Advances and Factors Influencing In Situ Combustion Effectiveness: A Review

Abstract

In situ combustion, as a technology for improving oil recovery efficiency, faces technical and economic challenges. Fire-driven oil recovery technology is renowned for its significant technical advantages, including wide reservoir applicability, efficient crude oil recovery rate, and lower extraction costs. It is particularly suitable for the recovery of high viscosity petroleum resources such as heavy oil and oil sands. However, due to the complexity of the fire-driven mechanism, there are still many problems in the engineering design of fire-driven reservoirs. In particular, the lack of intuitive and accurate understanding of the combustion and fire-driven process in the reservoir makes it difficult to take effective means to accurately judge the underground combustion conditions, monitoring and control of the fire-driven leading edge. This paper reviews the effects of permeability, oil saturation, gas injection rate, injection and extraction well spacing, and reservoir thickness. These findings can help to improve the stability and efficiency of fire-driven technology so as to realise better mining results in practical applications.

1. Introduction

In situ combustion is a type of EOR (enhanced oil recovery) technology aimed at reducing the viscosity of oil and improving its flowability by heating and burning hydrocarbon substances in the formation, thereby improving the flow [1] to production wells. This indicates that in situ combustion is an important branch of EOR technology and has a direct impact on the development of the EOR process. Heavy oil is indeed an important unconventional hydrocarbon resource that is difficult to extract due to its high viscosity and density. Currently, the world’s heavy oil reserves are mainly concentrated in areas such as the oil sands of Canada and the Orinoco Belt of Venezuela [2]. Fire-driven technology is a type of thermal recovery process in which combustion is sustained by igniting a fire in the formation and supplying oxygen (usually air), which generates heat to reduce the viscosity of the oil, increase its fluidity, and enhance recovery.

The effective utilisation of these resources requires the use of efficient extraction technologies, of which fire-driven technology is one. The fireline (combustion front) and the coking zone are key areas in the fire-driven process, and the speed and temperature of the fireline advance and the characteristics of the coking zone will directly affect the effectiveness and efficiency of the fire-driven process. In order to optimise the fire-driven process, researchers and engineers have investigated a variety of different fire-driven schemes and monitoring techniques, including controlling the combustion rate, improving the efficiency of oxygen delivery, and monitoring the subsurface temperature distribution. Environmental protection and production safety also need to be considered to ensure that the fire-driven process minimises the impact on the environment and ensures the safety of the operators. As technology advances and demand for unconventional oil and gas resources increases, extraction technologies for heavy oil and other hard-to-recover resources will continue to advance, thereby increasing the utilisation of these resources and supporting the long-term development of the global energy market [3].

Burning oil layers is a highly adaptable and fully utilised extraction technology for petroleum resources, and fire drive is an effective technology to improve oil recovery. It uses heavy components in the formation crude oil as fuel, air or oxygen rich gas as a combustion aid, and adopts methods such as self-ignition and artificial ignition to make the temperature of the oil reservoir reach the ignition point of the crude oil. The combustion aid is continuously injected to keep the crude oil in the oil reservoir burning, producing a large amount of heat in the combustion reaction. The oil reservoir is heated, causing the temperature of the oil reservoir to rise to 600~700 °C. The heavy components are cracked at high temperatures [4], and the injected gas, light oil produced by heavy oil cracking, gas produced by combustion, and steam are used to drive the flow of crude oil to the production well and extract it from the production well. The fuel for fire drive is usually considered to be a coke like substance that precipitates on the mineral matrix during the thermal cracking reaction process. The main mechanisms are high-temperature cracking, gas driving, and heating to reduce viscosity. Compared with other heavy oil extraction methods, the combustion oil layer has the following advantages [5,6,7]. First, the displacement medium injected underground is compressed air, which is not only inexpensive but also widely available. Second, during the burning process, a small amount of heavy components are burned, while the remaining oil undergoes high-temperature cracking to improve its quality. Third, it has high thermal energy utilisation efficiency. Fourth, it integrates various oil displacement mechanisms such as steam flooding, mixed phase flooding, and carbon dioxide flooding. Among various technologies for burning oil reservoirs, THAI fire-driven technology has broad application prospects due to its unique well network layout, which features a larger combustion chamber volume, higher sweep coefficient, shorter oil discharge distance, and stable hot line propulsion. The THAI process, on the other hand, has none of the disadvantages of the ISC, as it is a short-distance oil displacement process, and according to laboratory studies, it has very high oil recovery factors of up to 85% of oil originally in place (OOIP) [8,9,10,11,12]. Furthermore, the THAI process has been studied through numerical simulations with aims of providing design and operation procedures as can be found in multiple [13,14,15,16,17,18,19,20,21,22,23,24] articles.

It is of great significance to consider the factors affecting the effect of fire-driven combustion in the process of fire-burning oil reservoirs by studying the different ways of thermal recovery and to adjust the state of fire drive to further enhance the effect of fire-driven development.

2. Mechanism of Fire-Driven Combustion and Different Ways

2.1. Research on the Fire-Driven Reaction Mechanism

In situ combustion is an enhanced oil recovery method that uses the heavier components of crude oil as fuel. The process involves igniting the oil in the reservoir, either through spontaneous or artificial means, such as using electric fuses, burners, or pyrotechnics at the well bottom to create high temperatures for combustion.

Oxidants like air or oxygen-enriched gas are continuously injected into the reservoir through gas injection wells to sustain the combustion. This contact between the oxidant and the heavy oil fractions results in a heat-generating combustion reaction, creating a “fireline” that moves through the reservoir. As the fireline advances, it heats the crude oil, causing physical and chemical changes like viscosity reduction and breaking down the oil into lighter oils and gases, which are then pushed forward. The heavier components left behind are further broken down into coke, which serves as additional fuel to continue the process, reducing the need for fresh crude oil [25].

The technique employs various drive mechanisms like flue gas drive, gas mixed-phase drive, and superheated steam flushing to move the crude oil toward production wells. The success of in situ combustion depends on factors including reservoir properties, operational conditions, and monitoring techniques. Thus, careful planning, real-time monitoring, and adjustments are crucial for an effective fire-driven project to optimise oil recovery.

