The Catalytic Upgrading Performance of NiSO4 and FeSO4 in Case of Ashalcha Heavy Oil Reservoir

1. Introduction

The prompt growth of population and extensive urbanization requires alternative energy resources to the depleting conventional crude oil reserves in order to supply the energy demand. Heavy oil is considered as an alternative unconventional hydrocarbon resource to fill the gap of energy supply because of its abundant reserves worldwide and sustainability. Generally, crude oils with a density of higher than 920 kg/m³ and viscosity of over 30 mPa^.s are classified as heavy oil [1]. Such hydrocarbons typically contain a significant amount of high-molecular weight compounds such as resins and asphaltenes[2,3,4]. In addition, a vast amount of sulfur-, nitrogen-, and oxygen- containing compounds are concentrated in such macromolecules, which cause several problems during the production, transportation and processing of such crude oil [5,6,7]. Besides, the rich metal-containing complexes such as vanadyl and porphyrin, as well as other heavier metals in the structure of the heavy oils are prone to poison the active catalysts used during the crude oil processing in refineries. Due to the large macromolecules with high carbon content and relatively low hydrogen ratio, the yield of value-added fractions is significantly low in contrast to the conventional light crude oils [8,9]. Therefore, it is crucial to upgrade and improve the quality of such hydrocarbon resources before transferring to refineries. Moreover, it is important to obtain high-quality or at least partially upgraded crude oil in reservoir formations in order to ease mobility of crude oils through reservoir rocks and enhance the recovery efficiency.

Currently, steam-based enhanced oil recovery methods continue to be the predominant techniques for effectively producing heavy oil and natural bitumen [10,11,12]. The structural traps with impermeable cap rocks that block the migration of the crude oil and lead to the accumulation of it can serve as natural reactors to carry out upgrading processes, where the heat is supplied by steam, hot water etc. To date many attempts have been made to upgrade heavy oil in-situ by using the steam under relatively mild conditions, which was termed for the first time by Hyne et al. as an “Aquathermolysis” [13,14,15]. However, the efficiency of aquathermolysis reactions was very low and was not sufficient for effective viscosity reduction, especially for heavy oils with high asphaltene content. Early in 1986 Hyne and his colleagues implied that some metal ions, i.e., nickel and cobalt are believed to be involved in the cleavage of some bonds by hydrodesulfurization and hydrocracking processes, as well as water-gas-shift reactions to produce hydrogen intermediates [16]. Since then, many authors have tried to design different types of catalysts aiming to deliver to the oil reservoir formations, and transition metal-based catalysts have become of focal interest [17,18,19,20,21,22]. At present, the applied aquathermolysis catalysts can be roughly divided as water-soluble, oil-soluble and dispersed. Although the efficiency of the latter catalysts is high, the injection of such catalysts by requiring stable suspension to the downhole is cost-effective and challenging. Instead, many authors are focused on the developing oil-soluble catalysts based on different ligands due to the high sweep efficiency. Qin et al. synthesized oil-soluble iron oleate to improve aquathermolysis reactions [23]. The obtained laboratory data showed that the viscosity reduction degree was over 75% at 200℃, 24 hours and 0.3 % catalyst solution concentration. The authors noted that catalyst does not only promote a cleavage of C-S bonds, but also accelerates the scission rate of C-O and C-N bonds [23]. Zhao et al. synthesized oil-soluble catalyst based on 2-ethyl-hexanoic acid ligand, and evaluated its performance on upgrading heavy oil sample from Liaohe field[24]. The content of light fractions increased, while the ratio of resins and asphaltenes decreased after the catalyst assisted aquathermolytic upgrading. The authors reported decrease in the content of sulfur by 0.5% and increase in the atomic ratio of H/C from 1.63 to 2.0 [24]. Yi et al. carried out a comparison study to evaluate the efficiency of water-soluble catalysts and oil-soluble catalysts based on nickel (NiN) and iron naphthenates (FeN) on the aquathermolytic upgrading of resins and n-pentane asphaltenes, which were previously isolated from Liaohe heavy crude oil [25]. According to authors, the activity sequence of the catalysts was as follows: without catalyst < NiSO4 < FeSO4 < NiN < FeN, which means oil soluble catalysts had higher efficiency than water soluble ones in terms of aquathermolytic upgrading of resins and asphaltenes. Al-Muntaser et al. [26] and [27] Suwaid et al. studied the role of oil-soluble catalysts based on metal stearate on hydrogen donating capacity of decalin and upgrading of heavy oil under steam injection conditions. The authors imply that aquathermolytic upgrading using Ni-stearate demonstrated catalytic activity in terms of hydrogen production and facilitating the in-situ upgrading of hydrocarbons. Particularly, the viscosity of upgraded oil was reduced from 2034 mPa^.s to 1031 mPa^.s, measured in identical conditions. Moreover, significant decrease was obtained in resins and asphaltenes contents of heavy oil from 26.30 and 8.26% to 16.55 and 1.49%, respectively. Some other organic ligands such as octanoate and decanoate are also reported as an effective coupling compounds for the synthesis of oil-soluble aquathermolysis catalysts [7]. Although, the oil-soluble catalyst precursors exhibited high catalytic performance in aquathermolytic upgrading of heavy oil, implementation of organometallic catalysts in the fields can involve significant costs. The oil-soluble catalysts are expensive to obtain and require additional facilities for the synthesis. Solubility of the catalyst precursors in organic solvents is also challenging, as not all the ligands can provide excellent solubility in oil phase. It is crucial in terms of injectivity and distribution of catalyst precursors through the porous reservoir rocks without precipitation. Moreover, the synthesis of oil-soluble catalysts has environmental challenges due to the additional applied chemicals and disposal of used water, which has to be treated or utilized. The management of any by-products generated during the synthesis may also pose challenges in terms of environmental impact. These procedures without any doubt significantly increase the cost of oil-soluble catalyst precursors.

