Influence of Oil-Soluble Catalyst on Composition and Structure of Heavy Oil from Samara Region Field

Abstract

In this study, an examination was conducted of the influence of iron tallate on the composition and properties of highly viscous oil from the Strelovskoye deposit in the Samara region under thermal–catalytic treatment (TCT). The research revealed that the dynamic viscosity of the oil following TCT at 300 °C, with a measurement temperature of 20 °C, decreased by a factor of 8 in comparison to the initial sample and nearly 4.5 times compared to the control sample at the 96-h mark. The most promising results in reducing the pour point temperature to 7 °C were identified following a 96-h TCT at 300 °C. This reduction was attributed to the decrease in paraffin content facilitated by the presence of the catalyst. According to the ICP-MS results, the extraction of the catalyst with the oil amounted to only 1%. This indicates that during the implementation of TCT within the reservoir, the catalyst is likely to adsorb onto the rock surfaces.

1. Introduction

Due to the escalating global demand for energy resources and the dwindling conventional oil reserves, the production of heavy oil is expected to increase worldwide. Heavy oil constitutes no less than 25% of the world’s total oil reserves, representing a substantial portion of all known hydrocarbon resources. Particularly in regions like the Middle East, significant deposits are often associated with abundant reserves of light oil. While conventional extraction methods allow only a fraction of this oil to be recovered, the need for an effective approach to enhance reservoir oil recovery becomes even more pressing. In this context, steam injection emerges as a widely favored method for improving oil recovery, as it can significantly reduce oil viscosity. With viscosity lowered through high-temperature steam injection, the oil becomes more fluid, facilitating its enhanced extraction from unconventional heavy oil reservoirs [1].

A review of the research highlights the growing emphasis on refining aquathermolysis catalysts. This focus not only concerns the synthesis of innovative catalysts but also the strategic use of secondary products and waste materials from various industries. Notably, researchers are making concerted efforts to introduce catalysts in a nanoscale format, thereby ensuring easier access to the active centers of catalysts for the heavier components of high-viscosity oil [2,3].

Moreover, the presence of both a hydrogen donor and a dispersant plays a pivotal role in the process. As noted by prior researchers [4], a hydrogen source helps to scavenge free hydrocarbon radicals, preventing condensation and thus accelerating the reaction in the desired direction. At the same time, a dispersant aids in transforming the catalyst into an emulsion, ensuring its uniform distribution within the medium and increasing the contact area between the catalyst’s active sites and the heavy oil components. This reduces intermolecular forces, such as van der Waals forces and hydrogen bonds, thereby making the system more dispersed and the reaction more efficient.

In [5], the composition of heavy oil from the Republic of Tatarstan field and the peculiarities of structural and phase characteristics changes of its asphaltenes in a model hydrothermal–catalytic system in the presence of a natural catalyst, hematite (Fe₂O₃), were thoroughly studied. It was found that in the presence of steam, the catalyst intensifies the process of breaking down high-molecular-weight components of heavy crude oil, resulting in the formation of new light fractions. As the experimental temperature increases (210, 250, and 300 °C), and the water content in the reaction system decreases, the aromatics content of the asphaltenes increases, accompanied by an increase in the distance between aromatic layers and polyethylene chain fragments as well as a decrease in the size of the associates and the number of their aromatic layers. This occurs due to the destruction of heteroatomic bonds in asphaltene molecules, resulting in the cleavage of additional peripheral alkyl fragments, the destruction of vanadyl–porphyrin complexes, and an increase in the concentration of free radicals. At a temperature of 300 °C and low water content, asphaltenes undergo cracking, transforming into carbene–carboid compounds and subsequently into coke, which precipitates in the oil as solid particles.

Obviously, the delivery of catalysts in the form of dispersed powders is impossible in the reservoir. The catalyst particles would adsorb on the walls of the injection wellbore, failing to reach the bottomhole zone. Therefore, many researchers are working on the synthesis of catalysts for in situ applications and investigating their effectiveness in enhancing heavy oil recovery. Typically, catalysts are introduced in a nanoscale state or as oil-soluble precursors. Upon decomposition of the precursors in the reservoir, the active form of the catalyst is generated [6]. The active form primarily consists of oxides or sulfides of the corresponding metal.

