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. 2020 Dec 28;6(1):135–147. doi: 10.1021/acsomega.0c03974

Deep Insights into Heavy Oil Upgrading Using Supercritical Water by a Comprehensive Analysis of GC, GC–MS, NMR, and SEM–EDX with the Aid of EPR as a Complementary Technical Analysis

Richard Djimasbe †,‡, Mikhail A Varfolomeev †,‡,*, Ameen A Al-muntaser †,‡, Chengdong Yuan †,‡,*, Muneer A Suwaid †,‡, Dmitriy A Feoktistov ⊥, Ilfat Z Rakhmatullin §, Aleksei A Milovankin ⊥, Fadis Murzakhanov ‡,§, Vladimir Morozov ∥, Marat Gafurov ‡,§, Abdolreza Farhadian †,‡, Rustam R Davletshin #
PMCID: PMC7807475  PMID: 33458466

Abstract

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Upgrading of heavy oil in supercritical water (SCW) was analyzed by a comprehensive analysis of GC, GC–MS, NMR, and SEM–EDX with the aid of electron paramagnetic resonance (EPR) as a complementary technical analysis. The significant changes in the physical properties and chemical compositions reveal the effectiveness of heavy oil upgrading by SCW. Especially, changes of intensities of conventional EPR signals from free radicals (FRs) and paramagnetic vanadyl complexes (VO2+) with SCW treatment were noticed, and they were explained, respectively, to understand sulfur removal mechanism (by FR intensity and environment destruction) and metal removal mechanism (by VO2+ complexes’ transformation). For the first time, it was shown that electronic relaxation times extracted from the pulsed EPR measurements can serve as sensitive parameters of SCW treatment. The results confirm that EPR can be used as a complementary tool for analyzing heavy oil upgrading in SCW, even for the online monitoring of oilfield upgrading.

1. Introduction

Global energy consumption has been growing steadily in recent decades. Crude oil remains the main source of energy. According to BP Energy Outlook (2019 edition), crude oil represents more than 30% of total consumed energy.1 However, the excessive development of conventional oil resources in the last few decades has exerted tremendous pressure on the supply of crude oils. To close the gap between oil demand and its supply, it is necessary to expedite the production of unconventional oil reserves, such as heavy and extra-heavy oils, natural bitumen, and oil sands.2

Heavy oils are characterized by low API gravity and high viscosity that increase difficulties for their production, transportation, and processing.3 The low API gravity and high-viscosity values are essentially ascribed to the high content of heavy components (resins and asphaltenes) that usually contain a high amount of heteroatoms (such as S, N, and O) and metals.4 Thermal enhanced oil recovery (EOR) techniques usually can decrease oil viscosity, which makes them a good candidate for heavy and extra-heavy oil production.5 Among them, the most widely implemented technique is steam injection that includes steam driving, cyclic steam stimulation,6 steam-assisted gravity drainage, and so forth.7−10 The major problems encountered in the steam injection process are that they are limited by the depth of the reservoir11 and the recovered oil always has a high viscosity and density (sometimes even higher than that of the original oil in place) as a low recovery rate has been reached, making transport and refining difficult.12

Heavy oil upgrading using supercritical water (SCW) can be applied for both the in situ (for EOR especially in deep reservoirs) and ex situ upgrading process (for easy transportation, oil refinery, etc.). Using SCW allows us to reduce the viscosity and density of heavy oil and bitumen because of the hydrothermal cracking of high-molecular-weight components (such as resins and asphaltenes)13 by the cleavage of the aliphatic and alkyl–aromatics hydrocarbon groups and heterogeneous bonds including C–S, C–N, C–C, C–O, C–H, and so forth.14−16 In our previous work, it was also found that SCW decreased the viscosity of vacuum residues and heavy oil by 2530 and 124 times, respectively.17 The high upgrading performance using SCW is beneficial from its special properties. SCW is a thermodynamic state of water above its critical conditions (temperature 374.3 °C and pressure 22.1 MPa).18,19 It is a homogeneous phase that is able to increase mass transfer during the conversion process of heavy hydrocarbons due to its high diffusion and dissolving ability.20 Zhao et al. evaluated the in situ upgrading of heavy oil using subcritical and SCW and demonstrated that SCW injection improved oil recovery by 17% and reduced heat consumption by 34% compared to conventional steam flooding.15 Recently, upgrading of heavy hydrocarbons using SCW has been widely investigated, and it is a complex process that requires multiple methods and analyses including both calculation and experiments. Yoshii et al.21 found that the variation of dielectric constant (ε) depends on the elongation of the dipole moment of a molecule by molecular dynamics calculations. They detected that the decrease of the dipole moment of water from 2.60 D at ambient environment to 1.87 D at SCW conditions and ε of SCW varies from 73 to 21. Additionally, Kalinichev indicated that the low dipole moment of water may reflect the weak hydrogen bonding in SCW, which explains the weak attractive force between two ions of opposite signs, making SCW a nonpolar solvent.16 Zhao et al. investigated the upgrading of vacuum residues by SCW by elements (S, N, Ni, and V), viscosity, and SARA analysis, as well as average molecular weight analysis by gel permeation chromatography (GPC).22 Meng et al. studied the effect of temperature and pressure on the liquid yield during the upgrading process of Tumuji bituminous sand in sub-critical (Sub) and SCW conditions.23 Li et al. studied the effect of NaOH on asphaltene transformation in SCW by product yield analysis, gaseous products, and kinetic calculation.24 Watanabe et al. used a scanning electron microscopy (SEM) method to study the structure of coke formed during heavy oil upgrading in SCW.25 Ates et al. investigated the desulfurization of Arabian heavy oil in SCW conditions and the role of catalyst by a comprehensive analysis of gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) for gaseous and oil products.26 For a more understanding of SCW participation in the upgrading process of heavy oil, Hosseinpour et al. studied the hydrogen generation of SCW and its participation in the reaction such as a hydrogen donor by the isotopic labeling technique.27 Morimoto et al. have applied several methods to analyze the effect of SCW on upgrading reaction of oil sand, such as GC, 13C NMR, high-performance liquid chromatography (HPLC), and GC–MS. The global results revealed that a very small amount of water was involved in the upgrading reaction.28 Sato et al. investigated asphaltene upgrading in SCW with and without partial oxidation using GC-TCD for gaseous products, X-ray fluorescence (XRF) for liquid, and IR as well as TGA for the solid.29 Xu et al. investigated the role of SCW in pyrolysis of carbonaceous compounds by using GC for the gaseous phase, HPLC for the aqueous phase, and GC–MS for the organic phase.30 Kozhevnikov et al. analyzed the transformation of petroleum asphaltenes extracted from the heavy oil in SCW using FTIR and 1H NMR.31

