The Liquid Phase Oxidation of Light Hydrocarbons for Thermo-Gas-Chemical Enhanced Oil Recovery Method

Abstract

Heavy oil and natural bitumen resources in carbonate formations are huge and considered as the promising alternative energy resource to the conventional crude oils. However, the production of such resources is challenging due to the low permeability, high viscosity and significant content of resins and asphaltenes in the composition of heavy oil and natural bitumen. The combination of thermal, chemical and gas enhanced oil recoveries can be a promising method to unlock and upgrade heavy oil and natural bitumen in carbonate reservoirs. In this paper, we propose a novel in-situ liquid-phase oxidation of light hydrocarbons for a revolutionary thermo-gas-chemical enhanced oil recovery method, which can be applied in carbonate heavy oil reservoir formations. It is assumed that the oxidation process is carried out in a downhole well reactor, the products of which are a high temperature mixture of organic carboxylic acids and organic solvents. Here, we present the results of laboratory investigations of liquid-phase oxidation of n-hexane as a model compound imitating associated petroleum gases in the presence of Fe, Cr and Ni catalysts, which were introduced in the form of oil-soluble catalyst precursors. It was revealed that the oxidation process yields hydro peroxides, organic carboxylic acids (acetic, propionic and valeric acids), alcohols and ethers. The products of the oxidation process were justified by the results of FT-IR and GC-MS analysis methods. According to the results, Cr-based catalyst leads to the increase of CH₃-groups in the products. The oxidation process in the presence of nickel-based catalyst is compared with a control sample. The naphthalene was detected in the oxidation products of all experiments, the formation of which is explained by polymerization of benzene rings. In its turn, benzene is obtained due to dehydrocyclization of n-hexane on the surface of nanoparticles. However, iron-based catalyst showed the best catalytic performance in low-temperature oxidation of n-hexane in autocatalysis mode as the yield of acetic acid prevailed 52%. The given approach provides prolonged thermal and acid treatment of carbonate formations, where the evolved CO₂ gas will further assist in increasing the mobility of crude oil. Moreover, the produced alcohols, ethers and other hydrocarbons play the role of solvents, which dissolves polar and non-polar components of crude oil.

1. Introduction

Estimated in-place unconventional hydrocarbon resources in the world are more than 200 billion tons. More than 60% of the world’s oil reserves are concentrated in heterogeneous carbonate reservoirs and significant volume of reserves corresponds to the heavy and extra-heavy oil [1]. The share of unconventional hydrocarbon resources in Russia is 67%, which is concentrated in the Bazhenov and Tyumen formations, in the Achimov strata in the Khanty-Mansi Autonomous district (KhMAD), and in Domanic shale sweets located in Volga-Ural basin [2,3,4,5]. In this context, it is relevant to study and develop new and effective EOR/IOR techniques, as well as adapting the emerging oil recovery methods for such unconventional and complex reservoir [6,7,8]. The complexity of such reservoirs is due to highly heterogeneous and non-uniform properties within small sections of the reservoir, which make them difficult to characterize. Fracture channels often exist that range from 10 s to 1000 s of meters representing primary oil production pathways. These channels are characterized by high permeability comparing to the surrounding rock matrix, asserting significant impact on sweep efficiency. Moreover, the production of crude oils from such reservoirs is further complicated by significant content of paraffins, resins and asphaltenes, concentration of which determines the viscosity of crude oil. The low-permeable porous rocks can be improved by acidizing methods [7,8], while the issue of low mobility can be solved either by injection of solvents or steam [6]. However, the combined injection of acids with solvent or steam is challenging.

In recent years, energy, ecology and industrial communities focus on decarbonization processes, particularly on utilization of Associated Petroleum Gases (APG) and Wide Cut of Light Hydrocarbons (WCLH), which have cost-effective and environmental consequences. It is reported that associated petroleum gas flaring in oil field sites stands for 1.0 % of all global greenhouse carbon dioxide emissions, on the other hand, burning valuable nonrenewable natural resources. APG is a mixture of light hydrocarbons (methane, ethane, propane, butane and other hydrocarbons), which are dissolved in crude oil in reservoir conditions [9]. Its content varies from three to thousands of cubic meters per ton of crude oil. In-situ oxidation of APG and WCLH in downhole reactor produces a mixture of organic acids, high-temperature hydrocarbon solvents, which combines acidizing, solvent injection and thermal recovery techniques [10].