Due to the varying porosity and heterogeneity of oil-saturated formations, nanoscale catalysts based on transition metals are of particular interest. On one hand, they have a low cost, which is important since catalyst regeneration is not feasible during injection into the reservoir. On the other hand, they possess a developed surface area and good adsorption capacity. Organic salts of the respective metals can serve as the primary precursors, which, upon thermal treatment, form nanoscale particles of the active catalyst [7].

In [8], the researchers proposed a mechanical alloy, NiWMoC, containing crystals of NiC, WMoC, and WC (size ~30 nm), obtained by 240 h of mechanical treatment in a ball mill using metal powders (particle size 5–25 µm) and graphite (particle size 50 µm). Aquathermolysis at a temperature of 200 °C for 24 h, utilizing this precursor, resulted in an increased degree of viscosity reduction by up to 97% with an increase in the duration of the mechanical treatment.

Another possible method of catalyst synthesis is the reduction of iron and nickel chlorides by sodium borohydride in the presence of surface-modified silicon dioxide [9]. The catalyst SiO₂/Fe/Ni consists of silica with an average size of about 12 nm and iron and nickel nanoparticles (Fe/Ni) with sizes in the nanometer range. It was observed that the viscosity of heavy oil (aquathermolysis at 150 °C for 48 h with a catalyst mass fraction of 0.5%) decreased to 77.17% (from 184 to 42 Pa).

In [10], the complex action of mineral additives, hydrogen donors, and nanoscale catalysts based on transition elements Cu and Ni on heavy oil from the Ashalcha field was investigated under steam-thermal conditions in a periodic action laboratory reactor made of stainless steel, in the air atmosphere, within a temperature range of 280–375 °C and maintained at the specified temperature for 2.5 h. Mineral additives were introduced through the water phase at a concentration of 8 % of the oil. The research results showed that under the influence of thermobaric and catalytic factors, light hydrocarbons, including n-alkanes, were formed. At the same time, the presence of a hydrogen donor led to a reduction in the content of resins and asphaltenes and a decrease in the viscosity of the converted oil. However, the application of such nanocatalytic systems in reservoir conditions requires the study of their flow in porous media under the influence of forces of various natures and their deposition on particles of rocks with different mineral compositions.

The higher activity of inorganic metal compounds formed from oil-soluble precursors is presumably due to their smaller particle size [11]. This, combined with the solubility of such precursors in oil, allows them to penetrate small oil-saturated pores and spread throughout the reservoir along with the oil, unlike metal salts in the form of an aqueous solution. In this regard, a significant amount of research is devoted to the application of oil-soluble catalyst precursors such as oleates, naphthenates, acetylacetonates of molybdenum, iron, nickel, copper, iron complex compounds, and surfactants [12,13]. In all of the mentioned studies, a decrease in viscosity, density, molecular weight, sulfur content, resins, and asphaltenes, as well as an increase in hydrogen content and the formation of light hydrocarbons, was observed. In [14], catalyst precursors were used in combination with hydrogen donors and precursors, exhibiting a synergistic effect.

In [15], an oil-soluble catalyst was synthesized to catalyze the pyrolysis reaction of heavy oil in the presence of rock minerals, aiming to achieve an effective viscosity reduction and reduce production costs. By examining the reaction mechanism of a model compound along with the analysis of the aqueous phase, it was clearly demonstrated that depolymerization between macromolecules, chain breakage of heteroatoms, hydrogenation, ring-opening, and other effects primarily occur during the reaction. Consequently, these processes weaken the van der Waals forces and hydrogen bonds in asphaltene molecules, inhibiting the formation of a network structure in heavy oil and, as a result, effectively reducing its viscosity.