The concise review above shows that in order to understand the efficiency and the mechanism of heavy oil upgrading using SCW, a lot of work has been carried out and different analysis methods have been employed, including GC, 13C NMR, HPLC, GC–MS, XRF, IR, GC-TCD, and so forth. However, we can see that all these methods are step by step adopted by different researchers in their work in addition to those basic analyses (viscosity, density, SARA fractions, etc.) to better understand the upgrading process of heavy oil. We can say that these proposed methods are indeed helpful. Also, from the review of the literature given above, the upgrading process of heavy oil using SCW is dominant by a cracking reaction of heavy molecules of asphaltenes and resins, resulting in the formation of free radicals, which is a most probable route for the initiation of other reactions during upgrading process. Electron paramagnetic resonance (EPR) is known to be the most effective tool that is capable of providing quantitative information regarding the presence and concentration of a variety of radicals (paramagnetic species) within a sample under test.32 Being a nondestructive, rapid, and accurate tool, EPR can be used not only in the laboratory conditions but also for the online monitoring of oilfield upgrading by analyzing the changes of intrinsic properties for petroleum paramagnetic centers.33

Surprisingly, as it was reviewed above, though application of EPR for studying petroleum systems is known since 1956,34 EPR has been rarely used to investigate the heavy oil upgrading in SCW. In this study, we try to analyze heavy oil upgrading process under SCW conditions using conventional and pulsed EPR techniques at the X-band range (9–10 GHz) at room temperature together with other routine analysis (viscosity, SARA fractions, material balance, GC for gaseous products, GC–MS for aromatics, 13C NMR for resins and asphaltenes, and SEM equipped with energy-dispersive X-ray (EDX) for coke, etc.) to get a clearer picture about the upgrading process.

2. Results and Discussion

2.1. Products after HTU under SCW Conditions

The material balance of products after the SCW process is shown in Table 1. It is observed for N1 to N5 (15 to 120 min) that the yield of liquid products was changed from 84.7 to 71.3% with more production of gases from 9.8 to 13.8% and of coke from 5.5 to 14.9%, respectively. These results are in agreement with previous studies of Gudiyella et al.35 They also observed the gas and coke increase in yield, while the liquid product decreases. The high production of gaseous products from 15 and 120 min can be related to hydrothermal cracking by the cleavage of C-heteroatom bonds (such as C–S and others bonds) and of C–C bonds via β-scission of small alkyl aromatics and paraffins.36 Generally, the energy bond of C–C cleavage is about 60–80 kcal/mol that is far higher than that of H-abstraction (12–16 kcal/mol), which means that once alkyl radicals of (C1–C4) are formed, they can easily capture hydrogen to form gases.22 However, the increase of the reaction time increases the amount of coke, which occurred due to the dehydrogenation of naphthenic hydrocarbons, condensation, and polymerization of aromatics.37

Table 1. Yields of Gaseous, Liquid, and Solid Products after HTU of Heavy Oil.

medium sample number time, min coke, % upgraded oil, % gases, %
SCW + N2 N1 15 5.5 84.7 9.8
  N2 30 8.9 79.7 11.4
  N3 60 12.1 76.2 11.7
  N4 90 14.0 73.4 12.6
  N5 120 14.9 71.3 13.8
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2.2. Analysis of Upgraded Oil

2.2.1. Viscosity of Upgraded Oil

Table 2 presents the results of the viscosity of the initial and upgraded oil. After upgrading under SCW conditions, the viscosity of the heavy oil was significantly reduced from 2073 to lower than 30 mPa·s. It is assumed that the significant reduction of viscosity mainly resulted from the cleavage of C-heteroatom bonds (such as C–S and C–N) as well as some C–C bonds contained in the large molecules (resins and asphaltenes), which agree with the work.13,38 It is widely believed that even a small amount of bond cleavage (such as C–S bond) may lead to a great reduction of viscosity.14,39 The significant reduction of viscosity can be supported by the change in SARA fractions, that is, a significant increase of saturates and an obvious decrease in aromatics and resins (will be discussed in details in Section 2.2.4).

Table 2. Properties of Initial and Upgraded Oil under the SCW State.
time, min viscosity, mPa·s at 25 °C
N0 2073
N1 12.4
N2 15.1
N3 16.6
N4 26.6
N5 29.4
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2.2.2. Determination of Elemental Composition of Upgraded Oil

Table 3 provides the results of the elemental composition of the upgraded oil. Generally, the heteroatom (S and N) content was decreased after upgrading. The decrease of sulfur content is related to the cleavage of the C–S bond by generating H2S, consistent with GC results of gases (see 3.4), such as thiols (mercaptane), ethyl-mercaptanes, sulfide, and disulfide, which can be easily removed using SCW. Vogelaar et al.39 found that no thiophenic compounds have been converted in SCW at 460 °C, aromatic sulfides and thiophenic compounds are more stable, and their stability results from the delocalization of an isolated pair of electrons on the sulfur atom in the aromatic electron π system.40 Despite the significant reduction in the viscosity of upgraded oil, a small increase in carbon atom and a decrease in hydrogen content were observed. It is mainly due to the result of dehydrogenation. Also, Tan et al. pointed out that the dehydrogenation of polycyclic cyclo-paraffins occurs to produce aromatics instead of the cleavage of C–C under SCW conditions.41 For example, one typical reaction that can lead to the decrease of H/C is the condensation of aromatic hydrocarbons. Additionally, Guo et al. stated that during the thermal upgrading of heavy oil without adding hydrogen, the hydrogen from initial heavy oil will be redistributed into gases and light oil and the upgraded oil will have a low H/C and heaver coke. All these types of reactions eventually will cause an increase of aromatic structures in the upgraded oil, which can also be confirmed by the results of GC–MS in the later part.

