Research progress on chemical composition of petroleum acid

The research on acidic components of crude oil abroad started earlier. In the past, people paid more attention to the study of organic acids, mainly because carboxylic acids have always been regarded as the intermediate products of oil and gas forming crude oil from biological parent materials, and fatty acids play an important role in the process of organic-inorganic interaction.

Saturated naphthenic acids (Derungs, 1956) are the earliest compounds with definite structures in the acidic components of crude oil. Naphthenic acid accounts for more than 50% of all organic acids in crude oil (Zhu, 199 1). According to the structural types of rings, crude oil carboxylic acids can be divided into chain fatty acids, isoprenoid acids, monocyclic naphthenic acids, polycyclic naphthenic acids and aromatic carboxylic acids (Rochet and littmann,1955; Seifert and Tite,1970; Table 1-2), sometimes it may contain inorganic acids. Another kind of compound that may affect the acid value of crude oil is mainly weak acidic alkylphenol with low molecular weight. For example, Samadova and Gusenova (1993) found that the content of alkylphenol in Azerbaijan crude oil with high acid value is 2 ~ 7 times that of carboxylic acid. Mckay et al. (1975) comprehensively analyzed non-hydrocarbons (nitrogen compounds such as carbazole, amino compounds, sulfur compounds, etc. ), and it is considered that 28% of acidic compounds in Wilmington crude oil are carboxylic acids, 28% are phenols, 28% are pyrroles, and 16% are ammonia compounds. This can roughly reflect the composition of acidic compounds in crude oil.

Naphthenic acid is the main polymer organic acid in crude oil and petroleum products. It is a colorless liquid with unpleasant smell and is not volatile. Insoluble in water, soluble in organic solvents such as oil, benzene, alcohol and ether. Lochte and Littman( 1955) dissected the structure of naphthenic acid in crude oil for the first time, and found that naphthenic acid is the most important component in petroleum acid, and its content can reach more than 90%. The molecular weight of naphthenic acid is relatively large, from 100 to 1000, and the carbon number is from C7 to C7. Naphthenic acid is mainly composed of one ring, two rings and three rings, and there are also some tetracyclic and pentacyclic naphthenic acids. Among them, monobasic acids are the main ones, and the content of aromatic acids with aromatic ring structure is very low. Refining practice shows that the acid value of each distillate oil changes with the change of boiling range. The higher the boiling range, the greater the acid value, especially when the boiling point is greater than 300℃, the acid value rises sharply. Therefore, naphthenic acid components are mainly concentrated in heavy distillate oil above 300℃, and its average relative molecular weight is above 300. It is an excellent raw material for producing various oil additives, such as detergent dispersant for lubricating oil, rust inhibitor and dispersion stabilizer for fuel oil. The content of petroleum acid increases with the increase of naphthene content in crude oil. Generally, the content of petroleum acid is (mass fraction) 1% ~ 2%, fatty acid is below C6, C7-C 10 is a mixture of naphthenic acid and fatty acid, C 10-C 14 is alkyl naphthenic acid, and C 14 is alkyl naphthenic acid.

Table 1-2 Common Organic Acid Types in Crude Oil (Methyl Esterification)

With the development of geochemical testing technology, people have a deeper understanding of organic acids in crude oil. Tomczyk et al. (200 1) reported the distribution of petroleum acid types of crude oil from SanJoaquinVallay. Crude oil has undergone aerobic biodegradation (TAN=5. 19mgKOH/g). Through extraction and analysis, it is found that 40% (mass fraction) of esterified acidic components are not carboxylic acids, and only 10% of acidic compounds contain two oxygen atoms (carboxyl groups), while about 50% of carboxylic acids contain nitrogen heteroatoms and 25% contain sulfur atoms. Acidic sulfur-containing compounds such as mercaptan are easily oxidized in air, so they easily disappear. It has been suggested that amino acids derived from microorganisms may be the main source of acidic components in crude oil.

The development of electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR-MS) mass spectrometry provides a new way for the analysis of petroleum acids. Qian et al. (200 1) analyzed heavy oil samples in South America by high-resolution mass spectrometry, and found that the carbon number distribution range of monobasic acid in the sample was C 15-C55, with 1 ~ 6 fats. The identified acidic components in crude oil include various heteroatom combinations such as O 1, O2, O3, O4, N, N2, NO, NO2, s O, SO2, SO3, O2S and ns, and the molecular weight distribution range of acidic compounds is generally between 200 ~ 1000 Da (Hughey et al., 2000 Rogers, 2005), indicating that its composition is complex. Therefore, simply using conventional gas chromatography and gas chromatography-mass spectrometry to study highly volatile components such as phenols and alkyl acids can not fully understand the true face of acidic components in crude oil.

