1. Introduction
The last decade have witnessed an increase in heavy oil production utilizing thermal enhanced oil recovery methods as an energy efficiency techniques especially in the presence of fundamentally new class of reagents such as in-situ catalysts of aquathermolysis [
1,
2,
3,
4,
5,
6]. Many previous works and field experiments have demonstrated the effectiveness of such reagents [
7,
8,
9,
10]. Other works, suggest another way of improving the efficiency of in-situ aquathermolysis catalysts by exposing the reservoir to microwave radiation. In other words, the distributed catalyst particles within the formation may stimulate the reservoir coverage in the presence of microwave radiations [
6,
11].
Heavy oils reserves in the Russian Federation are estimated to range from 6 to 7 billion tons among which 55% are unconventional. In fact, heavy oils contain, in addition to resins and asphaltenes, a lot of heteroatoms such sulfur. Russian heavy and extra heavy oil fields are located in the Perm region, Tatarstan, Bashkiria and Udmurtia and their largest fields are presented by Van-Yeganskoe, Severo-Komsomolskoe, Usinskoe, Russkoe, Gremikhinskoe fields and others. It is worthy to note that more than 2/3 of these heavy oil reserves are located at depths of up to 2000 m and the significant parts are located in dense carbonate rocks [
12]. In terms of geological age, about 74% of these reserves belong to the Paleozoic era. Nonetheless, the Mesozoic deposits are characterized by the highest values of oil viscosity meanwhile the lowest viscosity values are observed in the Proterozoic deposits. Moreover, the maximum values of viscosity are characteristic of the transition from one geological era to another. This may be the result of the influence of transgressions and regressions of the world ocean [
13].
What we know about iron compounds is largely based on their wide use for catalytic aims whether in a heterogeneous or homogeneous form. Thus, in alkylation reactions, iron compounds (oxides, spinals, zeolites, and molecular sieves) are highly active and can outperform catalysts based on Al
+3. A radical cation mechanism of benzylation with the intermediate formation of Fe
2+ has been proposed for iron chloride supported on aluminum oxide in the works conducted by Salavati et al. [
14].
In recent years, there has considerable interest in catalysts based on γ-Fe
2O
3 nanoparticles supported on silicas of various structures using a solution of iron acetylacetonate in toluene. These compounds proved to be effective catalysts for a number of chlorolefin conversions, including isomerization of allyl chlorolefins, alkylation of benzene with allyl chloride and benzyl chloride [
15,
16]. In terms of activity and stability, they are in all cases superior to catalysts based on α-Fe
2O
3 prepared from the more common precursor, iron nitrate. For example, in benzylation, the most active samples turned out to be samples on an activated silica gel matrix with a high 18% iron content, which simultaneously contain γ-Fe
2O
3 and Fe
3O
4 nanoparticles with a size of 2–3 nm [
17]. Other uses of iron oxides include their wide use as catalysts in the Fischer-Tropsch synthesis, dehydrogenation, alkylation and other processes [
18,
19]. Moreover, nanosized iron oxides, stabilized on silica or in the bulk of polymer matrices, serve as active and selective catalysts for the conversion of halogenated hydrocarbons [
20,
21,
22]. On another hand, iron oxide catalysts are highly effective in hydrogenation reactions of highly condensed organic matter, such as brown coal [
23,
24], shales [
25], lignin [
26] or heavy oil [
26,
27]. Several authors have shown an increase in the content of light C
10-C
16 alkanes in the treated oil composition relatively to high-molecular homologues C
23-C
29. In addition, there has been shown a significant decrease in high molecular weight region intensity of the so-called naphthenic hump, reflecting the content of isoalkanes that undergo destruction [
28].
Much work on the potential of combining traditional thermal methods and microwave radiation has been carried out because of the promising effect of this application on enhancing heavy oil and bitumen recovery from reservoirs with low thermal conductivity and low permeability [
29]. Besides heavy oil and bitumen reservoirs, microwave treatment can be effective as well for shale rocks as reported by El harfi et al. [
30]. In general terms, an electric field oscillating at a high frequency promotes the cleavage of chemical bonds such as in petroleum pitch with a high sulfur content. The addition of a catalyst leads to the formation of hydrogen sulfide which can be further utilized for absorbing the microwave field. However, there are several works which demonstrated that the greatest ability to absorb microwave field is characterized by asphaltenes [
31,
32]. Taken together, the functioning of the catalysts and the effect of microwave radiation provide deep conversion of resins and asphaltenes, a decrease in the produced oil viscosity, and an increase in oil recovery factor. According to conditions totality for the use of catalytic electromagnetic heating mentioned in literature, the criteria for the applicability of the developed technology are summarized in
Table 1.