Moreover, the authors of [16] investigated the effect of dispersed catalysts on the hydrocracking of vacuum gas oil (VGO). The impact of oil-soluble compounds based on molybdenum, iron, and cobalt was assessed both in the presence and absence of a commercial first-stage hydrocracking catalyst, W−Ni/Al₂O₃−SiO₂. The authors identified a mechanism whereby high-molecular-weight molecules undergo cracking, either thermally or on the acidic sites of the applied catalyst. These disrupted molecules are not fully hydrogenated by the catalyst and instead react with one another, forming coke on its surface. As a result, this leads to the blockage of active sites and catalyst deactivation. However, the presence of dispersed active phases significantly enhances the hydrogenation of these coke precursor molecules, thereby reducing coke production.

In addition, the authors of [17] extensively discussed the progress made and key issues in catalyst development. It was found that the addition of an appropriate catalyst can provide an irreversible mechanism for catalyzing hydrodeoxygenation, hydrodesulfurization, and hydrodenitrogenation through the breakdown and removal of O, S, and N derivatives in oil, thus presenting a promising approach for catalytic conversion. Furthermore, the catalytic material must possess exceptional capabilities for breaking sulfur compounds, making it more prone to polymerization.

Finally, using an oil-soluble cobalt organic salt as a catalyst and formamide as a hydrogen donor, the authors of [18] investigated the aquathermolysis reaction of heavy oil by simulating steam-thermal conditions. The role of the hydrogen donor in affecting the viscosity, group composition, and component content of heavy oil was analyzed. FTIR spectroscopy was employed to examine structural changes in heavy oil after catalytic aquathermolysis with a hydrogen donor. The results clearly demonstrated that with an increase in the mass fraction of the hydrogen donor, the viscosity reduction rate increased, and the content of saturated and aromatic hydrocarbons grew, while the content of resins and asphaltenes decreased. Consequently, it was observed that the H/C ratio increased, while the sulfur content decreased.

2. Materials and Methods

2.1. Aquathermolysis Modeling Process in Laboratory Settings via High-Pressure Autoclave

The investigation of the non-catalytic and catalytic aquathermolysis phenomenon was conducted through laboratory modeling, utilizing a 300 mL high-pressure reactor manufactured by Parr Instruments (Moline, IL, USA). The process was carried out under inert conditions (nitrogen purging for 15 min prior to the experiment) to eliminate any oxidative reactions. The temperature in the reactor was maintained at up to 350 °C with an accuracy of 1 °C. Pressure was up to 138 bar with an accuracy of 0.1 bar, with stirrer rotation from 0 to 600 rpm with an accuracy of 1 rpm. The control experiments were performed without a catalyst, using a mixture of oil and water in a 70:30 ratio. Additionally, an iron tallate catalyst [19] was introduced along with the solvent RASPO-1 at concentrations of 0.4% by metal and 4% by weight of the oil, respectively. For an intensified process, 2% RASPO-1 and 2% hydrogen donor C4-155/205 (White-Spirit) were used at a temperature of 300 °C. The emulsion underwent thermal treatment for varying durations (ranging from 24 to 96 h) at both 250 °C and 300 °C, with daily intervals, all while maintaining a constant initial pressure of 8.5 atm.

Significantly, the RASPO-1 solvent serves as a dispersant for asphaltene–resin–paraffin deposits (ARPD) enriched with wetting agents, specifically designed to address the high solid paraffin content in the oil from the Strelovskoye field. By enhancing the oil’s thermal conductivity and inhibiting paraffin crystallization, dispersants play a crucial role in the process. Comprising a nonionic block copolymer of ethylene oxide and propylene oxide, RASPO-1 functions effectively as a solution. Moreover, RASPO-1 acts as an additive to the solvent, facilitating the removal of ARPD from oil wells and the surrounding area near the reservoir’s wellbore.

At the conclusion of the aquathermolysis process, both with and without the catalyst and solvent, the separation of oil from water was achieved through centrifugation using an Eppendorf 5804R laboratory centrifuge. The centrifuge operated at 5000 rpm for 120 min, effectively facilitating the extraction of the oil phase. The corresponding sample codes are provided in

Table 1. Codes of samples.