Table 3. Elemental Analysis of Upgraded Oil under the SCW Statea.
  elemental content, wt %
samples C* H* S* N* H/C
initial oil 83.45 11.35 4.21 0.36 1.63
N1 84.69 11.60 3.27 0.32 1.64
N2 84.82 11.24 3.20 0.25 1.59
N3 84.49 11.30 3.00 0.27 1.60
N4 84.73 10.88 2.48 0.25 1.54
N5 85.10 10.97 2.40 0.25 1.54
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a

CHSN* the sum of (CHSN) is less than 100% due to the amount of oxygen and metals are not considered here.

2.2.3. Microelement Composition of Vanadium and Nickel in the Initial Oil and Upgraded Oil

The change in the content of V and Ni before and after upgrading is shown in Table 4. The concentration of Ni and V in general is remarkably decreased after the HTU process. This means that Ni and V can be effectively removed in the SCW medium. Usually, V and Ni exist in the form of metal–organic complexes contained in fractions of resins and asphaltenes. Metallporphyrin is the main type.42 These metals in porphyrin can be removed by two reaction pathways, that is, (1) by successive hydrogenation several times until the macromolecule of porphyrin is broken (losing its porphyrinic character), followed by a hydrogenolysis step that fragments the ring and removed metal and (2) by reacting with OH and ring fragmentation process where the metal is removed from the core of porphyrin through both reaction with OH of SCW and the ring fragmentation phenomena due to the instability of metal-free porphyrins under reaction conditions.43 Wang et al.42 also indicated that driven by the π–π attractive interaction between aromatic sheets and high diffusivity of SCW, the coke-like self-assembly of metal-containing heavy aromatics occurs spontaneously and rapidly, thus significantly accelerating the condensation of metal-containing poly-aromatics to coke, by which the rate of demetalization is improved simultaneously. It can be assumed that the SCW both chemically and physically accelerates the removal of metals and makes the demetalization process easier. The removal mechanism of vanadium will be discussed in detail in Sections 2.2.8 and 2.3.1 according to the data obtained by EPR analysis.

Table 4. Results of Microelemental Analysis of V and Ni in Initial and Upgraded Oil.
  content in upgraded oil, wt %
elements N0 N1 N2 N3 N4 N5
V 0.0120 0.0035 0.0028 0.0009 0.0004 0.0005
Ni 0.00097 0.0004 0.0002 0.0002 0.0001 0.0001
accuracy, % relative 0.3 0.19 0.06 0.34 0.08 0.36
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2.2.4. SARA Fractions of Upgraded Oil

The results of SARA fractions of initial and upgraded oil are presented in Figure 1. After upgrading, the content of saturates was significantly increased from the initial value of 28.8 to 41.2, 62.3, 56.0, 50.1, and 43.0% for N1–N5. The increase of saturates is mainly due to the dealkylation of the alkyl aromatic structure and the hydrogenation of aromatics during the upgrading process of heavy oil. Also, the content of aromatics was decreased after upgrading, which on the one side can be attributed to the dealkylation or cracking of benzene ring as well as hydrogenation of aromatics that convert aromatics into saturates44 and on the other side can be resulted from their condensation reactions to form asphaltenes and eventually coke. For resins, its content was also significantly reduced after upgrading. The cleavage of C-heteroatom bonds as well as some C–C bonds in the large molecule of resins can be the most important reaction to convert resins to smaller molecules (gases, saturates, smaller resin structure, etc.), which is also one of the main reasons that why the viscosity was significantly decreased as mentioned in Section 2.2.1. Another reaction pathway to consume resins is the same like for aromatics, that is, their condensation reactions. However, the content of asphaltenes varied a lot in different runs. This can be because the thermal cracking of the heavy oil is very complex process that involves simultaneous occurrence of diverse parallel and sequential reactions, including cleavage, polymerization, condensation, dehydrogenation, and so forth. Asphaltenes behave more like an intermediate product. On the one hand, it is generated by condensation and polymerization reactions; and on the other hand, dehydrogenation in addition to condensation and rearrangement steps convert it to compaction products (coke). Zhao et al.22 concluded that until 1 h, the amount of resins, aromatics, and asphaltenes decreases with increasing reaction time, while the saturates increased until 1 h, which is similar to our results. However, after 1 h (reaction time), the change of SARA fractions in our case is different from their results. In their study, the SARA fractions tend to keep constant with increasing reaction time after 1 h, while they changed with increasing reaction time in our case, specifically, saturate content decreased after reaching its maximum at 1 h, aromatic, resins, and asphaltenes showed an increasing trend. The decrease in saturate content might be mainly caused by the intensive hydrothermal cracking reaction where a part of saturates are converted into gas because of the scission of long-chain alkanes, which is consistent with the increased gas yield (see Table 1).

Figure 1.

Figure 1

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SARA fractions of heavy crude and upgraded oil.

2.2.5. GC Analysis of Saturates

Table 5 shows the carbon number distribution of saturate calculated from Figures S1–S6 in the Supporting Information. In this study, to ease the interpretation of results, the saturates were grouped into two parts, that is, diesel–(C10–C20) and atmosphere residue (C21–C31). In general, the content of C10–C20 alkanes significantly increases and the content of C21–C31 decreases after SCW treatment compared to the initial heavy oil, as shown in Table 5. The increase of C10–C20 can be mainly ascribed to the scission of long-chain alkanes as well as the dealkylation of aromatics in the resins and asphaltenes. Notably, for run N3, the highest content of C10–C20 and lowest content of C21–C31 were observed among all reaction times studied in this work. In addition, comparing this result with the SARA fractions data and average molecular weight, one can find that at this time (1 h SCW treatment), the saturates also show the highest content and asphaltenes show the lowest content. Therefore, it can be assumed that the increase of saturates and the light fractions content (C10–C20) is a comprehensive result of whole thermal cracking reactions. For example, for asphaltenes, first, it can be destructed by the cleavage of side chain and ring-opening reactions; second, it can be generated through condensation and polymerization reactions; and third, it can be transformed into coke by dehydrogenation of cycloalkanes, addition to condensation and rearrangement steps. In this work, the average molecular weight of saturates in initial and upgraded oil was also calculated. For N0–N5 samples, the average molecular weight is 305.25; 276.94; 285.64; 252.56; 276.13; and 272.75 respectively. Consequently, the optimization of reaction time is very important for thermal treatment by SCW at certain conditions. It can be concluded that 1 h treatment is the best to obtain lower molecular weight saturates in bigger amounts.

