bitumen recovery and future prospects: A review

bitumen recovery and future prospects: A review

Applied Energy 151 (2015) 206–226 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Revie...

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Applied Energy 151 (2015) 206–226

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Review

Status of electromagnetic heating for enhanced heavy oil/bitumen recovery and future prospects: A review Achinta Bera, Tayfun Babadagli ⇑ University of Alberta, Canada

h i g h l i g h t s  A detailed critical review of electromagnetic heating was provided.  Most of the simulation and laboratory studies showed that the process is economically feasible.  A limited number of field applications showing the efficiency of the method were evaluated.  Microwave receptors (carbon, nano-metal oxides, and polar solvents) should be used for faster process.  Further research is needed to implement the use of metal-nanoparticles in this process.

a r t i c l e

i n f o

Article history: Received 22 December 2014 Received in revised form 14 March 2015 Accepted 8 April 2015

Keywords: Electromagnetic heating Aqueous/non-aqueous thermal recovery Enhanced heavy oil/bitumen recovery Radiofrequency/microwave heating Ohmic heating Electromagnetic induction heating

a b s t r a c t Thermal methods are inevitable in heavy oil/bitumen recovery. Different types of ‘‘aqueous’’ methods such as cyclic steam and hot water injection, in-situ combustion, hot water and steam flooding, and steam assisted gravity drainage have been widely applied over decades. Currently, non-aqueous heating methods, generally named electromagnetic, are in consideration as an alternative to the aqueous methods, which may not be applicable due to technical and environmental limitations. This technique still requires further research and field scale pilot applications to prove their technical and economic viability. In this paper, a critical discussion on the review of electromagnetic heating is presented. An attempt is undertaken to review most of the research works (computational and experimental as well as a limited number of field applications) performed over more than five decades. After evaluating aqueous and nonaqueous thermal methods, a comparative analysis is presented. Ó 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4. 5. 6.

7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Background of EM heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conventional thermal oil recovery methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Hot water flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Steam injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. In-situ combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic theory of radiofrequency and microwave (high frequency) heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mathematical models used in the EM heating method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other electrical EOR techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Ohmic heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. EM Induction Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field applications of RF-solvent heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature on EM heating methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Numerical simulation and modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

207 208 208 208 208 209 209 209 211 211 211 212 212 212

⇑ Corresponding author at: Department of Civil and Environmental Engineering, School of Mining and Petroleum Eng., 3-112 Markin CNRL-NREF, Edmonton, AB T6G 2W2, Canada. E-mail address: [email protected] (T. Babadagli). http://dx.doi.org/10.1016/j.apenergy.2015.04.031 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

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9. 10. 11. 12. 13. 14.

8.2. Laboratory experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3. Field applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4. General discussion on the applicability and feasibility of EM heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of nanotechnology in EM heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Economic feasibility of EM heating technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental impacts of EM heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages and disadvantages of EM heating method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further research and development challenges and opportunities on EM Heating method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and future development in EM heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Heavy oil and bitumen are considered one of the most crucial sources to meet future energy demand. The total accumulations of heavy oil and bitumen are 3396 and 5505 billion barrels of original oil in place (OOIP) and among these, 30 and 993 billion barrels of oil are prospective additional oil, respectively [1]. The above mentioned reserves are distributed among 192 heavy oil and 89 bitumen basins. The major portion of heavy-oil is in the Middle East and South America and thereafter followed by North America. North and South America have the largest bitumen reserves. Eastern Siberia also contains considerable reserves of heavy-oil and bitumen but sufficient data are not available. The details of the heavy oil and natural bitumen reserves are given in Table 1. The in-situ recovery of heavy-oil and bitumen is predominantly achieved by thermal methods. The main purpose of thermal recovery method is to reduce the viscosity of heavy oil/bitumen to mobilize it towards injection well. Conventional aqueous thermal recovery processes like steam injection, fire flooding, hot water injection and steam assisted gravity drainage (SAGD) are well known in this regard. Unfortunately, these methods may not fulfil the economic criteria and possess environmental constraints. Also, they may not be compatible with certain types of reservoirs (e.g., high clay contents, shales, deep reservoirs, heterogeneous formations, etc.). Recently, interest was devoted to electromagnetic (EM) heating with radiofrequency (RF) to overcome some of these problems. Radiation heating for enhanced heavy oil/bitumen recovery was first proposed by Ritchey [2], whose patent was filed in 1956. The main design was to transfer EM waves to well bore from the surface through coaxial system of internal and external tubing and casing. Over more than five decades, numerous computational and laboratory experiments have been carried out to propose

215 217 218 218 219 219 220 220 223 223 223

standard application procedures for field scale trials. Yet, economic and technical limitations exist to commercially apply this method. The main purpose of EM heating is to directly heat the reservoir to reduce oil viscosity. The basic principle of EM heating application in a reservoir is to transform EM energy to heat energy [3– 10]. In the EM heating, a broad range of radiofrequency energy from 300 kHz to 300 MHz can be applied through the reservoir. Since higher frequencies are not safe in laboratory, experiments were carried out using frequencies of few MHz or using commercial microwave ovens [7–10]. In general, the EM heating process relies on preferential absorption of EM energy as the means of increasing the temperature of dielectric materials. Different materials have different EM absorption properties. The ability of an EM wave to transfer energy to a medium depends on the molecular composition of the medium. If the medium contains mobile molecules with molecular dipole moments (such as water), then the passing EM wave will exert torques on the polar molecules and their dipole moments align themselves with the oscillating electric fields of the EM waves. As a result, the interaction of an oscillating polar molecule with its neighbours takes place and it generates frictional heat, which raises the temperature of the medium. The produced heat is then gradually distributed through the other dielectric materials present in the system. The level of temperature increase depends on the amount of EM energy absorbed by the irradiated materials. As mentioned above, electrical heating (especially EM heating) is an alternative to aqueous thermal enhanced oil recovery (EOR) methods for heavy oil/bitumen recovery from high clay content (shale reservoirs) or deep reservoirs. Additionally, this technology can work in geologically complex systems with relatively low cost and is environmentally friendly [11–14]. The present review article highlights the advantages of EM heating over other conventional thermal methods. A detailed discussion covering different types of thermal methods and their

Table 1 Discovered reserves and distributions of heavy oil/bitumen throughout the world [1]. Region

North America South America Europe Africa Transcaucasia Middle East Russia South Asia East Asia Southeast Asia and Oceania Total

Heavy oil (billion barrels)

Bitumen (billion barrels)

Discovered original oil in place

Prospective additional

Total original oil in place

Discovered original oil in place

Prospective additional

Total original oil in place

650 1099 75 83 52 971 182 18 168 68

2 28 0 0 0 0 0 0 0 0

652 1127 75 83 52 971 182 18 168 68

1671 2070 17 13 430 0 296 0 10 4

720 190 0 33 0 0 51 0 0 0

2391 2260 17 46 430 0 347 0 10 4

3366

30

3396

4511

994

5505

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applicability and efficiencies to recover oil was presented first. Then, a brief description of the theory of EM heating was introduced. After covering the advantage of EM heating over the other electrical heating methods, the possibility of the addition of nanoparticles during EM heating was analyzed. A brief description of further research and development challenges/opportunities of EM heating method was proposed for future research of this field. Finally, the future prospect of EM heating in EOR method was discussed. This review may be able to provide new information on the present status of EM heating method and how far will it be applicable at the field scale. It also serves as a new opener of further research in thermal heavy oil/bitumen recovery methods by EM heating. 2. Background of EM heating The EM heating was first proposed by Ritchey in 1956 in his patent ‘‘radiation heating for heavy oil recovery’’. The EM heating methods can be classified under three main categories depending on the frequency of the electrical current used for the heating. In case of low frequency current, ohmic heating or resistive heating takes place. The next range is EM induction heating where medium frequency is used to produce current. Alternative current flows through a conductor and it induces magnetic field in the surrounding area. Thus, a variation of magnetic field is produced and it induces secondary current, which circulates in the medium to generate heat. When high frequency (RF and microwave) radiations are used for heating purposes, the dipoles are formed by molecules and tend to align them with the electric field. As a result, the molecular movement produces heat in the reservoir. In the practice of oil production, the main purpose of the procedure is to heat up the reservoir by an RF antenna or by an induction coil inserted into the injector. As the reservoir temperature increases, oil viscosity reduces and starts to flow towards the producer well. In case of EM heating, the suggested practice is to use two horizontal wells (like the SAGD process) [15]. The upper horizontal well contains the RF antenna or induction coil and when it heats up the surroundings, the melted oil is collected through the lower producer well as shown in Fig. 1. It is also important to note that cyclic RF heating and continuous RF heating processes in a vertical well had been suggested for heavy oil recovery [16].

In general, no water is injected into the wells during RF heating to possibly create steam (or hot water) injection status (therefore, it can be named as ‘‘non-aqueous heating’’ for heavy oil recovery). On the other hand, the use of solvent during EM heating was suggested [17–19]. When solvent is used with the EM heating, oil can flow towards the production well more easily due to enhanced dilution resulting in better viscosity reduction. 3. Conventional thermal oil recovery methods Thermal recovery processes rely on the use of thermal energy in some forms to increase both the reservoir temperature, thereby reducing oil viscosity, and displace oil to a producing well. Three well-known processes, namely hot water flooding, steam injection, and forward in-situ combustion (fire flooding), have been proposed and commercially applied over the past 30–40 years. Thermal recovery processes are the most advanced EOR processes and contribute significant amounts of oil to daily production even though steam based applications are limited to shallow reservoirs, which are less than 3000 feet deep [20]. According to 2014 EOR survey, US thermal production (steam, in-situ combustion, and hot water injection) has reached 307,018 bbl/day [21]. 3.1. Hot water flooding A huge amount of water is required when injecting hot water into a heavy viscous oil reservoir to increase the production. Hot water reduces the viscosity of the crude oil allowing it to flow more easily towards production well. Hot water flooding or hot water injection is less efficient than steam injection due to lower heat content of hot water compared to steam even though water’s driving power (gas) could be higher than steam. 3.2. Steam injection Three common methods involving steam injection are cyclic steam stimulation (huff-and-puff method), steam flooding (steam drive), and SAGD (steam assisted gravity drainage). Cyclic steam injection (CSS) consists of three stages: injection of steam, soaking period, and production of oil. Steam is first injected into a well for a certain period of time to heat the oil around it, reduce its viscosity,

Fig. 1. EM heating method for heavy oil production (SAGD like design).