Sample	Code
Oil after TST 250 °C and 24 h	Str-K-250-24
Oil after TST with iron tallate 250 °C and 24 h	Str-Fe+RASPO-1-250-24
Oil after TST 250 °C and 48 h	Str-K-250-48
Oil after TST with iron tallate 250 °C and 48 h	Str-Fe+RASPO-1-250-48
Oil after TST 250 °C and 72 h	Str-K-250-72
Oil after TST with iron tallate 250 °C and 72 h	Str-Fe+RASPO-1-250-72
Oil after TST 250 °C and 96 h	Str-K-250-96
Oil after TST with iron tallate 250 °C and 96 h	Str-Fe+RASPO-1-250-96
Oil after TST 300 °C and 24 h	Str-K-300-24
Oil after TST with iron tallate 300 °C and 24 h	Str-Fe+RASPO-1+W-S-300-24
Oil after TST 300 °C and 48 h	Str-K-300-48
Oil after TST with iron tallate 300 °C and 48 h	Str-Fe+RASPO-1+W-S-300-48
Oil after TST 300 °C and 72 h	Str-K-300-72
Oil after TST with iron tallate 300 °C and 72 h	Str-Fe+RASPO-1+W-S-300-72
Oil after TST 300 °C and 96 h	Str-K-300-96
Oil after TST with iron tallate 300 °C and 96 h	Str-Fe+RASPO-1+W-S-300-96
Oil after TST with nickel tallate 300 °C and 24 h	Str-Ni+RASPO-1+W-S-300-24
Oil after TST with nickel tallate 300 °C and 48 h	Str-Ni+RASPO-1+W-S-300-48
Oil after TST with nickel tallate 300 °C and 72 h	Str-Ni+RASPO-1+W-S-300-72
Oil after TST with nickel tallate 300 °C and 96 h	Str-Ni+RASPO-1+W-S-300-96

.

2.2. Determination of Viscosity Characteristics

The viscosity–temperature characteristics of the Strelovskoye field oil were evaluated utilizing the FUNGILAB Alpha L rotational viscometer. This equipment was outfitted with a temperature-controlled jacket adapter, ensuring precise temperature regulation during the assessment. The desired analysis temperature was impeccably attained by maintaining the jacket’s temperature using the HUBER MPC K6 cooling thermostat.

For viscosity determination, a fitting spindle tailored to the viscosity range of the Strelovskoye field oil was chosen for integration with the FUNGILAB Alpha L rotational viscometer. The selected TL6 spindle encompassed a measurement range spanning from 20 to 200,000 mPa·s. The viscosity measurements were executed by introducing a 7.5 mL oil sample into the viscometer, covering a temperature spectrum ranging from 10 to 60 °C with a step of 10 °C. The RPM was determined for a given temperature and spring torque of 50 to 90%. The relative error of the FUNGILAB viscometer should not exceed ±1.0%. Repeatability was 0.2%.

2.3. Determination of Pour Point and Solidifying Point

Moreover, the determination of the pour point was conducted using an automatic TST-LAB-12 apparatus. This apparatus is specifically designed to analyze and determine the pour point of petroleum products in accordance with ASTM D 6749 and ASTM D 7683. The method involved preheating the oil sample and subsequently cooling it at a controlled rate until a temperature was reached at which the sample remained immobile. This temperature was then recorded as the pour point.

To determine the pour point and solidifying point, a 5 mL sample of oil was introduced into a small cylinder with a volume of 9 mL. The prescribed temperature program involved heating the sample to 45 °C, followed by cooling at a rate of −3 °C/min until reaching 20 °C. Subsequently, the cooling rate was adjusted to −1 °C/min to identify the temperatures at which the pour point and solidifying point occurred. The accuracy of temperature control for the sample was ±0.5 °C, with a temperature measurement range of −85 °C to +51 °C. The cooling bath temperature was maintained by an integrated refrigeration system.