Application of natural gas and associated petroleum gases in exothermic oxidation processes is very attractive due to the lack of conventional hydrocarbon resources. These gases are used not only in chemical industry, but also in enhanced oil recovery methods [11,12].

Many studies have been published on the application of transition metal compounds such as oxides and salts of manganese, cobalt, nickel, iron, copper, etc. for catalysis oxidation processes [13,14]. Metal salts may be water-soluble or organic with anion, which is characterized by the residue of organic acids with long aliphatic «tails», i.e., naphthenate or stearate [15,16,17,18]. More recent evidences propose bimetallic and trimetallic catalysts instead of mono-metal salts [19,20]. Application of such catalysts reduces the temperature of oxidation reactions. Hence, the selectivity of the oxidation products increases. For instance, authors of the article report the results of carried out n-butane oxidation under reservoir conditions (160–190 °C and 6 MPa) in the presence and absence of cobalt or manganese salts [21]. The high conversion degree and significant yield of acetic acid was observed after introduction of the given oxidation catalysts. In patent [22], the authors analyzed the process of butane conversion into the acetic acid in the presence of cobalt compounds (Co content varied from 0.5 to 20 wt%) at temperatures of 95–120 °C and pressure 0.7–3 MPa. It was found that high conversion degree of butane, as well as high yield of acetic acid was obtained at partial pressure of oxygen equal to 0.004–1 MPa. Colman and co-workers carried out liquid-phase oxidation of butane at 120–180 °C and 3.5 MPa in the presence of Co-based catalyst with the concentration of 0.12 wt% [23]. The partial pressure of oxygen varied in the range of 0.2–0.4 MPa. Oxidation processes are carried out in liquid phase by air or oxygen bubbling through initial organic reagent in which reaction products are gradually accumulated. The pressure is controlled in order to supply the reactants in liquid state.

In the given paper, this study, we evaluated the performance of liquid-phase oxidation of n-hexane imitating Wide Cut of Light Hydrocarbons (WCLH) in the presence of transition metal catalysts in high pressure and high temperature (HPHT) reactor in terms of producing organic acids, alcohols and ethers, which are considered as valuable products in thermo-gas-chemical treatment of heavy oil saturated carbonate formations.

There are several reasons for choosing n-hexane as a model of WCLH for oxidation processes:

-

The oxidation of solvents depends on the length of hydrocarbon chains: with an increase in the length of the hydrocarbon chains, its reactivity and oxidation ability improves. For instance, the oxidation of methane under atmospheric pressure initiates at 420 °C, ethane at 285 °C and propane at 270 °C. The increase in pressure reduces the initial oxidation temperature, i.e., methane at 10 MPa reacts with oxygen already at 330 °C. In the series CH₄ < C₂H₆ < C₃H₈ < C₅H₁₂–C₈H₁₈, the rate constant of hydrocarbon oxidation also increases, which suggests that with an increase in the molecular weight of hydrocarbons, oxidation reactions should be carried out at lower temperatures and pressures [24,25]. From propane to octane, the number of possible reactions that occur during the oxidation of corresponding alkane increases sharply because of isomerism. Therefore, oxidation of more heavier aliphatic hydrocarbons produces a wide range of products–oxygenates [26];

-

n-hexane is available and widely applied solvent in laboratory investigations;

-

Solubility of oxygen in hydrocarbons: oxygen is better dissolved in n-hexane than higher n-alkanes (Figure 1a), and aromatic hydrocarbons, alcohols and ethers (Figure 1b) [27].

2. Materials and Methods

The catalytic (Fe-, Cr- and Ni-based catalyst precursors) and non-catalytic oxidation (control experiment) of n-hexane was carried out in the presence of inert silica to prevent explosive consequences of reaction at temperature 180 °C and pressure 50 bar with continuous injection of synthetic air into the system. The catalyst precursors were dissolved in hexane with a concentration of 0.5% wt. The synthesis and formation of active form of catalyst precursors are discussed in detail in our previous publications [16,28,29]. The volumetric ratio of silica and n-hexane was 2:1. The schematic illustration of experimental set-up is presented in Figure 2. The gaseous products were condensed by condenser, and additionally in Dewar condenser, the refrigerant of which was liquid nitrogen with boiling point temperature of −197 °C. The condensed liquid, which is considered as oxidation products, was analyzed by FTIR spectroscopy and GC-MS. The oxidation products were composed of water and organic phases, as well as extracts after extraction of silica in soxhlet by hot chloroform solvent (Figure 2). The products were considered as an object of investigation and further analyzed in detail by analytical analysis methods.