However, this technology also has certain limitations. Traditional fire-driven technology may encounter problems of decreased thermal efficiency and reduced thermal wave and range in the later stages. There are difficulties in starting high-viscosity crude oil with fire-driven technology, which limits its application in certain heavy oil reservoirs. The presence of secondary water bodies in the later stage of steam injection may lower the peak temperature of the fire-driven combustion zone and expand the coverage range of the thermal front. There is a significant gas overlap phenomenon during the fire-driven process, which may lead to the presence of unburned coking zones at the bottom of the oil reservoir. Although most of the crude oil in the coking zone has been displaced, the remaining oil saturation is still below 20%, which affects the final recovery rate of the reservoir. At the same time, there is also some room for development. Coupling the advantages of steam flooding and fire flooding, a steam air composite fire flooding technology is proposed to improve the displacement effect. By optimising parameters such as the fire drive displacement mode, well network, well spacing, and gas injection rate, fire-driven technology is expected to further improve the final recovery rate of oil reservoirs. In the context of “dual carbon”, it is necessary to rely on improving the overall efficiency, optimising the energy consumption structure of oil fields, promoting the electrification and intelligence construction of oil fields, and vigorously developing low-carbon thermal recovery technology. It is necessary to vigorously develop efficient cold recovery technology, and promote the transition from hot recovery process to cold recovery process, in order to achieve quality improvement, efficiency enhancement, energy conservation and emission reduction in the development of difficult to recover heavy oil [26].

Burning the oil layer mainly utilises the partial combustion and cracking products of the oil layer itself as fuel, and it uses injected oxygen rich or air sources to ignite and maintain continuous combustion of the oil layer through electric heating and ignition methods. In this process, complex and multiple driving effects are achieved, ultimately pushing the live line continuously to the bottom of the production well. The oil displacement principle is as follows: under a certain well network mode, inject combustion-supporting gases such as air or oxygen into the injection well first so that the oil reservoir has sufficient relative permeability to provide sufficient oxygen for combustion and smoothly discharge the exhaust gas generated during the combustion process. Then, the formation is continuously heated by the igniter. Due to the low-temperature oxidation of the oil reservoir and the continuous accumulation of heat, coupled with the promotion of the igniter, the temperature of the combustion zone will continue to rise over time. When the temperature is higher than the ignition point of the crude oil, high-temperature oxidation will occur, instantly igniting the oil reservoir and causing it to ignite. At this time, sufficient ventilation intensity is controlled to gradually form a combustion zone with a certain area and a slowly advancing combustion front edge. That is, when the ignition line is established, the gas injection well will be stopped. Heat up, but continue to increase the injection volume, of the stable high-temperature combustion zone, slowly advancing from the injection well to the production well. High temperature causes crude oil in the near wellbore area to be distilled and cracked, resulting in complex chemical reactions of various high molecular weight organic compounds. The distilled light oil, water vapor, and combustion flue gas are driven forward and undergo heat exchange with the low-temperature zone at the front of the fire line, washing away the crude oil in the reservoir again. The residual coke after distillation and cracking deposits on the surface of the sand particles as fuel for fire-driven combustion, continuously generating the heat needed for oil recovery to maintain the forward combustion of the reservoir. Only after these fuels are basically burned out does the combustion front begins to move forward. During this process, a large amount of high-temperature gases and fluids are generated, including exhaust gases such as carbon monoxide, carbon dioxide, water vapor, gas-phase hydrocarbons, and condensate oil. At the same time, a series of complex oil displacement mechanisms such as thermal viscosity reduction, thermal expansion, distillation vaporisation, oil phase mixed displacement, gas displacement, and high-temperature changes in relative permeability are combined to drive crude oil toward production wells [27]. The basic technical diagram of the in-situ combustion method is shown in Figure 1.

SARA Freitag and Verkoczy proposed a method in 2005 [28] to divide crude oil into 14 pseudo-fluids. The reaction equation is shown as follows:

$$Asph+O_2\to H_{2}O+C{O}_2+OxdAsph$$

$$Resins+O_2\to H_{2}O+C{O}_2+OxdResAr$$

$$Arom+O_2\to H_{2}O+C{O}_2+OxdResAr$$

$$Sat+O_2\to H_{2}O+C{O}_2+Oxdsat$$

The above is the equation for low-temperature oxidation.

$$Coke+O_2\to H_{2}O+C{O}_2$$

$$OxdAsph+O_2\to H_{2}O+C{O}_2$$

$$OxdResAr+O_2\to H_{2}O+C{O}_2$$

$$Oxdsat+O_2\to H_{2}O+C{O}_2$$

$$Asph+O_2\to H_{2}O+C{O}_2$$

$$Resins+O_2\to H_{2}O+C{O}_2$$

$$Arom+O_2\to H_{2}O+C{O}_2$$

The above is the equation for high-temperature oxidation.

$$Asph\to H_{2}O+Arom+Sat+Lites+Coke$$

$$Resins\to H_{2}O+Asph+Arom+Sat+Lites$$

$$Arom\to Asph+Lites$$

The above is the equation for the cracking reaction. The explanation of vocabulary is shown in

Table 1. Explanations of specialized terms in relational expressions.

Acronym	Translate
Asph	Asphalt
Resins	Resin
Arom	Aromatic hydrocarbons
Sat	Saturated hydrocarbons
Oxd	The oxides of various substances
Coke	Coke
Lites	Light oil
LTO	Low-temperature oxidation reaction
HTO	High-temperature oxidation reaction
Crac	Cracking reaction

below.

2.2. Research on Different Modes of Fire-Driven Combustion

2.2.1. Dry Combustion

Based on the mechanism of fire-driven development and the arrangement of gas injection and production wells, fire-driven technology can be subdivided into dry fire-driven technology, wet fire-driven technology and horizontal well fire-driven technology. The dry positive combustion leading edge moves in the same direction as the air flow direction. The combustion starts from the gas injection wells, and the combustion leading edge moves from the injection wells to the production wells, and from the injection wells to the production wells, it can be divided into several zones such as the combusted zone, the combustion zone, the coking zone, the evaporation (cracking and distillation) zone, the light oil zone, the oil-rich zone, and the unaffected zone [29,30]. These zones move along the direction of air flow, as shown in Figure 2.