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and mobilize it. Once it is confirmed that enough steam has been injected, the well is shut-in for a period of time (few days to a few weeks). Then oil is produced from the same well by natural flow, i.e., by means of the pressure generated through steam injected. Production will decrease as the oil cools down, and, once the rate reaches an economically determined level, the same steps are repeated. The effectiveness of this process is questionable after the first few cycles. The ultimate recovery by CSS is lower (around 10–40% of the OOIP) [22,23], compared to SAGD, which has been reported to average around 50–60% [23–25]. Therefore, it is quite common for wells to be produced in the cyclic steam manner for a few cycles before being put on a steam flooding regime with other wells. In the steam flooding process, also known as steam drive, steam is injected through injection wells and the recovered oil is produced using producing wells. Two mechanisms act to improve the amount of oil recovered in this method. The first is to heat the oil to decrease its viscosity so that it flows more easily through the formation toward the producing wells. The second mechanism is physical displacement, which is similar to water flooding, wherein oil is pushed to the production wells. While more steam is needed for this method than for the cyclic method, it is typically more effective at recovering a larger portion of the oil. In the case of SAGD, high pressure steam is continuously injected into a horizontal well to heat the oil and reduce its viscosity. The heated oil is then drained into a lower well where it is pumped out. SAGD has been mostly popular in the oil sands and extra-heavy crudes of Alberta, and tested in Venezuela with limited success. Currently, SAGD is primarily applied in Alberta and its hybrid versions (e.g., injection of solvent with steam) are at the pilot stage. In 1993, worldwide production from cyclic steam and steam drive was more than 700,000 bbl/day. This amounts to 919,917 bbl/day in the USA, Canada, Brazil, and Norway in 2013 [21]. 3.3. In-situ combustion In this technique, air is injected into the reservoir. Once it is ignited using an igniter or spontaneously, heat is generated internally. The heat front progresses as air injection is continued, and heat and combustion gases generated enhance oil recovery. This process is also called fire flooding. Depending on the respective directions of front propagation and air-flow, the process is named forward and reverse combustion. In the forward combustion case, the combustion front advances in the same direction of air-flow and in the case of backward or reverse combustion, the front moves against the air-flow. In-situ combustion projects are not as popular as steam flooding. Field scale applications are reported in heavy oil sandstone reservoirs in Canada, India, Romania, and USA.

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4. Basic theory of radiofrequency and microwave (high frequency) heating Radiofrequency is usually related to the number of oscillation times of EM radiation per second. In general, all frequency ranges from 3 kHz to 300 GHz are referred to as radiofrequency. It should be mentioned that microwave is the general term used to indicate the application of extremely high frequencies (300 MHz–300 GHz). In general, microwaves are either transmitted by a material, absorbed or reflected depending on the properties of the material. These waves stimulate dielectric reactions and are able to cause atomic polarization and dipolar turning to polarization. When a dielectric material is polarized, temperature increases due to inner power dissipation of the material. Microwave heating does not rely solely on convection or conduction and can likewise heat objects internally regardless of whether or not physical contact is achieved between the microwave source and the sample. As a result, quick heating of the sample takes place provided that the sample absorbs the particular frequency of radiation that is applied [26,6]. Actually, in high frequency microwave heating, dielectric heating prevails and dipoles are formed by molecules tend to align themselves, resulting in rotational movement with velocity proportional to the frequency of alteration that generates heat. Fig. 2 shows microwave and conventional (resistive) heating processes schematically. The bulk phase of the container takes the heat from the source in conventional heating whereas, in case of microwave heating, molecules absorb radiation and, as a result, rotational movements of the molecules produce heat to increase the temperature of the system very fast.

5. Mathematical models used in the EM heating method It is essential to describe the mathematical models for heat flow through reservoir formation during EM heating for practical applications. This is actually governed by the equation of thermal energy conservation in the reservoir. Bientinesi et al. [27] described the initial and boundary conditions necessary for a 1D spherical geometry using COMSOL Multiphysics software [28– 30]. The required initial and boundary conditions are: Proper thermal equation selection for the model. Uniform temperature distribution for the entire domain. The boundary should be adiabatic. Equalization of initial temperature and constant temperature at the outer boundary.  A first order stationary equation for EM wave attenuation.  Boundary condition at the well where total power will be irradiated by the antenna.    

Fig. 2. Comparison of conventional (electrical) and microwave heating processes.

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Temperature distribution in RF heating of heavy oil reservoir using an antenna installed inside a wellbore obeys the equation of thermal energy conservation. The governing equation can be stated as follows [27]:

@T dwðxÞ  r  ðK eff rTÞ ¼ q ¼ aabs w @t dx

qeff C eff

ð1Þ

where qeff C eff and K eff are the effective heat capacity and effective thermal conductivity of the reservoir materials respectively, and q is the EM heat source term that describes the energy release per unit time and unit volume. Abernethy [31] first showed the mathematical solution and model to calculate the temperature distribution by EM heating for heavy oil production. Here, we briefly describe the model with the proper way to use the equations. It is important to clarify how the three heat transfer processes, i.e., radiation, convection, and conduction, contribute to distribution of temperature inside the reservoir. If we consider a linear homogeneous conductivity medium and the radiation propagation acting in the +x direction, then it follows the following relationship:

dwðxÞ ¼ aabs w dx

ð2Þ

where wðxÞ is the power density (W/cm2), aabs is the power absorption coefficient (1/cm), and x is positive coordinate (cm). The absorption power coefficient ðaabs Þ mainly depends on properties of the absorbing materials present in the medium and can be presented as follows [32]:

aabs ¼ 0:02ae (

ae ¼

x2 le 2

 1þ

ð3Þ

r2 x2 e2

)1=2

1=2 1

ð4Þ

ð5Þ

where r and h are the radius and height of the defined cylinder where power is radiated across it, r 0 is the wellbore radius, So is the specific heat of oil, qo is the density of oil, q0 is flow rate (cc/ s), qt is density (g/cc), and St is specific heat of total oil, water and rock (Cal/g/°C), respectively. The process can be modelled for the steady-state and transient conditions. The steady state temperature distribution can be expressed as:

hðrÞ ¼ T 0 þ

P 0 eaðrr0 Þ 4:18q0 q0 S0

Tðr; tÞ ¼ T 0 þ

ð6Þ

where hðrÞ is the resulting temperature distribution, T 0 is the initial temperature of the reservoir, and P0 is the total power radiated in watts. In case of transient temperature distribution, there are three solution possibilities such as transient temperature-constant flow, no flow, and increasing flow rate. The solutions are given below:

pffiffiffiffiffiffiffiffiffiffiffi o P0 ear0 n ar0 2 e  ea r þ2At 4:18q0 q0 S0

ð7Þ

0 S0 where A ¼ q20pqhK and K is the total conductivity of heat (Cal/s/ °C/cm). (2) For the transient temperature with no flow the equation is given as:

Tðr; tÞ ¼ T 0 þ

aP 0 eaðrr0 Þ t 4:18ð2phqt StÞ r

ð8Þ

(3) The transient temperature with increasing flow rate is critical to express the governing equation. Finite time step integration is necessary to develop the equation. Further attempts were made for more specific reservoir conditions. Recently, Davletbaev et al. [33] mathematically modeled heavy oil production by combined technology of RF-EM irradiation in hydraulically fractured well for tight heavy-oil reservoirs. They used the following equation to calculate flow rate of the oil:

"

#   kf @Pf  km @Pm  Q o ¼ 2hwf þ lo @x x¼wf =2 lo @y y¼wf =2

ð9Þ

where h is the height of the fracture and thickness of the formation, wf is the fracture width, kf and km are the permeability of the fracture and the matrix, lo is the viscosity at initial temperature, and P f and Pm are the pressure of the fracture and matrix. They also mathematically modeled for another condition of flow rate of oil from matrix to fracture for a reservoir with EM wave. The equation is given as:

qo ¼ 4

where ae is the electrical field absorption coefficient (1/m), x is angular frequency (x ¼ 2pf , f is frequency), l is permeability (H/ m), is permittivity (F/m), and r is conductivity (mho/m). The final generalized equation, which is responsible for temperature distribution Tðr; tÞ in order to ascertain the effect of radiative heating on the flow of well, can be expressed as follows:

   @T 1 aabs eaabs ðrr0 Þ r @T ¼ þ qo qo So dt 2prhqt St 4:18 @r

(1) In case of transient temperature with constant flow, temperature steadily increases at a constant flow rate. The following equation is the expression of transient temperature distribution in the case of a constant flow rate to the wellbore:

Z

xf 0

  Z wf =2 km @Pm  km @P m  dx  4 dy lo @y y¼wf =2 lo @x x¼xf 0

ð10Þ

where xf is the fracture half-length. Davletbaev et al. [34] also obtained the distribution of EM waves in case RF-EM heating. They discussed the concept of RFEM wave absorption in polar components. To properly describe the distribution of RF-EM wave and generation of heat (q), they used the following equation:



xe0 e0 tgd 2

E2

ð11Þ

where x is the frequency of EM waves (MHz), e0 is electrical constant, e0 is relative permittivity of the medium to liquid, tgd is dielectric loss (dissipation) tangent, and E is the electric field intensity. In this article, they used a model to describe a non-stationary operation of the production well and RF-EM treatment in the area of bottom-hole. A set of differential equations (Eqs. (3)-(7) in the original article) were used to properly describe the entire processes associated with RF-EM heating operation. Lui and Zhao [35] used mathematical formulations for a singlephase transient radial flow in the EM method. For modeling purposes, they assumed an isotropic reservoir. Under EM heating at reservoir conditions, the total energy conservation equation for 2D single-phase radial flow (in r–z coordinates) is given as follows:

ðM r ð1  ;Þ þ ;Mo So Þ

  @T @T 1 @ @T @T þ kr km r ¼ ðM o uo Þ þ @t @r r @r @r @z Q em expðaðr  r w Þ þ ae ð12Þ r

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where ; is the porosity, km is the effective reservoir thermal conductivity including rock and oil, M is the volumetric heat capacity, S is specific heat, u is the velocity of fluid, and ae is the EM absorption coefficient. The subscript o, w, and r represent oil, wellbore, and rock, respectively. They also formulated the oil–gas two-phase flow with EM heating. The following equation was used for conduction and heat source to represent the overall energy balance in the system:

@T ð;ðM w sw þ M nw snw Þ þ ð1  ;ÞM r Þ @t ! ! þ ðMw uw þ Mnw unw Þ  rT þ r  ðkm rTÞ ¼ Q em

ð13Þ

where s is the saturation and subscripts nw and w repents the nonwetting and wetting phase, respectively. At present, other research works on mathematical modelling and numerical simulation are also in the developing stage. The above given equations are the fundamental ones and can serve the primary knowledge of mathematical modeling of EM for heavy oil recovery under reservoir conditions. 6. Other electrical EOR techniques In general, the electrical current can be divided into three main types such as low-frequency ohmic heating or resistive heating, high frequency or radiofrequency/microwave heating, and medium frequency or inductive heating. The purpose of all of these methods is to reduce the oil viscosity and thereby to increase oil mobility towards the producing wells. Generation of heat in electrical heating depends on the use of frequency of electrical current. High frequency heating was described above and the other wellknown methods of electrical heating (inductive heating and electrical resistance heating) will be described briefly below. 6.1. Ohmic heating For electrical ohmic heating, a potential difference is applied between two electrical electrodes among which one acts as anode and the other as cathode. This low frequency heating is also known as the Joule heating. In practice, this method can be employed using two oil wells, one is anode and the other is cathode, in the reservoir. A potential difference is applied between these two electrodes and the current is allowed to pass through the formation water, which has good conductivity to transmit electricity [36].

