Toward Understanding the Effect of Electromagnetic Radiation on In Situ Heavy Oil Upgrading and Recovery: Background and Advancements

Abstract

Electromagnetic (EM) heating, like microwave radiation, is one of the newest and most promising thermal enhanced oil recovery (EOR) methods for producing oil from heavy oil and bitumen reservoirs. The basis of this method is reducing the viscosity of heavy oil to improve its movement toward the injection well. On the other hand, the given heat to the reservoir can, in situ, upgrade the heavy oil by cracking large molecules, reducing resin and asphaltene content, and so on. This study explained the method’s basic theory, mechanism, and governing equations. The background and recent developments in this field were reviewed. It found that using additional EM absorbing materials, like magnetic nanoparticles, polar solvents, and green ionic liquids, can improve the process’s efficiency. The limited field-scale applications of this method showed that this method is economically feasible and has fewer environmental challenges than conventional thermal EOR methods.

1. Introduction

Heavy oil refers to a type of crude oil with a high viscosity and density and low API gravity compared to conventional crude oil [1]. It is characterized by its thick, sticky consistency [2] and is often found in reservoirs that contain a mixture of bitumen, water, and other impurities. Heavy oil is typically more challenging to extract and refine than lighter crude oil grades [3]. The production of heavy oil involves various challenges due to its physical properties. Its high viscosity makes it resistant to flow, requiring specialized techniques for extraction [4, 5]. Additionally, heavy oil often contains higher amounts of sulfur, metals, and other impurities, which can complicate refining [6]. Despite these challenges, heavy oil reserves are abundant globally and significantly meet energy demands. Many countries possess substantial heavy oil resources [7, 8], including Canada’s oil sands, Venezuela’s Orinoco Belt, Russia’s Western Siberia region, and the United States’ Bakken Formation.

Moreover, heavy oil has both economic and environmental implications [9]. From an economic standpoint, heavy oil reserves provide an opportunity for energy security and economic growth in countries with significant resources. However, heavy oil extraction and refining processes can be costly due to the need for specialized equipment and technologies. From an environmental perspective, heavy oil production can have higher greenhouse gas emissions than conventional crude oil due to the energy-intensive extraction methods involved. Refining also produces more carbon dioxide emissions per barrel than lighter crude grades.

However, efforts are being made to develop technologies that can improve extraction efficiency and reduce the environmental impact of heavy oil production. Heavy oil recovery operation is the technique and process used to extract and produce heavy crude oil from reservoirs. The recovery of heavy oil involves several methods that aim to reduce the viscosity of the crude oil, allowing it to flow more easily through wells and pipelines. Hence, some common techniques utilized in heavy oil recovery include thermal methods [5], such as steam [10] or gas [11] injection and in situ combustion [12], as well as nonthermal methods like chemical flooding [13]. Thermal methods involve heating the reservoir to reduce the viscosity of the heavy oil [14]. Steam injection is one of the most widely used thermal recovery techniques, where steam is injected into the reservoir to heat the heavy oil and improve its mobility. Besides, in situ combustion involves igniting a portion of the heavy oil in place to generate heat and create a combustion front that moves through the reservoir. On the other hand, nonthermal methods focus on reducing viscosity through chemical means. Solvent injection involves injecting solvents such as propane or butane into the reservoir to dilute the heavy oil and improve its flow properties. Chemical flooding utilizes polymers or surfactants that interact with heavy oil to reduce viscosity or alter behavior. It is worth noting that selecting a specific recovery method depends on various factors such as reservoir characteristics, economics, environmental considerations, and technological feasibility. Each technique has its advantages and limitations, requiring careful evaluation before implementation. Nevertheless, heavy oil recovery is crucial in meeting global energy demands as conventional crude reserves decline [6]. But it poses significant technical challenges due to its unique properties. Ongoing research and development efforts focus on improving recovery techniques, reducing costs, and minimizing the environmental impacts of heavy oil production.

With the gradual depletion of lighter, easy-to-extract crude oil reserves, the industry increasingly focuses on heavy oil and bitumen, which are more difficult to produce. Heavy oil recovery is a critical and challenging aspect of petroleum production. One of the main challenges of heavy oil recovery is its high viscosity, which makes it difficult to flow under normal conditions. Various thermal enhanced oil recovery methods have been employed to reduce the viscosity of heavy oil, thereby facilitating its extraction. Among these methods, electromagnetic (EM) heating has emerged as a promising technique [15, 16]. EM heating is a promising technique for heavy oil recovery, offering a solution to the challenges associated with extracting heavy oil and bitumen from reservoirs. The principle behind EM heating for heavy oil recovery is to increase the temperature of the oil reservoir using EM energy. By doing so, the viscosity of the heavy oil is reduced, enabling it to flow more easily toward production wells. This method takes advantage of the fact that certain materials can absorb EM energy and convert it into heat [17]. EM waves can penetrate the reservoir, delivering heat more evenly and efficiently than other thermal methods like steam injection. The efficiency of EM heating-assisted oil recovery is influenced by various factors, including electricity costs, well type and completion status, water salinity, crude oil composition, irradiation time, power levels, and recovery scenarios. The arrangement of wells and the methodology for implementing the EM field within the reservoir significantly impact the efficacy of the process.