On the other hand, water-soluble catalysts are known as cheap, easy-to-handle metal salts with less impact to the environment and pose minimal health and safety risks. Such catalyst precursors do not require other hazardous organic additives and do not require regeneration or separation from oil bulk. Meanwhile, water-soluble catalyst precursors can be directly injected into the well by dissolving salts in water, which can significantly reduce the cost of implementation. Chen et al. synthesized a series of water-soluble metal complexes based on hydroxyl carbonyl acid and metal salts [28]. The authors evaluated the upgrading performance of the synthesized metal coordination complexes at 180°C for 24 h in case of heavy crude oil with no sand from Tazhong reservoir. The viscosity of heavy oil sample was reduced by 80% using only 0.1% of the iron (III) coordination complex. Deng et al. developed water-soluble catalytic complex based on Co (II) for steam-based upgrading of heavy oil sample from Huabei reservoir [29]. Introduction of 0.15% catalyst and 15% methanol to the aquathermolytic process carried out at 180°C for 24 hours, resulted to the heavy oil viscosity reduction degree of 85.3%, and the pour point was altered by 9.0°C. Clark et al. studied the influences of nickel and vanadyl salts, as well as transition metal cations on the aquathermolysis of sulfur-containing model compounds [15,30]. The authors found that certain cations were active for thiophane conversion, while others were reactive for tetrahydrothiophene, which was explained by the catalytic nature of the metals and chemical properties of the model compounds. The same authors studied the catalytic efficiency of the metal ions such as Fe⁺², Ru⁺², Os⁺³, Co⁺², Rh⁺³, Ir⁺³, Ni⁺², Pd⁺², Pt⁺², and Pt⁺⁴ on hydrodesulfurization of thiophene and tetrahydrothiophene during aquathermolysis processes[31]. The Pt (IV) solution revealed the best performance by reducing the content of sulfur at least by 40% (depends on the chemical nature of the used model compound). The iron (II) sulfate was used to promote hydrocracking of bituminous oil at higher temperature ranges 375-415°C [18]. Although the catalyst contributed to the rise of the asphaltenes content from 15% to 20%, the viscosity of upgraded oil was reduced from 2140 mPa^.s to 520 mPa^.s. It shows that the content of the asphaltenes in crude oil mixture is not the sole deterministic factor of viscosity alteration. Zhong et al. investigated the synergetic effect of water-soluble Fe (II) and hydrogen donor solvent – tetralin on the aquathermolytic upgrading of heavy oil sample from Liaohe [32]. The authors observed that with increasing Fe concentration, the viscosity of heavy oil sample was further reduced. If hydrogen donor and iron catalyst without tetralin contributed to the viscosity reduction of 40% and 60%, respectively. Then, the combination of the water-soluble catalyst and hydrogen donor reduced the viscosity of heavy oil sample by 90%. According to the temperature optimization study, the authors imply that the favorable temperature to carry out aquathermolysis reaction is 240°C. The researches on exploring and developing new catalytic complexes are continued to better the efficiency of the water-soluble catalysts. The stability and durability of the water-soluble catalysts is still poorly understood. Few researchers have addressed the problem of catalyst deactivation by the formation of the coke-like substances under the harsh conditions of aquathermolysis process. In this regard, developing water-soluble catalysts that can efficiently promote the aquathermolysis reactions, while suppressing unwanted side reactions and remaining active for a long period is challenging. Further focus should be placed on the design of water-soluble catalysts, optimization of operational conditions, and studying overall mechanisms of the catalytic aquathermolysis of heavy oil.

