compounds that are present in oil are prone to polymerise under acidic conditions [17]; and (3) acid-sludge is formed when a crude oil is exposed to acid. The stronger the acid concentration, the more acid-sludge is formed. The presence of small concentrations of Fe 31 considerably increases the amount of acid-sludge [1,2,18,19]. These observations suggest that the polymerisation theory based on Friedel± Crafts reactions is accurate. However, an analysis that reveals polymeric products has never been performed.
In the early-1960s, Lewis acids such as FeCl3 were shown to be capable of polymerising benzene to polyphenyl in the presence of small amounts of water [20]. However, a high concentration of FeCl3 was required in a ratio of 1:1 to benzene. FeCl3 was presumed to function both as a catalyst and as an oxidant. In contrast to benzene, the sterically hindered 1,3,5-trimethylbenzene could be dimerised and chlorinated, but not polymerised [21]. The more reactive compound, naphtalene, reacted at ambient temperature, but formed only 1,1 0 -bi-naphtyl. It was theorised that the
Fig. 6. Results for Tengiz crude-oil sample.
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
highest degree of polymerisation is associated with the most active propagating species, i.e. the carbonium ion that possesses the least delocalisation as in benzene [22]. The work by Kovacic et al. shows that few aromatic compounds can polymerise under ideal conditions. Later work by the group of Speight on reactions of asphaltenes with metal chlorides showed the elements Cl and O to be incorporated into the structure of asphaltenes, but no evidence of highermolecular-weight asphaltenes was provided [9]. In a later publication, the oxidation of bitumen in the presence of various metal salts was described. The results clearly indicated no increase in molecular weights of the asphaltenes [23]. On the contrary, lower-molecular-weight asphaltenes were identi®ed. This observation was attributed to enhanced intramolecular association of the oxidised, more polar, asphaltenes [23]. The results presented here also do not support the polymerisation theory. In addition, several theoretical arguments and arguments based on observations have been brought forward against this theory [3]. 4.2. Acid-sludge aggregation A parallel with asphaltene aggregation can be drawn. For this reason, the current theory on asphaltene aggregation is explained ®rst. A distinction must be made between aggregation of crude-oil components in the crude oil itself and aggregation of these components in THF. 4.2.1. Crude oil The presence of several permanent dipoles per molecule is the primary cause of asphaltene aggregation [24]. Yen et al. have postulated an aggregation theory for asphaltenes in which aggregates of various sizes exist, as shown in Scheme (2) [25,26] Free asphaltenes $ assembly $ cluster $ flocs
2
The free asphaltenes are the smallest, with a diameter of approximately 1.8 nm. Flocs have a diameter of approximately 5 mm, which is large enough to settle out. Although the shape of asphaltene molecules is far from idealÐlong alkyl chains with a hydrophilic headgroupÐthe initially formed assemblies could resemble a reversed micelle. A description in reversed micellar terminology has, in fact, been proposed by Yen et al. [27] for asphaltenes. Stack formation as occurs in asphaltene assemblies is another important mechanism [28]. Acid-sludge consists of particles that are similar in size to ¯occulated asphaltenes [4]. It is assumed that similar stages exist in acid-sludge formation as shown in Scheme (3). The term ªpresludgeº is used to identify components that are prone to form acid-sludge H1
Free presludge $ protonated presludge $ assembly $ cluster $ flocs
3
This theory is being explored with success. It states that the
39