Dry combustion technology is an oil recovery method that improves the recovery rate of oil reservoirs. Its core lies in using flames to heat and burn the crude oil in the reservoir, reducing its viscosity and promoting its flow, thereby improving the recovery rate of crude oil. In dry fire drive, the leading edge of the flame (i.e., the most advanced part of the flame) moves in the same direction as the flow of air or other injected gases. These areas gradually form as the flames advance. Dry fire-driven technology starts from the injection well, and the flame front moves toward the production well direction. Gas injection wells are responsible for injecting air or other gases to maintain combustion, while production wells are used to collect heated and partially burned crude oil. As the flame advances, crude oil undergoes physical and chemical changes. In the evaporation zone, light components in crude oil will evaporate. In the coking zone, heavy components will form coke. In the combustion zone, crude oil is directly burned. By reducing the viscosity of crude oil and creating driving force, dry fire-driven technology can effectively improve the recovery rate of oil reservoirs, especially in heavy oil and heavy oil reservoirs that are difficult to effectively exploit with traditional oil recovery methods. Dry fire-driven technology requires strict environmental and safety controls to prevent uncontrolled fires and reduce environmental impact. Although the initial investment in dry fire flooding technology may be high, it can improve oil recovery and reduce the cost per unit of crude oil in the long run. Dry fire flooding technology is an effective method to improve reservoir recovery, and it is especially suitable for reservoirs that are difficult to economically and effectively exploit with traditional oil recovery techniques.

2.2.2. Wet Forward Combustion

Wet forward combustion refers to the movement of the combustion belt from the injection well to the production well accompanied by the water phase drive. Compared with dry combustion, a large amount of heat is between the steam injection well and the combustion belt. Due to the high specific heat and high latent heat of vaporisation of the water phase, wet combustion can enhance the heat transfer and partially reach the front of the combustion belt. It is characterised by high heat utilisation but difficult operation. Wet combustion is more effective than dry combustion in driving oil. There are three main reasons: (1) steam belt oil drive is an important mechanism in the fire-driven process; (2) with the increase in the wet combustion water-to-gas ratio, the scope of the region where the oxidation reaction occurs to expand the temperature of the steam belt decreases, while the speed of convection leading edge increases, accelerating heat convection conduction, and the efficiency of the oil drive increases; (3) in the process of wet combustion, with the reduction in oxygen utilisation, the amount of air required to burn 1 m³ oil sands is lowered, the combustion leading edge velocity slows down, and the oil repellent efficiency [31] is almost unchanged.

Wet burning is also known as a combination of forward combustion and water drive [32]. The specific method is to inject water and gas into the injection well alternately or simultaneously; then, the water will be wholly or partially vaporised through the combustion leading edge to transfer heat to the front of the combustion zone to expand the volume of steam and hot water zones in front of the combustion zone to reduce the viscosity of crude oil. At low pressures, less fuel can be used to drive thick oil, and in some cases, a combination of these factors can reduce the air-to-oil ratio.

2.2.3. COSH

COSH (the combustion override split production horizontal well) is another way of combining gravity drainage and fire-driven technology, and the parallel type is generally in the form of straight-well–horizontal-well combination. The process was first proposed by KE Kisman in the early 1990s, as shown in Figure 3. Its operation process includes a combination of multiple vertical injector wells and horizontal production wells, and in general, the vertical injector wells are directly above the horizontal production wells.

The production [33] principle is as follows. (1) Oxygenated gas is injected into the formation through a row of vertical wells directly above the horizontal wells, and generally, the gas is injected into the upper part of the formation, and cold water is circulated in the injection wells to prevent damage to the wellbore due to excessive temperature. (2) Place exhaust wells away from production wells to vent flare gas, minimise O₂, and cool output gas. (3) Position horizontal production wells at the oil formation base for maximum gravity drainage; monitor to prevent high temperatures near wells. (4) If there is a poor thermal link between the injection and production wells initially, take action to establish it quickly to reach a high oxidation temperature. (5) Start with scattered combustion chambers around each injection well, expanding to form a continuous chamber as the process continues; heat crude drains to production wells by gravity, akin to SACD.

2.2.4. Vapour Throughput

Thermal recovery is a commonly used development method in China’s thick oil reservoirs, which includes steam throughput [34,35], steam drive [36,37], steam-assisted gravity drainage (SAGD) [38], and fire oil layer [39,40,41] and other methods. Among them, the steam throughput method is suitable for most of the thick oil reservoirs with good economic benefits, simple construction methods, and less influence by the non-homogeneity of the reservoir and the viscosity of the crude oil, which is a very widely used thermal recovery method for thick oil reservoirs in China at present [42,43].

(1)

Steam throughput

The steam throughput rate is a thermal recovery technology used to improve the recovery rate of heavy oil reservoirs. It involves injecting high-temperature steam into the oil well, then closing the wellhead to allow the steam to diffuse in the reservoir and heat the surrounding crude oil, reducing its viscosity and enhancing its fluidity. This process is called ‘stewing’. After a period of time, the wellhead is opened and oil extraction begins, which is a process called “throughput”. During steam throttling, when the oil production rate in one cycle drops below the critical value, the next cycle will begin, including steam injection, well stewing, and oil recovery. This process will be repeated until the accumulated oil-to-gas ratio reaches its limit, at which point other extraction methods will be considered. The characteristics of steam huff and puff technology include direct conversion to oil pumping at high temperatures, without the need for well killing operations, avoiding heat loss and the oil reservoir and well site pollution. In addition, the design of the long plunger and short pump barrel structure, combined with the steam injection hole, has sand control, sand discharge, and sedimentation functions, which are suitable for production in high angle and horizontal wells. The key technical indicators of steam throttling technology include the rated evaporation capacity, rated steam temperature, rated working pressure, and steam quality. The design thermal efficiency and operational thermal efficiency are important indicators for measuring the economic viability of this technology. Steam huff and puff technology can effectively improve the oil recovery rate of heavy oil reservoirs with an annual oil recovery rate that is three to eight times that of conventional oil recovery methods, and it has good economic benefits. During steam throttling, the viscosity of crude oil decreases and the flow resistance decreases after heating, which is the main mechanism for increasing production. The steam injected into the reservoir will raise the temperature of the formation near the wellbore, heating the reservoir and crude oil. Although steam preferentially enters the high permeability layer, the heating range gradually expands due to thermal convection and conduction, thereby increasing the recovery rate of crude oil. Overall, steam throughput is an effective technique for heavy oil recovery, which can significantly improve the recovery rate of reservoirs by precisely controlling the steam injection rate and well holding time. However, this technology needs to consider the permeability of the reservoir, the properties of the crude oil, and the wellbore conditions during implementation to ensure optimal production efficiency [42].