Two main EM heating techniques are used for heavy oil recovery: radio frequency (RF) and microwave heating. RF heating is the most commonly employed method and operates within the frequency range of 0.5 MHz - 1 GHz [18]. The heating mechanism in RF heating is predominantly dipolar [19]. As EM waves interact with the heavy oil, polar molecules within the oil are caused to oscillate rapidly, generating frictional heat. This heat reduces the viscosity of the heavy oil, allowing it to flow more freely. One of the critical advantages of RF heating is its ability to heat the reservoir uniformly, even in reservoirs with heterogeneous properties [2]. This uniform heating contributes to a more efficient oil recovery process. It helps avoid the risk of overheating, which may damage the reservoir or decrease the quality of the recovered oil. RF heating has been successfully applied in various heavy oil reservoirs worldwide, demonstrating its effectiveness and reliability.

Microwave heating is another EM heating technique used for heavy oil recovery. It operates at higher frequencies, typically around 2.45 GHz [20]. Microwave heating can heat the reservoir faster than RF heating but has a lower penetration depth. As a result, microwave heating is more suitable for near-wellbore heating [21], where it can rapidly increase the temperature of the oil near the production well. Additionally, microwave heating offers the advantage of selectively heating certain materials within the reservoir, such as water or minerals. This selective heating capability allows for greater control over the heating process and can be beneficial in specific reservoir conditions. The choice between RF and microwave heating depends on various factors, including reservoir characteristics, recovery objectives, and operational considerations. Each technique has advantages and disadvantages, and selecting the most appropriate method should be based on a comprehensive evaluation of these factors.

One significant advantage of EM heating over other thermal EOR methods is its environmental friendliness [2]. Unlike steam injection, which requires large volumes of water, EM heating consumes minimal water. This ability makes it an attractive option, particularly in arid regions where water scarcity is a concern. EM heating does not generate combustion gases, reducing its impact on air quality. This environmental aspect is increasingly important as the industry seeks to minimize its ecological footprint and adopt more sustainable practices.

Despite its potential benefits, the widespread adoption of EM heating for heavy oil recovery still faces particular challenges. One of the primary challenges is the high energy costs associated with this method. The generation and delivery of EM waves require significant energy inputs, which can affect the economic viability of the process. Efforts are underway to develop more energy-efficient technologies and optimize the use of EM heating in heavy oil recovery [22]. Technical difficulties in designing and deploying the necessary equipment for EM heating are another challenge. The equipment must efficiently generate and transmit EM waves while withstanding the oil reservoir’s harsh conditions (high temperature and pressure). Innovations in equipment design and manufacturing processes are continuously explored to overcome these technical obstacles. Another consideration is the potential interference of EM heating with other electronic systems. The generation of EM waves in the reservoir can interfere with nearby electronic equipment, such as communication systems or sensors. It is crucial to thoroughly assess and mitigate potential interference issues to ensure EM heating techniques’ safe and effective implementation.

Despite the extensive endeavors undertaken in this field, it is imperative to undertake novel investigations and explore alternative approaches to surmount the challenges associated with implementing this method on an industrial scale. Therefore, as the industry continues to evolve, EM heating has the potential to play a vital role in enhancing oil recovery efficiency and sustainability. Overall, this chapter acknowledges the existing challenges within this particular field and explores the potential for oil industry researchers and practitioners to implement this methodology on a larger scale in field settings.

2. Fundamentals of the EM heating process

2.1 Basic theory

EM heating is a process that utilizes EM radiation to generate heat in a material. Richey first proposed this process in 1956 to increase oil production [15]. This heating method is based on the principles of EM waves and their interaction with matter. EM heating methods can be divided into three main categories depending on the frequency of electric current: Ohmic heating or resistance heating at low frequencies (3 to 300 kHz), induction heating at medium frequencies (300 kHz to 300 MHz), and microwave heating at high frequencies (300 MHz to 300 GHz). Fundamentally, EM waves consist of electric and magnetic fields oscillating perpendicular to each other and propagating through space. These waves can transfer energy to matter when they interact with it. When an EM wave encounters a material, it can be absorbed, transmitted, or reflected depending on its properties [23]. A material’s absorption of EM waves converts the wave’s energy into heat. This phenomenon occurs due to the interaction between the electric field component of the wave and charged particles within the material. The charged particles, such as electrons or ions, experience forces due to the electric field oscillations, causing them to accelerate and collide with neighboring particles. These collisions increase kinetic energy and, subsequently, an increase in temperature. The absorption of EM waves and heat generation are influenced by polar substances like water [24], asphaltene, resin, and adsorbents present in the environment [25].

Consequently, heavy oil, tar sand, and oil shale exhibit a higher capacity for absorbing EM waves [26]. Applying EM heating on tight rocks increases pore-water pressure, thereby aiding in intricate fractures within reservoir rocks. Shale can absorb EM energy more than other rock types due to its elevated dielectric constant and physical structure [27]. Generally, the efficiency of EM heating depends on several factors, including the frequency of the radiation, the properties of the material being heated, and their interaction characteristics. The frequency of radiation determines its ability to penetrate materials; higher frequencies, like microwaves, can penetrate shorter into materials compared to lower frequencies, like RF. Appropriate EM radiation absorber materials are often used for efficient heating applications. For instance, water molecules strongly absorb microwave radiation [28] due to their dipolar nature, making microwaves an effective heating source for liquids containing water. In addition to absorption, other factors such as reflection and transmission also play a role in determining how efficiently EM waves heat a material. Reflection occurs when waves bounce off a surface without being absorbed or transmitted. Transmission is waves passing through a material without being significantly absorbed or reflected. Therefore, understanding the fundamentals of EM heating allows for designing and optimizing heating systems for specific applications.