2.4. Determination of Elemental Composition Using ICP-MS

The elemental composition of the oil samples was analyzed using an inductively coupled plasma mass spectrometer (ICP-MS) iCAP Qc (ThermoFisher Scientific, Dreieich, Germany). A precisely weighed portion of 250 mg from the oil sample was transferred into a Teflon autoclave using analytical balances with a precision of 0.1 mg. Inside the autoclave, 1 mL of concentrated hydrochloric acid (38%, extra pure) and 6 mL of concentrated nitric acid (68%, HNO₃, extra pure) were added using dosing devices after undergoing additional purification procedures. A blank sample, prepared without the oil sample and using the acid mixture only, was also included to account for background levels. The sealed Teflon autoclaves were then placed in an Ethos Up microwave digestion furnace (Milestone, Milan, Italy), where they were heated to 210 °C for 30 min, followed by a 10-min hold at this temperature. After cooling, the autoclaves were opened, and the resulting solution was quantitatively transferred into a flask and diluted to 50 mL with deionized water. A 3.5 mL aliquot from this solution was then further diluted to 10 mL with deionized water. The solution was analyzed on the mass spectrometer after adding an internal standard solution containing 5 ppb of In and bringing the final acid concentration to 3.2%. Prior to analysis, the mass spectrometer was calibrated using multi-element standards, with concentrations ranging from 1 to 100 ppb for each element. The concentration values obtained were recalculated to the initial concentration, taking into account the blank sample, sample mass, and solution dilution.

2.5. Employing MALDI Mass Spectrometry

The determination of molecular masses for asphaltenes and resins was accomplished through the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. MALDI, a gentle ionization technique, offers the ability to ionize substantial molecules without causing their degradation. This method hinges on the introduction of an ancillary substance, known as a “matrix”, which plays a pivotal role in mitigating the detrimental impact of laser emissions and facilitating the ionization of the target substance.

For MALDI mass spectroscopy, the state-of-the-art ULTRAFLEX III mass spectrometer by Bruker (Bruker Daltonics GmbH, Bremen, Germany) was enlisted. The following conditions were maintained during the mass spectrum recording: a Nd:YAG laser, emitting at a wavelength of 355 nm. The experiments were conducted under vacuum conditions spanning from 10⁻⁶ to 10⁻⁸ mbar (with 6.7 × 10⁻⁷ mbar in the source and 9.7 × 10⁻⁸ mbar in the analyzer). The temperature management was meticulous, with room temperature (20 °C) applied during standby and increased to 500 °C during data acquisition. The acquisition mode was set as linear, with no accumulation of mass spectra. Additionally, a metallic target was used in conjunction with 2,5-dihydroxybenzoic acid (DHB) as the matrix.

Throughout the investigation, the spectral data were procured, portraying Gaussian distributions of the molecular masses of compounds present in the analyzed samples. The molecular mass of the target substance was deduced by identifying the maximum peak in the spectra.

3. Results

3.1. Determination of Viscosity Characteristics

Figure 1 and Figure 2 illustrate the viscosity–temperature characteristics of the initial oil and oil samples after thermocatalytic treatment (TCT) with and without iron catalyst at a temperature of 250 °C and treatment durations of 48–96 h.

3.2. Results of Solidification Temperature and Pour Point Determination

Table 2. Results of solidification temperature and pour point determination for non-catalytic and catalytic aquathermolysis at 250 °C.

Test	Str-K-250-24	Str-Fe+RASPO-1-250-24	Str-K-250-48	Str-Fe+RASPO-1-250-48	Str-K-250-72	Str-Fe+RASPO-1-250-72	Str-K-250-96	Str-Fe+RASPO-1-250-96
Solidifying point, °C	14 ± 0.2	13 ± 0.3	13 ± 0.2	15 ± 0.3	13 ± 0.2	13 ± 0.3	12 ± 0.2	13 ± 0.3
Pour point, °C	15 ± 0.2	14 ± 0.3	14 ± 0.2	16 ± 0.3	14 ± 0.2	14 ± 0.3	13 ± 0.2	14 ± 0.3

and

Table 3. Results of determining the pour points and fluidity of oil from non-catalytic aquathermolysis at 300 °C.