IR spectra were recorded on a PERKIN ELMER Spectrum two with a UATR (Single Reflection Diamond). A total of 50 μL of a sample was placed on the diamond disk. In order to identify the necessary functional groups, dried until absorption bands from water disappear and then the spectrum was recorded from 4000 to 450 cm⁻¹ with a resolution of 4 cm⁻¹. The acquired data was corrected to the background spectrum.

Chromatograms of the products of oxidation were recorded using a chromato-mass-spectrometric system, including a Chromatec-Crystal 5000 gas chromatography with an ISQ mass-selective detector, Xcalibur software for processing the results. The chromatography was equipped with a capillary column 30 m long and 0.25 mm in diameter. Gas flow rate (helium) was 1 mL/min. Injector temperature was 310 °C. For the thermostat temperature program, there was a temperature rise from 100 to 300 °C at a rate of 3 °C/min followed by an isotherm until the end of the analysis. The voltage of the ion source was 70 eV, the temperature was 250 °C. Compounds were identified using the NIST electronic library of mass spectra and according to the literature data [30].

3. Results and Discussion

3.1. The Pressure and Temperature Profiles

The pressure and temperature profiles during oxidation of n-hexane are presented in Figure 3.

The process of liquid-phase oxidation of n-hexane in the absence of a catalyst proceeds more calmly in comparison with catalytic experiments. The rise in temperature and pressure in case of non-catalytic oxidation is relatively gentle. Hence, the oxidation of hydrocarbons is characterized by the long induction period due to the low selectivity to the value-added products [31].

However, all experiments were characterized by ramped temperature curve above 180 °C at the beginning of the experiment, which indicates conduction of autocatalytic oxidation process. The sharp increase in pressure and temperature was due to the formation of hydroperoxides (HPs) as the result of oxygen and n-hexane interaction with the bond cleavage in the molecules of the latter [32]. According to the chain reaction theory oxygen molecules join to hydrocarbons with the cleave of sole bondage and formation of HPs as the intermediates. HPs are unstable products and thereby under the oxidation temperature and in the presence of catalysts decompose with the -O-O- bonds. The dissociation energy of the given bond is equal to 30–40 kcal/mol. The obtained radicals after recombination reactions form molecular oxidation products [18,33]. The general mechanism of thermal oxidation reaction of hydrocarbons with oxygen is summarized by the following steps:

(1) Initiation of chains:	${\mathrm{RH}+\mathrm{O}}_2\to \mathrm{R}\bullet {+\mathrm{HO}}_2^{\bullet }$
	${2\mathrm{RH}+\mathrm{O}}_2\to 2\mathrm{R}\bullet {+\mathrm{H}}_2{\mathrm{O}}_2$
(2) Chain continuation:	${\mathrm{R}}^{\bullet }{+\mathrm{O}}_2\to {\mathrm{RO}}_2^{\bullet }$
	${\mathrm{RO}}_2^{\bullet }+\mathrm{RH}\to {\mathrm{ROOH}+\mathrm{R}}^{\bullet }$
(3) Branching of chains:	$\mathrm{ROOH}\to {\mathrm{RO}}^{\bullet }{+\mathrm{HO}}^{\bullet }$
	$2\mathrm{ROOH}\to {\mathrm{RO}}_2\bullet {+\mathrm{RO}}^{\bullet }{+\mathrm{H}}_2\mathrm{O}$
(4) Chain termination:	${2\mathrm{R}}^{\bullet }\to {\mathrm{P}}_1$
	${\mathrm{R}}^{\bullet }{+\mathrm{RO}}_2^{\bullet }\to {\mathrm{P}}_2$
	${2\mathrm{RO}}_2^{\bullet }\to {\mathrm{P}}_3$
Where, RH–hydrocarbon, ${\mathrm{R}}^{\bullet }$–alkyl radical, ${\mathrm{RO}}^{\bullet }$–alkoxy radical, ${\mathrm{RO}}_{2 }^{\bullet }$–peroxide radical, $\mathrm{ROOH}$–organic hydroperoxide, ${\mathrm{HO}}^{\bullet }$–hydroxyl radical, P1, P2, P3–molecular products.