The composition data of petroleum acids in different documents are quite different, one of the reasons is that the experimental analysis methods used are different, and more importantly, the composition of petroleum acids in different crude oil samples is different. Some recent research work mainly focuses on the description of advanced experimental methods of petroleum acid, and lacks systematic research on the composition of petroleum acid in crude oil.

The second is the correlation between the acidic components of crude oil and the total acid value of crude oil.

Table 1-3 shows the contents of acid components (AF) and methyl esters (FAMES) in crude oil samples analyzed by the author in the Canadian Geological Survey. As shown in figure 1-2, except for three crude oil samples (TK 10 1 S48 and TK7 13) obtained in Tarim basin, the contents of methyl esters (FAMES) separated from all other crude oils have a good correspondence with the total acid value (TAN). At the same time, Fourier infrared spectrum analysis shows that the acidic components before methyl esterification contain a large number of polar aromatic hydrocarbon components, which leads to poor correlation between the content of acidic components (AF) and the total acid value (TAN) of crude oil.

Table 1-3 Contents of Acid Fraction (AF) and Acid Methyl Ester Fraction (FAMES) in Crude Oil

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Figure 1-2 Comparison of methyl ester content separated from crude oil in China, Sudan and Canada with total acid value (TAN) of crude oil.

Because the viscosity of oil sands extract is beyond the technical range of conventional total acid value determination methods, it is impossible to obtain its total acid value data from commercial laboratories. In order to make up for this defect, the correlation between methyl ester content (FAMES) and total acid value (TAN) of crude oil was used to calculate the total acid value of oil sands extract (Table 1-4).

Thirdly, the composition of functional groups of petroleum acids.

In order to understand the composition characteristics of petroleum acid functional groups, the author analyzed some crude oil/oil sands extracts and their separated acid components and acid methyl ester components by Fourier infrared spectroscopy. Figure 1-3 shows the Fourier infrared spectra of the whole oil, acid components and acid methyl ester components of representative crude oil/oil sands extracts from different research areas.

Table 1-4 Calculation results of acid component (AF) and acid methyl ester component (FAMES) of oil sands extract and total acid value (TAN) of crude oil.

Fig. 1-3 Fourier infrared spectrogram (1) of the whole oil (a), acid component (b) and acid methyl ester component (c) of a representative crude oil with high acid value (TAN=4.68mgKOH/g) in Sudan.

Fig. 1-3 Fourier infrared spectra of the whole oil (a), acidic component (b) and methyl ester component (c) of Sudan's representative crude oil with low acid value (TAN=0. 15mgKOH/g) (II).

Fig. 1-3 Fourier infrared spectrum of the whole oil (a), acid component (b) and acid methyl ester component (c) of a typical high-acid crude oil (TAN=3.39mgKOH/g) in Bohai bay basin, China (Ⅲ).

Fig. 1-3 Fourier infrared spectrogram of the whole oil (a), acid component (b) and acid methyl ester component (c) of a representative low acid value crude oil (TAN=0.39mgKOH/g) in Bohai Bay Basin, China (IV).

Figure 1-3 Representative high acid value crude oil (oil sand extract; TAN= 15.40mgKOH/g) Fourier infrared spectrum (v) of whole oil (a), acidic component (b) and acid methyl ester component (c).

Fig. 1-3 Fourier infrared spectrum (VI) of the whole oil (a), acid component (b) and acid methyl ester component (c) of a typical crude oil with low acid value (TAN=0.23mgKOH/g) in western Canada basin.

(I) Crude oil/oil sands samples

As shown in figure 1-3, crude oil/oil sands samples all show similar Fourier infrared spectrum characteristics, as follows: ① Very strong aliphatic absorption peaks, corresponding to the extension (3100 ~ 2,800 cm-1) and bending (1460 and/respectively) of aliphatic groups. ② Aromatic hydrocarbon absorption peaks (about 1600 cm- 1 and 900-700 cm-1); ③ Some samples have absorption peaks in1800 ~1600 cm-1band, indicating the existence of oxygenates.