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Extensive analysis of this method, i.e., with current flow through the formation as a source of heat, was carried out by several researchers [37,38]; Sierra et al., 2001. Pizarro and Trevisan [38] conducted a low frequency electrical heating field trial in the Rio Panan field, Brazil. They observed oil production increase from 1.2 bbl/day to 10 bbl/day after 70 days of heating. A schematic diagram of the low frequency resistive heating is shown in Fig. 3. Although this method can improve the oil recovery, it suffers from certain limitations of use. During the heating, a huge amount of steam is formed from the reservoir formation water and therefore the amount of water decreases and heat dissipation is reduced. This eventually impedes the recovery. Harvey et al. [37] suggested using water injection along with this method. Resistive fluid injection prior to electrical heating would also exacerbate the heating effects in low-permeability zones.

6.2. EM Induction Heating EM induction heating is a technique where electrically conductive materials are placed in a variable magnetic field generated by an exciting winding called inductor. The exciting winding produces electromotive force (emf) to set up the flux in an electric machine or other apparatus. The current produced in this process is also called Eddy current. Due to the Joule effect (Pdissipated = I2R, where P is the power in Watt, I is the current in amp, and R is the resistance in ohm), this current dissipates heat in the material in which it is placed. The governing factors of the heating process are the specific heat of materials, the frequency of induced current, the permeability of the material, and the resistance of the material to flow current through it. A rare number of studies proposed different designs for the field applications of EM induction heating. In these applications, a number of inductors are installed at the bottom of producing well facing the production zone. Vermeulen and McGee [39] introduced a configuration of EM induction heating. The proposed schematic diagram can be seen in Fig. 2 of the relevant publication. More recently, Siemens AG worked on inductive heating technology for heavy oil recovery from oil sands. They specially designed inductive coil and applied medium frequency electric field depending on the reservoir condition to produce heat. Koolman et al. [40] performed sandbox test with inductive heating circuit within a box filled with mixture of sand and salt water solution. They used frequency compatible with the dimensions of the

Fig. 3. Schematic diagram of low frequency ohmic heating in a reservoir.

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sand box. To measure the temperature of different positions they placed fiber optical temperature sensors in the model as well as an infrared camera to capture the images of temperature distribution pattern. They also simulated the results and concluded that the EM induction heating is feasible for enhanced heavy oil recovery. 7. Field applications of RF-solvent heating Recently a new technology has been developed by a consortium composed of a few companies. They introduced a method called ‘‘Enhanced/Effective Solvent Extraction Incorporating EM Heating’’ (ESEIEH) in 2009 to improve oil recovery while mitigating environmental pollution (http://ccemc.ca/project/enhanced-solventextraction-incorporating-electromagnetic-heating/). The main mechanism of ESEIEH is to use two horizontal well pairs like SAGD configuration with addition an RF antenna. The antenna uses the electrical power to produce EM radiation, which will be absorbed by the dielectric materials of the reservoir and therefore heats up the bitumen to mobilize it. In the meantime, a solvent like butane or pentane is injected to dilute the bitumen. The main objective of ESEIEH is to manage the energy requirement in the extraction process with greater efficiency, control, and flexibility. Advantages of the technology over other thermal method like SAGD are listed as follows: (a) Energy Requirement: lower energy is required to control the process over other thermal methods. (b) Recovery Efficiency: reduced water saturation results in higher bitumen recovery. (c) Water Source: it does not require a huge amount of water to generate steam to heat the formation. (d) Fewer Emissions: reduce the greenhouse gas emission. (e) Applicability: can be applied to both clastic and carbonate formations. (f) CO2 Sequestration: emission can be captured by a central facility and results in better carbon management. The ESEIEH project has completed its first phase and based on promising observations, the next phase is set to be completed at the end of 2014. 8. Literature on EM heating methods Different papers appeared in literature covering reviews on the different aspects of EM heating for EOR [41–45]. The present paper is complementary to these works, focusing on the review of more recent literature as well as specific areas of the EM heating such as numerical simulation and modeling, laboratory experiments, field applications, and the use of nano-technology during EM heating. The journey to the use of EM heating for EOR started with Ritchey’s [2] patent for the production of heavy oil by applying radiation heating. The main design was to transfer EM waves to well bore from the surface through coaxial system of internal and external tubing and casing. Subsequently, Haagensen [46] filed another patent describing the method for increasing the temperature of wellbore using microwave technique through coaxial lines and a waveguide to a bottom hole. Numerous publications discussed the mathematics and modelling aspects of these two works. We begin the literature survey with these efforts. 8.1. Numerical simulation and modeling Most of the research works on EM heating for EOR were conducted based on the simulation and modeling beginning in the

mid-1970s. Abernethy [31] proposed a mathematical model to evaluate the temperature distributions and other physical effects resulting from the radiation of EM energy into an oil reservoir. He depicted the ideas about the steady-state temperature distribution, transient temperature-constant flow, transient temperature-no flow, and transient temperature-increasing flow from the developed mathematical model. He also stated that, at a relatively moderate power input (20 kW), 50–300% rate increments may be possible, depending on the power absorption coefficient. The theoretical conclusions of his study suggested that heat stimulations of the oil wells by radiation heating have great importance in flow rate as well as the temperature distribution characteristics. Although they are not directly related to the EOR aspects of EM radiation in porous media, several attempts were made on the propagation of EM waves through porous media for the purpose of reservoir delineation. The experience obtained through this type of reservoir characterization attempts turned out to be beneficial for the design of EM heating tools to be used for EOR. Freedman and Montague [47] examined log and laboratory data on core and or sidewall samples of three wells producing heavy oil from shaly sands with fresh water using EM propagation tool (EPT). They reported that using only down-hole logging data obtained from EPT and a porosity tool, one can identify oil bearing zones in shaly reservoir sands containing fresh water of variable and/or unknown water resistivity. They also reported the advantages of the EPT over conventional electrical logging devices. During the use of EPT, it does not require the knowledge of formation water resistivity, the cation exchange capacities of the sands and other auxiliary data not obtainable from down-hole logs. Further development was made by Witterholt and Kretzschmur [48] to map a reservoir under steamflood using EM waves with frequencies ranging from 1 MHz to 30 MHz. They reported that the baseline measurement will provide more quantitative values of the actual water–oil distributions throughout the reservoir by using this method. These saturation ratios, in turn, permitted more precise economic evaluation and process engineering design. In an attempt toward the use of EM heating purely for EOR, Hiebert et al. [49] developed a numerical simulator to study the process of electrically heating oil reservoirs consisting of several layers with different electrical resistivities. They used this simulator to study the effects of electrode placement on the final temperature contours resulting from electrically heating realistic reservoirs. They reported that the developed 2D simulator, MEGAERA can predict the electrical heating that will take place in typical multilayered oil-sand formations where the electrical properties may vary from layer to layer by several orders of magnitude. It can accurately and economically model many inherently 3D problems because it exploits the natural planes of symmetry that exist where large arrays of electrodes are used. In particular, their simulation studies showed that using conducting adjacent formations as extended electrodes permits relatively uniform electrical heating of an oil-sand formation with a well spacing of 50–75 m [164–250 ft.]. They finally concluded that uniform heating becomes much more difficult for those reservoirs where the rich oil-sand layer rests directly on a relatively poor conducting limestone layer. As seen, EM heating is efficient to work on carbonate reservoirs too. The effect of nature of the reservoir was investigated by Fanchi [50] for this purpose. He performed numerical simulation of reservoir heating using EM irradiation as a heating stimulation tool to assess the feasibility of the method for both sandstone and carbonate reservoirs. In the design of simulation, the properties of radiofrequency generator and waveguide, and electrical conductivity of reservoir fluid and reservoir depths were considered. He concluded that EM power attenuates exponentially in a linear, homogeneous, dielectric medium. The developed algorithm of the simulation worked

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nicely for hypothetical reservoirs. The hypothetical reservoirs are oil-bearing sandstone reservoir with connate water (Reservoir A) and oil-bearing carbonate reservoir (Reservoir B). Different estimations provided the information about the feasibility of EM radiation heating of Reservoirs A and B. The results of sandstone and carbonate reservoirs showed similar behavior, i.e., with an increase in electrical conductivity, the near-wellbore temperature increases. He noted that EM energy was absorbed by few feet of wellbore even at low electrical conductivity. He also reported that algorithm results are especially sensitive to input data such as electrical conductivity, water saturation, and relative electric permittivity. Finally, he concluded that EM heating process appears to be most applicable as either a well stimulation technique or a mobility enhancing process. At the same time more specific simulation study in field case was done by Pizzaro and Trevisan [38]. They performed numerical simulations using 2D, two-phase, numerical model and conducted field tests of electrical heating of an oil reservoir. They analyzed oil production against energy consumption for different electrode schemes and the results of simulations of the Rio Panon (Brazil) pilot test. They reported that field-test data from Rio Panon were matched reasonably well by numerical simulations. They concluded that wellbore damage was gradually removed with the application of resistive heating and an 86% increase in accumulated production can be achieved from a hypothetical undamaged well after a 1500-day operation. On the other hand Baylor et al. [51] carried out an improved calculation of oil production response to electrical resistance heating. They used two models to describe the electrical heating resistance power dissipation, namely radial power model and r–z power model. They reported that the new hand method is very efficient for electrical resistance heating and it has the following advantages: (a) (b) (c) (d) (e)

Easier to use. More accurate. Heated radius can be estimated. Power requirement can be estimated. Applicable for damaged wells.