Some authors report that polycondensation occurs following vaporization. Then, the remaining products decompose under cracking and oxidation reactions. These reactions under high pressure exhibit exothermic behavior [34].

However, the introduction of the transition metal-based catalysts intensifies the given process and reduces the induction period. This leads the reaction toward the formation of value-added oxidation products [35]. The chain oxidation is carried out by radical mechanism in the presence of oxygen and catalyst in the system:

ROOH + Meⁿ⁺ → ROO• + Me⁽ⁿ⁻¹⁾⁺ + H⁺

RH + Meⁿ⁺ → R• + Me⁽ⁿ⁻¹⁾⁺ + H⁺

The activity of the metals within the scope of our study was observed in the following order: Fe > Cr > Ni. It is important to note that iron-based catalyst supports the process of low-temperature oxidation of n-hexane in the autocatalysis mode. Nickel catalyst shows similar performance with control experiment, except the earlier formation of HPs.

The oxidation products of n-hexane through HPs may be alcohols, aldehydes, ketones and organic acids.

According to the oxidation reaction reported in [35], C-C bonds are cleaved:

RCH₂CH₂R + 2.5O₂→ 2RCOOH + H₂O

The destruction of C-C bonds during oxidation of n-paraffins is carried out mainly between secondary hydrocarbon atoms. Therefore, mainly acetic acid is obtained from n-alkane, from by products–methyl ethyl ketone and ethyl acetate. However, ketones are considered as acid precursors. The proposed scheme of carbonic acid yield during oxidation of n-alkanes in case of n-hexane:

C₆H₁₄ + 2.5O₂ → C₃H₇COOH + CH₃COOH + H₂O

C₆H₁₄ + 2.5O₂ → 2C₂H₅COOH + H₂O

C₆H₁₄ + 4.5O₂ → 2CH₃COOH + 2HCOOH + H₂O

The proposed general reaction:

3C₆H₁₄ + 9.5O₂ → C₃H₇COOH +2C₂H₅COOH + 3CH₃COOH + 2HCOOH + 3H₂O

In addition, the catalysts (most of all Fe-based catalyst) ease the transfer of oxygen vacancies between the main bulk and the surface of catalyst particles. Thus, increasing the activity of the catalysts. Some authors also report the enhancement of catalytic performance, most likely because of oxygen vacancies and varieties of Fe³⁺, Fe²⁺, Ni²⁺, Cr³ and Fe₂O₃, Fe₃O₄, NiO, Cr₂O₃ [36].

These substances were identified in the oxidation products by IR-spectroscopy and GC-MS analysis. To better understand objects, the transcription of legends corresponding to FT-IR and GC-MS graphs is presented in

Table 1. Transcription of shortcuts used in FT-IR and GC-MS spectra.

+Me	Water Phase	Organic Phase	Extract
Metals (Fe, Cr, Ni)	Aqueous phase of corresponding oxidation products accumulated after Dewar condenser	Organic phase of corresponding oxidation products accumulated after Dewar condenser	Liquid-phase organic extracts after extraction of silica in soxhlet by hot solvent

.

3.2. Evaluation of n-Hexane Oxidation Products by FT-IR Spectroscopy

The IR-spectra of catalytic and non-catalytic n-hexane oxidation products (organic and water phases) accumulated after Dewar condenser are presented in Figure 4. The behavior of all spectra is specific for normal-structure alkanes, particularly n-hexane-CH₃(CH₂)₄CH₃ [37]. The absorption band at approximately 3000 cm^–1 is overlapping due to stretching of C-H bonds. The spectra of water phases after catalytic oxidation show some peaks in the region of 3000–2700 cm^–1 corresponding to C-H absorptions in CH₃ and CH₂ [38]. Moreover, the wide peaks in 2500–3500 cm^–1 interval with the maximum peaks in 3300 cm^–1 are characteristic for νO–H absorption in carboxylic group of acids. Especially, the intensity of absorbance is significant in case of Fe and Cr. For the given metals, the significant absorbance in the range of 1705–1675 cm^–1 of water phases, which corresponds to the carbonyl group (C=O) is observed [39].