Table 5. Carbon Number Distribution of Saturates.
  n-alkanes, %
Samples ∑C10–C20 ∑C21–C31
N0 50.23 52.77
N1 63.61 36.39
N2 66.10 33.90
N3 79.67 20.33
N4 67.19 32.81
N5 66.01 33.99
boiling point, °C 216–342 356–458
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2.2.6. GC–MS Measurement of Aromatics

GC–MS results of aromatics are presented in Table 6. The aromatics are subdivided into mono-, di-, and poly-aromatics (MDPA) as well as unidentified types. In the case of shortest time reaction (15 min), the content of mono-aromatics is increased with the decrease of di- and poly-aromatics content compared with that of the initial heavy oil, which implies that the main reaction pathway is the hydrogenation of di- and poly-aromatics together with ring-opening reactions to produce mono-aromatics. It should be notable that the dealkylation of MDPA must occur simultaneously (even earlier) as it requires a lower energy than hydrogenation of aromatics. The most significant change that happened in mono-aromatics is the decrease of 1, 4-diisopropyl-2 methylbenzene content (from 30.7 to 2.02%) and increase of o-cymene concentration (from 13.9 to 58.3%). Therefore, it is reasonable to assume that the following reaction (in Figure 2) occurred during the thermal cracking process. However, when the reaction time increases to 30 min and longer, the content of mono-aromatics is significantly decreased (even disappeared at 30 min), while the content of di- and poly-aromatics is increased from 16.9 and 4.9% to 72.6–83.2% and 16.4–27.3%, respectively. This means that mono-aromatics were transformed into di- and poly-aromatics through the condensation process (mainly addition, cyclization, and dehydrogenation reactions).45,46 According to the thermal cracking behavior of aromatics, it is also reasonable to assume that the change in the content of mono-, di-, and poly-aromatics must be also affected by the cleavage of large molecules (resins and asphaltenes).

Table 6. Content of Aromatic Hydrocarbons in Initial and Upgraded Oil.
aromatic hydrocarbons initial oil N1 N2 N3 N4 N5
mono-aromatics, % 61.8 70.5 0.0 4.9 1.5 0.4
di-aromatics, % 21.9 16.9 72.6 73.8 77.2 83.2
poly-aromatics, % 16.3 4.9 27.3 21.2 21.2 16.4
others aromatics, % 0.35 7.72 0.0 0.0 0.0 0.0
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Figure 2.

Figure 2

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Possible reaction of 1, 4- diisopropyl-2 methylbenzene.

2.2.7. NMR Analysis of Resins and Asphaltenes of Upgraded Oil

The results of the 13C NMR spectra of resins and asphaltenes in initial and upgraded oil are shown in Figures 3 and 4. The 13C NMR spectra contain many distinct signals that can be attributed to different typical regions and thus provide information about the primary (methyl groups −CH3)—Cp, secondary (methylene −CH2−) plus quaternary—Csq, tertiary (CH groups)—Ct, and aromatic—Car carbons. The values of Cp, Csq, Ct, and Car were obtained by integrating the corresponding regions in the 13C NMR spectra.47 Also, aromaticity factor (FCA) was calculated from FCA = Car/(Car + Cal), where Cal = Cp + Csq + Ct (the total aliphatic carbon content). For resins, after upgrading, a noticeable decrease of Csq (−CH2−) was observed, which can be resulted from the thermal decomposition of lateral chains. This is consistent with the SARA analysis where the resin content was obviously decreased because of the cleavage of C-heteroatom bonds as well as some C–C bonds in the large molecule of resins. In addition, Car and FCA were increased after upgrading, which manifests that the upgraded resins are more aromatized in comparison with the initial oil. For asphaltenes, the same changes in the Csq, Car, and FCA such as resins were observed according to Tables 7 and 8fig.

Figure 3.

Figure 3

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13C NMR spectra of resins fraction of original and upgraded oil.

Figure 4.

Figure 4

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13C NMR spectra of asphaltenes fraction of original and upgraded oil.

Table 7. Molar Fraction (%) of Carbon Groups (Cp, Csq, Ct, Car) and Aromaticity Factor FCA (FCA = Car/(Car + Cal), Where Cal = Cp + Csq + Ct) of Resins before and after the SCW Upgrading Process.
  molar fraction (mol %)
group types initial oil N1 N2 N3 N4 N5
Cp 10.4 11.3 11.6 9.7 7.7 10.6
Csq 38.8 26.9 25.5 25.8 28.7 25.9
Ct 8.4 7.8 10.3 8.7 11.2 6.7
Car 42.4 54.0 52.6 55.8 52.4 56.8
FCA 0.42 0.54 0.52 0.56 0.52 0.57
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Table 8. Molar Fraction (%) of Carbon Groups (Cp, Csq, Ct, Car) and Aromaticity Factor FCA (FCA = Car/(Car + Cal), Where Cal = Cp + Csq + Ct) of Asphaltenes before and after the SCW Upgrading Process.
  molar fraction (mol %)
group type initial oil N1 N2 N3 N4 N5
Cp 15.1 9.0 5.5 6.5 10.1 8.4
Csq 36.4 26.7 21.2 19.1 29.6 33.8
Ct 8.5 7.2 8.5 5.3 5.7 11.5
Car 40.0 57.1 64.8 69.1 54.6 46.3
FCA 0.40 0.57 0.65 0.69 0.54 0.46
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2.2.8. EPR Spectra Analysis of Upgraded Oil