®nal stage, acid-sludge, is basically a ¯occulated material that results from aggregation of protonated crude-oil components. Whether assemblies and clusters truly exist is under investigation. This work shows that the basic constituents have molecular weights in the same order as the original crude-oil components. The onset of acid-sludge, as detected by microscopy, is characterised by particles .300 nm. This is the same value as calculated for centrifugation at 3000 rpm. Thus, particles ,300 nm remain in the nonsludging phase, whereas particles .300 nm settle out. Upon contact with a water-based acid such as 15% HCl, crude oil can form acid-sludge, but the transport of HCl to the oil phase is required [3,4]. As a result, acid-sludge formation requires time. When crude oils are mixed with acids dissolved in organic solvents (TCA/toluene and FeCl3/ dioxane), acid-sludge forms almost instantaneously. The interaction of an acid (Brùnsted or Lewis) with some crude-oil components results in protonated species or metal complexes which are more polar. These complexes can be caused by simple acid±base interactions. However, electron donor/acceptor complexes can also form [28,29]. It has been demonstrated that HCl is incorporated into the structure of acid-sludge, but the nonsludging phase also contains a substantial amount of HCl acid [3]. This HCl acid can be simply dissolved in the oil phase or incorporated into assembles and clusters. Large aggregates, at least up to the ¯occulation stage, are evidently formed in the acidsludge itself. We propose that the large ¯occulates are formed from the smaller clusters which, in turn, are formed from assemblies. We hypothesise that the smallest assemblies are formed from protonated crude oil±acid complexes. Protonation of basic sites of, for example, nitrogen-based crude-oil components, renders surfactant-like species that have a natural tendency to aggregate. This process could resemble reversed micelle formation of ordinary surfactants in organic solvents [30]. Stack formation, as occurs in asphaltenes, is another important mechanism. In addition, the number of polar sites or hydrophilic groups is relevant, as well. Compounds with fewÐor just oneÐhydrophilic group and a bulky nonpolar group could aggregate as classical surfactants aggregate in organic media [30]. If a component contains several polar sites, such as asphaltenes, the solubility of these components in the crude oil will be small and the components will be prone to aggregate even when unprotonated. 4.2.2. Tetrahydrofuran When various samples are dissolved in THF, the aggregation behaviour is altered. THF is a semipolar solvent, in contrast to a crude oil, which is basically nonpolar. The fact that acid-sludge dissolves very well in THF shows that the ¯occulated material dissociates. In fact, a new equilibrium is established that is quanti®able through SEC. Thus, aggregates at the assembly or cluster level could still be present in THF. Because the basicity of THF is low, crude oil±HCl complexes probably remain largely undissociated. This may
40
M. Rietjens, M. Nieuwpoort / Fuel 80 (2001) 33±40
not be the case with FeCl3-induced acid-sludge because THF can form complexes with Fe(III) [31]. Furthermore, compared with HCl, TCA is less polar and more soluble in THF, and crude oil±TCA complexes may be dissociated more than their HCl equivalents. These effects are tentatively shown in the data. Although the upper values of the molecular weights are equal for all acid-sludges, the molecular weights based on peak values are smaller for acidsludges derived from TCA and FeCl3. Because the peak values represent the majority of components, this ®nding points to a shift in Scheme (3) toward dissociation. The polar fraction is relatively rich in compounds containing heteroatoms such as nitrogen, which are generally basic in character, whereas the nonpolar fraction, by comparison, is relatively poor in these compounds. The acid-sludge of the nonpolar fractions shows the highest increase in molecular weight. It is not unreasonable to assume that compounds in this fraction are more surfactant-like: nonpolar alkyl and/or aryl groups with very few polar groups. Upon protonation, such compounds are more likely to aggregate as classical surfactants aggregate in organic solvents, whereas compounds carrying several polar groups distributed over the molecule are less likely to aggregate. This explains the higher molecular weight observed. In contrast, the polar fractions and acid-sludge generated from the polar fractions are more soluble in a