They also reported that the r–z power model shows that hot spots occur near the ends of the electrode. These hot spots may limit the maximum power and accompanying rate response for some field applications. Shortly after this, an interesting field case simulation of nine runs was presented by Wadadar and Islam [52]. They performed numerical simulation of EM heating of Alaskan tar sands (Ugnu formations) using horizontal wells and reported that using horizontal well in conjunction with EM heating a significant amount of oil (40% of the oil in place for tar sands) can be recovered. A total of nine simulation runs were performed to study the effects of rock-fluid properties and electric power input. It was reported that, with 50% increase of electrical power, oil production increased 125% and the recovery was even higher for heavy oil of moderate viscosity. Another simulation study on horizontal well heating with electrical method was presented by McGee and Vermeulen [53]. They proposed a semi-analytical model to calculate the temperature distribution along the horizontal well that is electrically heated. They reported that, for a long horizontal well, heat transfer to the adjacent reservoir by conduction is more significant than heat transfer by convection and electrical heating. It was also stated in their report that the safe operating magnitude of the current in the horizontal well, determined by the allowable temperature rise, is limited by the cooling effect of the produced fluids. They finally analysed the cost of the production and concluded that the electrical costs per meter of produced oil associated with

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operating the well at a peak temperature will depend on the stimulated production rate and economic conditions of the day. Microwave heating was also numerically modelled for possible applications in enhanced heavy oil recovery. In an attempt to this, Soliman [54] developed numerical and analytical models for EOR using the microwave technique considering the heat loss into adjacent strata and no heat loss. He concluded that the use of microwave in EOR for heating oil is feasible and it can improve the oil production up to a factor of two using microwaves power level of 100 kW. He also discussed the limitations of this method during the field scale applications. Due to produced steam by this heating method, permeability to oil was decreased yielding a decrease in oil production. The physics of EM heating and the possible application for oil recovery was also presented by Sahni et al. [6]. They introduced a numerical model for heat transfer to heavy oil reservoirs through EM waves and compared low frequency resistive (ohmic) heating and higher frequency EM heating (microwave frequency). Then, they modelled pre-heating of reservoir with low frequency current using two horizontal electrodes before injecting steam. The simulation results showed that the electrical pre-heating significantly accelerated early production and resulted in better cumulative oil production compared to the non- pre-heated case. In a similar attempt, Vermeulen and MacGee [39] studied hydrocarbon recovery by in-situ EM heating and its impacts on environment through numerical modeling. They discussed the importance of electrical heating for reducing viscosity of heavy oil as well as environmental bioremediation. Their report indicated that, with increasing temperature, the semi-volatile component of soil can be removed from soil. They later presented the mechanism of electrical heating recovery process in terms of fundamental equations and numerical solutions. They described the heat and mass transfer mechanisms associated with a specific application of electrical heating named ‘‘Electro-Thermal Dynamic Stripping Process (ETDSP)’’ for the production of bitumen from the oil sands. Their results indicated that electro-thermal heating provides rapid and uniform heating and covers a larger area in the reservoirs. The cost of the method increased due to purchasing cost of electrical power and the capital cost was observed to be dependent on the operation of the electrical stimulation equipment. Presently, improved technology reduces the capital costs of electrical stimulation methods. Another numerical study of in-situ oil production was presented by Wacker et al. [55]. They studied the reservoir behavior under EM heating for in-situ production of heavy oil and bitumen using a commercial numerical simulator (CMG-STARS). They also used the simulation tool ANSYS coupled with STARS to study the impact of EM heating in oil recovery. EM heating method was observed to be more acceptable than the SAGD method from economic feasibility, energy efficiency, and environmental footprint points of view. Downhole electrical heating in Venezuela fields was also simulated for possible application of this method. Ovalles et al. [56] carried out a case study on downhole dielectric heating for three conceptual reservoirs in Venezuela with three kinds of crude oils [medium (24°API), heavy (11°API) and extra-heavy crude oil (7.7°API)]. The heavy and extra-heavy crude oil conceptual reservoirs were associated with shallow Lake Maracaibo and Orinoco Basin, respectively. They reported that radiofrequency can recover 86% oil when the reservoir is heated with 140 MHz radiofrequency at 50 kW over 10 years of period. The simulation results showed that the acceleration of oil production takes place due to radiofrequency and microwave heating, which can noticeably reduce the viscosity of oil. Thus, according to their conceptual numerical simulation, it is possible to apply potentially in field condition of medium, heavy and extra-heavy crude oil reservoirs. A few years later, a new idea was implemented in the field of electrical heating by Rangel-German et al. [57] performing numerical simulations of

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electrical resistance heating using mineral-insulated cable and novel heater-well arrangement. They used 2D and 3D (heterogeneous) reservoir simulation models to illustrate the output of electrical resistance heating and reported that this method can enhance the recovery by many folds. Based on the economic analysis of the heating process using mineral insulated cable, they stated that the cost of electricity is about 1.25 USD per barrel of incremental oil. Later on, another field case study and numerical investigation was presented by Gasbarri et al. [58]. They studied the effect of electrical heating of bottom-hole on production and recovery factor of extra-heavy Orinoco Oil Belt reservoirs. They carried out numerical simulations and analysis of influential parameters of horizontal wells with different crude oils of 8.1, 10, 12, and 15°API gravity. On the basis of the simulation results, they concluded that EM heating process can increase the oil recovery rate up to 60%. In electrical heating systems, several factors are responsible for oil recovery. Flow behavior of phases has an important role on temperature distribution during electrical heating. To clarify the effects of flow behavior on temperature distribution, Carrizales et al. [59] proposed a model for single-phase flow and computed the temperature distribution and enhanced productivity using EM heating in a well. They considered both counter-current and co-current flow but the flow was co-current when electrical energy was incorporated. They solved the mathematical model using steady state solutions and transient solutions. Their findings suggested that the EM heating is more efficient than resistive heating. Most of the cases EM heating was designed like SAGD wells. Therefore, it is important to compare the efficiencies of SAGD and EM-SAGD methods. In an attempt for this, Koolman et al. [40] studied the improved technology of the effect of EM heating in combination with steam assisted gravity drainage (EM-SAGD) for bitumen recovery from oil sands. They used a simulation tool for comparison of the two processes like SAGD and EM-SAGD. The EM-SAGD process gave better oil recovery and low energy consumption. Different EM heating methods of varying frequency ranging from 60 Hz to >300 MHz were investigated on the basis of technical feasibility, capital expenditure, reservoir feasibility. The simulation results showed that, for the thick reservoir, 10% more recovery was possible after 20 years of production compared to SAGD. For the shallow reservoir, the EM-SAGD process used 20% more steam and 10% more boiler capacity. In this type of reservoir, three times more production was obtained after 20 years of production. On the other hand, thin reservoir used the same capacity of boiler energy and also produced 38% more bitumen by using the same amount of steam in the EM-SAGD method. They reported that EM-SAGD process has low environmental impacts in terms of greenhouse gas emission and water usage. Their simulation study and small sandbox test also indicated the feasibility of the process. Another comparative study was carried out by Wang et al. [60] using a simulation tool developed by CMG. They simulated three types of reservoir cases. For 5 m pay zone the simulation results showed higher oil production in case of electrothermal method over the normal heating method. For 15 m pay zone with bottom water, electrothermal process recovered 14% of bitumen after 4 years whereas SAGD process only 1% as the high steam oil ratio (SOR) forcedly terminated production. SAGD was not suitable for low permeability pay zone and bitumen recovery deviated 38% from 3000 mD to 300 mD. In case of electrothermal method, the value was only 10%. This was due to the fact that electrothermal methods heated up formation in-situ by absorbing the radiation into formation water. In this method no external fluid was necessary. They also observed that, under favorable conditions, electrothermal methods have potential to recover bitumen from thin reservoir, which is not economically possible by the SAGD process. They finally showed that low permeability

affects the recovery in case of SAGD but has only a small effect on electrothermal methods. It is also important to make comparisons between high frequency and low frequency heating methods. Bogdanov et al. [61] performed the comparative analysis based on the numerical simulation of high frequency heating and low frequency heating processes. Production and thermal efficiency of EM heating method was evaluated and the results showed that production of bitumen by EM heating method was more than other conventional methods. Simulations were also conducted to investigate the effect of power and frequency variations on recovery. It was shown that high power led to enhanced oil production and better thermal efficiency. They suggested that EM heating at 330 kW with 10 MHz frequency source effectively enhances oil recovery. Water injection and production pressure variation with EM heating method were also applied using simulations. The water injection with medium rate with EM heating yielded improved production rate. They concluded that depending on numerical analysis, EM heating has promising efficiency to recover bitumen compared to other thermal method like SAGD. Comparative studies were also performed for steam injection methods other than SAGD and EM heating methods. To present a comparative overview of these two methods Das [62] studied the EM heating in viscous oil reservoir. He described the advantages and disadvantages of EM heating over steam injection and reported that heat penetration is less in the case of EM heating over steambased heating, and that volumetric sweep efficiency can be improved by EM heating in the low permeability area whereas fluid displacement technique might not be effective. He also carried out simulation of EM heating for viscous oil reservoir using CMG STARS thermal simulator and concluded that most heating takes place near the electrode area. It is important to recognize the other advantages of EM heating method over the conventional methods. For example, EM heating can be applied to ease the production of oil rather than heating the reservoir. An interesting study establishing a finite element model of an alternating current heating of hollow sucker rod was presented by Li-ying et al. [63]. They simulated the temperature of inner and outer wall of the hollow sucker rod and analyzed the factors affecting the temperature. They reported that the frequency and heating time can influence the temperature of the hollow sucker rod and the temperature could be increased by changing the heating time and frequency. Different studies showed inter-relationships among factors by numerical simulation. For better understanding the phenomenon, McGee and Donalson [64] proposed a model for radial heat transfer that can compare the resulting temperature distribution, time to achieve a heated volume at some distance away from the wellbore, and the power density in the reservoir between the different electro-thermal methods. They discussed the influences of well spacing, power input, near well bore heating and water vaporization on the production of oil in EM heating process. They concluded that convection can improve the energy efficiency by a factor of two over the conduction method and, in convection, well spacing is higher than that of the conduction method and less time is required to achieve the required temperature in case of convection. A similar 2D multiphase radial model describing three phase flow (water, oil and steam) and heat flow in a reservoir with confining formations was presented by Carrizales et al. [65]. They used Lagrange-quadratic finite elements in the environment provided by COMSOL Multiphysiscs software and applied the model on different reservoir types to simulate EM heating method. The simulation results showed that, after 3 years heating, oil recovery by EM heating reached 18% whereas only 2% recovery was obtained by cyclic steam stimulation. Lastly, simulations were carried out for an extra heavy oil reservoir with an initial viscosity of 12,115 cP

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and 7°API gravity at 100°F. For this type of extra heavy oil reservoir, EM heating produced more than 63% of cumulative oil compared to cyclic steam stimulation confirming that EM heating gives better results than the other conventional heating processes. They also reported that energy consumption is very small in the EM heating process. For further understanding the RF-EM heating method, Davletbaev et al. [66] presented a multi-layer, two-dimensional mathematical model of radio-frequency EM reservoir heating. Their modeling results indicated that the temperature of the medium can be enhanced by applying EM heating up to 100 °C. After performing a sensitivity analysis of the damping coefficient of the EM waves, they suggested that the effects bottom-hole temperature and heat/mass transfer in the reservoir can be controlled by setting the output performance of the RF generator and the pressure difference between the reservoir and bottom hole. For an economic feasibility investigation, Bogdanov et al. [67] presented a study on EM heating of heavy oil recovery methods. The simulation and modeling results were discussed with different issues like preheating, production efficiency factor, fluid flow pattern, power and frequency variation and pressure variations and maintenance. They concluded that EM heating power field and its evolution can be computed more precisely by using the coupled codes and simulator in oil production. They reported that the numerical analysis shows the promising efficiency of oil recovery by EM heating comparable than other thermal recovery techniques. More updated software was used to predict the efficiency of EM heating method. Mustafina et al. [68] studied the numerical simulation of inductive EM heating for heavy oil recovery using ANSYS Emag and CMG STARS. They considered two-dimensional pay zone completely surrounded by impermeable barriers and two horizontal well pairs of producer (lower well) and inductor (upper well) just like SAGD process. They showed that the obtained results can help understand the drive mechanisms responsible for oil recovery during EM heating process. Recently, several new conceptual designs of EM heating were proposed including mathematical models. Abdulrahman and Meribout [30] designed an antenna array for EOR under oil reservoir constraints with experimental validation. They performed simulation studies at different frequencies (915 MHz, 2450 MHz, and 5800 MHz) for three different designs of the array of EM sources under the constraint of the dimensions, dielectric constants, and geophysical properties of oil reservoirs. For implication of the microwave heating, they used several models like heat generation and distribution model, power distribution model, dielectric loss factor updating models, and mass balance and fluid flow model. In their experimental study, two tanks with different outer (75  75  60 cm) and inner (60  60  60 cm) dimensions were used. Experiments were performed with 2450 MHz frequency and 1080 W input power and 700 output power. The experimental results were perfectly matched (about 95%) with the suggested model produced by COMSOL Multiphysiscs. Jha et al. [69] proposed a microwave assisted gravity drainage (MWAGD) model. The model was composed of 1 km horizontal well and a vertical well with downhole micro-wave antenna. The separation distance between the microwave generator and horizontal well was 15 m. The lower horizontal well acted as a production well and the upper vertical well was the microwave source. When microwave antenna was switched on, the microwaves penetrated into the formation and got absorbed by the dielectric materials of the reservoir. By changing the molecular orientation, heat was produced and the formation was heated. The produced heat reduced the viscosity of heavy oil and the oil was collected through the lower well. In another attempt, the effect of different gas injection during EM heating was investigated by Liguo et al. [70]. They studied the feasibility of producing heavy oil by gas and electrical heating assisted gravity drainage (GEHAGD). They analysed production