The primary purpose of the EM heating method is to convert EM energy into thermal energy, increase the temperature of the reservoir, and optimally spread the heat inside the formation to crack the heavy molecules in the crude oil and reduce its viscosity. This work makes it possible to produce more and better quality oil during EM heating in heavy oil reservoirs. Various well configurations have been proposed for this process in field operations, such as using two horizontal wells (like the SAGD process) and cyclic and continuous RF heating in a vertical well. For instance, Figure 1 shows the implementation of EM heating in the vertical well configuration.

Figure 1.

EM heating implementation in the horizontal well configuration [6].

2.2 Mechanisms and governing equations of EM heating

EM heating finds applications in several fields due to its numerous advantages, such as industrial processes like oil recovery operations [29]. One significant advantage is its efficiency in transferring heat directly to the material being heated without needing a medium like air or water. This direct transfer minimizes energy losses and allows precise control over the heating process. Furthermore, EM heating offers rapid and uniform heating compared to conventional methods like conduction or convection. Generating heat within the material allows faster heating rates and more consistent temperature distribution. Microwave heating is a selective process. This process is very suitable for reactions that occur in the cross section of two different phases (liquid/liquid and solid/liquid). These features make this technology a suitable option for use in in situ processes. Additionally, EM heating can be easily controlled by adjusting parameters such as frequency, intensity, and duration of the applied EM field. This flexibility enables precise temperature control and reduces the risk of overheating or thermal damage to sensitive materials.

However, the mechanisms behind EM heating are based on the principles of EM induction [30] and dielectric heating [31]. EM induction is the process by which an electric current is induced in a conductor when exposed to a changing magnetic field. Michael Faraday first discovered this phenomenon in the early 19th century. According to Faraday’s law of EM induction, the magnitude of the induced current is directly proportional to the rate at which the magnetic field changes. In EM heating, an alternating current (AC) is passed through a coil or conductor, creating a time-varying magnetic field around it. When a conductive material is placed within this changing magnetic field, eddy currents are induced within the material due to Faraday’s law. These eddy currents flow in closed loops within the material and generate heat through resistive losses. The amount of heat generated by eddy currents depends on several factors, including the material’s electrical conductivity, magnetic permeability, and the frequency and intensity of the applied magnetic field [6]. Materials with higher electrical conductivity, such as metals like copper or aluminum, exhibit more significant resistive losses, generating more heat.

Another mechanism involved in EM heating is dielectric heating. Dielectric materials are nonconductive substances that can store electrical energy when subjected to an electric field. When these materials are exposed to an alternating electric field, their molecules align themselves with the changing direction of the field. As these molecules continuously reorient themselves with each change in polarity of the electric field, frictional forces are generated within the dielectric material due to molecular interactions. These frictional forces result in molecular vibrations and rotations, leading to energy dissipation in heat. The amount of heat generated through dielectric heating depends on different factors, including the material’s dielectric constant and loss tangent, as well as the frequency and intensity of the applied electric field. For instance, dielectric materials with higher loss tangents, like water, exhibit greater energy dissipation, generating more heat [32]. The primary relation of dielectric heating of a material in the process of EM wave radiation is as follows:

$$Q=\omega .{\epsilon }_r^{"}.{\epsilon }_0.E^2$$E1

where Q is the amount of dielectric heating, ω is the frequency, ε_r” is the imaginary component of the relative permeability coefficient, ε₀ is the vacuum permeability coefficient, and E is the electric field strength (with appropriate power and intensity). So, it is clear that if the material has a more significant dielectric loss component, it can be heated more and better in an EM heating process. The process of EM heating in the context of oil recovery can be described using Maxwell’s equations, heat transfer equations, and the equation of continuity. These equations together can describe the process of EM heating in oil recovery. However, it is important to note that solving these equations for a real-world application can be very complex and may require numerical methods. The actual process may also involve multiphase flow and other considerations, further complicating the equations. Overall, the governing equations of the EM heating method are summarized in Table 1.

Name	Equation	References
Gauss’s Law for Electric Fields	∇ · E = ρ / ε₀	[33]
Gauss’s Law for Magnetic Fields	∇ · B = 0	[33]
Faraday’s Law of EM Induction	∇ × E = −∂B/∂t	[34]
Ampere’s Law with Maxwell’s Addition	∇ × B = μ₀J + μ₀ε₀∂E/∂t	[35]
Heat Transfer Equation - Fourier’s Law	q = −k∇T	[36]
Ohm’s Law	J = σE	[37]
Power Density Equation	P = σE^2	[38]
Heat Generation Equation	Q = J · E	[36]

Table 1.

Summary of equations of EM heating.

