In 2016, the global use of enhanced oil recovery (EOR) technology produced 1.17×10⁸ t of oil. Among the total production, heavy oil produced by thermal recovery was 6583×10⁴ t, accounting for 56.2%; gas flooding was 3671×10⁴ t, accounting for 31.4%; and chemical flooding was 1453×10⁴ t, accounting for 12.4%. EOR oil production in the United States (US) is dominated by heavy oil thermal recovery and gas flooding, accounting for 98.7%. The US gas flooding oil production surpassed thermal recovery in 2006. Furthermore, this production has increased year by year, reaching 2346×10⁴ t in 2016, accounting for 64.0% of global gas flooding oil production. Over 90.0% of the oil produced by gas flooding abroad mainly depends on carbon dioxide flooding, while other gas flooding media (such as nitrogen, natural gas, flue gas, air, etc.) are less used in oilfields.

At present, the newly increased proven oil reserves in China are mainly in low-permeability, ultra-low-permeability and tight oil reservoirs, and there are prominent problems such as “injection and production difficulties” when applying water injection technology. These kinds of reserves will be the main resources for increasing reserves and production for a considerable period of time in the future. Nanoscale gas molecules are easier to be injected into the reservoir to supplement energy and complete the oil displacement process. At the same time, compared with water, gas has a greater compressibility and greater elastic energy, which can be obtained by decompression expansion. The reserves of natural gas and carbon dioxide in China that can be used to enhance oil recovery are limited. Meanwhile, due to the relative position of gas and oil reservoirs, it is difficult to promote large-scale industrialization over long distances. There are also some technical and economic problems in carbon dioxide recovery from other industries. The cost of nitrogen is high for nitrogen injection oil recovery technology. On the contrary, air can be sourced locally without being restricted by region, space and climate. The composition of air is stable, and the air source is abundant. For reservoirs with extremely scarce water resources and water-sensitive reservoirs in areas, such as deserts and the Gobi, air is the most popular gas flooding medium. According to statistics, the cost of purchasing air per ton of oil is zero, and the cost of oxygen-reduced air is 400 to 600 yuan. The purchase cost of carbon dioxide, natural gas, and nitrogen per ton of oil are more than 1200, 4000, and 2000 yuan, respectively. Compared with other gas flooding media, air has obvious economic advantages.

In terms of air injection technology for light oil reservoirs, several mechanism studies have been conducted by researchers worldwide since the 1960s^[1,2,3,4]. Field tests have also achieved good results and formed relatively complete supporting technologies. In the 1970s, research and experiments on air-injection-enhanced oil recovery technology in China were carried out in Shengli, Daqing, Baise and other oilfields. Due to the limitation of the air injection experiences and operation methods, the research and experiments were mainly focused on measuring the oxygen consumption rate. Although the field tests did achieve certain results, they were small in scale and did not have a clear understanding of the oil flooding mechanism of air injection. Meanwhile, the safety risks were large, and most of the tests were mainly based on air foam control and flooding, so it can be said that those field tests were not true air injection field tests.

In recent years, China National Petroleum Corporation (hereinafter referred to as CNPC) has developed air injection indoor mechanisms and field practices in Daqing, Changqing, and Dagang oilfields. Combining these with specific reservoir conditions, air injection (including oxygen reduction air) technologies for enhanced oil recovery have been developed. The field test results show that as long as the oxygen content is controlled within a certain range, the hidden danger of explosion can be effectively controlled; by optimizing the injection and production process, using anticorrosive materials and adding inhibitors, the corrosion of oil production pipe strings can be effectively prevented.

In terms of air injection in situ combustion technology for heavy oil reservoirs, laboratory research and field tests have been carried out in the oilfields of Yumen (1958), Xinjiang Heiyoushan (1958), Liaohe Korqin (1996), Shengli Zhengwangzhuang (2003) and others^[5,6,7,8]. In 2005, PetroChina carried out a pilot test of in situ combustion in the Du 66 block of Liaohe Oilfield. The annual oil production from the in situ combustion of 105 well groups reached 24×10⁴ t in 2018. In 2009, the pilot test of high-temperature in situ combustion in China's Xinjiang Hongqian oilfield block No. 1 achieved high-temperature ignition. The reservoir experienced steam stimulation and steam flooding in the early stage, with a stage oil recovery rate of 29.0%, and was in an abandoned state before the in situ combustion pilot test. The main purpose of the test was to explore the alternative development method in the later stage of steam injection recovery of heavy oil reservoirs. At the end of 2018, the oil recovery during the in situ combustion process was 34.6%. The success of the experiment with a long recovery history confirmed that high-temperature air-flooding in situ combustion technology can be a strategic replacement technology for steam injection in heavy oilfields^[9].