(2)

Steam driven

Steam drive is also one of the most commonly used development methods in heat-injection carriers, which is generally adopted as a replacement development method when the field becomes inefficient or ineffective after multiple rounds of steam throughput. Steam drive refers to the continuous injection of steam into the formation from an injection well to drive off the heated crude oil in the reservoir, while the production well is continuously open for production [44].

(3)

Superheated steam extraction

Superheated steam is a new type of heat carrier with a dryness of up to 100%, and the higher enthalpy of superheated steam makes it more effective in heating crude oil in reservoirs. At the same time, compared with ordinary steam, superheated steam can strengthen its distillation effect on crude oil so that the oil driving efficiency is significantly improved [45].

(4)

Steam-Assisted Gravity Drainage (SAGD)

In 1994, Dr. Butler was the first to propose SAGD technology [46,47], and the technique has been applied on a large scale in Liaohe and Xinjiang [48]. SAGD belongs to a special kind of steam drive, which relies on the gravity of crude oil for oil release. SAGD technology can be realised in three ways: double horizontal well SAGD, straight horizontal well SAGD and single well SAGD.

(5)

Steam-Assisted Gas Push (SAGP)

In the middle and late stages of SAGD development, problems such as high water content and pressure reduction in the steam chamber have significant impacts on steam thermal efficiency, and SAGP becomes an effective replacement technology. SAGP is based on SAGD, and it simultaneously injects non-condensable gases such as N₂, CO₂, and CH₄, and it also makes use of non-condensable gases to improve the steam thermal efficiency of SAGD by keeping heat, preserving pressure, and suppressing the steam over-coating, and thus it achieves the purpose of controlling the shape of the steam chamber and maintaining steam pressure, which can further improve the recovery rate of thick oil reservoirs [49,50]. A schematic diagram of the implementation of SAGD technology is shown in Figure 4 [51].

According to the relative layout of injection wells and production wells, steam assisted gravity drainage (SAGD) technology can be divided into two forms: “traditional” and reverse SAGD. In traditional SAGD, two wells are drilled from the same well site, with the injection well being approximately 5 m higher than the production well. Next, steam is injected into the injection well under a certain pressure, and with the help of gravity, hot oil containing light fractions flows towards the wellbore position of the production well. The steam-assisted gravity drainage (SAGD) technology involves drilling from two well sites within a block with an average distance of about 1 km between the two sites based on the horizontal section of the well. The production well is drilled first, followed by the steam injection well, ensuring that the horizontal section of the steam injection well is directly above the horizontal section of the production well. During the SAGD process, a large amount of steam needs to be injected into the reservoir: typically up to several hundred tons per hour. In addition, it is necessary to ensure that a suitable temperature is maintained inside the injection well to fully heat the crude oil for optimal results (see Figure 4).

2.2.5. Catalysts

In high-temperature oil reservoirs, crude oil can self-ignite with air injection. However, for cooler reservoirs with less oxidisable oil [52], external additives are needed to ignite the reservoir. Chemical ignition, an artificial method, raises the reservoir’s temperature by steam injection, followed by additive injection to induce intense oxidation upon contact with air or oxygen, thus igniting the reservoir [53].

Additives have been proven effective for enhancing the combustion performance of viscous oils and are economically valuable. Various additives are used at different oxidation stages, such as metal salts for low-temperature oxidation acceleration, iron salts for coke deposition improvement, and metal oxides for high-temperature combustion enhancement. The effects of these additives are depicted in Figure 5.

However, a single additive is often insufficient, and synergistic effects are observed when combining different additives, offering greater improvements than individual additives. The synergistic use of additives is a key development direction [54,55,56,57,58].

As shown in

Table 2. The main promising catalysts for mitigating the temperature oxidation processes.

Composition of the Catalyst	Effect	Results
Clay mixtures of 3 wt%	Catalytic	Controls the combustion front
Quartz and mica containing clay	Catalytic	Mica leads to a decrease in activation energy
Clinochlor and talc containing clay	Inhibitory	Leads to a delay in the reaction of isomerisation and decomposition as well as in the reaction of oxidative cracking
Kaolinite, montmorillonite, mica, and clinochlor containing clay	Catalytic	Controls the combustion front but does not lead to a shift at the stage of high-temperature oxidation
The level of calcite and dolomite	Catalytic	Provides light ignition and significantly reduces the activation energy
Copper stearate	Catalytic	Shifts the combustion reactions to a lower temperature range, especially during high-temperature oxidation. Increases the efficiency of coke combustion.

, the main promising catalysts for slowing down the temperature oxidation process are displayed.

This figure shows the effect of additives on the heat released during oxidation at different temperatures and how additives alter the heat distribution during the oxidation process. There are two curves in the figure, representing the situation without additives (black line) and with additives added (blue line). In low-temperature regions, additives (such as oil soluble metal salts) can enhance low-temperature oxidation heat release and reduce ignition temperature. This means that additives help to initiate the oxidation process at lower temperatures, thereby reducing the energy required to start the fire drive. In the “coke deposition” stage, this refers to the possibility that certain components in the crude oil may be converted into coke during the oxidation process. Additives can improve the deposition of coke, reduce its blockage of reservoir pores, and thus enhance the permeability of the reservoir. Adding iron salts can improve coke deposition and help crude oil quickly enter the high-temperature oxidation zone. This indicates that iron salts help improve the thermal efficiency during the fire-driven process. Metal oxides can enhance high-temperature oxidation and increase the intensity of oxidation reactions. This helps to convert crude oil more effectively at high temperatures and improve oil recovery. The use of additives shifts the various stages of the oxidation process toward lower temperature regions, which means that the entire oxidation process can be carried out in a lower temperature range, thereby improving the efficiency of fire drive. The Y-axis represents the heat released during the oxidation process, which is measured as the volume fraction of carbon dioxide (CO₂). This reflects the intensity of oxidation reactions at different temperatures. Overall, this graph illustrates the important role of additives in the process of fire flooding, as they can optimise oxidation reactions and improve the efficiency and effectiveness of fire flooding. By adjusting the type and dosage of additives, the temperature distribution during the fire flooding process can be controlled, thereby improving the recovery rate of crude oil.