Where B represents the magnetic flux density (T), ρ is the electric charge density (C/m³), ε₀ indicates the permittivity of free space (8.85 × 10⁻¹² F/m), μ₀ is the magnetic permeability (H/m), J represents the current density vector (A/m²), σ is the electrical conductivity (S/m), q is related to the heat generated (W), k is the thermal conductivity (W/m.K), T represents the temperature (K), P is the power density (W/m³), and Q indicates the heat generation rate (W). In general, EM heating mechanisms can be divided into the following three general categories:

• Magnetic Loss: It refers to the loss of different energies inside ferromagnetic compounds, such as iron and nickel. The time-varying magnetic field of microwave waves, H(t), creates magnetic spin impulses inside the magnetic material. The temperature of materials increases due to the combination of residual energy loss and eddy currents as heat inside them.

• Dipolar Polarization: It is caused by the electric field component of microwave waves, E(t), due to the continuous rotation of polar molecules to align with an oscillating magnetic field. Therefore, the temperature of the fluid increases due to the pushing, pulling, and collision of these rotating molecules with other neighboring molecules.

• Ionic Conduction: The energy dissipated as heat is due to the collision of moving charged particles (such as ions) within the solution.

3. EM absorber materials to improve heating efficiency

One of the problems of the microwave radiation process is the low penetration depth of the waves inside the formation. On the other hand, crude oil is not a good absorber for microwaves. The greater amount of radiated energy is absorbed by increasing the amount of polar components like asphaltene and resin in crude oil. Therefore, it is necessary to use EM-absorbing materials to increase the efficiency of the EM heating process and heat propagation over a larger reservoir area. EM absorbers are materials designed to absorb and dissipate EM energy. These materials find applications in various fields, including heavy oil recovery in the petroleum industry.

Nanomaterials have gained significant attention in recent years due to their unique properties at the nanoscale. When it comes to heavy oil recovery, nanomaterials can be used as EM absorbers to enhance the efficiency of the recovery process [39]. The absorption of EM waves by these nanomaterials is based on several mechanisms. One common mechanism is dielectric loss, where the nanomaterials possess high dielectric constant and loss tangent values [40]. It allows them to effectively absorb and convert the incident EM energy into heat energy. Another mechanism is magnetic loss, where the nanomaterials exhibit high magnetic permeability and loss tangent values. It enables them to absorb and dissipate EM energy through magnetic interactions. Not every nanoparticle can absorb EM waves. Meanwhile, metal nanoparticles, especially magnetic nanoparticles such as iron (Fe), nickel (Ni), and cobalt (Co), can be suitable candidates for optimal absorption of EM waves. Several factors must be considered to design effective EM absorbers for heavy oil recovery. Firstly, the nanomaterials should have a high absorption capacity over a wide range of frequencies relevant to the heavy oil recovery process [41]. It ensures they can efficiently absorb different types of EM waves generated during the recovery operations. Additionally, the nanomaterials should possess good thermal stability and chemical resistance to withstand harsh conditions encountered during heavy oil recovery processes. They should also be compatible with other components used in the recovery system. Furthermore, these nanomaterial-based EM absorbers must have a high dispersion ability in heavy oil reservoirs. It allows them for uniform distribution within the reservoir, maximizing their absorption efficiency.

Moreover, polar solvents can be used as EM absorbers to improve the efficiency of heavy oil recovery. Polar solvents are substances with a permanent dipole moment due to polar bonds or functional groups [42]. This dipole moment allows them to interact with EM fields. When exposed to an EM wave, polar solvents can absorb some EM energy and convert it into heat through molecular interactions. In heavy oil recovery, EM absorbers can enhance the heating process. Heavy oil is characterized by its high viscosity, which makes it difficult to extract from reservoirs. When the solvent is used in the EM heating process, the oil can move more easily toward the production well due to its increased dilution and reduced viscosity. EM waves can be directed toward the reservoir, where the solvent molecules absorb them using polar solvents as EM absorbers. This absorption leads to localized solvent heating and subsequently increases its temperature. The increased temperature of the solvent helps reduce the viscosity of heavy oil, making it easier to flow and recover from the reservoir. This heating process can also lead to other beneficial effects, such as reduced interfacial tension (IFT) between oil and water, improved rock surface wettability, and enhanced oil mobility within the reservoir [43]. The choice of polar solvents as EM absorbers depends on their ability to efficiently absorb EM radiation in a specific frequency range. Depending on their molecular structure and properties, different polar solvents may have varying absorption characteristics.

Besides, ionic liquids are a class of materials that consist of ions, which are charged particles and are typically liquid at or near room temperature [44]. They have gained significant attention recently due to their unique properties and potential applications in various fields, including heavy oil recovery. In heavy oil recovery, ionic liquids can act as efficient EM absorbers. EM waves are composed of electric and magnetic fields that oscillate perpendicularly. When these waves encounter a material, they can be absorbed, reflected, or transmitted depending on the material’s properties. Ionic liquids possess several characteristics that make them suitable for EM absorption in heavy oil recovery processes. Firstly, they have high electrical conductivity due to charged ions [45]. This conductivity allows them to interact with EM waves effectively and absorb their energy. Secondly, ionic liquids can be tailored to have specific properties by selecting appropriate cations (positively charged ions) and anions (negatively charged ions). This tunability enables researchers to design ionic liquids with optimal EM absorption properties for heavy oil recovery applications.