(2) Acidic components separated from crude oil/oil sands samples

As shown in figure 1-3, compared with the original crude oil/oil sands sample, the acidic components of crude oil/oil sands sample correspond to the extension (3 100 ~ 2800 cm- 1) and bending (1460 and1377 cm) of aliphatic groups. While the absorption of oxygen-containing functional groups (1800 ~ 1600cm- 1 band) and aromatic groups (about1600 cm-1band and 900 ~ 700 cm-/band).

(3) Methylation products of acidic components in crude oil/oil sands samples

As shown in Figure 1-3, after esterification, the methyl ester component of crude oil/oil sands samples is greatly reduced compared with the acidic component of crude oil. The expansion and contraction of aliphatic groups (3 100 ~ 2800 cm- 1), bending (1460 and 1377 cm- 1) correspond to rotational vibration (720 cm- 1). However, the absorption peaks of various oxygen-containing functional groups (1800 ~1600 cm-1) are significantly enhanced, and the methyl ester component of Sudan crude oil with high acid value is rich in oxygen-containing groups such as carbonyl, polycyclic quinone and phenol. The methyl ester component of crude oil in Bohai Bay Basin lacks polycyclic quinones, while the methyl ester component of high acid value oil sands extract in western Canada is mainly sulfur-containing and oxygen-containing compounds.

The relationship between the acid value of crude oil and the characteristics of organic functional groups reflected by the Fourier infrared spectra of crude oil itself and its acid methyl ester components will be discussed in the following chapters.

Fourthly, high-resolution mass spectrometry reveals the elemental composition and compound types of petroleum acids.

The relative molecular weight of acidic compounds in crude oil is generally less than 1000, mainly distributed between 200 and 800, and the number of carbon atoms in the corresponding molecule is generally distributed in C 10-C60-C60. The relative molecular weight of acidic compounds in different crude oils is quite different, and the average molecular weight distribution is between 420 and 550. Taking the high-resolution mass spectrometry analysis of crude oil from Well Huan 127 in Liaohe Oilfield as an example, this paper expounds the relevant progress in the study of petroleum acid composition and compound types. Figure 1-4a is the Fourier transform mass spectrum of crude oil, with mass-to-charge ratio as the abscissa and relative abundance as the ordinate. Figures 1-4b and 1-4c are partial enlarged views of figure 1-4a. In figure 1-4b, we can see the mass difference of mass spectrum peak series 14.0 1565. These compounds have the same number of heteroatoms, but differ in CH2 methylene units. As long as one of the molecular components is identified, other compounds can be easily identified. The molecular formula of the compound can be determined within the error range of 10-6 by accurate molecular mass. At the same time, according to the peak intensity of adjacent isotope mass spectra, the reliability of identification results and whether different compounds overlap can be verified. The No.8 peak in the figure 1-4c is the 1-4c isotope peak of the No.4 peak.

Interpretation of (1) crude oil high resolution mass spectrometry data

Three levels of composition information can be obtained from the results of high-resolution mass spectrometry analysis (take the crude oil from Well Huan 127 as an example, as shown in Figure 1-5): the molecular composition type, that is, the combination mode of atoms such as C, H, O, N and S in the molecule, and the composition of main elements (C, H) is generally expressed as molecular condensation degree and molecular weight. Compounds of the same type are divided into different groups according to molecular unsaturation, that is, the number of double bonds and rings in the molecule. The relative distribution of compounds of the same type with different condensation degrees is reflected by the size of Z in the molecular general formula CnH2n+ZOoNnSs (Figure1-5b). The molecular composition of the same group of compounds is different from N-CH2-,and its distribution characteristics reflect the molecular weight distribution of this group of compounds (as shown in figure 1-5c). In addition to the conventional structural types of O2 molecules, the acidic components of petroleum also include various heteroatom types, such as N 1, NO, N 1O2, O 1, O3 and O4. The molecular condensation degree of O2 compounds ranges from 0 (fatty acid) to -34. The relative abundance of compounds with different condensation degrees tends to normal distribution, but the carbon number distribution of compounds with different condensation degrees is inconsistent.

Figure1-4Huan 127 Oil Electrospray Mass Spectrometry.