With this in mind we tried to evaluate the performance of nickel and iron sulfates as water-soluble metal salts on the aquathermolytic upgrading of heavy oil sample produced from Ashal’cha reservoir at temperature of 300°C, for 24 hours. We hope that our research will be beneficial for decisive makers in solving the problems regarding the applications of water-soluble catalysts to enhance the production of heavy oil and upgrade it in-situ.

2. Materials and Methods

The object of the given study was a heavy oil sample produced from Ashal’cha oil field, which is located in the Republic of Tatarstan, Russia Federation. The density and dynamic viscosity of the oil sample under normal conditions is 0.951 g/cm³ and 3280 mPa^.s, respectively. The average content of high-molecular components is about 45 %wt., and the content of sulfur can reach as high as 5 %wt.

The iron and nickel salts with assay of more than 98%, which were manufactured for laboratory uses were purchased from LLC «ChemTrade» (Russia Federation). The catalytic aquathermolysis processes using water-soluble catalysts NiSO₄ and FeSO₄ are conducted in a high pressure and high temperature batch reactor with stirrer (HPHT), which is regulated by a control unit connected to a personnel computer for monitoring and recording the kinetics and the progress of the process. The scheme of the HPHT reactor is illustrated in Figure 1. The model system was initially purged with nitrogen for 20 minutes at room temperature to provide inert ambient of the process before the temperature in the reactor was raised up to 300°C in less than 1 hour and maintained for 24 hours. The HPHT reactor is then cooled down at room temperature. The oil: water mass ratio was selected 70:30 in order to simulate the process of aquathermolysis in steam stimulated production techniques. The concentration of the catalyst was 2 %wt. from oil bulk.

2.1. Elemental Analysis of Oil Samples

A Perkin Elmer 2400 Series II (Perkin Elmer, Massachusetts, USA) Analyzer was used to evaluate the changes in the mass content of carbon, hydrogen, nitrogen and sulfur in the composition of oil sample before and after thermo-catalytic upgrading. The difference between the one and the sum of the CHNS content can be attributed to the content of oxygen. The results of the given analysis method allow to estimate the atomic ratio of H/C, which reflects the hydrogenation processes during aquathermolytic upgrading. Moreover, it provides information regarding the hydrodesulfurization, hydrodenitrogenation and hydrodeoxygenation of crude oil samples.

2.2. FT-IR of Crude Oil Samples

Fourier-transform infrared spectroscopy analysis was performed using a Vertex 70 FT-IR spectrometer (Bruker, Ettingen, Germany) in order to identify the changes in the functional groups of heavy oil under the influence of the catalytic systems. The infrared absorption characteristic peaks were identified in the wide range of 400 – 4000 cm^-1 wavenumbers.

2.3. Atmospheric Crude Distillation Unit

The crude oil atmospheric distillation was conducted in an ARN-LAB-03 distillation unit as per ASTM D86 [33]. The heavy crude oil samples (initial and after upgrading) were evaluated by fractionation of them into light-end and heavy-end hydrocarbons. The light-end hydrocarbons referred to gasoline and diesel fractions with the boiling point ranges of initial boiling point (i.b.p) – 200°C and 200°C – 300°C, respectively. The heavy-end hydrocarbons correspond to the fractions, the boiling point of which are higher than 300°C.