The use of microwave exposure is possible on any heavy hydrocarbon feedstock containing resinous-asphaltene substances. The limit value of viscosity indicated in the table is set for the entire array of the analyzed sources. In fact, the minimum value of oil viscosity exposed to microwave heating in the presence of catalysts is reported in the works of Taheri et al. [
32]. It has been shown that nanodispersed catalysts can perform two functions: On one hand, efficiently absorb a microwave field with the generation of thermal energy and on another one provides chemical conversion of heavy hydrocarbons molecules. Actually, the most effective in terms of their ability to absorb microwave fields are graphite, FeS
2, Fe
3O
4, Co
3O
4, NiO
x characterized by a resistivity of 10
−4–10
−2 Ohm m and providing local heating within 1019–1305 °C at an industrial radiation frequency of 2.45 GHz and a power of 800 watts. Our knowledge of the effect of the dispersion of catalyst particles on the process of converting microwave radiation into thermal energy and the influence of the electric and magnetic components of the electromagnetic field on the process of chemical conversion of hydrocarbons is largely based on very limited data. The aim of the present research was thus to study further this subject by determining the most effective catalysts with the ability to absorb the microwave field. These catalysts have been found to be primarily oxide-sulfide compounds of a number of transition metals such as Fe
3O
4, NiO
x, NiS
2, FeS
2, etc. This work will be devoted for investigating the magnetite effect on microwave absorption. According to the literature data, magnetite has the greatest ability to absorb the microwave field. The resistivity of magnetite is 10
−4–10
−2 Ohm·m. which makes the heating reaches 1258 °C near the particles [
33].
3. Results and discussion
The composition of non-hydrocarbon gases is shown in
Table 4. In all four experiments, oxygen, nitrogen, and carbon dioxide were identified. It has been found that the presence of a magnetite catalyst leads to a significant increase in oxygen content. Moreover, the highest carbon dioxide content was observed in the longest experiment (No 4).
In addition, carbon monoxide and hydrogen were not recorded in all experiments. It is worthy to note that carbon dioxide was the predominant component in gas products. However, the results showed no correlation in the content of carbon dioxide from the conditions of microwave exposure and the presence of a catalyst. In the presence of catalyst in experiments No 1 and No 3, gases of the methane series C5 and higher prevailed, among which—propane, n-butane, i-butane, n-pentane, i-pentane, etc have presented the significant amount. Besides, the methane content was several times higher in experiments No 1 and No 3 in the presence of catalyst compared to the non-catalytic experiment. What’s more, the content of ethane and ethylene naturally increased in a series of experiments No 1 and No 3 as the radiation power decreases. From another hand, the presence of the catalyst leads to significant increasing in the content of pentane molecules. In contrast, the content of butane molecules practically was independent of experimental conditions meanwhile the propane content was at maximum in the experiment without catalyst and it has been found that the largest amount of saturated hydrocarbon gases is formed in the experiment No 3 in the presence of the catalyst.
The oil containing rock extracts have been analyzed after experiments of microwave exposure, and the obtained SARA group analysis has been presented in (
Table 5).
The original composition of oil is characterized by a high content of resinous-asphaltene substances (more than 50 wt.%) and a low content of saturated hydrocarbons (17.2 wt.%). In all scenarios of microwave processing, an increase in the content of saturated hydrocarbons by 2 wt.% or more is achieved. At the same time, the content of aromatic hydrocarbons decreases in all cases, but to varying degrees. To the greatest extent, the content of aromatic hydrocarbons was observed for the longest experiment (No. 4)—up to 17.0 wt.%. Our data indicate that the lightest fractions have been removed from the rock sample as a result of microwave energy conversion into thermal energy which led to an increase in the content of heavy components. What’s more, the obtained results showed a decrease in the content of asphaltenes by 1.3–1.6 times in all experiments with catalysts (No2). This has been provided with a commensurate increase in the content of resins. It should be noted that the ratios of saturates to aromatics is 1.14 for catalytic process. This result is more effective than other alternative noncatalytic methods [
35].