Furthermore, the viscosity of ionic liquids can be adjusted by modifying their chemical structure [46]. This property is crucial in heavy oil recovery as it allows the ionic liquid to penetrate through the reservoir efficiently and improve the mobility of heavy oil. The mechanism behind EM absorption by ionic liquids involves the conversion of EM energy into thermal energy. When an EM wave interacts with an ionic liquid, it induces molecular motion within the liquid due to its high electrical conductivity. This molecular motion generates heat through frictional forces, leading to the dissipation of EM energy as thermal energy. Several benefits can be achieved by using ionic liquids as EM absorbers in heavy oil recovery processes. Firstly, the absorbed EM energy can help increase the temperature within the reservoir. This temperature rise reduces the viscosity of heavy oil and enhances its flowability, making it easier to recover. Secondly, using ionic liquids can improve the efficiency of EM heating methods, such as EM induction or microwave heating. By absorbing and converting more EM energy into heat, ionic liquids can enhance the overall heating efficiency and reduce energy losses.

4. Background and advancements in EM-heating oil upgrading and recovery

4.1 EM heating for in-situ upgrading of heavy oil

in situ upgrading of heavy crude oil using EM heating is a technique employed in the oil industry to enhance the recovery and processing of heavy oil reserves [22]. The in situ upgrading process begins with the drilling of wells into the heavy oil reservoir. EM heating systems are installed in these wells, typically in electrical conductors or antennas. RF energy is transmitted through these antennas into the reservoir formation, generating heat within the oil-bearing rock. The EM waves induce molecular polarization and agitation within the heavy oil, causing it to heat up. This temperature increase reduces the oil’s viscosity, making it easier to flow through the reservoir and toward the production wells. The heated oil can then be pumped to the surface for further processing. One of the critical advantages of in situ upgrading using EM heating is its ability to selectively heat the oil and the reservoir rock while minimizing heat loss to the surrounding formation [47, 48]. This targeted heating helps avoid excessive energy consumption and ensures efficient oil extraction. The studies conducted in the insitu upgrading of crude oil using EM radiation are divided into several general categories, as shown in Figure 2.

Figure 2.

Classification of conducted studies in in situ upgrading using EM radiation.

Several numerical models have been presented in the EM heating process to predict fluid movement, heating area, temperature distribution, and so on. Studies have shown that the rate of heating increases with rising frequency [15]. Higher EM frequencies can overcome problems caused by medium discontinuities (fractures) that cause EM waves to propagate. The process can be controlled and adjusted to optimize the heating profile, depending on the characteristics of the reservoir. in situ upgrading using EM heating offers several potential benefits. It can increase the recovery factor of heavy oil reserves by improving oil mobility and reducing the need for costly diluents or solvents. It also enables the extraction of heavy oil that would otherwise be uneconomical to produce using conventional methods.

Furthermore, heating can facilitate partially upgrading heavy oil within the reservoir. The elevated temperatures can induce chemical reactions that lead to the decomposition of asphaltenes and the reduction of sulfur and metal content, resulting in a higher-quality oil product. Asphaltene induces alterations in the rheological properties of crude oil, leading to elevated viscosity, diminished mobility, compromised oil quality, and reduced production in reservoirs. When subjected to EM radiation, asphaltene particles within this oil variety assimilate the energy from these waves and undergo fragmentation into smaller hydrocarbon chains, thereby causing a decrease in viscosity. In this regard, it has been proved that the variations in crude oil viscosity are nonlinearly correlated with both the power of EM waves and the proportion of asphaltene in crude oil [49]. This in-site upgrading can reduce the costs of transporting and refining heavy oil and mitigate environmental concerns related to high sulfur content. However, it is essential to note that in situ upgrading using EM heating is still an emerging technology, and specific challenges limit its widespread application. These include the high power requirements, potential technical difficulties associated with installing and maintaining EM heating systems, and the need for comprehensive reservoir characterization to optimize the heating process.

4.2 Recent advantages in EM heating oil recovery

EM radiation technology utilizes the interaction between EM waves and hydrocarbon molecules to enhance oil recovery. The EM spectrum encompasses a wide range of wavelengths, from radio waves with long wavelengths to gamma rays with short wavelengths. When EM waves interact with hydrocarbon molecules, they induce molecular vibrations and rotations, increasing temperature. The process involves transmitting high-frequency EM waves into the reservoir through antennas or electrodes on or near the wellbore. These waves penetrate deep into the reservoir, where they encounter hydrocarbon molecules. As a result, energy is transferred from the EM waves to the hydrocarbons, causing them to heat up. Low-frequency waves tend to penetrate deeper but are more prone to attenuation due to absorption by water or conductive minerals present in the formation [50]. It is crucial to select frequencies that match well with the dielectric properties of both hydrocarbons and surrounding rock formations to optimize heating efficiency. It ensures that most of the energy is absorbed by hydrocarbons rather than being wasted on nonproductive regions.