Recently, basic research on in situ combustion has mainly focused on the characteristics of high-temperature in situ combustion reservoir zones, the oxidation kinetics of heavy oil, the coke deposition process and oxidation characteristics, and high-temperature in situ combustion well patterns^{[10,11,12,13,14,15,16,17,18,19]}. However, there is a lack of systematic research on the relationship between different reservoir air injection recovery methods and the oxidation characteristics of heavy oil at different temperature intervals.

In this paper, light oil and heavy oil samples from the block where the in situ combustion pilot experiment has been conducted are used to carry out a thermogravimetric-differential scanning calorimetry joint experiment to analyze the characteristics of the crude oil oxidation reactions from 30 °C to 600 °C and to reveal the reaction mechanisms. Combined with the results of field tests, the oxidation temperature interval of crude oil is divided and the recovery methods of air injection under different reservoir conditions are proposed.

After the air is injected into the reservoir, complex exothermic reactions between crude oil and the oxygen in air occur. The reaction mechanism and thermal effects change with the temperature. During the development of air injection technology, different recovery methods have corresponded to different reaction temperature ranges, and the recovery mechanism is controlled by the crude oil oxidation mechanism in this temperature range. To clarify the reaction mechanism between air and crude oil in different temperature ranges and its influence on the reservoir recovery method, light oil and heavy oil samples from typical blocks of the air injection pilot test area were selected for crude oil oxidation thermal analysis experiments.

In this paper, the TGA/DSC (Mettler Toledo, TGA/DSC 1) synchronous thermal analyzer was used to study the high-temperature oxidation process of crude oils with different viscosities. The conversion rate of crude oil samples (derived thermogravimetry, DTG) and the exothermic rate of crude oil samples (differential scanning calorimetry, DSC) were measured simultaneously. During the experiment, the protective gas was nitrogen, and the flow rate was 79 mL/min; the reactant gas was oxygen, and the flow rate was 21 mL/min. After the two gases were mixed in the reaction chamber, they crossed the surface of the crucible and reached the sample layer by diffusion. The oxygen concentration on the surface of the material was 21%, and the pressure was ambient pressure.

A light oil sample from the Haita Basin of Daqing Oilfield and a heavy oil sample from the Gaosheng Oilfield of Liaohe were selected to prepare simulated oil sands; the viscosities of the two crude oils at 50 °C were 23 mPa•s and 1878 mPa•s, respectively. Dehydration and impurity removal were in accordance with the processing steps specified in the industry standard (SY/T6316-1997 Analytical approach of fluid physical property for heavy-oil reservoirs: Crude oil viscosity measurements)^[20]. The water content of the dehydrated oil sample after treatment was less than 0.5%. A homogeneous mixture of 45 mg SiO₂ particles and 5 mg pure oil was used as the sample. The heating temperature range was set to 30-600 °C, and the heating rate was 10 °C/min. The mass and exothermic changes in oil sand samples were measured.

According to the measurement results, the DTG and DSC curves are presented in Fig. 1. As shown in Fig. 1, the oxidation reaction of crude oil for the whole temperature range of air injection recovery can be divided into 4 intervals: dissolution and expansion, low-temperature oxidation (LTO), medium-temperature oxidation (MTO) and high-temperature oxidation (HTO). Each zone presents the different characteristics of crude oil oxidation reactions (Fig. 1).

1.2.1. Dissolution and expansion interval

The upper temperature limit of this zone is approximately 120 °C^[13]. In this temperature range, after the air is injected into the reservoir, dissolution and expansion are dominant. The DSC curve cannot determine the exothermic phenomenon of the crude oil reaction, indicating that the crude oil reacts with oxygen in this area is not obvious. The DTG curve shows that there is a low weight loss rate of crude oil samples in this area. This small change in the weight loss rate is mainly caused by the volatilization of light hydrocarbons.