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performance and temperature and gas distribution during this process based on the experimental and simulation results. They concluded that the GEHAGD process can improve the production by six fold compared to other production methods. They presented different oil production rate with various methods. Maximum oil production rate of cold production was 104 ml/min. They concluded that synergistic effect of gas mixed and electrical heating make the GEHAGD process more promising than the other processes. 8.2. Laboratory experiments Compared to computational models, a limited number of laboratory experiments were conducted to verify the effectiveness of the electrical heating methods. In this section, they are discussed with the outcomes and findings of the experiments and their importance in further applications at the field scales. Persons [71] conducted experiments using microwave attenuation technology to measure the water saturation and oil flow in core holder. Microwave scans were performed for Berea sandstone sample with dry, water-saturated, and oilfield condition. Results showed that the waterflood profile and oilfield profiles were very similar as the immiscible displacement acting in both cases and relative permeability relationship is controlled by displacement too. Coincidentally, it was observed that water saturation decreases with microwave signal. The microwave scans also indicated nonuniformity of the rock at the end of the water flood. Thin-film thermocouple detector worked well to analyse the sensitivity of temperature change. Based on these observations on the effects of microwave on the process, he concluded that this technology has potential to apply as a secondary and tertiary oil recovery process. Later on, a new concept in EM heating process was proposed by Chakma and Jha [12]. They presented a study on EM heating of scaled thin heavy oil reservoir pay zones. They also studied the combination of EM heating and gas injection with horizontal wells and reported that higher EM frequencies provide faster heating rate and can overcome problems associated with the discontinuity of the media through which EM waves must propagate. They stated that heat loss can be minimized with the use of higher frequencies and it is not necessary to heat the entire pay zone for moderately heavy oil reservoirs (less than 1000 mP s). The frequency of the EM source affected, i.e., for 5 MHz, 10 MHz, and 20 MHz frequencies, the overall recovery went up to 29%, 32%, and 37% OOIP, respectively. They observed oil recoveries as high as 45% of OOIP using EM heating and gas injection compared to primary recovery rates of less than 5%. On the other hand, Islam and Chilingarian [72] showed that 80% of the oil in place can be recovered if the EM process is combined with gas injection from a top horizontal well. After analyzing the affecting factors of wave propagation. Further laboratory study was conducted to investigate the effects of frequency and power on oil recovery. To test the effects of these parameters on oil recovery, Kasevich et al. [73] studied the heating characteristics of the diatomaceous earth and the changes in its dielectric properties during heating. The laboratory study included 1 kW, 50.55 MHz heating source and a 200 W, 144 MHz heating source. In their study, a high frequency source was used to heat soil samples in a modified 55 gallon drum with an electric monopole. The results indicated that radiofrequency heating can increase the temperature up to 150 °C of the diatomaceous earth. A heavy oil sample from Bakersfield area was heated using high radiofrequency at 400 W and the maximum temperature goes up to 150 °C after 49 min heating. They also measured the dielectric constant and conductivity with temperature changing from 50 °C to 295 °C. The dielectric constant of the sample increased steadily with temperature and attained the value of

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dielectric constant of sand. In case of field test, a radiofrequency system with 25 kW and 13.56 MHz source was used. They concluded that radiofrequency performs as theoretically envisioned but it requires proper design of equipment for suitable down hole operation. They also stated that reasonable cost fiber optic systems should be developed if continuous temperature monitoring in the presence of radiofrequency energy is required. EM irradiation also affects crude oil properties. Gunal and Islam [74] performed experimental studies on EM heating and ultrasonic irradiation to alter crude oil properties also discussing the impacts of EM heating on recovery of oil and gas from gas hydrate and carbonate reservoirs. Their results showed that the efficiency of EM heating is very high and can be used to heat near the wellbore with minimal energy input. Supply of power has an effect on heating time. They noted two different modes of temperature rise, the first one is exponential and another is logarithmic at higher power irradiation. Gunal and Islam [74] also studied the effect of water in the EM heating process and concluded that the carbonate formation as well as crude oil can be heated with EM irradiation without the presence of water. Presence of water in crude oil changed the shape of the temperature rise curve and 10% water made a steeper slope of the temperature curve at the high temperature region. They observed that crude oil-carbonate and water-carbonate rock mixtures had different temperature profiles. In case of 10% water and 90% carbonate rock, the temperature profile was totally different than that of 10% crude oil and 90% carbonate rock. Another combination of 5% crude oil, 5% water, and 90% carbonate rock had a diverse temperature rise when EM irradiation was applied. Similar observations on the effect of water in the electromagnetically heated systems were made by Chakma and Jha [12]. They showed that water saturation and the salinity of brine affect the wave propagation. It was also confirmed that the presence of water is essential for continuous propagation of the EM wave and higher salinity increases the conductance of the EM wave. Upgrading of crude oil is possible by using EM radiation or microwave energy. Gunal and Islam [74] observed that the presence of asphaltenes causes permanent changes in crude oil rheology when exposed to EM irradiation due to polar nature of asphaltene molecules as there is a non-linear dependence of crude oil viscosity on both EM power and asphaltene concentration. Later, Jackson [75] performed experiments on upgrading of heavy oil using variable microwave energy at a facility in North Carolina. The experiments were performed with a variation of heating times, frequency, and different additives like activated carbon, molybdic acid, and iron powder. It was reported that less than 2 wt.% coke was formed by heating for anhydride molybdic acid and iron powder mixture. The liquid products were promising and the best results for the liquid volumes were found to be 77%, 79%, and 90%. The effect of frequency on the performance of heating and fluid production was also investigated. For this purpose, 6700 MHz and 6100 MHz frequencies were chosen. The results showed that 6700 MHz frequency was more efficient to translate the energy to the sample. It was also proved that energy consumption was much lower than other conventional processes. EM energy or microwave radiation can also be used for oil–water separation process which has great impact on oil recovery. For this purpose, Vega and Delgado [76] studied the treatment of waste water–oil emulsion using microwave radiation. They concluded that microwaves are able to destabilize the water–oil emulsion in two ways. Firstly, they reduce the viscosity of continuous phase and break the outer film droplets to allow them for coalescence. Secondly, they rearranged the charge distribution of the water molecules, moving them around the ions. They reported that by using microwave technology breaking of water-in-oil emulsion is very easy. The same methodology can also be applied to clean water blocking. Li et al. [77] conducted laboratory experiments

on clean up water blocking of gas reservoir by microwave heating and showed that microwave heating performed better than conventional formation heating technologies. Microwave technology rapidly increased the temperature of the reservoir and cleaned up aqueous phase and water blocking. It even improved gas permeability and hence enhanced the production. They also studied the effects of microwave heating on the composition of sandstone. The results showed that, after 100 min microwave heating, there were no characteristic changes in the quartz, feldspar, amazonstone, kalifeldspath, calcite, and dolomite. In case of clay, there were also no change of kaolinite, illite and chlorite properties after microwave heating. The experimental results also indicated that microwave heating can clean up water phase quickly and improve permeability significantly. Gas permeability can be increased 102– 248% and 107–266% after 1 min and 15 min microwave heating. Microwave heating also changed water saturation. Microwave heating was used for different applications in oil industry including removal of drill cutting during drilling. Robinson et al. [78,79] presented a study of microwave treatment of oil-contaminated drill cuttings at pilot scale. The drill cuttings contained 6% water and 7% oil. An experiment was performed at microwave power levels of 15 kW and 30 kW and the material throughput was varied between 110 and 570 kg/h. The belt speed of the machine was managed at 10–52 mm/s and residence times within the electric field were maintained 1–5 s. The results showed that residual oil content increases with increasing material drill cuttings throughput. The effect of microwave power input on the process was also investigated in this study and it was observed that when 30 kW of microwave power was applied, more material throughput was necessary to achieve the same residual oil level. Robinson et al. [78,79] also reported that the microwave treatment of cutting could reduce the residual oil level down to 1% environmental discharge threshold. In this process, the energy consumption was reasonably low (70–100 kW h per tonne). A sequence of laboratory works was presented by a group of authors to test the effect of electromagnetic heating on the behavior of heavy-oil and bitumen. In the leading paper, Bjorndalen et al. [80] proposed several heat transfer models to study the temperature rise in petroleum fluids under microwave radiation and validated their models using experiments. They reported that a slight variation between the experimental and numerical results is due to non-homogeneous mixture. Later, Bjorndalen et al. [81] performed experiments to clarify the effects of irradiation on petroleum sludge and concluded that this technique can be applied to recover oil from the sludge. In a succeeding study, Bjorndalen et al. [82] investigated the effects of temperature rise due to microwave heating time on paraffin wax concentration. Their results indicated that with an increase in the density of the fluid, there is a decrease in the temperature change with time. Later, Hascakir et al. [83,84] performed experimental and numerical studies to measure the upgrading capability of electrical heating in the presence of iron-micro-particles. Their retort experiments showed that the viscosities of two different crude oils can be reduced 86% and 63% after addition of 0.5% iron powder. They used these results to construct a field scale model and defined the economic limits of the process [83,84]. Hascakir et al. [7] also conducted microwave experiments with three crude oil samples in a sand pack. Results indicated that 60% water saturation generates more temperature than 40% and 20% cases. They reported that higher initial water saturation shows higher recovery by microwave heating. They also examined the effect of initial wettability of the sample on oil recovery by EM heating method and showed that water wet and mixed wet systems give better results than oil wet system. Effect of heating time on oil production was also tested in this work. It was observed that with increasing heating time oil production also increases and high porosity and