Test	Str-K-300-24	Str-Fe+RASPO-1+W-S-300-24	Str-K-300-48	Str-Fe+RASPO-1+W-S-300-48	Str-K-300-72	Str-Fe+RASPO-1+W-S-300-72	Str-K-300-96	Str-Fe+RASPO-1+W-S-300-96	Str-Ni+RASPO-1+W-S-300-24	Str-Ni+RASPO-1+W-S-300-48	Str-Ni+RASPO-1+W-S-300-72	Str-Ni+RASPO-1+W-S-300-96
Solidifying point, °C	14 ± 0.1	12 ± 0.2	13 ± 0.3	7 ± 0.2	12 ± 0.3	9 ± 0.1	13 ± 0.2	7 ± 0.3	9 ± 0.1	10 ± 0.2	9 ± 0.1	10 ± 0.3
Pour point, °C	15 ± 0.1	13 ± 0.2	14 ± 0.3	8 ± 0.2	13 ± 0.3	10 ± 0.1	14 ± 0.2	8 ± 0.3	10 ± 0.1	11 ± 0.2	10 ± 0.1	11 ± 0.3

present the results of the solidification temperature and pour point determination for non-catalytic and catalytic aquathermolysis of oil at 250 °C and 300 °C. The solidification temperature of the initial oil was measured at 14 °C.

3.3. Results of the Elemental Composition Determination Using ICP-MS

To evaluate the extent of catalytic metal leaching, a microelement analysis was conducted on the oil samples following 96 h of thermocatalytic treatment at 300 °C. The results of the elemental composition determined by ICP-MS are presented in

Table 4. The results of the elemental composition.

Samples	Fe, ppm	Ni
Initial oil	1.77	105.18
With iron tallate 300-96	8.78	-
With nickel tallate 300-96	-	283.91

.

• Estimated calculation of iron leaching:
• 67.2 g of oil × ((8.78 − 1.77) ppm/1,000,000) = 0.000471072 g of Fe metal.
• The amount of iron carboxylate added to the oil is 1.4 g.
• Molecular weights: M (Fe(C₁₇H₃₃COO)₂) = 618 g/mol; M (Fe) = 56 g/mol.
• Mass fraction of iron in carboxylate: ω (Fe) = 56/618 = 9.1%.
• Mass of iron in the loaded carboxylate:
• 1.4 g Fe(C₁₇H₃₃COO)₂ − 100%
• x g Fe − 9.06%
• x = 0.12684 g.
• The percentage of iron leached into the oil is 0.000471072/0.12684 g × 100% = 0.37%.
• Thus, approximately 0.4% of the catalyst was leached with the oil.
• Approximate Nickel Loss Calculation:
• 67.2 g of oil × ((283.91 − 105.18) ppm/1,000,000) = 0.00017873 g of nickel.
• The amount of nickel carboxylate added to the oil is 1.4 g.
• The molecular weights are as follows: M (Ni(C₁₇H₃₃COO)₂) = 621 g/mol; M(Ni) = 59 g/mol.
• The mass fraction of nickel in the carboxylate, ω (Ni) = 59/621 = 9.5%.
• The mass of nickel in the loaded carboxylate is as follows:
• 1,4 г Ni(C₁₇H₃₃COO)₂ – 100 %
• x г Ni − 9.5 %
• x = 0.133 g
• The amount of nickel leached into the oil is calculated as follows: 0.00017873 g/0.133 g × 100 % = 0.13%.

3.4. Changes in the Molecular Mass of Resins and Asphaltenes

Figure 3 displays data on the molecular weight of the resins in the initial oil, after the control experiment, and after the catalytic treatment with iron- and nickel-based catalyst precursors in the presence of solvents at 300 °C TST.