1.2.2. Low-temperature oxidation interval

The lower limit temperature in this zone is approximately 120 °C, and the upper limit is approximately 200 °C. The oxidation heat effect of crude oil in this interval is weak, and no obvious exotherm is observed in the DSC curve. The DTG curve shows that the conversion rate of crude oil samples is still caused by the volatilization of light hydrocarbon components. The LTO reaction mainly occurs in this interval. Although the exothermic rate of the LTO cannot be observed using DSC curves, the accumulation of reaction heat can still increase the reservoir temperature under adiabatic reaction conditions^[21]. The main reason is that oxygen compounds such as alcohols, aldehydes, ketones, and acids generated by the oxygenation reaction undergo further oxidation reactions to generate peroxides and that the decarboxylation of peroxides generates CO₂ and CO, releasing a certain amount of heat^[1]. The reaction equations in this interval can be simplified as:

1.2.3. Medium-temperature oxidation interval

The lower limit of the temperature in this interval is approximately 200 °C, and the upper limit is approximately 400 °C. In this interval, crude oil and oxygen undergo a MTO reaction. The DTG curve and DSC curve indicate that the conversion rate and the exothermic rate of crude oil samples are significantly changed. In this interval, crude oil reacts with oxygen to generate light hydrocarbons, CO₂, CO, H₂O, etc., releasing tremendous heat at the same time. The MTO reaction includes a polycondensation reaction and a bond-breaking reaction. In addition to generating light oil, CO₂, CO, and H₂O, a certain amount of coke is also generated (the coke produced in the oxidation atmosphere is called oxidized coke). Because the mass of the crude oil (reaction fuel) is far greater than the mass of coke produced by the MTO reaction, the heat released by MTO reaction is also relatively large, which can form a thermal front in the reservoir that is different from that of high-temperature in situ combustion.

In the MTO stage, the heavy oil molecules undergo further oxidation, forming oxygen-containing functional groups and releasing heat^[22]. After oxidation, a portion of heavy oil molecules are cracked to form low-carbon-number small-molecule compounds and are finally converted into light oil^[23]; the other part is formed into larger molecules through crosslinking and polymerization between oxygen-containing func-tional groups and finally converted to oxidized coke^[24], with carbon oxide and water being formed simultaneously in the reaction process. Based on the above studies, the reaction process can be expressed as shown in Fig. 2.

1.2.4. High-temperature oxidation interval

The lower limit of the temperature in this interval is 400 °C. When the reaction temperature is higher than the lower limit, the DTG curve and DSC curve show a second peak of the conversion rate and the exothermic rate of crude oil samples. The corresponding reaction is the solid coke oxidation reaction. This interval is the HTO interval. After the reaction temperature is higher than 400 °C, the crude oil undergoes a thermal cracking reaction to generate pyrolysis coke and light hydrocarbons. Pyrolysis coke mainly comes from the resins and asphaltenes, which have a large molecular weight, high viscosity, and complex aromatic ring structure^[25,26]. The cracking reaction does not require oxygen. After the temperature is higher than 400 °C, the linear alkyl groups and the carbon-hydrogen bonds are easily thermally cracked to form small molecular substances, which are converted into cracked oil and cracked gas^[27]. The free radicals formed by cracking are in an unstable state. They are easily combined with stable polycyclic aromatic hydrocarbons and then transform into polycyclic aromatic hydrocarbons with a larger number of aromatic rings. After dehydrogenation and reforming, they are finally converted into pyrolysis coke^[28] (Fig. 3).

The oxidized coke formed by MTO and the pyrolysis coke formed by HTO react with O₂ to generate CO₂, CO, H₂O and release substantial heat during the HTO interval (the heat from the oxidation of oxidized coke is defined as Q1, and the heat from the oxidation of pyrolysis coke is defined as Q2), and the reaction equations can be expressed as:

The oxidation reaction of oxidized coke:

The oxidation reaction of pyrolysis coke:

The total heat released by the HTO reactions could be defined as:

where R is the mass percentage of oxidized coke in the total coke mass involved in HTO:

During the development of high-temperature in situ combustion, oxidized coke and pyrolysis coke coexist in the reservoir. Liu et al.^[29] showed that pyrolysis coke has a higher hydrogen content, and its oxidizing activity is better than that of oxidized coke. The heat value is also higher than that of oxidized coke.