In addition to the above factors, the SARA (saturated hydrocarbons, aromatic hydrocarbons, colloids, asphaltene) components of crude oil also have a significant impact on the ignition effect of fire flooding [59]. Understanding the oxidation characteristics of various components in heavy oil is of great significance for guiding the selection of additives. Saturated hydrocarbons are more prone to spontaneous combustion due to their lower molecular weight and lower self-ignition point. Aromatic hydrocarbons are easily oxidised during the low-temperature oxidation process of crude oil, converting into colloids and asphaltenes. At the same time, some colloids and asphaltenes in crude oil will crack into coke, providing necessary fuel for high-temperature combustion. Colloids have stronger low-temperature oxidation activity than other components, and the combustion characteristics of coke generated by crude oil oxidation are very similar to those of coke generated by asphaltene. This further confirms that the fuel required for crude oil combustion mainly comes from the cracking of asphaltene [60].

Therefore, for heavy oil with low content of aromatic hydrocarbons, resin, asphaltene and other heavy components, in order to ensure sufficient fuel during combustion, additives such as iron salts or tin salts that can improve coke deposition need to be added [61]. For heavy oil with low saturated hydrocarbon content, it is necessary to introduce oil-soluble metal salt additives that can reduce the self-ignition temperature and enhance the low-temperature oxidation heat release effect. This type of additive selection helps optimise the combustion efficiency and effectiveness during the fire-driven process.

The most promising catalysts for controlling the temperature oxidation process are detailed in various tables from cited studies. Currently, additive evaluation and screening are primarily conducted through indoor experiments, such as TG-DSC, reactors, combustion pools, and other methods to assess the additives’ impact on fire oil layer combustion efficiency and reaction kinetics. A comparison of indoor evaluation methods is shown in Figure 6, where TG, combustion cell, and reactor experiments are used to evaluate additive effects by comparing reaction kinetic parameters.

Differential Scanning Calorimetry (DSC) and porous media thermal effect monitoring experiments are both based on the principle of exothermic oxidation to evaluate the performance of additives. The DSC experiment evaluates by comparing the enthalpy changes of reactions, while the porous media thermal effect monitoring experiment evaluates by comparing the thermal effect changes of oil samples in porous media. The combustion tube experiment can provide key fire drive parameters such as oxygen utilisation rate, fuel mass concentration, propulsion speed and distance of the combustion front, and they can simulate convection, heat loss, and fluid flow in the reservoir, making the experimental conditions more closely related to the actual reservoir environment. However, this experimental procedure is more complex, costly, and has a longer experimental period, consuming more time and materials. Therefore, it is more suitable for evaluating the effectiveness of additives rather than being the preferred experimental method for screening additives.

2.2.6. Toe-to-Heel Air Injection

THAI (toe-to-heel air injection) is a fire-driven extraction method using horizontal well technology, which is particularly suitable for unconventional oil resources such as heavy oil and oil sands. The following are the general steps and characteristics of the THAI process.

In THAI, pairs of horizontal wells are typically used with one used as an injection well (toe) and the other as a production well (heel). This configuration allows crude oil to be extracted over shorter distances, which reduces the distance the oil has to be transported and the corresponding pressure loss compared to conventional vertical wells. Prior to the start of air injection, the oil formation around the injection well needs to be preheated to ignition temperature by heat transfer, convection, or other heating methods to ensure that the oxidation reaction can proceed smoothly. Once the proper temperature is reached, the continuous injection of air begins to oxidise with the crude oil in the subsurface. The heat generated during this process helps reduce the viscosity of the crude oil and drives the flow of crude oil to the production well. Crude oil cracks at high temperatures, producing heavy coke deposits that form a coking band that acts as a fuel to support the ongoing combustion reaction. The heat generated by combustion not only reduces the viscosity of the crude oil but also activates the rock extensions and flue gases, which are further propagated downward to the unheated crude oil areas.

Due to gravity and convection currents caused by temperature differences, the heated and reformed crude oil flows to the horizontal production wells and is brought to the surface through the production system. As shown in the Figure 7, due to the short distance between injection and production wells, movable oil does not need to undergo long-distance migration as in the case of conventional fire drives, thus reducing oil loss and improving recovery efficiency [6,9,10,12,62,63,64,65,66]. The advantages of THAI are its higher recovery rates, lower surface facility requirements and smaller environmental footprint. In addition, because the combustion leading edge is closer to the horizontal production well, the pressure and temperature can be managed more efficiently, improving safety. The direction of the arrow in the vertical well is the direction of air flow.

It can also achieve a significant amount of in situ upgrading through thermal cracking, producing the upgraded oil to the surface. This process operates in a gravity stable manner by restricting drainage to narrow moving areas. This results in the flow of mobile fluid directly entering the exposed portion of the horizontal production well. This process can be operated as a new technology, as a follow-up technology to existing technologies, or as a collaborative process that requires high thermal efficiency advantages in primary production. This is achieved by concentrating the energy required for oil mobilisation, recovery, and thermal upgrading in the reservoir.

3. A Research on the Factors Influencing Fire-Driven Combustion

The stability during the combustion process of fire drive is closely related to factors such as reservoir properties, well layout, and gas injection rate. However, the impact of these influencing factors on the stability of fire-driven combustion is not yet clear.