When the magnetic field intensity of the instrument used in the experiment is 7.0T, the resolution of about 100000 can be obtained near the mass number of 450Da. This resolution can not accurately analyze the molecular formula composition of all compounds in crude oil, but it can still get reliable results for compounds with relatively high abundance. In order to ensure the reliability of the interpretation results, only N, NO, NO2, O, O2, O3 and O4 compounds with high relative abundance can be characterized, and their relative contents can be determined semi-quantitatively according to their relative abundance in mass spectrometry.

As shown in Figure 1-5, the content of O 1 compound in most samples is not high, but the relative abundance of O 1 compound in some samples exceeds O2, and the maximum Z value of identified CnH2n+ZO compound is generally -6, which is exactly the same as that of alkylphenol. As it has been proved that phenolic compounds are ubiquitous in petroleum, it can be determined that O 1 compounds in crude oil are mainly phenols, that is, O atoms are bonded to aromatic rings in the form of hydroxyl groups.

Figure 1-5 Composition information of compounds reflected by high-resolution mass spectrometry

O2 compounds show the strongest abundance in most crude oils, and the Z value of compounds with molecular formula CnH2n+ZO2 ranges from 0 to -34. Compounds containing two oxygen atoms in the molecule can be carboxylic acids or diols. Because ether and ketone are difficult to ionize under the condition of negative ion ESI, the molecule of O2 compound in crude oil contains at least 1 hydroxyl. At the same time, because the minimum molecular condensation degree of diol is Z=2, and the maximum z value seen in crude oil samples is 0, it can be inferred that the O2 compound in crude oil is mainly carboxylic acid.

The content of O3 and O4 compounds in petroleum is generally low, with 1 hydroxyl group and 1 carboxyl group or 2 carboxyl groups in the molecule. Because the identification of O3 and O4 compounds in negative ion ESI mode needs to consider the relationship between O 1 and O2 (Smith et al., 2006), it is necessary to be cautious in the identification of these compounds.

Because basic nitrides can't ionize under the condition of negative ion ESI, N-type compounds seen in high-resolution mass spectrometry of crude oil samples are mainly non-basic nitrides. The non-basic nitrides identified by conventional chromatography-mass spectrometry in petroleum and petroleum products mainly include pyrrole, indole, carbazole and benzocarbazole, but the first two are unstable and generally do not exist in crude oil. The maximum z value reflecting the molecular condensation degree of N compound is generally-15, corresponding to alkyl carbazole. From the point of thermodynamic stability, N compound with z =- 15 is also the most likely carbazole. At the same time, N compounds show the advantages of Z =-2 1 and Z =-27 in most crude oils, which is just consistent with the molecular composition of benzocarbazole and dibenzocarbazole compounds. Therefore, it can be inferred that the N compounds in crude oil are mainly pyrrole non-alkaline nitrides. The lower limit of Z value of compound N is -43, but it is mainly distributed between-15 ~-27, that is, the molecular condensation degree is between carbazole and dibenzo-carbazole.

NO and NO2 compounds can be interpreted as nitride molecules with 1 hydroxyl or carboxyl groups, but due to the lack of single molecule composition data of these compounds, their structural types are not clear at present.

(2) Classify crude oil according to the composition and compound types of petroleum acid elements.

The author studied the crude oil samples from Liaohe, Bohai, Tahe, Xinjiang and Sudan oilfields in China, and found that all the samples contained N 1 and O2 compounds, and their sum accounted for 80% of the quantitative relative abundances of seven compounds in most crude oils, such as O2, N 1, NO, N 1O2, O 1, O3 and O4. Taking O2 as an example, its relative abundance accounts for 1% ~ 93% of seven compounds. At the same time, the molecular condensation degree and carbon number distribution of petroleum acids in crude oil with similar relative abundance of different types of compounds are quite different. According to the heteroatom type, condensation degree and carbon number distribution revealed by high resolution mass spectrometry, we divide crude oil into five types. The composition characteristics of petroleum acids with different composition types are as follows.

1. crude oil

Naphthenic acid is dominant, mainly monocyclic naphthenic acid. The representative sample is the shallow crude oil in Area 9 of Xinjiang, the reservoir depth is 6 18 ~ 606m, and the carbon number distribution of total acid value10.7 mg KOH/g ... O2 compound is shown in Figure 1-6. Most crude oils with high acid value have the characteristics of class A composition. The relative abundance of O2 is more than 50%, and the fatty acid content is low or very low. Naphthenic acid is mainly bicyclic, and monocyclic and tricyclic rings also have high relative abundance. The main peak of carbon appears near C25.