Air is a new oil displacement medium with a wide range of sources, low cost and high oil displacement efficiency. Air is suitable for low/extra-low permeability reservoirs, medium-high permeability reservoirs and buried hill light oil reservoirs. Additionally, it is suitable for original heavy oil reservoirs as well as heavy oil reservoirs after steam recovery. Air injection recovery technology shows the characteristics of a high recovery, low cost, energy savings, water savings, environmental friendliness, etc. and shows broad application prospects. This technology also has unique advantages in the effective use of low-quality oil and dense oil and presents the most potential to become a strategic recovery technology in the future.

At different temperature intervals, air and crude oil have different oxidation reaction characteristics. If the temperature of the reservoir (air injection in light oil reservoirs) and the temperature of the combustion front (air injection in heavy oil reservoirs) are different, the main reaction mechanism between air and crude oil is different, and the air injection recovery method is also different. After years of continuous research, four main types of air injection recovery methods and technologies have been developed: light oil oxygen-reduced air injection, light oil air injection, heavy oil medium- temperature in situ combustion and heavy oil high-temperature in situ combustion.

According to the differences in reservoir temperature, air injection in light oil reservoirs includes two main technologies: oxygen-reduced air injection and air injection. 1) When the reservoir temperature is lower than 120 °C, the oxygenation reaction between air and oil presents a very low heat release. It is difficult to accumulate reaction heat under reservoir conditions, and the oxygen cannot be fully consumed. If the oxygen content at the production well is greater than 10%, there will be a risk of explosion. The main operating strategy of this type of reservoir is to reduce the oxygen content in the injected air to less than 10%, adopting the oxygen-reduced air injection technology^[13]. 2) When the reservoir temperature is higher than 120 °C, LTO gradually becomes the main reaction type. The heat produced by LTO is accumulated, thus, increasing reservoir temperature, reducing crude oil viscosity, and increasing crude oil fluidity. When the light oil reservoir is in this temperature range, air injection technology can be used for oil recovery safely. When the reservoir temperature is about 120 °C, the choice of the reservoir air injection method (Oxygen-reduced air injection or air injection) needs to further consider the specific reservoir mineral catalyst, oil oxidation characteristics, reservoir pressure, injection-production well spacing, and reservoir fracture conditions.

The combustion front temperature could be controlled by using different ignition methods in heavy oil reservoirs. When the combustion front temperature is lower than 400 °C, the MTO reaction of crude oil mainly occurs. The fuel is mainly crude oil in the reservoir. The recovery method is medium- temperature in situ combustion. When the combustion front temperature is higher than 450 °C, the coke HTO reaction mainly occurs. The fuels are oxidized coke produced by MTO and pyrolysis coke produced by thermal condensation in the temperature range of 400-450 °C. These two kinds of coke burn quickly and release significant heat at 450 °C, forming a stable combustion front. This recovery method is high-temperature in situ combustion. Table 1 presents the oxidation mechanisms and recovery methods of crude oil in different reservoirs.

Since 2009, PetroChina has successively carried out a number of oxygen-reduced air injection recovery tests in low-permeability, water-sensitive, high-water-cut, and other types of reservoirs^[30], expanding the industrial application of oxygen-reduced air injection EOR methods. Based on these tests, the corresponding reservoir engineering methods, supporting injection and production technology, and ground engineering supporting technology are developed to ensure the safe and efficient operation of oxygen-reduced air injection projects.

Reservoirs suitable for oxygen-reduced air injection are widely distributed, with many types, and their single-well gas injection capacities differ greatly. The main factors such as the pressure, gas volume, and oxygen content of oxygen-reduced air in different blocks are different. PetroChina has developed an oxygen-reducing air integrated device, which has made breakthroughs in key technologies such as pressure control, flow matching, intelligent joint control, and chain protection. The device initially forms a standard injection pressure of 15-50 MPa, a flow rate of (3-20)×10⁴ m³/d, and an oxygen concentration of 2% to 10%, being skid-mounted and serialized complete equipment. The technology provides a guarantee to the industrial application of oxygen-reduced air injection.