The continuous wave (cw) EPR spectrum of oil is defined by the superposition of a single “free” radical (FR) line (a linewidth of 6 G and isotropic g-factor g ≈ 2.0040) and paramagnetic vanadyl-porphyrin (with the skeleton of VO2+) complexes of axial symmetry with the parallel and perpendicular components of g- and A (hyperfine because of the interaction of electron and 51V nuclei with the nuclear moment I = 7/2) tensors, as shown in Figure 5. In our notations, the values of (g, A)∥ correspond to the orientation perpendicular to VO2+ plane (out of plane), along the direction c while (g, A)⊥ corresponds to the orientation in the VO2+ plane, ab plane see Figure 6 and refs.48,49

Figure 5.

Figure 5

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cw EPR spectrum of initial heavy oil at T = 297 K. Position of FR and hyperfine components for VO2+ complexes are marked.

Figure 6.

Figure 6

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Schematic representation of a skeleton of single vanadyl porphyrin molecule.

Figures 7 and 8 represent cw EPR spectra and ESE spectra, respectively. The parameters obtained from these spectra are presented in Table 9.

Figure 7.

Figure 7

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Central part of cw EPR for initial oil and upgraded oil.

Figure 8.

Figure 8

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Central part of field-swept ESE EPR for initial and upgraded oil.

Table 9. Parameters for Fitting the cw EPR Spectrum of Oil at T = 297 K.
VO2+ g⊥ = 1.9860 g∥ = 1.9661 A⊥ = 160 MHz A∥ = 470 MHz
FR g = 2.0040 ΔH(G) = 0.64 mT % Gaussian   ΔH(L) = 0.48 mT % Lorentzian
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In the EPR spectra, the most obvious change is the sharp decrease of VO2+ intensity with increasing reaction time, which might be attributed to either the decrease in concentration of VO2+ or changing in the valence state of vanadyls in the complexes, or both. However, because of the fast relaxation time of VO2+, no ESE from vanadyl was observed at room temperature and no relaxation measurements at room temperature could be done for VO2+. The intensities of the FR also decrease with the reaction time generally, but they do not change as significantly as VO2+ intensity for N1, N2, and N3 (Table 10). Furthermore, the intensity ratio of VO2+ to FR was calculated to avoid inaccuracies in determining the absolute concentrations of paramagnetic centers often caused by the evaporation of light fractions or variations in density, viscosity, dielectric properties, and so forth. The VO2+/FR ratio also shows a similar decreasing trend such as VO2+ intensity, which confirms that VO2+ complexes indeed decreased and eventually disappeared with increasing reaction time.

Table 10. FR, VO2+ Intensities, Width of the FR EPR Signal, and Electronic Relaxation Times for FR at T = 298 K in Initial and Upgraded Oil.
sample intensity/g IPP FR intensity/g IPP VO2+ intensity ratio IPPVO2+/FR width FR BPP (G) FR T2e (ns) FR T1e (μs)
(N0) 1.976 0.998 0.51 6.4 375 36
N1 1.168 0.133 0.11 4.9 170 17.5
N2 1.308 0.12 0.09 5.0 185 1.9
N3 1.306 0.065 0.05 5.2 170 2.0
N4 0.456 0.04 0.08 4.9 150 2.4
N5 0.54 0 0 5.7 170 2.5
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If one compares the change in intensities and T1e time for FR, it can be found that although the FR intensities do not vary significantly, the T1e time remarkably decreases from 36 to 17.5 and 1.9 μs from N0 to N1 and N2, respectively. The decrease of intensities of FR can be attributed to the radical recombination reaction, resulting in the increase of coke formation and the reduction of liquid products, which accords with the material balance results in Section 2.1. The significant decrease of T1e time indicates that the treatment under SCW conditions destroyed the environment of FR even at short reaction time.50 According to the literature data,49 the FR is mainly stabilized in large molecules with condensed poly-aromatic hydrocarbons sheet such as resins and asphaltenes. The destruction of the environment of FR might be due to the thermal decomposition of lateral chain of resins and asphaltenes (most likely from resins according to SARA fraction analysis in Section 2.2.4) as well as ring-opening reactions that make the environment of FR less condensed. Simultaneously, it can be observed that the linewidth of FR significantly decreases after upgrading mainly due to the loss of the left (low-field) wing of the FR signal that is conventionally ascribed to the sulfur-containing radicals.49

This confirms the elemental results of sulfur removal in upgraded oil in Section 2.2.2.

Based on the observation in Section 2.2.3, it should be noted that the EPR method has large interest and can determine free radicals and paramagnetic complex metals amount and state as well as allow determination of the heavy metal content in the upgraded oil.

2.3. Coke Analysis

2.3.1. EPR Analysis of the Solid Product (Coke)

As mentioned in Section 2.2.8, the sharp decrease of VO2+ intensity can be caused by either the removal of VO2+ from oil or the variation of the valence state of vanadium in the complexes, or both of them. If VO2+ complexes were removed from the oil, they must have existed in coke. Therefore, we also analyzed the EPR spectra for coke (Figure 9). The EPR and ESE spectra of coke obtained after upgrading are presented in Figure 10 and Table 11, respectively. Unlike in the liquid upgraded oil, for the coke, the ESE signal from VO2+ complexes was obtained at room temperature. The ability of detection of the ESE signal from VO2+ in coke can be ascribed to the restricted motional mobility of vanadyls in coke compared to the upgraded and initial oils.51,52 Considering the theory of relaxation of paramagnetic centers in oil systems is still not developed, and therefore, here no deep theoretical analysis of the revealed experimental findings is presented.

Figure 9.

Figure 9

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Central part of cw EPR of coke.

Figure 10.

Figure 10

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Central part of the field-swept ESE EPR of coke.