The peaks in the FT-IR spectra of water phases after oxidation in the presence of Fe and Ni catalysts in the wavenumber ranges of 1460–1400 cm⁻¹ are specific for the absorbance of CH₂–group. However, the given band can overlay the absorbance of aromatic rings including condensed ones (two and more rings) [38]. Characteristic peaks in the ranges of 1100–950 cm⁻¹ of all samples correspond to the formation of simple C-O-C ether groups. The significant amount of ethers is detected in the oxidation products in the presence of Ni catalyst. Moreover, Ni-based catalyst along other catalysts promotes generation of alcohols, the presence of which is justified by the smooth hump in the ranges of 3600–3000 cm⁻¹, which corresponds to the νO-H absorption in the alcohol molecules. It was found that oxidation in the presence of a chromium-based catalyst leads to the shorter-chain acids, which quickly evaporate with water and therefore are poorly detected in IR spectroscopy. Our findings appear to be well substantiated by the FT-IR spectra of silica extracts, which were characterized by absorption in the range of 3000–2750 cm⁻¹ corresponding to the C-H in CH₃ and CH₂ groups.

The FT-IR spectroscopy of catalytic and non-catalytic products, which were adsorbed on the silica surface are presented in Figure 5.

There was νO–H absorbance in acids which was characterized by wide peaks in the region of 2500–3500 cm⁻¹ with the maximum in 3300 cm⁻¹. In the FT-IR spectra of all samples the peak 1460–1440 cm⁻¹ corresponds to the deformational absorbance of CH₂-groups. However, in case of Cr-based catalyst, the number of terminal CH₃ groups increases, which is due to the formation of short-chain acids or the appearance of isomerism in the molecules. However, the given band can also overlap an absorption band of variable intensity, corresponding to the condensed aromatic rings C_ar-C (two adjacent atoms). The proposed idea is justified by the results of GC-MS analysis, where the presence of naphthalene in the extracts was confirmed. In addition, for all samples, a strong peak is observed in the range of 1750–1705 cm⁻¹, which indicates a high content of carbonyl groups (C=O), which indirectly may indicate a high content of acids in the extracts. Two characteristic peaks are observed in the region of absorption bands 890–830 cm⁻¹ in the extract samples obtained after the oxidation of n-hexane in the presence of catalysts based on Fe and Cr. These peaks may correspond to the absorption of O–O peroxides, the presence of which indicates the amount of unreacted HPs residues. The control extract sample, as well as extract samples in the presence of Cr and Ni, is characterized by the presence of a group of peaks in the region of 1200–900 cm⁻¹, which may be associated with the formation of carboxylic acids esters [40]. Moreover, this band may indicate the formation of anhydrides. However, in the presence of alcohols, anhydrides are unstable and form the corresponding organic acids. Thus, the evidences from this study intimate that addition of Fe-, Cr-based catalysts promote hexane oxidation process with the formation of organic acids, alcohols, and ethers. Although, Ni-based catalyst leads the latter product to a greater extent.

3.3. The Composition of n-Hexane Oxidation Products by GC-MS

The liquid oxidation products (adsorbed in silica in Figure 6 and

Table 2. The composition of liquid oxidation products in the presence and absence of catalysts for silica extracts (estimated from the GC-MS spectra provided in Figure 6).