Combustion stability refers to the spontaneous occurrence of high-temperature combustion reactions when fuel comes into contact with oxygen in the air without the need for external energy supply; that is, the heat released by the combustion reaction is sufficient to maintain its own combustion. Through extensive literature research and summarisation of previous work, the conditions for maintaining stable combustion in a fire-driven environment can be summarised as follows [67,68,69]. (1) During the ignition stage of fire drive, it is necessary to ensure a sufficiently high preheating temperature and a certain oxygen supply to avoid long-term low-temperature oxidation reactions in the early stages of ignition, which can cause a large amount of resin and asphaltene components in the crude oil to deposit, block pores, and prevent stable expansion of the fire chamber. (2) During the fire-driven process, the air injection speed should not be too high, and it is necessary to ensure that all the injected oxygen participates in the oxidation reaction and is consumed to avoid oxygen breakthrough in the production well. (3) The gases generated during the combustion process and the crude oil that has been modified and reduced in viscosity by heating can be smoothly released into the production well, while oxygen is completely consumed in the firewall and coking zone.

Under laboratory conditions, stable combustion generally manifests in the following three aspects.

(1) The combustion front advances smoothly. The live line is the surface where fuel and oxygen come into contact and undergo intense exothermic reactions, and it is an important parameter for studying the process of fire-driven oil displacement. The advance speed of the live line is significantly affected by factors such as reservoir properties, crude oil properties, gas injection rate, and injection production pressure difference. In the one-dimensional combustion tube experiment, under the condition of constant injection velocity, if the hot wire advances slowly or cannot advance, it indicates that the combustion reaction has not spontaneously occurred.

(2) The concentration (volume percentage concentration) of each component in the produced gas does not experience significant fluctuations. The concentration of gas components produced is an important monitoring indicator for fire-driven indoor experiments. Due to factors such as reservoir characteristics, crude oil properties, and gas injection rate, the fluctuation range of the concentration of each component in the produced gas during different experimental processes may vary. In one-dimensional combustion tube experiments, due to the fact that the volume of the combustion chamber remains relatively constant, the combustion area remains relatively constant, and the oxygen consumption per unit time remains relatively constant, the fluctuation of the concentration of each component in the produced gas can to some extent characterise the stability of combustion. In the three-dimensional fire drive experiment, if the oxygen content increases sharply in a short period of time, it indicates that the injected air enters the production well directly along the high permeability channel without sufficient contact with the fuel, resulting in unstable combustion reactions.

(3) The fire-driven experiment achieved a high recovery rate. The main energy source of the fire-driven process is the heat released during high-temperature combustion. A more stable combustion process is beneficial for improving the recovery rate, so the recovery rate also reflects the stability of the combustion process to a certain extent.

The following text observes the stability of fire flooding through the study of factors such as permeability, oil saturation, gas injection rate, injection production well spacing, and reservoir thickness.

3.1. Permeability

Permeability is the ability of a rock to allow the passage of fluids under certain differential pressure conditions. In the fire-driven process, the heat released from the combustion reaction increases the reservoir temperature, greatly reduces the viscosity of crude oil, and enhances the fluidity of the crude oil. Under the gas-driven as well as gravity-driven effects, the fluid in the reservoir is accompanied by a complex flow, and the permeability has a significant effect on the flow resistance. The fire-driven process needs to continuously provide sufficient oxygen to the combustion area to maintain combustion, and the output gas as well as the viscosity-reduced crude oil also needs to flow into the production well through the drainage channel, which requires the reservoir to satisfy a certain fluid-passing capacity, and with the increase in permeability, the fireline propulsion speed gradually increases. This is due to the fact that a larger permeability has a better fluid flow capacity, and the crude oil is more easily driven out of the gravel voids, which accelerates the advancement speed of the fireline [70]. Therefore, permeability has a significant effect on the oil driving effect of fire-driven combustion.

3.2. Oil Saturation

Oil saturation refers to the ratio of the volume of pore space occupied by crude oil to the total pore volume of the rock in the oil formation. In the process of fire-driven THAI, crude oil is used as the main fuel for combustion reaction, and the nature of crude oil and oil saturation have a great influence on the combustion reaction: if the oil saturation is too low, the heat released from the combustion reaction will not be able to maintain the stable combustion, and the size of oil saturation will also have a great influence on the flow of fluids in the rock voids, which will, to a certain extent, affect the combustion stability and oil-driving effect of fire-driven THAI.

We selected T2, T4, T6, T8, T10 and T12 for the study, and the temperature versus time curves for each group of experiments are shown in Figure 8 and Figure 9 [70].

It can be seen that in the experiment with 8.0% oil saturation, the maximum temperature reached 470 °C, but it lasted for a short time and the temperature quickly and sharply decreased. At the initial stage of ignition, the experiment can maintain the combustion reaction for a period of time due to the energy supply from the igniter, but the heat generated by combustion is not enough to maintain the stable advance of the fire line backward, the combustion reaction stops very quickly, and the temperature decreases rapidly, which leads to the failure of the experiment. This indicates that too low oil saturation cannot meet the heat demand of the combustion reaction and cannot ensure combustion stability during the experiment. In the other three groups of reactions, the maximum temperature reached about 600 °C and lasted for a long time, indicating that all three groups of experiments occurred during the intense combustion reaction.

3.3. Injection Speed

The gas injection rate in THAI fire-driving experiments is critical for maintaining stable combustion. If the rate is too low, it can lead to insufficient oxygen supply and flame extinguishment. Conversely, a high rate may cause excessive pressure, leading to oxygen breakthrough into horizontal wells and gas flaring.

Yalong Zhu’s [70] analysis of experimental data shows that a low injection rate initially results in successful ignition with low oxygen content, but this quickly rises and stabilises around 10%. This indicates high-temperature combustion at the experiment’s start, but due to oxygen scarcity and heat loss, the reaction’s heat output is insufficient to maintain high temperatures, causing the combustion to gradually cease and shift to low-temperature oxidation, as shown in the Figure 10, Figure 11 and Figure 12.

Higher injection rates facilitate oxygen breakthrough into production wells, causing gas flushing, as evidenced by comparing oxygen breakthrough times at 10 L/min and 12 L/min rates, which resulted in 1100 min and 800 min of high-temperature stable combustion, respectively. Numerical simulations also show that increased injection rates lead to earlier oxygen breakthrough and gas flushing. This is because higher rates increase combustion chamber pressure, requiring a thicker “oil wall” to prevent gas flushing, making it easier for oxygen to penetrate into production wells.