2.4. Dynamic Viscosity of Crude Oil Samples

The dynamic viscosity of oil samples was measured using a Fungilab rotational viscometer Alpha L series combined with a temperature maintaining unit MPC K6 from Huber (Offenburg, Germany). The temperature dependent viscosity values were measured in the temperature ranges of 20 to 80 °C with a step of 20 °C.

2.5. SARA-Analysis

The SARA analysis is a widely employed technique for characterizing different fractions of petroleum. It involves the chromatographic separation of crude oil into four primary categories based on polarity: saturates, aromatic compounds, resins, and asphaltenes. Asphaltenes were isolated from the maltenes by the addition of n-heptane. The oil-to-solvent ratio was 1:40 by weight. It is important to note that the applied solvent determines the nature of the asphaltenes isolated from the oil [34]. The n-alkane soluble components – maltenes are then separated by passing their mixture through a chromatographic column with an adsorbent. In the given study previously calcined aluminum oxides are used as the adsorbent. The chromatography column is tapped with a rubber tube after a portion of aluminum oxide is poured into the column for its sealing, and n-heptane is carefully poured into the upper part of the column. The n-heptane is passed through the column until the residue discolors. The expiring fraction of Saturates is collected. When the upper part of the aluminum oxide is almost dry, toluene is added to the column. The collecting flask is changed and the Aromatics fraction is collected. The toluene is passed through until the expiring solvent is discolored. Then a 150:50 toluene/methanol mixture is passed through the column in order to elute Resins fraction. The solvents of all three fractions are distilled off and evaporated under vacuum till constant weight of fractions was reached.

2.6. Gas Chromatography-Mass Spectroscopy (GC MS)

The Saturates and Aromatics fractions obtained from the SARA analysis were further analyzed using a GC MS analysis method. The complex system was composed of a gas chromatography “Chromatech-Crystal 5000” manufactured in Yoshkarala, Russia, and a mass-selective detector ISQ produced in USA. The results obtained were processed using the XCalibur application. For the experiments, a capillary column with a length of 30 m and a diameter of 0.25 mm was employed. The carrier gas used was helium, flowing at a rate of 1 mL/min, and the column temperature was set to 310°C. The thermostat regime was adjusted as follows: starting with a temperature rise from 100°C to 150°C at a heating rate of 3°C/min, followed by a further increase from 150°C to 300°C at a heating rate of 12°C/min, and then maintaining this temperature isothermally until the end of the analysis. In terms of the GC MS settings, the Electron energy was set to 70 eV, and the ion source temperature was maintained at 250°C. To identify the molecules and compounds, the NIST Mass Spectral Library and relevant literature sources were used.

4. Conclusion

This study was devoted to the catalytic performance of water-soluble catalysts NiSO₄ and FeSO₄ in in-situ upgrading of heavy oil by means of aquathermolysis process. The obtained experimental data revealed that the catalysts contributed to the intensification of the C-S, C=C bonds scission, resulting in an increase in light components while significantly reducing heavy components. At the same time, the upgraded oil is further diluted with light hydrocarbons, which led to reduce its viscosity. Among the both catalysts used, the FeSO₄ catalyst demonstrated the highest degree of viscosity reduction, reaching 60%. Following the iron (II) sulfate assisted aquathermolytic upgrading, the composition of the hydrocarbons underwent significant changes such that the content of saturated and aromatic hydrocarbons increased from 29.1 %wt and 32.6 %wt to 36.3 %wt and 40.2 %wt., respectively. In contrast, the content of resins decreased by 17%, while the amount of asphaltenes decreased by 12.9%. The initial boiling point of catalyst-assisted upgraded oil decreases from 168 ºС to 154 ºС, and also leads to an increase in gasoline and diesel fractions by 13% and 53%, respectively. NiSO₄ catalyst led to an increase of gasoline and diesel fractions by 40% and 9%, respectively. After catalytic aquathermolysis, the IR spectra of the heavy oil showed that hydrogenation and heteroatom removal reactions took place during catalytic aquathermolysis process, leading to a decrease in aliphaticity, branching and sulfurization spectral coefficients, while condensation and oxidation coefficients were increased. The thermal decomposition products of metal salts were identified by SEM-EDX and XRD as oxides and sulfides of corresponding metals.