Figure 4 offers a comprehensive dataset on the molecular weight of the initial oil’s asphaltenes. This includes measurements post-control experiment and after catalytic treatment with iron- and nickel-based catalyst precursors. These experiments are conducted with the presence of solvents and exposure to a 300 °C TST.

4. Discussion

As shown in Figure 1, the dynamic viscosity of the oil decreases with increasing treatment duration. Moreover, the highest effectiveness is observed after 96 h of TST with the use of a catalyst. Specifically, the dynamic viscosity at the measurement temperature of 20 °C decreased by a factor of 6 compared to the initial sample and by 6% compared to the control experiment. These viscosity measurement results indicate a positive influence of the catalyst on the rheological properties of the oil after hydrothermal–catalytic treatment. It is well known that oils with a high content of resins and asphaltenes experience a significant increase in viscosity. Therefore, by considering the changes in the compositional groups, it can be concluded that the catalyst contributes to the disruption of the associated complexes of resin and asphaltene molecules, thereby reducing the oil viscosity. Consequently, the decrease in viscosity indicates that the original oil undergoes significant changes due to cracking chemical reactions, leading to the breakdown of large molecules and the formation of lighter saturated hydrocarbons and aromatic compounds [20]. Furthermore, when asphaltene–resin clusters are broken down, catalysts are expected to be more accessible to asphaltene molecules, thus enhancing their effectiveness [21].

As shown in Figure 2, the highest effectiveness is observed after TCT for 96 h with the use of an iron tallate catalyst. The dynamic viscosity at the measurement temperature of 20 °C decreased by a factor of 8 compared to the initial sample and by nearly 4.5 times compared to the control sample at 96 h.

Reactive hydrogen from the hydrogen donor, represented by C4-155/205, during aquathermolysis can inhibit polymerization between low-molecular-weight radicals formed during cracking and reduce the viscosity of the oil [22]. Additionally, TCT can influence the decomposition and depolymerization of asphaltenes in the high-viscosity oil of the Strelovskoye field, which contributes to the reduction in oil viscosity.

Gases formed during the reaction, such as hydrogen and carbon dioxide, play a crucial role in reducing oil viscosity. This is because hydrogen, in addition to acting as a hydrogen donor, is released from water and can reduce viscosity through hydrogen transfer reactions with the oil [23]. The introduction of a mixture of RASPO-1 and C4-155/205 alters the molecular mobility of the non-distillate fractions of crude oil, leading to a reduction in both the viscosity and aggregative stability of these non-distillate components. Furthermore, when hydrogen donors are used, additional hydrogen is available for hydrogenation reactions. The hydrogen donor molecules undergo dehydrogenation, transferring hydrogen atoms to the heavy hydrocarbons in the oil, thereby improving the quality of the cracking products and minimizing the polymerization of heavier molecules via a free-radical mechanism. As a result, coke formation can be reduced, while the yield of light and medium distillates can be significantly increased, achieving a more efficient use of hydrogen compared to simple hydrocracking [24].

As shown in Table 2 and Table 3, the best results in reducing the pour point temperature were observed after 48 and 96 h of catalytic aquathermolysis at 300 °C, leading to a decrease of 7 °C. This can be attributed to the presence of the catalyst, which effectively reduces the paraffin content. In accordance with the crystallization theory, oil solidification occurs due to the formation of a crystalline phase as the temperature decreases. Consequently, the formed crystals grow, coalesce, and form a crystalline network, effectively trapping the liquid phase and causing a solidifying point in the oil [25]. Therefore, the crystallization theory is frequently employed to investigate the low-temperature properties of oils and petroleum products.

