1. Introduction
Heavy oil can be upgraded catalytically via aquathermolysis viscosity reduction and catalytic oxidation technologies. Using them may result in an irreversible drop in the viscosity of heavy oils [
1,
2,
3]. Injecting a cracking catalyst into a heavy oil reservoir at high temperatures (>200 °C) decreases the apparent viscosity of the heavy oil [
4,
5]. In contrast, catalytic oxidation reactions occur at low temperatures, often below 160 °C. Catalytic oxidation technology integrates various types of oil recovery methods, such as oxidation modification and self-emulsification. In terms of the mechanism, the essence of low-temperature catalytic oxidation technology lies in the removal of heteroatoms, which is demonstrated by effectively breaking the heteroatom side chain of polycyclic aromatic hydrocarbon and weakening the hydrogen bond between the lamellar structures of heavy oil [
6]. This can effectively increase the viscosity reduction rate of heavy oil, improving the quality of crude oil and enhancing the flowability of heavy oil. The technology has the advantages of low cost, low temperature, mild reaction conditions, and easy operation, effectively promoting the recovery of heavy oil. The key to low-temperature catalytic oxidation technology is the preparation of special catalysts to improve the efficiency of the low-temperature oxidation reaction of heavy oil, as well as the oxidation of C-R (R = S, N, O) bonds into alcohols, aldehydes, or carboxylic acids, which are located on the side chains of polycyclic aromatic hydrocarbon of heavy oil [
7,
8,
9].
Among the catalysts that can be used for the catalytic oxidation of heavy oil, transition metal-based catalysts are particularly prominent, especially ferric-based catalysts. The outer valence electron configuration of the iron element is 3d
64s
2; when the outermost electrons are lost, an empty orbit is created, and the ferric ion is obtained, which is electrophilic. If a heteroatom containing lone pair electrons is nearby, the ferric ion nucleus will attract the lone pair electrons, which will rapidly fill the hole. Thus, the metal active center of the catalyst attacks the C-R (R = S, N, O) bonds, and the reactive oxygen seals the end forming oxygen-containing groups. Catalysts such as ferric oleate, iron naphthenate, ferric dodecylbenzene sulfonate, ferric acety-lacetonate, metallic oxide nanoparticles, and so on, have been used in aquathermolysis and the catalytic oxidation of heavy oil, showing better modification effects and better viscosity reduction [
10,
11,
12,
13,
14,
15,
16,
17]. Zhou [
18] compared γ-Al
2O
3 and Fe
3O
4 nanoparticles with a traditional displacement agent for heavy oil upgrading. They found that Fe
3O
4 nanoparticles can promote the upgrading of heavy oil, improving the oxygen consumption rate of heavy oil catalytic oxidation at low temperatures. Li [
19] found that α-Fe
2O
3 has a strong effect on asphaltene modification, reflected in the reduction of S and N content, the reduction of aromatic degree, and the reduction of viscosity from 161,180 mPa·s to 45,882 mPa·s.
However, although nanoparticle catalysts exhibit good catalytic modification performance, their application is limited due to their characteristics of easy agglomeration and problematic injection formation. Therefore, the catalytic performance of the ferric-based organic complex needs to be further studied because they easily come into contact with oil and can be easily injected into the formation.
In addition, an oxidant plays an indispensable role in the process of heavy oil modification. Previously, researchers have conducted a series of catalytic oxidation studies using air as an oxidizing agent in the process. However, if the petroleum reservoir is fractured or a cavity reservoir, the injected air will have insufficient contact with the heavy oil, reducing the efficiency of the reaction. At the same time, gas is prone to leak out. If a large amount of air is concentrated in the producing well, and the oxygen content exceeds the safety threshold (5%), the well becomes highly susceptible to explosion [
9].
Based on this, hydrogen peroxide has gradually gained attention as a highly efficient oxidant, but it is easily decomposed and has poor stability, which poses safety risks and limits its application conditions. Therefore, there is a need to switch to other relatively safer oxidants, such as organic peroxides, most of which are more stable than inorganic peroxides. Previous research has shown that tert-butyl hydroperoxide can make complete contact with heavy oil and boost the upgrading effect under the action of a catalyst.