Table 11. FR and VO2+ Intensities, FR Width, and Relaxation Times for FR and VO2+ at T = 298 K for Coke.
sample Ipp/mass FR (a.u.) Ipp/mass VO2+ (a.u.) intensity ratio VO2+/FR FR Bpp (G) FR T2e (ns) FR T1e (μs) VO2+T2e (ns) VO2+T1e (μs)
N1 4.1 0.23 0.057 6.5 160 1.6 90 2.3
N2 3.7 0.23 0.062 6.8 160 1.6 80 2.0
N3 4.0 0.2 0.046 6.56 180 1.7 100 2.9
N4 3.7 0.17 0.043 6.71 150 1.5 90 3.7
N5 3.1 0.09 0.03 6.07 190 2.1 110 5.3
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Considering the values of electronic relaxation times for VO2+ in coke are of the same order of magnitude as for FR (Table 12) while usually in heavy oils and oil asphaltenes, VO2+ relaxation times are up to 10–100 times shorter in comparison to FR.50,53

Table 12. Yields of Gaseous Products after SCW Treatment of Heavy Oil at 400 °Ca.
gases, vol % N1 N2 N3 N4 N5
<keep-together>∑C1–C4*</keep-together> 87.9 88.6 94.5 89.3 86.7
ethylene 0.07 0.07   0.07 0.05
butene 0.2 0.3 0.3 0.25 0.14
H2 0.5 0.4 0.4 0.3 0.35
CO2 4.4 4.5 4.7 5.17 5.6
H2S 0.01 0.008 0.05 0.07 0.05
non-identified gases 6.7 5.4 0.05 4.84 7.1
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a

∑C1–C4* is the sum of C1–C4 hydrocarbons.

In coke, we also obtained both FR and VO2+ signals in all the samples. This means that some of VO2+ complexes were indeed removed from the heavy oil to coke without being destroyed during the upgrading process. In Section 2.2.3, we mentioned that metals in porphyrin can be removed by two chemical reaction pathways. Apparently, the direct removal of VO2+ complexes does not belong to any of them. It is likely that the removal of VO2+ complexes is caused by the condensation of metal-containing poly-aromatics to coke. However, we also found that the absolute and relative amount of VO2+ in coke and in the upgraded oil is lower than that in initial heavy oil. This means that the transformation of valence state of vanadyl also occurred simultaneously. The use of EPR techniques helps us to better understand the mechanism of metal removal.

Simultaneously, we found that the values of the electronic relaxation times for FR in N1–N4 samples for coke (Table 11) are almost the same as for the N2–N5 samples of upgraded oil (Table 10), indicating the same nature and structure of FR in both coke and upgraded oil. As for the upgraded oil, these times are much lower than in initial oil, indicating the possibility of using FR relaxation times for both oil and coke as indicators of SCW treatment. Interestingly, the values of electronic relaxation times for VO2+ in coke are of the same order of magnitude as for FR (Table 11) while usually in heavy oils and oil asphaltenes VO2+ relaxation times are up to 10–100 times shorter in comparison to FR.50,54 For now, it is difficult to explain these observed interesting phenomena, considering that the theory of relaxation of paramagnetic centers in oil systems is still not developed. Nevertheless, the obtained EPR results show that the values of the electronic relaxation times both for FR and vanadyls can serve at least as sensitive parameters to follow SCW treatment, even for the online monitoring of oilfield upgrading.

2.3.2. High-Resolution Field-Emission Scanning Electron Microscope of Coke

Field emission scanning electron microscopy (FESEM) equipped with energy-dispersive EDX analysis system was used to analyze the structure and elemental composition of coke. Figure 11 shows the morphology and elemental composition of coke after the upgrading process, respectively. As shown in Figure 11, there are various types of coke particles being existed after 15 min SCW treatment. With the increase of reaction time, the morphology of coke becomes more uniform and compact, which means that the formed coke is more condensed. Also, it can be observed that the bulk coke is consisting of many aggregates of small particles. As indicated by Bai et al.,37 coke precursor is first generated by the condensation reactions of asphaltenes, and the asphaltene molecule is highly dispersed in the continuous phase at the high-density SCW condition; thus, the generation of coke precursor and the growth of coke occurred independently in each asphaltene molecule. As a consequence, small coke particles were eventually generated and aggregated together, resulting in the final bulk coke.

Figure 11.

Figure 11

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Morphology of obtained coke. (a) 15 min, (b) 30 min, (c) 60 min, and (d) 120 min.

2.4. Gaseous Product Analysis

The composition of obtained gaseous products after the HTU of heavy oil at SCW conditions is presented in Table 12. The gas is mainly composed of C1–C4, H2S, CO2, H2, and alkenes. The C1–C4 gases were mainly generated by the Rice–Kossiakoff (R–K) mechanism of alkyl radicals after the successive decomposition of β-scission,42,43 while the olefins gas production was due to the deep thermal cracking of saturates to short olefins and di-olefins.37,55,56 Regarding the content of CO2, it generally increases with reaction time, which can partly arise from methane reforming reactions with the steam producing syngas product, the reaction can be simply described as follows57

2.4. 1
2.4. 2

Another important reaction to generate CO2 is the decarboxylation of the carboxylic acids in the heavy oil. Both the reactions can lead to the increase of CO2. Also, we can see that the content of H2S was slightly increased with reaction time, which is consistent with the removal of sulfur evidenced by elemental analysis in Section 2.2.2. This also implies that some part of sulfur in heavy oil was removed through sulfur hydrogenation,26 while another part was transformed into coke that is verified by EPR and EDX analysis of coke.

3. Conclusions

In summary, by a comprehensive study of the heavy oil upgrading process under SCW conditions, the following conclusions can be drawn:

The effectiveness of heavy oil upgrading by SCW was verified by a significant viscosity reduction from 2073 to lower than 30 mPa·s, as well as a considerable increase of saturates fraction, and reduction of aromatics and resins.

The main reason for the decrease in the viscosity of heavy oil can be attributed to the cleavage of C-heteroatom bonds (such as C–S and C–N) as well as some C–C bonds contained in the large molecules (resins and asphaltenes).