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permeability also enhance the oil recovery. They concluded that the heating pattern of microwave technique should be continuous type. Effect of microwave heating on viscosity reduction was also investigated by Jha et al. [69]. They performed laboratory experiments with crushed sandstone mixed with heavy oil collected from Mehsana oil field (Gujarat, India) using microwave heating for heavy oil recovery and studied the effect of porosity, wettability, initial oil and water saturation, and permeability on the process. They discussed the viscosity variation with temperature during EM heating. They used 16°API crude oil with crushed sandstone in a 7 cm long cubical stainless steel case. The microwave was used at 3 GHz frequency and 100 W power. The viscosity of the crude was 300 cP at 70 °C and 10 cP at 110 °C. It was confirmed from their conclusions that the reduction rate of crude viscosity is higher than the other thermal methods. Kovaleva et al. [17,18] compared electrical and electromagnetic heating experimentally. They used electrical and radiofrequency electromagnetic (RF-EM) heating with 6 kW generator and an operating frequency of 81.36 MHz on mass and heat transfer processes in a multi-component hydrocarbon system during miscible injection for heavy oil recovery. Their results showed that RF-EM heating gives better results than electrical heating process. It was also noticed that RF-EM heating started much earlier than electrical heating. The enhanced efficiency of RF-EM heating can be demonstrated on the basis of heavy components desorption in the RF-EM heating case. They reported that the RF-EM field influences the oil recovery more pronouncedly than electrical heating. For experimental analysis of the adsorption of asphaltenes and resin in porous media, silica sand model was used under static conditions. In this experiment, 1 cm diameter plastic tube was filled with 970 cP viscous oil and silica sands were used with this oil making small holes at the bottom to collect the produced oil under heating condition. The effect of silica sand size (0.2 and 0.6 mm) was investigated in this experiment. Results indicated that 0.2 mm silica sand could recover 100% adsorbed oil which is more than 0.6 mm silica sand and the polar components of oil were desorbed from rock surface during RF-EM heating as observed through atomic force microscopy images. Different types of electrical methods would yield different efficiencies that eventually determine the economic feasibility. To compare the efficiencies of the methods, Alomair et al. [9] performed laboratory experiments using three types of unconventional thermal heating methods such as electrical resistant electrodes, EM inductors, and microwave heating for EOR. The EM heating was performed with a sand pack (2 in. by 12 in.) core holder using a crude oil of 17.3 API and 540.52 cP viscosity. It was reported that 55 h heating at 45 °C recovered 51.7% oil and 58.7% was recovered after 47.7 h heating at 65 °C. 15 h heating at 85 °C, however, yielded much higher (67.8%) oil recovery. In case of electrical resistance heating, 10.43% oil was recovered after 60 h heating. With increasing voltage, heating time decreased and 20.79% oil was recovered after 42.4 h heating. In case of microwave heating different power levels of 10%, 30%, 70%, and 100% were applied in the experiments. At power levels of 10% and 30%, the total recoveries were 37.24% and 38.44% for heating times 240 s and 230 s, respectively, whereas 70% and 100% power levels were able to recover 42.3% and 45.8% in 290 s and 270 s, respectively. They reported that oil recovery can be increased 30% depending on the power used in the experiment. The average power consumptions were 39, 2570, and 3.775 W hr/cc for EM induction, electrical resistance and microwave, respectively. They concluded that microwave heating for oil recovery is better than other electrical heating methods based on the increased oil recovery and power consumption. It is obvious that there exist an critical power and heating time for economic limit and determination of this is a crucial task.

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Recently, Bientinesi et al. [27] conducted laboratory experiments on EM heating of nearly 2000 kg of oil sands in a sandbox up to 200 °C, using a dipolar antenna. They applied 2.45 GHz frequency to irradiate oil sand with variable power (1–2 kW) and recorded the temperature of the oil sand and the boundary and in several specific points of the setup throughout the experiment. From the temperature distribution of EM heating after 100 °C, a decrease in heating rate was observed due to evaporation of connate water. Temperature rise followed a monotonic path during EM irradiation of oil sand due to dielectric heating phenomenon. They concluded on the basis of the experimental results that EM irradiation is capable of heating oil sands, even above the boiling temperature of connate water. They also performed numerical simulation with a simplified model and showed that the presence of a tight shell could help to reach uniform heating of large volumes of reservoir. This process can avoid the risk of high temperatures at the wellbore and therefore make the recovery method much less sensitive to local dielectric properties of the reservoir materials. 8.3. Field applications Field scale applications of the EM technique are highly limited. Earlier reports on this date back to 1980s. Only few field trials of EM heating for heavy oil recovery have been conducted in the USA (California and Utah), Canada (Alberta and Saskatchewan), and Russia (Bashkortostan and Tatarstan). Sayakhov et al. [85–87] presented a brief description of the first field test of radiofrequency heating in the Ishimbayskoye heavy-oil field (Bashkortostan) and the Yultimirovskoye bitumen field (Tatarstan). A detailed description of these field studies was precisely documented by Mukhametshina and Martynova [44] with limited information on the performance efficiency. Field trials of RF-EM heating was also conducted in Bakersfield, California, USA. Kasevich et al. [73] reported that temperature increased during the RF-EM heating from 293 K to 393 K for 20 h duration of the test. Bridges et al. [88] filed a patent for in-situ heat processing of hydrocarbonaceous formation by extensively studying different experiments at Illinois Institute of Technology Research Institute (IITRI). In 1983, they implemented their IITRI technique of RF-EM heating in the Avintaquin Canyon field, Utah, USA. Two different field tests applying EM heating were also reported in Canada. In 1986, the first EM heating was implemented in the Wildmere Field, Alberta to increase the heavy oil production [89]. After applying EM heating in the well, the production rate was increased from 0.95 tonnes/day to 3.18 tonnes/day. Another well in this field also showed a very high production rate of 4.77 tonnes/day increased from 1.59 tonnes/day after the EM heating application. Another reported case was two-well pilot in a field in the Lloydminster area (Sparky formation), Saskatchewan, Canada in 1988–1989 [90]. There were three pilot tests conducted at Northminister, Lashburn, and Wildmere. The Northminister and Lashburn wells produced oils of 13.7°API and 11.4°API, respectively. The first pilot well (in Northminister) achieved an increase of production rate from 10 m3/day to 20 m3/day by application of EM heating [90]. The second pilot well (in Lashburn) also increased its production rate from 5 m3/day to 9 m3/day when EM power was used. The Wildmere project was also conducted but it was not continued due to casing installation failure and sanding problem. In both Northminister and Lashburn cases, the inflow performance relationship (IPR) curves were generated to predict the production rate and pump speed. For Northminister, the IPR curves indicated that when pump speeds were maintained at 1.0, 1.5, and 1.9 spm, the average oil production rates were found to be 7.5,

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10.0, and 12.0 m3/day, respectively. In case of Lashburn, the stimulated IPR curve showed EOR pattern. They concluded that EM heating was suitable for enhanced heavy oil recovery. It was also noted that the peak stimulation ratios of Northminister and Lashburn pilot projects were 1.27 and 3.75, respectively. Radiofrequency heating pilot tests were also implemented in a tar sand formation in Utah. Sresty et al. [91] conducted field scale experiments in a pilot plant as well as laboratory tests to recover bitumen from tar sand deposits near Vernal (Utah) using radiofrequency heating. In their experimental trials, the recovery methods investigated include replacing the heated bitumen with sodium silicate solutions, gravity drainage, and autogenously developed steam and hydrocarbon gases. They then conducted two smallscale field experiments in the Asphalt Ridge tar sand deposit near Vernal and reported that about 25 m3 [33 cu yd] of the deposit was heated with RF energy, and about 35% of the total in-place bitumen was recovered during the 3-week test period. But their laboratory experiment showed 50–80% bitumen recovery depending upon the proper application of frequency and methods. They finally stated that the process is economically feasible and attractive. The ratio of energy recovered to energy expended in the process ranges from 3 to 12 depending on the total quantity of bitumen recovered and other process conditions. They focused on the environmental concerns, which were minimal because the process was based on in-situ recovery. Recently, Bottazzi et al. [92] discussed the successful application of downhole electrical heating (DHEH) for enhanced heavy oil recovery in offshore Congo. The main goal of the study was to evaluate the possible application of this technology through the reservoir modeling. The actual objectives of the study were to describe the results of the reservoir modeling, activities of the completion planning, and the different techniques used during the operation. They presented a comparison between cyclic steam stimulation/SAGD and DHEH method. The production was 7.6% more than the SAGD/cyclic steam stimulation method (20% enhanced production was observed over cold production). It was observed from the simulation results that DHEH is more efficient to increase the temperature of the surrounding fluid and the viscosity of the fluid was decreased and production rate was increased by many folds. Due to reduction of viscosity, the production rate increased and less friction was formed inside the production tubing and pump worked efficiently with lower back pressure. They concluded that the capital expenditure and DHEH installation program can be implemented with less cost than the electrical submersible pump, and therefore the method could be economically feasible.

combined process can deeply heat the layer and increase the displacement area. They also presented a model to describe the transfer of heat and mass within the ‘‘well/reservoir/rock environment’’ system. Their analyses of the mathematical model and calculation results showed that solvent steadily percolated into the formation and its distribution was monotonic. EM heating also influenced the distribution behavior in the mixing zone very critically. Their proposed work on 2D mathematical model of a 3-stage procedure of combined RF-EM/solvent treatment for heavy oil production concluded that this process is economically feasible. They reported that the optimal solvent volume is 7.5–12.5% of the total pore volume and the duration of first treatment stage is 27 and 60 days correspondingly. The combined process showed six times greater temperature than the process without solvent injection. Studies on EM heating using mathematical models were discussed by different authors. Peraser et al. [93] studied the EM heating for EOR mathematically. They clarified the effects of EM power level and frequency on heat penetration into reservoir, temperature, and oil production with the help of heat transfer and fluid flow applications. Modeling approach showed that, after 1 year EM heating, temperature was raised to 212°F from initial reservoir temperature of 120°F. The reduction of viscosity was found to be 97% (from 3062 cP to 98.9 cP). They observed that using EM heating heavy oil production rates can be increased by more than 200%, due to viscosity reduction and temperature increment. They made a comparison between cyclic steam stimulation and EM heating method. The results showed that, after 3 years of heating, cyclic steam stimulation can produce 37,000 bbl of oil and EM heating can produce 80,000 bbl of oil. This makes the technique potentially applicable in Alaskan heavy oil reserves. Eventually, Lui and Zhao [35] compared the performance of EM heating and SAGD processes. Based on the simulation results, they concluded that EM adsorption coefficient plays an important role in estimating the penetration of heat to the reservoir. They presented a model for single phase radial flow and oil–gas two-phase linear flow under EM heating using a commercial finite element simulator (COMSOL). A comparison was made between this modeling approach and electrical resistive heating by a commercial finite difference simulator (STARS-CMG). The results indicated that heating distribution in case of COMSOL model was almost double than that of the STARS model. They also carried out a sensitivity analysis and reported that the higher the frequency (2450 MHz), the higher the cumulative oil production. This was only valid up to 915 MHz and after that a very high temperature was generated inside the reservoir. Finally, they concluded from the sensitivity analysis that permeability change has a low impact on oil production based on EM heating than steam injection.