Figure 1- 6A carbon number distribution of O2 compounds in crude oil

2. Grade B crude oil

Naphthenic acid is dominant, mainly tetracyclic naphthenic acid. According to the distribution characteristics of Z =-8 and Z =- 10 in the Z-value distribution diagram of O2 compounds, Class B crude oil is divided into two subclasses.

Among them, tetracyclic and pentacyclic naphthenic acids in B- 1 crude oil are dominant among O2 compounds, and tetracyclic and pentacyclic naphthenic acids are the main O2 compounds in a wide range of carbon numbers. The representative crude oil sample is taken from Well Qing 5 in Liaohe Oilfield, with the reservoir depth of 2050.6 ~ 2073. 1m, the total acid value of crude oil is 1.86mgKOH/g, and the carbon number distribution of O2 compound is shown in Figure 1-7.

Figure 1-7b- 1 carbon number distribution of crude oil O2 compound.

B-2 sub-crude oil is mainly naphthenic acid with low condensation degree in low carbon number region, while tetracyclic and pentacyclic naphthenic acids in C30 and higher carbon number region have obvious advantages. The representative crude oil sample is the third member of Shahejie Formation in Well Wa 70 in Liaohe Oilfield, with the oil depth of1434.3 ~1457.6m and the total acid value of 4.48mg KOH/ g ... The carbon number distribution of O2 compound is shown in Figure 1-8.

Figure 1-8b-2 Carbon number distribution of O2 compounds in crude oil.

The z values of tetracyclic and pentacyclic naphthenic acids are -8 and-10, respectively, but the compounds with z =-8 and-10 can also be aromatic carboxylic acids, and the structural types of these two compounds cannot be distinguished by high-resolution mass spectrometry, although the B- 1 and B-2 subclasses of crude oil are both Z =-8 and z =-/kloc. On the other hand, the O2 carbon number of the latter is distributed on two curves, z =-8 and z =- 10, and there is an obvious jump after it is higher than C30, which may be related to the high abundance of tetracyclic and pentacyclic naphthenic acids, which may be mainly hoppa acid, while tetracyclic naphthenic acid may be related to steroid acids (or its isomers).

3. Grade C crude oil

C the organic acids in crude oil are mainly fatty acids. According to the relative abundance of N and O2 compounds in crude oil, C crude oil can be divided into two subcategories.

C- 1 sub-crude oil is mainly composed of O2 compounds and fatty acids. The representative sample is crude oil from Gao 10 1 Well 2 168.6 ~ 2 162.9 m interval in Liaohe Oilfield. The carbon number distribution of O2 compound with total acid value of 3.76 mg KOH/g is shown in Figure 1-9.

Figure 1-9 C- 1 carbon number distribution of O2 compounds in subclasses of crude oil

C-2 crude oil is dominated by N-type compounds, containing more NO and NO2 compounds, and O2 is dominated by fatty acids. The representative sample is crude oil from 3996-4050m interval of Shu 1 16 well in Liaohe Oilfield. The total acid value of crude oil is 1 1.9mgKOH/g, and the carbon number distribution characteristics of O2 compounds are similar to those of crude oil C- 1 subclass.

The relative abundance of fatty acids in C crude oil is much higher than naphthenic acid, which shows the advantages of C 16 and C 18 fatty acids, but it is not necessarily the main peak. Fatty acids with high carbon number are widely distributed and relatively abundant. The abundance of nitrogen compounds in FTMS spectra of this kind of crude oil is high or low, and most samples with high nitrogen abundance contain high abundance of NO and NO2 compounds.

4. Grade D crude oil

Compared with oxygenated compounds, the abundance of nitrogenous compounds in Class D crude oil is absolutely dominant, and the main component is N 1 compound. The representative sample is the crude oil of 5965-6000m interval of Well S77 in Tahe Oilfield. The total acid value of crude oil is 0.77mgKOH/g, and the carbon number distribution of O2 compounds is shown in Figure 1- 10. Among them, the content of O2 compound is very low, and the curves of Z=0 and z =-2 show obvious advantages of C 16 and C 18.

Figure 1- 10d Carbon number distribution of O2 compounds in crude oil.