Our research group has previously synthesized Ni-based and Fe-based catalysts, which have demonstrated superior catalytic modification properties for heavy oil [
20,
21]. In order to explore the catalytic effect of ferric-based organic complex, four kinds of ferric-based catalysts have been selected to investigate the catalytic oxidation performance of heavy oil from the Tahe oilfield in Xinjiang, with the reaction temperature ranging from 100 °C to 180 °C. This study provides a theoretical framework for improving heavy oil recovery and lowering viscosity at low temperatures.
3. Experiment
3.1. Materials
Commercially available chemical reagents of analytical grade were used in the experiment without any additional purification processes prior to use. Tert-butyl hydroperoxide solution (TBHP, AR, Macklin, Shanghai, China) was used as the oxidant, 1,2,3,4-tetralin (THN, AR, Macklin) was used as the proton donor, and iron naphthenate (AR, Sia reagent), ferric oleate, (AR, Macklin), EDTA–FeNa (AR, Bide Pharmatech, Shanghai, China), and EDDAH–FeNa (C18H16N2O6FeNa, AR, Coolaber, Beijing, China) were used as catalysts. This study used heavy oil samples from the Tahe Oilfield in Xinjiang, with a viscosity of 38,010 mPa·s at 50 °C. The group composition data are as follows: asphaltene 21.96%, resin 8.24%, saturate 22.63%, aromatic 47.17%, crude oil initial water content: 14.5%.
3.2. Low Temperature Catalytic Oxidation Experiment and Characterization
The heavy oil was first introduced into the reactor, then into the catalyst, oxidant, proton donor, and water, and the oil–water ratio was controlled at 7:3. The temperature ranged from 100 °C to 180 °C, and the reaction time was 24 h.
After the reaction, the oxidized oil was cooled to room temperature and dehydrated to determine its group composition and viscosity. The rates of the corresponding viscosity reduction were calculated. The effects of the amount of catalyst and oxidant and temperature on the properties of the heavy oil were investigated using the control variable method.
The heavy oil content before and after the reaction was determined through group composition analysis according to the ‘SY/T 5119-2016 Analysis method for family compositions of rock extracts and crude oil’. The functional groups in the oxidized oil before and after the reaction were analyzed by a Fourier transform infrared detector (FT–IR, Nicolet6700, Thermo Scientific™, Waltham, MA, USA). The saturates before and after the reaction were analyzed by gas chromatography–mass spectrometry (GC–MS, 7890A/5975C). Elemental analysis of resin and asphaltene was performed using a Vario EL III automatic elemental analyzer. Deuterated chloroform was used as a solvent to dissolve resin and asphaltene for testing on an AVANCE III HD 400 MHz nuclear magnetic resonance spectrometer (1H-NMR), with trimethylsilane (TMS) serving as an internal reference. The viscosity of heavy oil before and after the reaction was measured by a Bohler Fly Rheometer DV3T-RV at 50 °C, and the viscosity reduction rate was calculated using Formula (3):
where Δη is the viscosity reduction rate, %, η
0 is the initial viscosity of heavy oil, mPa·s, η is the viscosity after the reaction, mPa·s.
4. Conclusions
The catalytic oxidation reaction of Tahe heavy oil has been carried out using four kinds of iron-based catalysts: iron oleate, iron naphthenate, EDTA–FeNa, and EDDHA–FeNa, in combination with the TBHP oxidant and a proton-donating acid to form a catalytic system. EDDHA–FeNa is selected as the best catalyst for catalytic oxidation among the four ferric-based catalysts, achieving a viscosity reduction rate of 78.57%. Remarkably, the metal center iron ion of EDDHA–Fe has an empty orbit and is electrophilic. These metal centers provide an empty orbit for the heteroatoms of heavy oil macromolecules, facilitating the binding of reactive oxygen species to heteroatoms and breaking the C–R (R = S, N) bond, especially the C–S bond, thereby increasing the viscosity reduction rate.