8.4. General discussion on the applicability and feasibility of EM heating 9. Application of nanotechnology in EM heating Davletbaev et al. [34] discussed the improvement of recovery efficiency of heavy oil/bitumen by radiofrequency and EM heating methods. They analysed the mechanism and dynamics of the radiofrequency and EM heating for several field scale applications in Russia. According to their simulation results, bottom-hole temperature and heat/mass transfer effects in reservoir can be controlled by setting output performance of RF generator and by the difference between bottom-hole and reservoir pressure. Mathematical model results showed that higher water cuts (>30%) are possible with electric heating. It was also reported that paraffin deposition can be controlled as RF-EM energy was lost along the wellbore during heating. In a subsequent work, Kovaleva et al. [17,18] studied the combined effect of solvent injection with EM heating for EOR using the improved version of this mathematical model and analyzed the electro, thermo, and hydrodynamic aspects of EOR during this process. They reported that the

It is a well-known fact that nano-metal oxides have potential to absorb microwaves and thus increasing the system temperature where it is used. No field application has been conducted with nano-metal particle incorporating EM heating but different laboratory experiments have been carried out by several researchers with active metal-oxide nanoparticles to improve heavy oil recovery. Greff and Babadagli [94,10] performed microwave heating experiments on glass beads saturated with crude oil in presence of iron oxide, copper, and nickel. The range of nano-metal particle concentrations was 0.1–1 wt.% and they obtained a reduction in viscosity (up to 10–20%) with increasing concentration up to a critical concentration. The Ni particles showed the best performance out of these three and the recovery increase was significant at lower microwave powers (the effect of Ni particles on recovery was shown visually in Figs. 11–13 of Greff and Babadagli [10].

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A similar trend in lowering oil viscosity with addition of nanometal particles was also reported by Hamedi Shokrlu and Babadagli [95,96]. A critical concentration of nano particles exists that minimizes the viscosity and among the above mentioned metal types, nickel nano catalyst showed the best result. Hascakir and Akin [97] performed laboratory experiments with iron nano-powder, iron chloride, and iron oxide and reported that iron nano powder gives the highest viscosity reduction. Hamedi Shokrlu and Babadagli [96] discussed the mechanism of reduction of oil viscosity by metal oxide nano-powder with the variation of concentration. There were two physical or chemical processes behind the variation of viscosity with nano-metal concentration. The positive one is that with addition of metal nanoparticle up to a certain limit, it decreases the viscosity of crude oil due to attraction of the maximum asphaltene molecules on the metal surface, making the bulk oil less viscous. On the other hand, with increasing metal ion concentration, the structural complexity of the asphaltene molecules was increased through coordination reactions and, as a result of this, the viscosity of crude oil increased. Table 2 shows the uses of nano-metal particles in laboratory studies for oil recovery experiments. Several laboratory-based researches were conducted for heavy oil upgrading by catalytic pyrolysis reactions in microwave. Nano-metal oxides have capabilities of absorbing microwave frequency yielding an increase in the temperature of the system. This results in a decrease in the viscosity of crude oil. In many aquathermolysis reaction of crude oil, nano-metal catalysts were efficiently used [100–106].

10. Economic feasibility of EM heating technique Due to excessive cost, mainly emerging from the cost of electricity used to generate EM waves, the economics of this process is critically important. This, on the other hand, is dictated by the oil prices. Interestingly, based on today’s oil prices (mid-2014), most of the studies reported that EM heating is economically feasible for oil production. If we consider the energy consumption for different electrical heating methods, it can be concluded that the microwave heating consumes the lowest energy [9]. Note that the local efficiency of the microwave heating process is very high due to its effectiveness caused by high frequency. On the other hand, if we consider the ‘‘global efficiency’’ of the process with respect to burning of oil and gas in the combustion unit, the

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efficiency of the boiler to produce steam, the efficiency to the turbine and generator, and the transmission and distribution, then EM heating or microwave technology is not as effective as stated in literature. For example, Khan et al. [107,108] presented a concept of global efficiency in utilization of energy for producing oil. It is well known that the electrical energy has to be converted into microwave or radiofrequency electromagnetic waves for applications in transformer or microwave. Considering all the governing factors including the cost of the process, the ‘‘total efficiency’’ of microwave heating method is very low. This should be considered in designing oil field applications of EM heating. Table 3 shows the general efficiency of different processes; i.e., nonaqueous-electrical and aqueous heating (efficiency is defined as the ratio of the useful work done by a machine, engine, or device to the energy supplied to it and as a percentage). Lack of field projects on EM heating makes this comparison very superficial. As widely accepted, in case of aqueous heating method, steam oil ratio (SOR) is the primary measurement of effectiveness of heavy oil recovery. For a non-aqueous method, it is computed by calculating the input energy expressed as an equivalent barrels of steam quantity. Bogdanov et al. [61] showed a comparison of effective SOR for different in-situ production technologies of oil sand as described in Table 4. Another economic analysis on electrical heating with nano iron and iron oxide was done by Hascakir et al. [83,84]. They showed that with addition of nanoparticles, the cost is in the range of 15–25 $/bbl, and without nanoparticles, it increases to 30–40 $/ bbl for three types of oil shale samples. Koolman et al. [40] discussed the capital expenditure (CAPEX) of different electrical heating methods. They analysed the CAPEX for all electrical and EM heating methods and concluded that depending on the CAPEX, resistive heating and inductive heating are better options for thermal heavy oil recovery process. Microwave and radiofrequency heating often exceed the desired economic criteria for heavy oil recovery. Table 5 summarizes the CAPEX description for different types of electrical heating methods.

11. Environmental impacts of EM heating A number of studies on the subject discussed the positive and negative aspects of using high frequency heating for oil recovery. The use of EM heating in soil remediation processes is more practical than other techniques. As stated by McGee and Vermelen

Table 2 List of nanoparticles used in laboratory experiments for oil recovery. Sl. no.

Author(s)

Nanoparticles used

Brief description

1

Hascakir et al. [7]

Iron oxide, Ferric chloride, Iron powder

2

Jackson [75]

3

Hascakir and Akin [97]

Activated carbon (AC), AC with NaOH, AC with iron powder, Molybdic acid anhydride with iron powder Fe, Fe2O3 and FeCl3 powder

The upgrading of heavy oil by microwave heating with nano-metallic additives was done. Iron powder showed the best results to reduce viscosity of oil AC yields best result over the others

4

Greff and Babadagli [94] Greff and Babadagli [10] Hamedi Shokrlu and Babadagli [96] Davidson et al. [98]

Iron nano powder, iron (III) oxide nano powder, copper micro powder Iron nano powder, nano iron oxide, nano nickel

Neto et al. [99]

Acidic BEA zeolite catalyst

5 6

7

8

Nickel Particle with 100 and 5000 nm

Single-domain Fe3O4 (10 nm)

Three types of oil shales (OS1, OS2, and OS3) were used to study the oil recovery by EM heating using Fe, Fe2O3 and FeCl3 powder as microwave receptors. 0.1% Fe and 0.5% Fe2O3 are best dose for OS1sanple for oil recovery Breaking of asphaltene molecules was done. Iron nano powder shows better results EM heating with nano metal catalyst has been conducted and nano-nickel catalyst shows better result Nano sized nickel performs better way than micro one

They conducted experiment with nanoparticles dispersed in water, hydrocarbon liquid, embedded thin and solid film dubbed ‘‘nanopaint’’ to study the heating rate under EM condition Oil shale transformation has been studied using nan catalyst

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Table 3 Efficiencies of the different process compared to the radiofrequency/microwave heating. Process

Efficiency, %

Oil and coal boiler Turbine Generator Transmission and distribution Microwave (itself) Microwave (overall or total)

35 70 80 90 50 <10

Table 4 Comparison of effective SOR for different in-situ production technologies of oil sand [16]. Types of heating methods

Effective SOR

Low frequency heating Conductive heating SAGD heating RF heating (no pressure enhancement) RF heating (with pressure enhancement)

1.8–2.5 2.2 3.3 1.8–2.2 1.4

[109], many contaminants such as gasoline and other volatile organic compounds can be removed very fast from soil through microwave heating. In a different application of microwave heating, Martinez [110] showed that microwaves can resonate the cellular membranes of the tree and thereby interrupt the water circulation. As a result, the balance of electricity-charged particles in the plant may also be disturbed and mineral management of the trees may also be affected and thereby the soil organisms. Other researchers also reported that there are harmful environmental effects associated with microwaves with respect to the killing of bacteria, microorganism, plants, toxic emission, and biological misbalance from heavy oil recovery from reservoirs [111,112,74,113]. 12. Advantages and disadvantages of EM heating method The advantages of the electrical methods are as follows: 1. The EM heating method is more energetically efficient than other aqueous thermal heating methods. 2. It is also efficient to work in shallow wells where other aqueous thermal methods like steam injection cannot work. 3. This method does not require huge amount of water supply like steam stimulation method. 4. It can also work in heterogeneous reservoirs even in the high permeability zones or fractured area. 5. The production of EM heating does not depend on the application of electrical power. 6. It is a time-saving process (within shorter time compared to other heating processes) wherein it can increase the temperature and therefore enhances the production rate. 7. Heat loss can be reduced by controlled use of EM heating process.

8. In EM heating, less amounts of greenhouse gas is emitted compared to other steam-based methods. Therefore it is environmentally accepted. On the other hand, in addition to some environmental issue, this method has some disadvantages including: 1. This method is only applicable for near-well bore heating, typically in vertical wells. 2. Electrodes or antenna might suffer from corrosion problem in case of high salt concentration reservoir. As a result, the cost of the technique would not be feasible. Even conducting a field trial for this method is more expensive than the other electrical methods as described by Koolman et al. [40]. Therefore, it cannot meet the economic feasibility criteria for pilot scale application for heavy oil recovery. 3. In the case of high frequency radiation, the penetration depth is low. Therefore heating area in the reservoir will be reduced. 4. EM heating method also suffers from few environmental issues. These issues have been already discussed in Section 11. As seen, the main disadvantages are the economic and environmental issues. More research work is needed to propose economically and environmentally viable EM heating at the field scale.