5. Grade E crude oil

E-grade crude oil contains a lot of halogenated hydrocarbons. The representative sample is crude oil from TK 10 1 well 4557~4563m in Tahe Oilfield. The total acid value of crude oil is as high as 20.0mgKOH/g, and the carbon number distribution of O2 compound is shown in Figure1-1.

There are strong peaks separated by 58 mass units in the mass spectrum. The number and relative abundance of peaks and the distribution characteristics of two related mass units in peaks are consistent with those of halogenated hydrocarbons, but the exact molecular composition of these compounds is still uncertain. Although this kind of crude oil shows high acid value, the abundance of O2 compounds is very low, and its distribution characteristics are similar to those of crude oil with low acid value D.

Fig. 1- 1 1E FTMS mass spectrum of crude oil.

The heteroatom types, O2 condensation degree distribution and N 1 condensation degree distribution of several representative crude oils are shown in Figure1-12 ~1-14 respectively. The types of heteroatoms in petroleum acids are complex, including N, NO, NO2, O, O2, O3 and O4, among which N and O2 are the most abundant compounds, and the relative abundances of different types of compounds in different crude oils are obviously different. If geological factors are not considered, there is no obvious correlation between the total acid value of crude oil and the content of a petroleum acid compound.

Fig. 1- 12 heteroatom composition of petroleum acid in typical crude oil

Verb (abbreviation for verb) Molecular composition of acidic oxygenated compounds

(A) Research status

The composition of oxygenated compounds can provide important information for studying the origin and biodegradation of crude oil, which has long been concerned by researchers. Although many types of compounds have been identified in some sediments or low-mature crude oil, the composition of oxygenates in crude oil is still unclear for two main reasons: first, it is difficult to separate oxygenates, and the content of oxygenates in crude oil is very low, and the molecular weight distribution and polarity are very different, so it is difficult for traditional separation methods to achieve high-purity separation under the premise of ensuring recovery; On the other hand, the reason is that there is no suitable means of analysis and characterization, and oxygen-containing compounds (such as carboxylic acids) are highly polar and need to be derivatized before chromatographic analysis. Gas chromatography is the most effective method to separate monomer compounds at present, but it can only analyze compounds with relatively small molecular weight in crude oil, and there are many isomers of oxygenated compounds, so it is impossible to separate monomers even on high performance capillary gas chromatography columns.

Fig. 1- 13 concentration distribution of O2 compounds in petroleum acid of typical crude oil.

Figure 1- 14n 1 Condensation distribution of compounds in typical crude oil and petroleum acid.

Petroleum acid composition is one of the hot spots in petrochemical research in recent years. The latest research results on petroleum acids are mainly based on the molecular type distribution results of mass spectrometry, and the composition characteristics and distribution law of petroleum acids are obtained by soft ionization mass spectrometry. Because the composition of petroleum acid is very complex, there are few research documents on the monomer compounds of petroleum acid, and the analysis method is generally gas chromatography-mass spectrometry.

At present, oxygenates identified from crude oil include ethers, alcohols, ketones, carboxylic acids, phenols and esters. Some heterocyclic oxygenates (such as dibenzofurans) are enriched in aromatic hydrocarbon samples, which are very easy to detect, while other oxygenates are difficult to separate. In recent years, small molecular phenolic compounds have been used in the study of petroleum migration, and most C0-C3 phenolic monomer compounds have been accurately identified by standard samples. The content of alcohols and ketones in crude oil is very low. At present, the structures of normal fatty alcohols and ketones are mainly identified.

Carboxylic acid compounds are the most abundant oxygenated compounds in crude oil, and the related research reports are the most. Anna Ding et al. (2004) identified monobasic acid (C 10-C33), dibasic acid (C 10-C25), palmitic acid, phytic acid, hopanic acid (C30—C33) and steroid acid (C27—C29) in Daqing crude oil. Steroids and terpenoid carboxylic acid compounds with relatively high abundance exist in some immature crude oils, biodegradable crude oils (Jaffe and Gallardo, 1993), sediments (Azevedo et al., 1994) or products of ruthenium ion oxidation of asphaltenes (Wang Peirong, 2002).