An obvious increase in saturates content together with a higher content of C10–C20 and a lower content of C21–C31 is mainly resulted from the dealkylation of side chains attached to aromatics, resins, and asphaltenes and the ring opening of heteroaromatic cycles.

For aromatics, in the early reaction time, the main reaction pathway is the hydrogenation of di- and poly-aromatics together with ring-opening reactions to produce mono-aromatics, while with increasing reaction time, mono-aromatics were transformed into di- and poly-aromatics through the condensation process (mainly addition, cyclization, and dehydrogenation reaction).

After upgrading, the aromaticity of resins and aromatics is increased mainly due to the dealkylation of alkyl chains in the molecule of resins and asphaltenes, which is evidenced by the change in the SARA fractions as well as the destruction of the environment of FR.

Upgrading of heavy oil in the SCW shows effectively a potential to remove a part of sulfur and sulfur-containing compounds, which is supported by the significant decrease in the linewidth of FR because of the loss of the left (low-field) wing of the FR signal (sulfur-containing radicals).

SCW treatment is favorable for the demetalization of heavy oil, and V and Ni can be effectively removed. The removal of VO2+ complexes occurs in two ways: being directly transferred into coke without being destroyed during the upgrading process because of the coke-like self-assembly of metal-containing heavy aromatics driven by the π–π attractive interaction and being transformed to other valence states of vanadyl and removed by a series of chemical reactions.

A part of original FR in the heavy oil can react easily in the beginning of SCW treatment by radical recombination reaction. However, some of them can participate in the reaction only when their environment is destroyed first.

In general, the obtained results show that EPR can be used as a complementary tool for analyzing heavy oil upgrading by SCW. The values of the electronic relaxation times of intrinsic paramagnetic centers are sensitive to the SCW conversion demonstrating an opportunity to follow the effectiveness of SCW treatment not only by tracking the intensity of the VO2+ radicals but also by the measuring T1e and T2e for FR and VO2+. Taking into account that electronic relaxation times are of about 3 orders of magnitudes shorter than nuclear ones (microseconds vs seconds), EPR relaxometry may potentially serve as a fast (comparing to the well-known NMR relaxometry) technique for analyzing oil treatment processes in both ex-situ and in-situ upgrading for enhanced oil recovery of hard-to-recover hydrocarbons.

4. Experimental Section

4.1. Materials

The heavy oil was obtained from Tatarstan heavy oil deposit (Tatneft Oil Company, Russia). The physical properties and chemical composition of the heavy oil are shown in Table 13. The heavy oil has a high sulfur content of about 4.2%. Solvents n-heptane (99.6%), toluene (99.8%), and isopropyl alcohol (99.5%) were provided by Eco.1 Company (Russia) for SARA analysis. The chloroform purchased from Eco.1 was also used as a solvent to collect liquid and solid products after upgrading processes.

Table 13. Elemental Composition and SARA Fractions of the Studied Heavy Oil.

  SARAb fractions, wt %
 organic elemental content, wt %
viscositya (mPa s) saturate aromatic resin asphaltene C H S N
2073 28.8 44.3 21.0 5.9 83.7 11.4 4.2 0.4
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a

Rotational viscometer (Brookfield DV-II Pro) was used to measure viscosity at 25 °C and atmospheric pressure.

b

SARA fractions were analyzed according to ASTM D-4124.

4.2. Upgrading Experiments in the High-Pressure Reactor

The experiments of hydrothermal upgrading (HTU) of heavy oil using SCW were carried out in a 4575/76 HP/HT reactor produced from Inconel Alloy 600 (Parr Instruments, USA) shown in Figure 12. The temperature and reaction time of experiments are, respectively, 400 ± 2 °C and from 15 to 120 min (run N1, N2, N3, N4, and N5, respectively) (this time does not include the heating time from room temperature to 400 °C). Using the Aspen tech Hysys V9 program, the density of SCW (0.141–0.164 g/cm3) was determined.58 In this study, 100 g of heavy oil (denoted N0) and 50 g of distilled water were loaded in the reactor. Nitrogen was used to remove impurities and provide an initial pressure of 10 bars at 25 °C. After that, the heating rate of the reactor is set up to 8 °C/min until a desired temperature of 400 °C. The heating of the reactor remains constant to reach the desired reaction time. The experiment was ended by turning off heating system, and the reactor is cooled down until reaching room temperature in 30–40 min. The evolved gas was analyzed by a gas chromatograph Chromatec-Crystal 5000.2 (Chromatec, Russia) using GOST 32507–2013 (ASTM D 5134-98 (2008), MOD).59 Extraction and separation processes were performed to get liquid oil and solid products after SCW treatment. The viscosity and SARA fractions were analyzed according to the methods described in footnotes of Table 13. The yield of upgraded oil (x1) was obtained by eq 3

4.2. 3

where M1 and M2 (g) are the mass of initial crude oil and upgraded oil, respectively.

Figure 12.

Figure 12

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Scheme of a high-pressure autoclave reactor.

The yield of coke (x2) was calculated using eq 4

4.2. 4

where M3(g) is the mass of solid residues.

The yield of gaseous products (x3) was obtained by eq 5

4.2. 5

4.3. Elemental and Microelemental Analysis of the Initial and Upgraded Oil

The elemental composition (C, H, N, and S) of the initial and upgraded oil was analyzed using PE 2400 Series II CHNS/O Analyzer. For this measurement, 1.5–2.0 mg of samples was encapsulated in tin or aluminum vials that were inserted automatically from the integrated 60-position auto sampler or manually using an automatic single-sample injector according to ASTM (P/N 0240-1289). The samples were burned at the heating process up to 1100 °C. Oxygen was used as combustion gas.