In summary, too low an injection rate hampers high-temperature combustion stability, reducing crude oil extraction and recovery rates. Too high a rate increases the pressure differential, leading to greater oxygen breakthrough, gas flushing, and larger dead oil zones, also reducing recovery rates. Therefore, finding the optimal injection rate is crucial for maximising oil recovery in THAI fire-driving processes.

3.4. Distance Between Injection and Extraction Wells

The distance between the gas injection well and the start of the horizontal well in THAI fire-drive oil extraction significantly impacts the process’s efficiency. This spacing affects gas flow in the reservoir, influencing both the direction of fire cavity expansion and crude oil movement. It also determines where gas flushing occurs, which is a key issue in THAI fire-drive technology.

The numerical models used in the study resemble three validated models from previous research, which were all created with the CMG STARS simulator. The model’s dimensions, well layout, and grid block count are depicted in Figure 13 with the reservoir divided into 30 blocks in one direction, 19 in another, and 7 in the third. Model A is shown in Figure 13a, while Models B (using electric heating) and C (using steam) are shown in Figure 13b. The simulator can simulate the dynamics of multiphase, multicomponent flow and heat transport after discretisation [71].

In Zhu Yalong’s research, an in-depth analysis was conducted on the role of horizontal wells in the process of fire flooding. In the study, the horizontal distance from the end of the horizontal well to the injection well was defined as X, and the vertical distance was defined as Y. By changing the length of the horizontal well, the spacing between injection and production wells can be adjusted [71]. In order to investigate the influence of different X values on the fire drive effect, researchers conducted three sets of 3D THAI (toe-to-heel air injection) fire drive simulation experiments, corresponding to wells with lengths of 48 cm, 45 cm, and 42 cm, respectively. In these simulations, the distance from the injection well end to the model end is fixed at 50 cm, while the X values are set to 2 cm, 5 cm, and 8 cm, respectively. The experimental results indicate that adjusting the X distance has a significant impact on the stability and efficiency of fire drive. A smaller X distance may result in earlier gas removal, affecting combustion efficiency. A larger X distance helps to improve combustion stability, as it reduces the risk of gas migration, avoids the formation of dead oil zones, and maintains stable high-temperature combustion for a longer period of time. The duration of stable high-temperature combustion observed in the experiment increases with the increase in X distance, reaching 200, 700 and 1100 min, respectively. This indicates that by carefully designing the distance between horizontal wells and injection wells, the fire-driven process can be optimised, and the recovery rate of the reservoir can be improved. In addition, studies have found that too small or too large X/Y distances can lead to shortened stable combustion length, reduced oil recovery rate, increased injection extraction pressure difference, and the formation of dead oil zones, thereby hindering combustion stability. Therefore, determining the optimal X and Y distance is crucial for the success of a fire drive project. Through these findings, Zhu Yalong’s research provides valuable guidance for the application of fire-driven technology, especially in designing well locations and optimising fire drive parameters, as shown in the Figure 14, Figure 15 and Figure 16.

In the experiment, the oxygen content and oxygen utilisation rate under different experimental conditions have a significant impact on combustion stability and efficiency. The average oxygen content in experiment 3 was the lowest, only 3.37%, but its oxygen utilisation rate was the highest, reaching 54.58%. In contrast, experiment 1 had the highest oxygen content at 10.43%, but the lowest oxygen utilisation rate was only 10.23% [70]. These results indicate that a higher X distance (which may refer to the distance between the burner and the oil reservoir or the distance between the combustion front and the production well) helps to improve combustion stability. A smaller X distance may result in earlier gas removal, affecting combustion efficiency. The stable duration of high-temperature combustion in the experiment increases with the increase in X distance, reaching 200, 700, and 1100 min, respectively. This further confirms the positive impact of X distance on the duration of combustion. However, inappropriate adjustment of the X/Y distance (which may refer to the ratio of the distance between the combustion front and the production well to the thickness of the oil reservoir) may lead to a shortened stable combustion length, decreased oil recovery rate, increased injection extraction pressure difference, and the formation of dead oil zones, all of which may hinder combustion stability. In fire-driven technology, the oxygen utilisation rate is a key parameter that reflects the efficiency of oxygen during the combustion process. The study on the boundary between high-temperature and low-temperature oxidation conversion shows that by adjusting the initial ignition temperature and ventilation intensity, a stable high-temperature oxidation front can be quickly established, thereby improving the efficiency of the fire-driven process. In addition, during the multi-layer fire-driven process, due to the large number of layers and the significant differences in physical properties among different oil layers, the strategy of layered gas injection can be used instead of general gas injection to meet the requirements of critical ventilation intensity, further optimise combustion stability, and improve recovery efficiency. In summary, the experimental results emphasise the importance of adjusting the X/Y distance and oxygen content appropriately during the fire flooding process to improve the combustion stability and oil recovery rate. By optimising these parameters, the efficiency and economy of fire-driven technology can be significantly improved.

3.5. Oil Thickness

Wei Yiguang [71] used the method of artificially increasing the cover layer in the process of loading oil sands to change the thickness of the oil layer, and they simulated the effect of THAI fire-driven oil in different formation thicknesses with the cover layer thickness of 6–10 cm. At the early stage of fire ignition, with the injection of air, the fire cavity development expands along the radial direction, presenting the “sphere” or “nest shape”. The fire cavity developed along the radial expansion with the air injection at the beginning of the ignition, showing a “spherical” or “nesting head shape”. With the advancement of the fire line, the longitudinal expansion of the fire cavity is accelerated under the effect of gravity oil drainage and pressure difference between injection and extraction, the dragging and pulling effect of horizontal wells on the advancement of the fire line is obvious, and the effect of gravity oil drainage is stronger. The pore cavity formed by oil drainage is filled by the injected air, which further causes the injected air to advance upward diagonally, and the fire cavity then advances upward diagonally.

As shown in

Table 3. Results of the experiments with different “thickness” value.