(II) Structural identification of oxides in crude oil with high acid value

Petroleum acids in crude oil with high acid value were separated by modified alumina adsorption column, and the composition of methylated acidic compounds was analyzed by gas chromatography-mass spectrometry. The total ion flow chromatogram of typical methyl petroleum acid is shown in figure 1- 15, and the spectral characteristics of the three samples correspond to representative crude oils with different acid values. The composition of petroleum acids in different crude oils is very different, and the carboxylic acid compounds identified in this section are actually their corresponding methyl esters. The identified monomer compounds mainly include fatty acids, naphthenic acids, aromatic carboxylic acids and lactones.

Figure 1- 15 Total ion flow chromatogram of typical methyl oleate (IS- 1 and IS-2 are internal standards; C24 is a normal fatty acid; X is the pollution peak)

1. fatty acid

According to the results of FTMS mass spectrometry, the compounds with Z value of 0 in the molecular formula of petroleum acid CnH2n+ZO2 mainly correspond to fatty acids. Fatty acids are widely found in crude oil, but their relative concentrations are quite different from naphthenic acids. The crude oil from Gao 1 well in Liaohe Oilfield is rich in CnH2n+ZO2 compounds, and the mass chromatogram of its fatty acid methyl ester is shown in Figure 1- 16. M/z74, m/z88 and m/z 102 represent normal, α-and β-methyl substituted long-chain fatty acids, respectively. Normal C 16 and C 18 fatty acids show strong relative abundance in the mass spectrum of m/z74, which is very easy to identify. The carbon number of normal fatty acids is between C9 and C34. Basinic acid and phytic acid are the base peaks on the mass spectra of m/z88 and m/z 102, respectively. Isoprene-like long-chain carboxylic acids are high in several low-mature crude oil samples, with carbon numbers ranging from C 17 to C2 1, while other isomeric fatty acids are relatively low, so the molecular structure is difficult to identify.

Figure 1- 16 high 1 well quality chromatogram of fatty acid methyl ester.

C 16 and C 18 unsaturated fatty acids were identified in some samples with high fatty acid content. The mass spectrum of the latter is shown in figure 1- 16. When analyzing unsaturated fatty acids of C 16 and C 18 by FTMS, we can see the abnormal carbon number distribution of Z =-2 series compounds. These compounds are theoretical. If the unsaturated fatty acids of C 16 and C 18 are brought in by pollution, then the normal contents of C 16 and C 18 may also be uncertain, because these two compounds are as easy to be introduced by pollution as unsaturated fatty acids, and the poor experimental repeatability of these two compounds in different laboratories can also be used as the basis for this inference.

2. aromatic carboxylic acids

There are few literature reports about aromatic carboxylic acids. Haug et al. (1968) found several aromatic carboxylic acid series with one ring and two rings in the shale extract of GreenRiver. Watson et al. (2002) simulated the biodegradation process of petroleum in the laboratory, and separated alkylbenzene carboxylic acid series compounds from crude oil samples at the initial stage of degradation, and thought that aromatic carboxylic acids were biodegradable products. Some samples studied by the author are rich in aromatic carboxylic acids, such as DST2 layer of PL 19-3-2 well in Bohai Bay Basin and Xing 603 well in Liaohe Oilfield. There are many kinds of aromatic carboxylic acids, including mono-pentacyclic aromatic acids, and the aryl skeleton structure corresponds to aromatic compounds. Figure 1- 17 ~ 1-24 is the mass spectrum diagram of aromatic carboxylic acids in crude oil of Well Xing 603 in Liaohe Oilfield, which are alkyl benzoic acid, alkyl naphthalene carboxylic acid, tricyclic aromatic carboxylic acid, tetracyclic aromatic carboxylic acid, pentacyclic aromatic carboxylic acid, monocyclic aromatic steroid acid and tricyclic aromatic steroid acid respectively. Alkylbenzene carboxylic acids have the widest distribution range, and the series of C0-C 18 alkylbenzene carboxylic acids can be clearly determined in the mass spectrum.

Fig. 1- 17 quality chromatogram of alkyl benzoate.

Fig. 1- 18 quality chromatogram of alkyl naphthalate.

Fig. 1- 19 mass chromatogram of alkyl tricyclic aromatic carboxylate.

Fig. 1-20 mass chromatogram of tetracyclic (pyrene) aromatic carboxylic acid alkyl ester.