Microelemental analysis was performed to analyze the possible removal of vanadium and nickel by a Clever C31 energy dispersive XRF spectrometer equipped with a high-resolution detector (135 eV) with electric cooling on a Peltier element. The basic parameters of the XRF spectrometer are as follows: Rh anode material, 0.125 mm beryllium window thickness, 5–50 kV tube voltage, 20–1000 mA tube current, and 50 W maximum powers. A layered sample with a small thickness was prepared in the bottom of measuring cell that is a polyethylene terephthalate film with a thickness of 3.6 μm. The presence of elements in the sample was determined by the signal from the characteristic lines (Kα) of each element.60

4.4. EPR Measurements of Upgraded Oil and Coke

EPR spectra were obtained in stationary, continuous-wave mode at a frequency of 9.6 GHz (X-band) on a Bruker Elexsys E580 series spectrometer at room temperature. The modulation frequency and amplitude, constant low-pass filter time, and microwave power were adjusted to avoid over modulation, distortion, or saturation of the EPR signals, respectively (microwave power P = 2–25 μW, modulation amplitude M = 0.1 G at 100 kHz). The concentration of paramagnetic centers was estimated at 25 °C by comparing the integrated intensities of spectra simulated with the Easy Spin utility of test and reference samples (a series of Cu-DETC solutions with the known concentrations). Pulsed EPR spectra were recorded by the sweeping of the external magnetic field B0 and detecting an electron spin echo (ESE)

4.4. 6

where the duration of π/2 pulse was equal to 16 ns (π = 32 ns), the minimal delay between pulses τ = 240 ns. To determine the time of transverse (spin–spin) relaxation of T2, the same pulse sequence (6) was applied by increasing the time interval τ between the pulses with a step of 4 ns from 200 ns to the desired value. The dependence of ESE as a function of time 2τ in a fixed magnetic field can be estimated by eq 7

4.4. 7

where the spin–spin relaxation time T2 can be obtained. To measure the longitudinal (spin–lattice) relaxation time T1, the inversion-recovery three pulse sequences was used eq 8

4.4. 8

where τ was fixed to 240 ns. The time T was changed with a step of 32 ns (initial value T = 900 ns). The dependence of the integral intensity of the ESE versus T can be estimated by eq 9

4.4. 9

where the spin–lattice relaxation time T1 can be determined.53

4.5. GC Measurement of Saturated Fraction

The molecular distribution of saturated fractions was determined using gas chromatography. Gas chromatograph Agilent 7890B with a flame ionization detector was used for this task. It was equipped with a capillary column with a length of 30 m and a diameter of 0.32 mm. The flow rate of the carrier gas (nitrogen) was 1.5 mL/min. Injector temperature was 310 °C. The column temperature was increased from 100 to 150 °C at a rate of 10 °C/min and from 150 to 325 °C at a rate of 3 °C/min, followed by isotherm at 325 °C until the end of the analysis. Compounds were identified using an electronic library “Agilent ChemStation”.

4.6. GC–MS Measurement of Aromatic Fraction

The fraction of aromatics was examined using a gas chromatography/mass spectrometer system, which included the gas chromatograph “Chromatec-Crystal 5000” with a mass selective detector ISQ (USA). The Xcalibur software has been used for processing the results. The chromatograph is fitted with a capillary column, 30 m long, with a diameter of 0.25 mm. The speed of the flow of carrier gas (helium) was 1 ml/min. The temperature of the injector was 310 °C. The temperature program of the thermostat was as follows: temperature increase from 100 to 150 °C at a speed of 3 °C/min and from 150 to 300 °C at a speed of 12 °C/min, followed by its isotherm to the end of the analysis. Electron energy of the mass detector was 70 eV; the temperature of ion source was 250 °C. The compounds have been identified through the electronic library of the NIST spectra database and according to the data of the literary sources.61

4.7. NMR Spectroscopy Measurement of Resins and Asphaltene Fractions

The resins and asphaltenes of initial and upgraded oil were studied by a Bruker AVANCE–III–HD-700 NMR spectrometer. Locking and stalling of the field were performed using the deuterium D2O signal in a glass capillary placed in a 5 mm NMR tube. The fractions of resins were dissolved in deuterochloroform to get a diluted solution. 1H NMR spectra were recorded at 25 °C (pulse program zg30), the acquisition time was 4.7 s, the pre-scan delay was 6.5 μs, the relaxation time between scans was 2 s, the spectrum width was 12.0 ppm (6000 Hz), and 400 scans were accumulated. 13C NMR spectra were recorded using 90 °C pulses with inverse-gate broadband proton decoupling (pulse program zgig). The relaxation time between the pulses was 9 s (and the acquisition time was 3.5 s), the spectrum width was set at 220.0 ppm, and the number of scans was 3200. An exponential digital filter with the 10 Hz parameter line broadening (lb) was applied to process 13C NMR spectra prior to Fourier transformation. The measurements were carried out at a temperature of 25 °C. All NMR spectra were integrated after baseline correction, and an average of at least on-screen integration values were taken for each calculation.62−65

4.8. High-Resolution FESEM Analysis of Coke

FESEM (Merlin Carl Zeiss) equipped with an AZtec X-MAX energy dispersive spectrometer was used to study the formed solid residue (coke) after the HTU. The surface morphology was taken at an accelerating voltage of 5 keV and a probe current of 300 pA. Microprobe elemental analysis was performed at an accelerating voltage of 20 keV. The sounding depth was about 1 μm.

Acknowledgments

F.M. and M.G. acknowledge the support from the Russian Science Foundation (Project # 19-12-00332).

Glossary

Abbreviations

EOR

enhanced oil recovery;

SAGD

steam-assisted gravity drainage

CSS

cyclic steam stimulation

SCW

supercritical water

HTU

hydrothermal upgrading

SARA

saturate, aromatic, resin, and asphaltene

MDPA

mono-, di-, and poly-aromatic

FR

free radical

FTIR

Fourier-transform infrared spectroscopy

EPR

electron paramagnetic resonance

GC

gas chromatography

GC–MS

gas chromatography–mass spectrometry

NMR

nuclear magnetic resonance

FESEM

field emission scanning electron microscopy

ESE

electron spin echo

EDX

energy-dispersive X-ray

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03974.

  • GC chromatograms of saturated hydrocarbons of initial heavy Ashalcha oil and upgraded oil at different times of 15, 30, 60, 90, and 120 min (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c03974_si_001.pdf (178.4KB, pdf)

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