№	Component	Retention Time, min	Content, %
			Blank Sample (without Catalyst)	Catalyst
				Fe	Cr	Ni
1	Formic acid	3.02	1.0	-	1.3	-
2	1-Hexadecanol	3.08	1.6	-	2.8	5.3
3	Acetic acid	3.13	7.7	14.4	3.0	4.0
4	1-Hexanol	3.20	5.3	-	1.6	2.8
5	Propionic acid	3.30	7.3	-	1.8	4.0
6	Valeric acid	3.58	14.7	11.4	10.8	18.9
7	Hexanoic acid	3.71	2.3	10.5	-	7.3
8	Heptanoic acid	3.78	2.3	-	1.6	3.3
9	Nonanoic acid	3.95	2.9	-	4.5	7.0
10	Decanoic acid	4.00	1.1	-	-	-
11	Propionic acid, hexyl ester	4.06	6.5	-	7.7	10.6
12	Undecanoic acid	4.19	4.2	-	-	-
13	Nonanoic acid, hexyl ester	4.23	1.7	-	-	-
14	Dodecanoic acid	4.32	0.9	-	-	-
15	Tridecanoic acid	4.51	1.9	-	-	-
16	2-Hexadecanol	4.58	2.1	2.5	-	-
17	Tetradecanoic acid	4.70	4.9	4.4	2.6	8.4
18	Naphthalene	5.15	31.7	56.8	62.3	28.4
	Total		100	100	100	100

) were further analyzed quantitatively via GC-MS spectroscopy; the results of which were in accordance with FR-IR results.

According to the GC-MS spectra of extracts, the significant content of naphthalene (from 30 to 60%) was detected in the products of liquid-phase oxidation of n-hexane in the presence and absence of catalysts, which is in agreement with the results of FT-IR analysis (see Figure 5). We propose that naphthalene was formed due to the polymerization of benzene rings. In its turn, the formation possibility of benzene from n-hexane during its thermal oxidation on the surface of catalysts particles is high enough. One of the paths of benzene formation is dehydrocyclization of n-hexane at temperature in the presence of catalysts [41]:

$${\mathrm{C}}_6{\mathrm{H}}_{14} \stackrel{\mathrm{t}, \mathrm{cat}}{\to }{\mathrm{C}}_6{\mathrm{H}}_6+4{\mathrm{H}}_2$$

Comparison of catalytic and non-catalytic experiments show that generation of various compounds such as alcohols (9.0%), esters (8.2%) and carboxylic acids (approximately 51.0%) after non-catalytic oxidation of n-hexane is higher. The products of catalytic oxidation are characterized by low-molecular components. Particularly, iron-based catalyst is more selective in production of organic carbonic acids with various numbers of hydrocarbon skeleton –n (acetic (14.4%), valeric (11.4%), hexanoic (10.5%), tetradecanoic (4.4%)). The absence of peaks corresponding to alcohols and esters are due to the mild reaction conditions (180 °C and 50 bar) for non-catalytic experiments. The composition of oxidation products in the presence of nickel is similar to the control sample both qualitatively and quantitatively except high molecular acids and esters, which were identified in the composition of catalytic products and correlate with the changes in temperature and pressure profiles (see Figure 3).

The iron-based catalyst showed high performance in oxidation of n-hexane. This catalyst maintained autocatalytic process with tendency of self-increasing temperature up to 250 °C. In addition, the selectivity of the given catalyst in terms of acetic acid is very high in contrast to the Cr and Ni catalysts (14% versus 3% and 4%, correspondingly (see Table 2)).

The GC-MS results of water oxidation products are presented in Figure 7 and

Table 3. The composition of liquid oxidation products (accumulated after Dewar condenser) in the presence of catalysts (estimated from the GC-MS spectra provided in Figure 7).

№	Component	Retention Time, min	Content, %
			Catalyst
			Fe	Cr	Ni
1	Formic acid	4.44	-	2.8	-
2	Acetaldehyde	5.14	1.9	26.7	17.9
3	Methyl formate	5.3	-	3.3	-
4	Propanal	6.08	-	8.0	3.5
5	Methyl ester of acetic acid	6.11	1.8	-	-
6	Methyl alcohol	6.40	26.7	14.1	42.0
7	Acetone	6.86	2.3	20.0	8.2
8	Ethanol	7.19	10.0	5.1	18.6
9	2-butanone	9.00	-	2.9	-
10	1-Propanol	10.58	4.4	-	3.7
11	1-Butanol	14.73	3.2	-	-
12	Acetic acid	24.73	37.7	-	-
13	Benzaldehyde	29.21	0.9	-	-
14	2,5-hexadione	30.98	2.9	-	-
15	Phenol	33.83	3.9	-	-
16	1,1-Ethanediol diacetate	35.09	-	1.8	-
17	Dibutyl ether decanedionic acid	37.55	3.0	8.1	4.9
18	Pyrene	39.18	1.3	-	1.2
	Total		100	100	100

. It was established that iron catalyst showed the highest performance in oxidation processes in contrast to the catalysts based on other metals. The formation of acetic acid (37.7%) was specific for the experiments with the given catalyst.