To solve the problem of accelerated water cut rate, comprehensive decline and natural decline increase after the reservoir entered the medium water content period, pilot tests of oxygen-reduced air injection were conducted and expanded in December 2009 at the Jingli Oilfield and Changqing Jingan Oilfield. At the end of 2013, a large-scale industrial test with 15 injection wells and 63 production wells was completed. The test showed that oxygen-reduced air injection is superior to water flooding in terms of the supplementary reservoir energy, as well as improving water flooding and expanding the area of plane sweep. The field test achieved good results (Fig. 4). Sixty oil wells were examined in the test area, and reduced-oxygen air injection was proven to be effective in 95.2% of these wells. At the end of 2019, the cumulative oil increase during the test period was 6.3×10⁴ t, and the final recovery rate was expected to increase by 10%.

Light oil reservoirs with high reservoir temperatures can be directly recovered by air injection. The accumulation of the oxidation heat of crude oil in the reservoir is used to increase the local temperature, thereby achieving the effective con-sumption of oxygen. Air injection oil recovery integrates multiple flooding mechanisms such as gas flooding, reservoir energy supplementation, and low-temperature oxidation. The initial stage of air injection is mainly to maintain or increase reservoir pressure and promote the gas flooding effect. Since the heat generation can be effectively accumulated, the later thermal effect is also an important flooding mechanism. Nitrogen flooding has no thermal effect, so there is only one oil production peak because of gas flooding. However, air injection recovery has a thermal effect for oil production peaks after the gas flooding peak^[31].

Air injection is suitable for high-, medium-, low-, and ultra-low permeability reservoirs (including sandstone, conglomerate, carbonate and other types) with temperatures greater than 120 °C. Only a compressor is needed to continuously inject air into the oil layer, so the ground process is simple. Compared with oxygen-reduced air injection, air injection has no oxygen-reducing process, which lowers the investment and cost due to oxygen reduction and improves the economic benefits of air injection. It is necessary to monitor the concentration of oxygen and hydrocarbon gases at the compressor outlet, gas injection wellhead, oil production wellhead, etc., especially for gas injection wells and gas breakthrough production wells, which need to be precisely regulated during injection to reduce the risk of explosion. On the other hand, the injection of high-pressure, high-temperature, and high- oxygen content gas promotes oxygen corrosion of the gas injection well tubular string, and it is necessary to strengthen the corrosion monitoring and anticorrosion measures.

To make better use of the thermal effects in air injection for crude oil oxidation, the United States has performed abundant research and practiced using light oil reservoir air injection recovery technologies. The screening reservoir temperatures are mostly above 80 °C, and good results have been obtained. The Liaohe Oilfield carried out an air injection test in the Jin 625 block (reservoir temperature 123 °C). The test results show that the 7 well groups in the test area cumulatively increased oil by 3.4×10⁴ t, indicating a good recovery effect and application prospect.

For ordinary heavy oil reservoirs with a low viscosity, chemical ignition can be used to heat the reservoir to 200-400 °C. In this temperature range, the MTO reaction releases tremendous heat. By continuously injecting air, the MTO reaction front maintained at 200-400 °C is advanced in the ground, forming a medium-temperature in situ combustion of heavy oil.

MTO in situ combustion mainly uses chemical ignition. The temperature of the combustion front formed is relatively low (generally lower than 400 °C), which has a weaker effect on the upgrading of crude oil. Oil reservoirs with a low oil viscosity are suitable for this kind technology.

The Du 66 block in the Liaohe Oilfield is a typical ordinary heavy oil reservoir. After years of steam stimulation, it has entered the late stage of recovery. In 2005, a pilot test of 7 well groups was carried out. A significant increase in production and good economic benefits have been achieved. As of the end of 2018, the number of on-site air injection in situ combustion well groups has reached 112, with an annual oil production scale of 24×10⁴ t, which is an increase of 16×10⁴ t compared with conventional steam stimulation. The Du 66 block in the Liaohe Oilfield is currently the largest straight- well in situ combustion base in China with significant recovery effects.

Air injection high-temperature in situ combustion technology has a wide range of adaptability and can be used in both ordinary heavy oil reservoirs and super/ultra-heavy oil reservoirs with high resin and asphaltene contents. This technology can be used as the initial recovery method of heavy oil reservoirs, and it can also be applied to heavy oil reservoirs at the later stage of steam injection to further improve the recovery efficiency. To date, the high-temperature air injection in situ combustion generally uses the electric ignition method to ignite the oil layer. During the process, heat transfer and complex physical and chemical changes are accompanied. Many oil recovery mechanisms, such as oil upgrading, steam flooding, hot water flooding, gas flooding, etc., are included.