The obtained data indicate the occurrence of the destructive hydrogenation process at the carbon-heteroatom bonds in the asphaltene structure. In this case, a part of the aromatic hydrocarbon fraction can act as a hydrogen donor with a corresponding decrease in solubility and their transfer to the resin fraction. The result of a variety of chemical reactions is a decrease in the content of asphaltenes and a decrease in their molecular weight, which increases the filtration capacity of the oil fluid in the porous medium of the reservoir rock. Previously, filtration experiments were carried out for such catalysts [
9]. this effect can be explained by the destruction of asphaltene aggregates and a decrease in their molecular weight [
36]. As a result, the rheological parameters of the oil change [
37].
A wide range of literature has reported that oil yield increases in the presence of iron or nickel oxides nanoparticles as a result of asphaltenes destruction [
38,
39].
In order to determine the content of non-extractable organic matter in the composition of the oil-containing rock and the effect of the microwave field in the presence of catalyst on the possible additional generation of coke-like substances like carbene-carboids (compaction products during asphaltene conversion reactions), thermogravimetric study of rock samples after oil extraction was carried out. Weight loss during thermal analysis of the extracted rock samples is presented in
Table 6 and the obtained general thermograms are presented by
Figure 3 and
Figure 4.
It has been found that the absence of the catalyst causes the least increase in the content of non-extractable organic matter in the rock. This fact is related to the intensive processes of resinous-asphaltene compounds destruction especially at the level of peripheral groups which are the most condensed fraction, and hence leads to a decrease in their solubility in the organic medium and eases their adsorption on the mineral skeleton surface.
4. Conclusions
A series of experiments was carried out on a sample of oil-containing rock under various modes of microwave exposure in the presence and absence of nanoscale magnetite. The advantage of this laboratory design (with a small amount of substance loaded) was the ability to quickly reach the temperature necessary for the pyrolysis reaction at a relatively low power level. It should be noted that a feature of the experimental setup used was a significant inhomogeneity of the microwave field in the reactor volume, leading to inhomogeneity of heating inside the reactor volume. In order to ensure more uniform heating, it is necessary to optimize the electrodynamic input system of the reactor radiation from the point of view of reducing the reflected power from the study object. The composition of gaseous products of oil transformation in the composition of the rock has been studied in addition to identifying oxygen, nitrogen, and carbon dioxide in all four experiments. It has been found that oxygen content increases noticeably in the presence of magnetite catalyst and the highest carbon dioxide content has been observed in the longest experiment. Moreover, the methane content was found to be several times higher in the presence of catalyst compared with the experiment in its absence. On the other hand, the content of ethane and ethylene naturally increases in a number of experiments as the radiation power decreases. The largest amount of saturated hydrocarbon gases was found to be formed in the experiment with a catalyst.
The group composition of the extracted oil was studied after the experiments, and it has been found that the original composition of oil is characterized by a high content of resinous-asphaltene substances—more than 50 wt.% and a low content of saturated hydrocarbons—17.2 wt.%. In all scenarios of microwave processing, an increase in the content of saturated hydrocarbons by 2 wt.% or more is achieved. At the same time, the content of aromatic hydrocarbons decreases in all cases, but to varying degrees. It has been shown that the content of aromatic hydrocarbons to a greatest extent, was associated with the longest experiment—up to 17.0 wt.%. As a result of the conversion of microwave energy into thermal energy, the lightest fractions were removed from the rock sample and the content of heavy components increased. What’s more, the obtained results showed a decrease in the content of asphaltenes by 1.3–1.6 times in all experiments with catalysts (No 2). This has been provided with a commensurate increase in the content of resins. The obtained data indicate the occurrence of the destructive hydrogenation process at the carbon-heteroatom bonds in the asphaltene structure. In this case, a part of the aromatic hydrocarbon fraction can act as a hydrogen donor with a corresponding decrease in solubility and their transfer to the resin fraction. The result of a variety of chemical reactions is a decrease in the content of asphaltenes and a decrease in their molecular weight, which increases the filtration capacity of the oil fluid in the porous medium of the reservoir rock.
The content of non-extractable organic matter in the rock sample after experiments and after oil extraction was determined. It has been found that the absence of the catalyst causes the least increase in the content of non-extractable organic matter in the rock. This fact is related to the intensive processes of resinous-asphaltene compounds destruction especially at the level of peripheral groups which are the most condensed fraction, and hence leads to a decrease in their solubility in the organic medium and eases their adsorption on the mineral skeleton surface.