Cover Thickness	Oxygen Breakthrough Time	Recovery Ratio	Peak Combustion Temperature (°C)
6	25	26.7%	466
6	25	26.7%	466
10	2	3.07%	495

, when the thickness of the formation decreases, the expansion of the fire cavity is more influenced by the boundary of the formation, and it is easy to reach a balance between the heat generated by combustion and heat loss, forming a coking zone with a temperature of about 200–300 °C and a thickness of 1–2 cm. The dragging effect of horizontal wells on the development of fire cavity is more obvious, and the thick oil and flue gas subjected to heat and viscosity reduction are produced from the horizontal wells under the dual effects of gravity and injection and extraction differential pressure with faster oxygen breakthrough and shorter expansion distance of the leading edge. When the thickness of the 3D large-size THAI fire physical simulation model is larger, it is easy to achieve better fire-driven effect; when the thickness of the formation is smaller, it is easy to have gas scrambling.

4. Conclusions

In view of the research on the technology of flaring oil reservoirs, a review of the current techniques of flaring oil reservoirs has been carried out, exploring the effect on reservoir recovery under different factors. Fire-driven as an enhanced oil recovery (EOR) technology improves reservoir recovery by burning crude oil in the subsurface to reduce the viscosity of the remaining crude oil. During flaring, the subsurface crude oil undergoes physical and chemical property changes that are critical to improving the quality of heavy or ultra-heavy oil. The high temperatures generated by flaring the formation directly cause a decrease in the viscosity of the crude oil, as the increased temperature makes the large hydrocarbons in the crude oil more active, thus reducing the friction between them.

Lower oil saturation typically results in lower oxygen utilisation, as some of the oxygen passes directly through the reservoir without participating in the combustion reaction. At the same time, lower oil saturation also leads to lower recovery because the combustion front does not propagate evenly and efficiently, leaving more residual oil.

In order to achieve optimal fire drive results, it is necessary to find an appropriate balance of well spacing between ensuring adequate combustion and avoiding early gas breakthrough. In addition, advances in monitoring technology can help adjust gas injection strategies and production operations in real time, leading to precise control of the fireline and effective management of coking zones.

Permeability, oil saturation, gas injection rate, well spacing and reservoir thickness are key geological and engineering parameters that affect the research and application of fire flooding technology. They have a direct impact on the effectiveness and efficiency of fire flooding. The permeability determines the distribution and flow rate of the fire drive fluid in the reservoir, thereby affecting the effectiveness of fire drive. Usually, lower permeability can lead to poor fire drive performance, so high permeability is beneficial for the implementation of fire-driven technology. Oil saturation is an important indicator of water saturation before fire flooding, which determines the initial water content of fire flooding and thus affects the production dynamic characteristics in the early stage of fire flooding. High oil saturation is beneficial for improving the recovery rate of fire flooding. The gas injection rate is one of the important factors affecting the effectiveness of fire flooding. A faster combustion speed can improve the efficiency of fire drive and reduce the cycle of fire drive, but it also increases the risk of fire and explosion. The gas injection rate has a significant impact on the recovery rate, and there exists an optimal gas injection rate that maximises the recovery rate. The distance between injection and production wells affects the area sweep coefficient of fire flooding, and a reasonable well spacing can improve the efficiency and recovery rate of fire flooding. A large well spacing is beneficial for burning oil reservoirs. The thickness of the oil layer affects the thermal efficiency and oil displacement efficiency of fire flooding. Thick oil reservoirs are more suitable for the application of fire-driven technology because they can provide greater heat transfer and oil drive space.

Fire-driven technology is particularly suitable for heavy oil reservoirs, especially those developed by steam injection, and it has good adaptability, making it a highly promising alternative production technology. Fire-driven technology has application prospects and potential in heavy oil old areas and difficult to produce reserves.

Although fire-driven technology has obvious advantages in improving the recovery rate of heavy oil reservoirs, it also has some limitations, and its future development potential is worth paying attention to. During the process of fire flooding, the direction of advance of the combustion front is difficult to control, which can easily cause unidirectional fire breakthrough and fire line overlap, resulting in low horizontal and vertical sweep coefficients, thereby affecting the recovery rate. Fire-driven technology requires continuous injection of air to maintain high-temperature oxidation, but in practical operation, this requirement is often difficult to meet. For heavy oil reservoirs in the later stage of development, the engineering design of fire-driven technology becomes more complex due to the scattered distribution of remaining oil and the heterogeneity of the reservoir. Due to the combustion process occurring inside the oil reservoir, there are significant limitations in quantifying the process and a lack of comprehensive understanding. Fire drive has strict requirements for on-site operation and management, and there are difficulties in rigorously evaluating pilot tests. The separation methods of gas produced by oilfield fire drive have certain limitations in engineering applications, such as the high equipment investment and energy consumption of chemical absorption method; The pressure swing adsorption method and membrane separation method have problems such as small processing capacity and easy material deactivation.

At the same time, there is also great potential for the development of fire-driven technology in the future. Traditional fire-driven technology can be combined with other engineering or production measures to form many new fire-driven technologies, such as water drive forward combustion technology that combines forward combustion and wet combustion characteristics, as well as toe-to-heel fire-driven technology (THAI) and top combustion gravity drive technology. The fire-driven technology improves the recovery rate by optimising the geometric structure of the well network, and it proposes a combustion-assisted gravity drive technology consisting of uniformly distributed injection vertical wells and production horizontal wells. Fire-driven technology is a green and low-carbon thermal extraction technology that does not consume water resources and natural gas, and it emits less carbon dioxide than steam injection technology. It meets the strategic goals of carbon peak and carbon neutrality and has strategic significance. By combining the characteristics of forward combustion and wet combustion in water drive forward combustion technology, the thermal efficiency and recovery rate can be improved. The integrated, innovative, efficient, and green technology for the utilisation and storage of fire-driven exhaust gas has revealed the mechanism of low-permeability conglomerate fire-driven exhaust gas extraction. The on-site test results show that the mobile electric ignition technology provides an efficient ignition method for the in situ combustion of oil reservoirs, which will play an important role in achieving the high-temperature combustion of oil reservoirs and improving the fire drive effect.

In summary, these parameters are of great significance for the research of fire-driven technology. They not only affect the efficiency and safety of fire drive but also determine the economy and applicability of fire-driven technology. The application of fire-driven technology mainly focuses on the development of heavy oil reservoirs and difficult to produce reserves. Although the fire-driven technology has certain limitations, it still has great potential and development prospects in the future development of heavy oil reservoirs through technological innovation and optimisation.