Moreover, some alcohols, such as propanol and butanol, were detected in the water phase products of oxidation process in the presence of iron catalyst. The ketones such as acetone (20.0%) and 2-butanon (2.9%), and aldehydes were produced in the presence of chromium catalyst. The production of significant amount of methyl alcohol was specific for the experiments with nickel catalyst. The presence of acetaldehyde in the water phase products of oxidation processes in the presence of nickel and chromium catalysts (26.7% и 17.9%, respectively) indicates that they are not active in terms of obtaining carboxylic acids, particularly acetic acid.

Thus, the most effective catalyst in terms of oxidation of light hydrocarbons in case of n-hexane is iron catalyst. It provides the highest yield of useful product-acetic acid, the total yield (extract + aqueous phase) of which in the presence of the latter is 52.1%.

The comparison of Table 2 and Table 3 revealed the differences in the liquid composition of n-hexane oxidation products extracted from silica and accumulated in Dewar condenser. The silica extracts mainly contain organic acids, alcohols, ethers with long hydrocarbon chains and significant content of naphthalene. In contrast to silica extracts, products of accumulated in Dewar condenser are characterized by low-molecular alcohols, aldehydes and ketones in addition to the low-molecular organic acids. Moreover, insignificant content of polyaromatic hydrocarbon compounds–pyrene was observed.

We believe that our research will serve as a first step toward establishing the optimum conditions for catalytic oxidation of WCLH. Under the proposed conditions, the catalytic efficiency of used nanoparticle precursors was confirmed and hence, it is considered as an attractive and feasible method to scale up to field technology. Moreover, the presence of carboxylic acids can provide thermal and acid treatment of a carbonate formation with the release of carbon dioxide, which will further increase oil mobility. The generated weak organic acids will provide a prolonged acidizing treatment, which will improve the hydrodynamic characteristics of the formation and increase the permeability and porosity of the reservoir. Alcohols, ethers and other hydrocarbons can act as solvents that dissolve and dilute the polar and non-polar components of oil. Thus, the viscosity of oil can be reduced and the oil recovery is enhanced.

4. Conclusions

In this study, we demonstrated the application of the products of in-situ catalytic (based on Fe, Cr and Ni) oxidation process of light hydrocarbons in case of n-hexane carried out in a downhole well reactor as a combined thermochemical method to enhance heavy oil recovery from carbonate reservoirs.

A sharp increase in temperature (from room temperature up to 270 °C) and pressure (from atmospheric up to 140 bar) was observed during oxidation processes, which is explained by the formation of hydroperoxides.

The FT-IR analysis of oxidation products showed the presence of the following functional groups: carbonyl (C=O) corresponding to the wavelength interval of 1705–1675 cm⁻¹; O-H oscillations in carboxylic acids groups corresponding to the peak at 3300 cm⁻¹; C-O-C in simple ethers corresponding to the wavelength interval of 1100–950 cm⁻¹. Unreacted hydroperoxides (O-O, 890–830 cm⁻¹) were identified in experiments with Fe and Cr catalysts.

The results of GC-MS analysis justify the results obtained from FT-IR analysis method. The identified oxidation products are the mixture of various chemical compounds such as organic acids (acetic, propionic, valeric, dodecanoic), alcohols (methanol, ethanol, propanol, butanol, hexanol), ketones (acetone and 2-butanone) and various ethers of the upper mentioned acids.

It was established that Cr- and Ni-based catalysts exhibited less activity in the oxidation processes in contrast to the Fe-based catalyst, as significant content of aldehydes (26.7% and 17.9%, respectively) were identified in the composition of the oxidation products. The Cr-based catalyst promotes the formation ketones (the content of acetone–20%), while the NI-based catalyst–alcohols (the content of methanol–42% and ethanol–18.6%).

The Fe-based catalyst is the most effective because of promoting low-temperature oxidation of n-hexane in autocatalysis mode. The yield of value-added product–acetic acid is more than 50%.