As illustrated in Figure 3, the molecular weight of the resins in the initial oil is approximately 1200 a.m.u. As thermal treatment is applied, there is a noticeable shift in the ion peaks to the left, bringing them down to approximately 1150 amu. Furthermore, during the transition from the control experiment to catalytic treatment, a reduction in the intensities of fragment ions is observed. This shift gravitates toward lower molecular weights, favoring lighter fractions like saturated and aromatic hydrocarbons. After undergoing a 96-h treatment, the average molecular weight decreases to around 1100 amu. In addition, Figure 3c,d indicate a diminishing intensity in resin fragment peaks, reaching 900 arbitrary units for iron tallate and 700 arbitrary units for nickel tallate.

Based on the results obtained from MALDI spectroscopy, it can be deduced that there is a reduction in the molecular weight of asphaltenes following catalytic treatment. The initial oil’s asphaltene fractions exhibit molecular weights within the range of 1600–1700 a.m.u., with notably high peak intensities. Subsequently, the MALDI spectra peaks from the control experiment conducted at 300 °C show a noticeable leftward shift, extending into the range of average molecular weights from 1500–1600 a.m.u. In parallel, in the asphaltene fractions enriched with iron and nickel tallates, a distinct migration of ion peaks toward molecular weight values between 1300 and 1400 a.m.u. is observed. The peak intensities drop significantly, falling below 700 arbitrary units for the iron tallate sample and below 800 arbitrary units for the nickel tallate sample. This shift is primarily attributed to the redistribution of heavier fragment ions toward resins and aromatic compounds.

Due to the high content of asphaltenes in heavy oil, van der Waals forces exist between the stacked structures, leading to a mutual association among the asphaltenes, which intuitively results in high viscosity and poor flowability. The addition of catalysts disrupts the structure of the asphaltenes, leading to partial, permanent depolymerization and partial loose bonding. As a result, some asphaltene units depolymerize and separate, significantly reducing the viscosity of the heavy oil [15]. The catalyst acts on the heteroatoms in the side chains of aromatic compounds, breaking the hydrogen bonds between certain high-carbon hydrocarbons, which leads to the rupture of C-S, C-O, and C-N bonds.

5. Conclusions

In conclusion, our thorough laboratory modeling has provided valuable insights into the thermal steam treatment (TST) process, examining its effects both with and without the catalyst under various conditions, including different durations (ranging from 24 to 96 h with daily intervals) and temperature settings (250 and 300 °C).

At 20 °C, the dynamic viscosity of the oil exhibited a significant reduction after thermal catalytic treatment (TCT) at 250 °C, showing a six-fold decrease compared to the initial sample. Furthermore, after TCT at 300 °C, the dynamic viscosity decreased by an impressive eight-fold compared to the initial sample and nearly 4.5 times less than the control sample after the 96-h experiment.

Of particular significance, the most notable reduction in the pour point, reaching an impressive low of 7 °C, was observed following the extended 96-h TCT at 300 °C. This change was directly attributed to the catalyst’s role in reducing the paraffin content.

Consequently, it can be concluded that thermocatalytic treatment results in a reduction of the molecular mass of resins and asphaltenes in samples treated with iron and nickel tallates.

Based on all of the above, it can be concluded that the catalyst promotes hydrogen transfer reactions from naphtheno-aromatic components of the hydrogen donor to free radicals, ensuring their saturation and preventing recombination. In the presence of the catalyst and hydrogen donor, the processes of hydrogenolysis of C-S bonds, hydrogenation of aromatic rings, and partial degradation of C-C bonds in the structures of resins and asphaltenes are enhanced, which leads to a decrease in viscosity.

To sum up, the comprehensive findings from our study highlight the unequivocal and favorable impact of thermal–catalytic treatment on the oil’s physicochemical attributes. These insights emphasize the considerable potential of in situ catalysts and their ability to significantly enhance the operational efficiency of high-viscosity paraffinic oil reservoirs, as demonstrated in the context of the Strelovskoye field.

The conducted research confirms the promising prospects of using catalytic complexes to improve the efficiency of unconventional hydrocarbon resource development. Further progress in this field could focus on studying the adsorption process of the active catalytic phase particles onto the mineral surface of the reservoir rock and determining the influence of the hydrogen donor solvent composition on the effectiveness of the catalytic complex.