Air injection high-temperature in situ combustion continuously injects air into the oil layer through the gas injection well. By forming a high-temperature combustion front that stably expands above 450 °C, crude oil is pushed from the air injection well to the production well. A high oil saturation wall in front of the combustion front usually exists in the high-temperature in situ combustion process, which can block high water-containing channels and cracks in the reservoir and then achieve a high-efficiency vertical spread of the oil layer by the high-temperature combustion front.

The Xinjiang Oilfield Hongqian No. 1 well block has carried out steam stimulation and steam flooding recoveries. The cumulative oil production during the stimulation period was 11.3×10⁴ t, and the stage recovery was 26.5%; the cumulative oil production during the steam flooding period was 1.0×10⁴ t, and the stage recovery was 2.4%. The production was discontinued for 10 years after the steam flooding was completed (Fig. 5). In 2009, PetroChina carried out a pilot test of air injection high-temperature in situ combustion in this reservoir, using a linear well pattern of 13 injection wells and 37 production wells. Comparing core photographs before and after the air injection in situ combustion in well h2118A in the test area (Fig. 6) found that there were significant differences in lithology and residual oil saturation in the entire oil reservoir after the in situ combustion. All the reservoirs spread longitudinally, and the remaining oil saturation of the entire core was less than 5%. The ability of the high-temperature in situ combustion to comprehensively improve the longitudinal spread is not available in other flooding methods. As of 2018, the cumulative oil production in the test block was 14.7×10⁴ t, the stage recovery was 34.6% (Fig. 5), and the cumulative air-oil ratio was 2789 m³/m³. The energy saving and emission reduction effect is obvious. Compared with steam flooding, this method can save 16.2×10⁴ t of standard coal and 12.5×10⁴ t of carbon dioxide. Air injection high-temperature in situ combustion technology can be used as a strategic replacement technology for steam injection to recovery heavy oil blocks with a long recovery history.

The oxidation reactions of crude oil over the whole temperature range of air injection have the following characteristics: below 120 °C, no oxidation reaction of crude oil occurs and no obvious thermal effect; from 120-200 °C, crude oil and oxygen mainly undergo the LTO reaction, consuming oxygen and releasing a certain amount of heat; from 200-400 °C, the polycondensation and bond-breaking reactions of the crude oil occur generating CO₂, CO, H₂O, and oxidized coke, releasing more heat than LTO; and at higher than 400 °C, the crude oil cracking and coke oxidation reactions occur, producing pyrolysis coke and releasing abundant heat simultaneously.

The full temperature range air injection recovery method has been discussed. For light oil reservoirs with a reservoir temperature of less than 120 °C, oxygen-reduced air injection can be used to effectively supplement reservoir energy. For light oil reservoirs with a reservoir temperature is higher than 120 °C, air injection can be directly carried out, and the oxygen can be consumed by the LTO reaction. Ordinary heavy oil reservoirs can be recovered by air injection medium-temperature in situ combustion with chemical ignition to heat the reservoir to 200-400 °C. Ordinary heavy oil reservoirs and reservoirs of super/ultra-heavy oil with high resin and asphaltene contents can be recovered by air injection high-temperature in situ combustion with the electrical ignition method to heat the reservoir to over 450 °C.

PetroChina's research and field tests over the past 10 years have confirmed that air is a high-efficiency, low-cost, and green type of oil recovery medium. Oxygen-reduced air injection, air injection, medium-temperature in situ combustion and high-temperature in situ combustion have shown the characteristics of an enhanced recovery, low cost, energy savings, water savings and environmental friendliness. These technologies have broad application prospects.

The research results of this article have received large support from the relevant oilfields and research institutes. Research literatures by many experts and oilfield test results are quoted in this article. We are sincerely thankful for all these contributions.

m_coke1—mass of oxidized coke, g;

m_coke2——mass of pyrolysis coke, g;

Q₁—heat release of the oxidation reaction of oxidized coke, J/g;

Q₂—heat release of the oxidation reaction of pyrolysis coke, J/g;

Q_HT—total heat release of HTO per gram coke, J/g;

R—mass percentage of oxidized coke in total coke involved in HTO, %;

x—carbon number of oil and its oxygenated products;

y—hydrogen number of oil and its oxygenated products;

z—oxygen number of oil and its oxygenated products;

α—carbon number of coke;

β—hydrogen number of coke;

γ—oxygen number of coke.