3. Naphthenic acid

Figure 1-23 ~ 1-25 is the mass chromatogram of common naphthenic acids. Hopanic acid is an important acidic compound with molecular marker significance, which was discovered and identified earlier. The formation of hopanic acid is considered to be the product of hopane biodegradation. Undegraded crude oil generally does not contain hopanic acid. With the increase of biodegradation, the content of hopanic acid increased, but when biodegradation was very serious, hopanic acid was degraded and disappeared. Previous studies have shown that hopanic acid exists in biodegradable crude oil, while the content of hopanic acid in undegraded and severely degraded crude oil is low. Hopanic acid was detected in most of the crude oil samples we studied, but their relative composition changed greatly.

Fig. 1-2 1 mass chromatogram of tetracyclic aromatic carboxylic acid alkyl ester.

Fig. 1-22 mass chromatogram of pentacyclic (benzopyrene) aromatic carboxylic acid alkyl ester.

Figure 1-23 mass chromatogram of monoaromatic sterane ester

Fig. 1-24 quality chromatogram of triarylstearic acid

Figure 1-25 Identification of compounds corresponding to the mass chromatograms of tricyclic terpanes and hopanic acids is shown in Table 1-5.

Table 1-5 identification table of hopanic acid

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4. Lactones

C10-c18 fatty acid lactone compounds were identified in most oil sands samples in western Canada basin. The mass spectra of these compounds are as shown in figure 1-26, with characteristic fragments of m/z57, m/z7 1 and m/z85, which are easily misjudged as normal alkanes. However, the significant difference between its mass spectrum and n-alkanes is that the fragment strength with high mass-to-charge ratio has obvious steps between m/z85 and m/z99. These compounds are formed by intramolecular dehydration of β-,χ-or δ-hydroxy acids, and are unlikely to be natural products in mature crude oil.

Figure 1-26 m/z85 mass chromatogram and mass spectrogram of lactone.

Six, two-dimensional chromatography/mass spectrometry identification of petroleum acids

Two-dimensional chromatography is a new analytical technique (Phillips and Liu, 1992) developed abroad in the early 1990s. This technique was first applied to the analysis of environmental samples, and the chromatographic analysis ability of complex mixtures was greatly improved by using dual chromatography (Dalluge et al., 2003; Zrostlikova et al., 2003). Using the detector of time-of-flight mass spectrometer, the speed of mass spectrogram acquisition can reach 500 pieces per second, which meets the requirements of rapid data acquisition in sample analysis. By combining the hardware conditions of these chromatograms with the dedicated mass spectrum deconvolution software, the mass spectra of single compounds separated in the analysis process can be obtained. Because the composition of methyl oleate in crude oil is extremely complex, Hao et al. (2005) first conducted a method test with three commercial naphthenic acid samples from Canadian Synthetic Oil Company, Acros Company and Fluka Company.

Figure1-27 Two-dimensional color reconstruction of fluka naphthenic acid standard sample by full ion flow chromatography (bottom) and acyclic normal fatty acid two-dimensional mass spectrometry (top)

One-dimensional color analysis of industrial naphthenic acid samples after methyl esterification and quaternary butyl methylsilylation was carried out by predecessors. The modified full ion flow chromatography usually has a large bulge, which makes it impossible to separate three naphthenic acid samples produced by three companies. However, using two-dimensional chromatography technology, we can get many chromatographic peaks with good resolution (Figure 1-27). As can be seen from the figure 1-27, using m/z87, m/z10/,m/z 15, m/z 129 and m/z 143. Similarly, use m/z 127, m/z14, m/z 155, m/z 169, m/z 183, m. Moreover, the fingerprint characteristics of three kinds of naphthenic acid standard samples are obviously different in the distribution of these compounds. By selecting the mass spectra of compounds with specific (x, y) retention time and using library search, the structures of these compounds can be characterized (figure 1-29). It is difficult to completely separate the bicyclic and polycyclic fatty acids (z =-4, -6 and -8) in these naphthenic acid samples into single compounds by two-dimensional chromatography, thus providing sufficient structural information. The practical application of two-dimensional color analysis of crude oil and oil sands samples still needs a lot of laboratory methods and experiments.

Fig. 1-28 reconstructed mass chromatograms of two-dimensional color acyclic normal fatty acids (Z=0) and monocyclic long-chain fatty acids (z =-2) of three naphthenic acid standards.

Fig. 1-29 reconstructed mass spectra of two-dimensional colored acyclic normal fatty acids (Z=0) and monocyclic long-chain fatty acids (z =-2) and mass spectra of single compounds of naphthenic acid standard samples.