A further advantage of EM heating oil recovery is its compatibility with various reservoirs. Unlike steam injection or hot water flooding, which are primarily suitable for high-permeability reservoirs, EM heating can be applied to high-permeability and low-permeability formations [51]. This versatility makes it viable for various reservoir conditions, expanding its applicability in different geographical locations. Moreover, recent advancements in EM heating oil recovery technology have focused on optimizing the design and configuration of the EM heating system. Researchers have been exploring different antenna geometries [52], such as dipole or loop antennas, to enhance energy transfer efficiency into the reservoir. Improving antenna design makes achieving higher power densities within the formation possible, resulting in faster and more effective heating. In addition to optimizing antenna design, researchers have also been investigating various methods for controlling and monitoring the temperature distribution within the reservoir during EM heating. Real-time temperature monitoring using fiber optic sensors or distributed temperature sensing (DTS) systems allows for a better understanding of the heating process. It enables adjustments to be made to optimize oil recovery [53]. This level of control and monitoring is crucial for ensuring efficient and safe operation of the EM heating system.

Another recent EM heating oil recovery advancement is integrating this technology with other enhanced oil recovery (EOR) techniques. For instance, researchers have further explored combining EM heating with chemical flooding methods, such as nanoparticles, to improve oil recovery efficiency [54, 55]. Nanoparticles exhibit a pronounced affinity for EM waves and possess notable dielectric constants [56, 57, 58]. Consequently, employing these particles as catalysts to augment heat production and expedite reaction rates represents a viable approach for enhancing oil recovery and achieving more efficient upgrades. Nevertheless, the synergistic effects of these combined techniques can lead to enhanced oil displacement from the reservoir and increased ultimate recovery. As an illustration, Table 2 exhibits a selection of nanoparticles employed with EM radiation to enhance the oil recovery.

Nanoparticles	Effectiveness	References
ZnO	Increasing oil recovery factor	[59]
TiO2, TiO2-Fe3O4	Crude oil viscosity reduction, changing wettability	[39]
γ-Al2O3	Temperature rising and oil upgrading	[60]
Fe, Fe2O3, Fe3O4	Viscosity reduction	[22]
Ni, NiO	Viscosity reduction and increasing oil recovery	[56]
Carbon Nanocatalysts	Oil temperature rising and viscosity reduction	[61]
Nano Ferro	Increasing temperature and oil recovery factor	[62]
Mn2O3	Increasing oil recovery factor	[63]
Fe3O4-MWCNT	Improve EM absorption, increase oil recovery	[39]
Fe3O4-NiO	Crude oil viscosity reduction, changing wettability	[41]

Table 2.

Summary of nanoparticles-assisted EM radiation effect on the oil recovery process.

EM properties of nanoparticles, such as dielectric constant, loss tangent, magnetic saturation, and electrical conductivity, play an essential role in the ability of nanoparticles to absorb the energy radiated to them optimally. Nanoparticles transfer the irradiated EM energy to the surrounding environment (injection fluid and oil) through hybrid heating mechanisms, including conductive, convective, and even radiation (due to the very high temperature of the particles). Two general ways to improve the ability of a material to absorb EM waves have been reported: one is to reduce their size to nanoscale, and the other is to hybridize them with other materials. Recently, the use of magnetic nanohybrids such as iron oxide-multiwalled carbon nanotubes (Fe₃O₄-MWCNT) as strong EM absorbers has been investigated. The hybridization of Fe₃O₄ nanoparticles with MWCNT adds the dielectric loss mechanism caused by the polarization of carbon nanotubes to the magnetic loss mechanism of microwave absorption of Fe3O4 nanoparticles. It makes the nanohybrid absorb the radiated EM energy’s electric and magnetic terms. Therefore, during the process of EM radiation, they will heat up more and better and transfer it to the surrounding medium optimally.

One of the recent advances in using EM heating in oil reservoirs has been the simultaneous injection of a solvent (such as butane or pentane) under EM radiation. Using solvent causes it to dilute the crude oil, reducing viscosity and improving the movement of oil in the porous medium. It will increase the production rate of crude oil and, at the same time, reduce environmental pollution. The primary purpose of this method, which is known as Effective Solvent Extraction Incorporating EM Heating (ESEIEH), is more control and flexibility in managing the energy required in the thermal EOR production methods from shale oil and bitumen reservoirs. This method can be implemented in a wide range of conventional and unconventional reservoirs without needing a large volume of water to produce steam and with less energy. This method reduces greenhouse gases such as carbon dioxide (CO₂) emissions (generally ∼50–70%). Production of a cheap solvent with high polarity, which has a high ability to absorb EM energy, can increase the efficiency of this method.

Ionic liquids are a new class of green solvents that have received much attention recently. As green solvents, these compounds have an excellent potential to overcome the disadvantages of conventional methods of increasing oil production, which should be specially investigated. Ionic liquids have advantages such as low vapor pressure, high density, high thermal and chemical stability, and the ability to synthesize various ionic and cationic components. Ionic liquids increase the amount of oil recovery by reducing the surface tension forces between oil and water and changing the wettability of the reservoir rock from oil-wet to water-wet. Due to their high polarity, these liquids have shown an excellent ability to absorb EM waves, which can prevent asphaltene precipitation.

Lastly, recent advancements in computational modeling and simulation tools have significantly contributed to developing EM-heating oil recovery [64]. Numerical simulations allow engineers and researchers to predict and optimize the performance of EM heating oil recovery systems under different reservoir conditions. These models consider reservoir heterogeneity, fluid properties, and EM field distribution, providing valuable insights into system design and operational parameters.