semi-polar solvent such as THF, and have less tendency to aggregate. The higher molecular weights observed for the untreated asphaltene fractions con®rm the tendency of asphaltenes to aggregate, even in a solvent such as THF. When acid complexes were formed, no further increase in molecular weight was observed. Conversely, an increase in molecular weight was observed for the acid-sludge of the de-asphaltenated fractions. Resins are part of the de-asphaltenated fraction, and their propensity to aggregate is low. As with the nonpolar fractions, their tendency to form aggregates increases greatly when acid complexes are formed. In both cases, the acid complexes generated acquired surfactant-like properties and formed aggregates. Recently, it was found that sludge contains a much higher resin content than untreated crude oils, con®rming the ®ndings reported here [2]. 5. Conclusions The ®ndings reported in this paper do not indicate that acid-sludge is polymeric in nature, but they do indicate that acid-sludge consists of aggregates. Acid-sludge particles can be easily dissolved in THF, and were shown to consist of relatively small-molecular-weight components. Increased molecular weights were identi®ed, but could be explained by the aggregation hypothesis. The most
pronounced increases in molecular weight observed were among the nonpolar and de-asphaltenated fractions. Upon the formation of acid complexes, these components had surfactant-like properties and formed aggregates. The most polar species, asphaltenes, and polar fractions have a natural tendency to aggregate, even when unprotonated. Upon formation of acid complexes, this propensity is slightly enhanced, which explains the minor increases in molecular weights that were observed for these fractions. References [1] Moore EW, Crowe CW, Hendrickson AR. J Petrol Technol 1965:1023. [2] Wong TC, Hwang RJ, Beaty DW, Dolan JD, McCarthy RA, Franzen AL. Soc Petrol Engrs, Prod Fac 1997:51. [3] Rietjens M, Nieuwpoort M. European Formation Damage Conference, 31 May 31±1 June 1999, The Hague. Soc. Petrol. Engrs, Paper 54727. [4] Rietjens M. European Formation Damage Conference, 2±3 June 1997, The Hague. Soc. Petrol. Engrs, Paper 38163. [5] Wu CS. Handbook of size-exclusion chromatograph. New York: Marcel Dekker, 1995. (chap. 8). [6] Andersen SI. J Liq Chromatogr 1994;17(19):4065. [7] Sjoblom J, Urdahl O, Hoil H, Christy HH, Johansen EJ. Progr Coll Polym Sci 1990;82:131. [8] Later DW, Wilson BW, Lee ML. Anal Chem 1985;57:2979. [9] Speight JG. Fuel 1970;50:175. [10] Braam WGM. Scheidingsmethode: chromatogra®e. Groningen: Wolters-Noordhoff, 1992. (chap. 5). [11] Young RJ, Powell PA. Introduction to polymers. London: Chapman & Hall, 1991. (Sections 3.17.1±3.17.6). [12] Bergmann JG, Duffy LJ, Stevenson RB. Anal Chem 1971;43:131. [13] Snyder LR. Anal Chem 1969;41:1223. [14] Reerink H, Lijzenga J. Anal Chem 1975;47:2160. [15] Altgelt KH. In: Altgelt KH, Gouw TH, editors. Chromatographic science, vol. 11. New York: Marcel Dekker, 1979. (chap. 12). [16] Olah GA. Friedel±Crafts chemistry. New York: Wiley, 1973. [17] Badger GM. The chemistry of heterocyclic compounds. New York: Academic Press, 1961. [18] Jacobs IC, Thorne MA. Soc. Petrol. Engrs, Paper 14823. [19] Houchin LR, Dunlap DD, Arnold BD, Domke KM. Soc. Petrol. Engrs, Paper 19410. [20] Kovacic P, Koch FW. J Org Chem 1963;28:1864. [21] Kovacic P, Koch FW. J Org Chem 1965;30:3176. [22] Kovacic P, Wu C. J Org Chem 1961;26:759. [23] Moschopedis S, Speight JG. Fuel 1978;57:235. [24] Maruska HP, Rao BML. Fuel Sci Technol Int 1987;5(2):119. [25] Yen TF. In: Yen TF, Chilingarian GV, editors. Asphaltenes and asphalts I, development in petroleum science 40. Amsterdam: Elsevier, 1994. (chap. 5). [26] Dickie DP, Yen TF. Anal Chem 1967;39:1847. [27] Lian H, Lin JR, Yen TF. Fuel 1993;73:423. [28] Taylor R. Electrophilic aromatic substitution. New York: Wiley, 1990. (chap. 2). [29] March J. Advanced organic chemistry. 3rd ed. New York: Wiley, 1985 (p. 74±7, 485). [30] El-Seoud OA. In: Hinze WL, editor. Organized assemblies in chemical analysis, vol. 1. Greenwich (CT): JAI press, 1994. [31] Cotton FA, Wilkinson G. Advanced inorganic chemistry. 4th ed. New York: Wiley. (p. 759).