5. Economic justification and environmental issues

5.1 Economic feasibility

The economic feasibility of EM heating for heavy oil recovery depends on several factors, including the cost-effectiveness of the technology compared to alternative methods, the availability and cost of energy sources, and the specific conditions of the oil reservoir. It involves assessing the investment required for implementing the technology, including equipment costs, installation, and operation. The potential increase in oil production and associated revenues must also be considered. Comparative economic evaluations should be conducted to determine how EM heating performs against other extraction methods, such as steam injection or solvent-based processes. EM heating requires significant energy to generate the EM field and heat the reservoir. The availability and cost of energy sources play a crucial role in the economic feasibility of the technology. The energy can be supplied by electricity, natural gas, or other fuels. Hence, the cost of these energy sources will impact the overall operational expenses of the EM heating system.

Moreover, the proximity and accessibility of energy infrastructure to the oil field should also be considered. Also, the heavy oil deposits’ geological and reservoir characteristics influence EM heating’s economic feasibility. Factors such as the reservoir’s depth, thickness, and permeability affect oil recovery efficiency using EM heating [2]. The presence of water or gas zones within the reservoir can also impact the performance and economic viability of the technology. Detailed reservoir studies, including core analysis and simulation modeling, are essential to assess the potential benefits and limitations of EM heating in a specific reservoir. The economic feasibility of EM heating may also depend on the scale of the project and the anticipated duration of oil production. Pilot projects may have different economic considerations than large-scale commercial operations [65]. Furthermore, the time required to recover the investment and achieve a positive return on investment (ROI) should be carefully evaluated. The longevity and sustainability of the EM heating system should be assessed to determine its economic viability over the expected project lifespan.

Most importantly, the economic feasibility of any oil recovery technology, including EM heating, is influenced by regulatory requirements and environmental considerations [15]. Compliance with environmental regulations, such as emissions control and water usage, can impact the costs of implementing and operating the EM heating system. Nevertheless, potential environmental benefits, such as reduced greenhouse gas emissions compared to other extraction methods, may also have economic implications, including potential carbon credits or incentives. It is important to note that the economic feasibility of EM heating for heavy oil recovery can vary significantly depending on the specific project and regional factors. Detailed feasibility studies and economic analyses, considering all relevant parameters and uncertainties, are necessary to determine the viability of implementing EM heating technology in a particular oil field.

5.2 Environmental impacts

EM heating requires significant energy to generate and maintain the EM field. This energy is typically obtained from fossil fuel sources, contributing to greenhouse gas emissions and climate change. The combustion of fossil fuels used to generate electricity for EM heating can release pollutants such as sulfur dioxide, nitrogen oxides, and particulate matter into the atmosphere [66]. These pollutants can contribute to air pollution and have detrimental effects on human health and ecosystems. Besides, the extraction process may involve injecting chemicals into the ground along with EM heating to aid in oil recovery. If not properly managed, these chemicals can contaminate groundwater sources, potentially affecting drinking water supplies and harming aquatic ecosystems. On the other hand, the construction and operation of EM heating facilities may require clearing land for infrastructure development, including well pads, pipelines, and power transmission lines [67]. This land disturbance can lead to habitat loss and fragmentation, impacting local flora and fauna.

In some cases, EM-heating heavy oil recovery techniques can induce seismic activity [68] due to the injection of steam or other fluids into the ground. It can potentially lead to earthquakes or ground subsidence, causing damage to infrastructure and posing risks to nearby communities. Furthermore, the heavy oil recovery process generates waste materials such as spent solvents, produced water, and solid residues. Proper management and disposal of these wastes are essential to prevent soil, water, and air contamination. However, to mitigate these environmental impacts, it is crucial to implement appropriate environmental management practices such as using renewable energy sources for electricity generation, implementing water recycling and treatment systems, employing proper waste management techniques, and conducting thorough environmental impact assessments before initiating EM-heating heavy oil recovery projects.

5.3 Challenges, future development, and opportunities

The development of EM heating for heavy oil recovery presents challenges and opportunities. Scientifically understanding these aspects is crucial for advancing the technology and realizing its potential in the oil industry. Hence, some of the critical challenges and opportunities associated with the development of EM-heating for heavy oil recovery have been described:

1. Static and Dynamic Experimental Analysis:

   • Studying the primary mechanism of the EM heating process and the kinetics of its reactions in the reservoir can help understand the behavior of the heavy oil and bitumen phases in the porous medium.

   • Finding the effect of different parameters, such as power and frequency, on oil recovery requires conducting different dynamic studies. Therefore, micromodel and core injection experiments can help to understand how the injection fluid moves in the porous media under EM radiation.

   • EM wave radiation can affect crude oil production mechanisms, such as wettability alteration and surface tension reduction, which must be carefully investigated.

   • These waves can provide the energy needed to crack large molecules (such as asphaltene). Therefore, the effect of EM radiation on the quality of crude oil and asphaltene deposition requires more studies.

   • The effect of EM wave radiation on rock mechanics, such as mechanical resistance and surface properties of reservoir rock, plays a vital role in formation damage issues.

2. Synthesize strong EM absorber materials;

   • The best type of nanoparticle that is most effective in this process is not yet known. Synthesis of hybrid nanoparticles can significantly help increase nanoparticle efficiency in the EM heating process.

   • The instability of nanoparticles in harsh reservoir conditions reduces the efficiency of the heating process and damages the formation by reducing the permeability of the reservoir rock. Developing an optimal method to modify the surface of nanoparticles prevents the aggregation and agglomeration of nanoparticles and increases their efficiency in absorbing EM energy.

   • The uniform dispersion of nanoparticles inside the reservoir improves the heating speed. Due to its high polarity, water can be a suitable candidate as an injection fluid. However, the role of other solvents as dispersion mediums and carriers of nanoparticles in the absorption of EM waves should be investigated.

   • Salinity reduces nanofluids’ stability and increases EM waves’ absorption power simultaneously. Therefore, the effect of the salt type and the base fluid salinity on this process should be explored.

   • Different production techniques are currently used depending on the type of nanoparticle to be synthesized (metal, metal oxide, etc.) for cost-effective synthesis of nanoparticles. The use of plant extracts, waste, and disposable or recycled materials, as well as the use of simple methods, can reduce the cost of producing nanoparticles. However, researchers and industrialists should consider conducting additional studies to propose a suitable method for producing nanoparticles on an industrial scale and in high tonnage.

3. Reservoir Characterization and Modeling:

   • Accurately characterizing the reservoir properties and understanding their influence on the EM heating process is a significant challenge. Variations in the reservoir’s electrical conductivity, permeability, and fluid saturation can impact heat distribution and ultimate recovery efficiency.

   • Developing reliable simulation models that can capture the complex physics of EM heating in heterogeneous reservoirs is vital. Incorporating accurate reservoir data and accounting for multiphase flow behavior, heat transfer, and EM properties are essential for predictive modeling and optimization.

   • The mechanism of EM heating using nanoparticles is complex. Therefore, to find the dominant mechanism and the determining stage of the heating rate, more research (especially numerical studies) should be done in the future.

4. Engineering and Design Challenges:

   • EM heating requires the design and implementation of efficient and robust heating systems. Developing EM heating technologies that can withstand the harsh conditions of oil reservoirs, such as high temperatures, pressures, and corrosive environments, is a significant engineering challenge.

   • Designing effective antenna systems that can generate and distribute EM fields uniformly throughout the reservoir is critical. Achieving optimal antenna placement and configuration to maximize heat transfer and minimize energy losses is a complex engineering task.

   • Maintaining the integrity and insulation of the wellbore and surrounding formations during EM heating operations is essential. Addressing challenges related to wellbore sealing, electrical insulation, and potential formation damage is crucial for successful implementation.

5. Energy Efficiency and Optimization:

   • Enhancing the energy efficiency of EM heating systems is an ongoing challenge. Minimizing energy losses during the generation and transmission of EM fields is critical to improve the overall efficiency of the process.

   • Optimizing the power and frequency of the EM field to achieve efficient heating while minimizing energy consumption is an essential area of research. Understanding the complex interactions between the EM field and the reservoir’s properties is necessary for effective optimization.

   • Using additional materials injected into the reservoir can help reduce the amount of energy required. Therefore, the optimal design and synthesis of EM-absorbing materials on an industrial scale can lead to maximum absorption of radiated energy and heat distribution in a larger reservoir volume. Magnetic hybrid nanoparticles and green ionic liquids can be suitable options.

6. Scale-Up and Field Deployment:

   • Scaling up EM heating from laboratory-scale experiments to field applications poses practical challenges. Ensuring the scalability and robustness of the technology while maintaining cost-effectiveness is a critical consideration.

   • Field deployment of EM heating systems requires careful planning, including well configuration design, installation, and operational considerations. Addressing logistical challenges, such as power supply, equipment maintenance, and monitoring, is crucial for successful implementation.

7. Environmental and Regulatory Considerations:

   • Understanding and addressing potential environmental impacts associated with EM heating, such as induced seismicity, groundwater contamination, and emissions, is essential. Developing mitigation strategies and complying with regulatory requirements are essential for the sustainable development of the technology.

   • Exploring potential synergies between EM heating and environmental goals, such as carbon capture and storage (CCS) or utilizing renewable energy sources, presents opportunities for enhancing the environmental performance and acceptance of the technology.

6. Conclusion

Electromagnetic (EM) radiation is an alternative technology for the future of thermal enhanced oil recovery processes for optimal heating of heavy oil and bitumen reservoirs through different mechanisms, including magnetic loss, dipolar polarization, and ionic conduction. In this process, by converting electromagnetic energy into thermal energy, the large molecules of crude oil, such as asphaltene and resin, are broken, and the viscosity of crude oil is reduced. In this method, the quality and the amount of crude oil can be improved simultaneously. Crude oil is not an appropriate absorber for EM radiation like microwaves. Therefore, using EM absorbent materials such as magnetic nanoparticles and nanohybrids, green ionic liquids, and polar solvents to accelerate the heating process and increase the stimulated area inside the reservoirs and the efficiency of the process is contributory. Numerical and laboratory studies and a limited number of pilot-scale applications in America, Canada, and Russia have shown that this method is economically feasible. By addressing challenges and capitalizing on opportunities, EM heating has the potential to contribute to the efficient recovery of heavy oil resources, thereby enhancing energy security and reducing environmental impacts in the oil industry.