13. Further research and development challenges and opportunities on EM Heating method As seen through the review presented in this paper, most of the reported studies on EM heating are computational. There have been many attempts on deriving analytical models to calculate the heat distribution and recovery potential with respect to power and frequency. Similar attempts were made using existing commercial numerical simulators. All these approaches are successful in estimating the total heated area and ultimate effect of EM heating on recovery improvement even considering heterogeneity and fractured nature. This kind of simulation, however, needs to be fed with experimental data on the fluid behavior under EM heating. Despite numerous attempts on this, the phase behavior heavy-oil and bitumen under this type of unconventional heating needs to be understood. Effects of EM waves on the wettability in unconventional reservoirs (shales and even carbonates) require further clarifications. Most of the laboratory experiments for recovery performance estimation were conducted at the core scale (order of centimeters) without considering the effect of reservoir characteristics and upscaling issues. Further research is necessary for better understanding the process through larger scale lab experimentation (order of meters). Application of this technology also requires new developments and lab scale tests on RF antenna design, power input modeling and efficiency increment of reservoir materials to absorb RF waves during reservoir heating for enhanced heavy oil recovery. One of the critical aspects of EM heating is the cost. Field scale applications indicate that commercial equipment is available for

Table 5 Comparison of CAPEX for different EM heating [40]. EM heating methods

Frequency range

Converter position

Dissipation materials

CAPEX (estimated) million US$/megawatts

Resistive heating 1 Resistive heating 2 Inductive heating Radiofrequency heating Microwave heating

60 Hz 60 Hz <300 kHz 0.3–300 MHz >300 MHz

On On On On On

Tubing Reservoir and tubing/electrodes Reservoir Reservoir Reservoir

<0.13 <0.13 1.27 6.35–12.70 >12.70

surface surface surface surface with downhole waveguide and antenna surface antenna and down hole antenna

Table 6 Tabulated form of literatures on laboratory experiments of electrical/EM heating. Experimental model

Subject of study

Method of heating

Oil recovery

Exp. temp.

Oil type

Rock type

Porosity

Permeability

Chakma and Jha [12]

20 cm  20 cm  10 cm Plexiglass filled with sand

Effect of oil viscosity, EM frequency, gas injection pressure, electrode distance, salinity, temperature on oil recovery

Radiofrequency (5, 10, and 20 MHz)

60 °C

Moderately heavy oil (1000 and 2000 mPa s)

Reservoir sand





Kasevich et al. [73]

55 gallon drum with diatomaceous earth and soil



Diatomaceous earth and soil





Crushed carbonate in core holder with crude oil and water

Radiofrequency (50.55 MHz, 1 kW and 144 MHz, 200 W) Microwave (2450 MHz, 700 W)

150 °C

Gunal and Islam [74]

Effect of radiofrequency on temperature increase of soil, diatomaceous earth and Bakersfield sample Effect of EM wave on temperature increase and crude rheology

29%, 32% and 37% of OOIP for 5, 10, and 20 MHz and up to 45% of OOIP depending on condition applied –



70–80 K

UAE formation oil (5.27 cP at room temperature)





Jackson [75]

Heavy oil sample in Erlenmeyer flask

Microwave (6700 ± 507.5 MHz)



Up to 330 °C

Plover lake crude (11,300 @ 20 °C)





Ovalles et al. [56]

300 ml stainless steel reactor

Upgradation of oil using microwave energy with additives Effect of microwave heating on oil recovery and model developed

Carbonate rock (Jabel Hafeet region of Al-Ain, United Arab Emirates) –

Microwave (2.45 GHz)

76% for extra heavy oil and 86% for heavy oil

150–200 °C

Silica sand

Graphite core holder (5.2 cm diameter and 8.5 cm height)

Effect of microwave for oil production and temperature increase

Microwave (2.45 GHz, 1400 W)

Up to 80% oil recovery depending on rock properties

Up to 170 °C

42% (Venezuelan medium crude oil reservoir), 36% (Lake Maracaibo heavy crude oil reservoir) and 32% (Orinoco basin extra-heavy crude oil reservoir) 18% (Banti Raman), 21% (Camurlu), 6% (Garzan)

350 mD, 1000 mD, 12000 mD respectively

Hascakir et al. [7]

Venezuelan medium crude oil (24°API, 15 cP @ 150°F), Lake Maracaibo heavy crude oil (11°API, 2163 cP @ 115°F), and Orinoco basin extra-heavy crude oil (7.7°API, 2700 cP @ 130°F) 592 cP @ 150 °C (Banti Raman), 700 cP @115 °C (Camurlu) and 33 cP @ 179 °C (Garzan)

Kovaleva et al. [17,18]

Sand pack model in Polyvinyl chloride pipe

Effects of electrical and EM-RF heating on oil production

Radiofrequency (81.36 MHz, 6 kW) and electrical

Up to 100% depending on the silica size

Jha et al. [69]

Cubical stainless steel case with crushed sandstone Berea sand pack model

Effect of temperature on viscosity reduction under microwave irradiation Effect of different electrical heating method on oil recovery

Microwave (3 GHz, 1 kW)



57 °C (0.6 mm silica) and 70 °C 90.2 mm) Up to 110 °C

Electrical resistive, EM induction, Microwave (2.45 GHz)

Sandbox model

Effect of microwave on oil recovery Effect of microwave on oil recovery in presence of nano metals

Microwave (2.45 GHz, 1–2 kW) Microwave (2.45 GHz, 1 kW)

10.34–20.79% (Electrical resistive), 17.8–34% (EM induction), 24.8–29.4% (Microwave) – Up to 90% depending on condition

Alomair et al. [9]

Bientinesi et al. [27] Greff and Babadagli [10]

Buchner filter funnel model

Varandei crude oil (970 cP @ room temperature)

Silica sand



58 mD (Banti Raman), 40 mD (Camurlu), 3 mD (Garzan) –

Mehsana crude oil (ranges from 50–450 cP @ 70 °C)

Sandstone





Three operating temperatures like 45 °C, 65 °C, 85 °C

Heavy oil (17.5°API, 540.52 cP @ room temperature)

Berea sandstone, Glass beads, Silica sand

35%

13 D

Up to 200 °C



Oil sands

40% sand void grade



More than 200 °C

North Alberta crude oil (120 cP @ 60 °C)

Glass beads (40 mm diameter)





Limestone

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Author (s)

221

222

A. Bera, T. Babadagli / Applied Energy 151 (2015) 206–226

Table 7 Tabulated form of literatures on numerical simulation of electrical/EM heating. Author (s)

Subject of study

Method of heating

Oil recovery

Temperature

Oil type

Rock type

Porosity (%)

Permeability

Abernethy [31]

Mathematical model development to evaluate the temperature distribution in EM heating Numerical simulation of EM heating of Athabasca oil sand Development of numerical simulation model to study the EOR by electrical heating Simulation of EM heating of a reservoir of Ugnu, Alaska Solution of oil flow in reservoir under microwave heating Simulation of EM heating for application in oil recovery

Radiofrequency













Electrical













Electrical

98.6°F

Heavy oil (2452 cP @ 100°F)

Reservoir rock

27

4D

EM

86% increase in production after 1500 days operation 40% of OOIP

Up to 200 °C

Shale and oil sand

25

200–3000 mD

Microwave



Reservoir rock

20

50 mD

Electrical resistive and Microwave

80

50 °C (Formation temperature) 300°F

10,000–100,000 cP @ reservoir conditions Crude oil (53 cP @ 122°F)

Reservoir sand

36

2500 mD

Vermeulen and McGee [39]

Simulation of electrical and induction heating for oil recovery

Electrical resistive and EM induction

30

10,000 mD

Comparison of the two processes like SAGD and EM-SAGD using simulation tool Simulation of EM heating in few viscous oil reservoirs Oil recovery calculation by simulation in EM heating

EM induction

140–160 °C (Inductive heating) and 120–180 °C (resistive heating) More than 8000 K (numerical result) 90–100 °C

Reservoir rock

Koolman et al. [40]

24110 m3 and 29550 m3 by EM inductive and electrical resistive heating 38% additional bitumen recovery (thin reservoir) 100 bpd after 3 years heating

Viscous and low viscous oil (9541 cP and 33 cP at initial reservoir conditions respectively) Dead oil (10,000 cP @ 20 °C)

Bitumen

Oil sand





Reservoir sand



500 mD

100–280°F

Reservoir rock

38

Bogdanov et al. [61]

Simulation on EM process to verify its capability in oil recovery

EM (10 MHz, 330 kW)

200, 400, 600, and 800 °C (Set point temperatures)

Crude oil (560 cP @ 15 °C

Reservoir rock

30

1000 mD (horizontal) and 100 mD (vertical) 3000 mD

Gasbarri et al. [58]

Numerical simulation on electrical heating

Electrical (1.6  107 and 5  107 BTU/Day)

16% (Oil recovery factor for EM heating) 2–3 m3/day (vertical well) and 0.1–0.2 m3/day (horizontal well) Increases recovery factor up to 60%

Viscous oil (3500 cP @ reservoir condition) Heavy oil (3780 cP @ 100°F)

Around 350°F

Four types of oil (8.1, 10, 12, and 15°API)

32

7000 mD

Bogdanov et al. [67]

Numerical simulation of EM drive for heavy oil recovery

EM



Crude oil (1000 cP @ 10 °C)

30

3000 mD

Godard and ReyBethbeder [114] Mustafina et al. [68]

Simulation of radiofrequency heating in sand-water mixture Simulation on EM induction heating

Radiofrequency (10 MHz)

4–5.5 GJ/m3 (ratio of energy consume to oil produced) –

Orinoco oil belt reservoir rock Reservoir rock

Up to 400 °C



Silica sand









Bitumen (107 cP @ 10 °C)

Reservoir rock with different thickness

30

3000 mD

Hiebert et al. [49]

Pizzaro and Trevisan [38]

Wadadar and Islam [52] Soliman [54]

Sahni et al. [6]

Das [62]

Carrizales et al. [65]

EM

EM

EM induction

reservoir heating. Continuous heating through high density injection wells while producing from the same wells (SAGD like design) may not be economically feasible even though it is the only option for shallow (low pressure) bitumen containing reservoir. Hence, single well applications will be more suitable but this requires further attempts on well design to resist temperature generate by high energy EM waves. This also requires optimal design of injection with also additional pressure supply through solvent vapor (gas or liquid solvent injection) or steam (water injection).

One of the ways to reduce the cost is to use additional materials that help reduce the amount of heat energy needed. One option to achieve this is to use compatible nano-metals that are capable of absorbing EM waves and act as a capacitor. This type of materials also helps distribute the heat more effectively, i.e., to a larger volume in shorter period of time. Selection of – inexpensive – metals and introducing them into the reservoir are critical problems to work on. Use of solvent to better distribute heat is another option but minimization of the solvent use and its negative effects (low











heat capacity of gas solvent) need further clarifications. The use of solvents and nano-metals in tight systems after fracturing them (shales) and naturally heterogonous systems (carbonates) require different research methodologies than that of the applications in high permeability and homogeneous sands (oil shales). All these require optimization studies and to achieve this robust numerical/analytical models are needed with correct description of the physics of the process and accurate data of the phase behavior of heavy crude under EM field.







Up to 300 °C 35% bitumen production (but 50% and 80% of the total depending on the temperature of heating and the recovery method) Radiofrequency (13.56 MHz, 40– 75 kW) Vernal, Utah Sresty et al. [91]

295–309 K for Lashburn 20 m3/day production rate (Northminster pilot) and 20 m3/day production rate (Lashburn) Davidson [90]

EM with 30 kW power

Bitumen (100 cP @ 100 °C)

Reservoir rock Sparky formation (reservoir rock) Tar sand deposit – Spencer [89]

Wildmere Field, Alberta Northminster area and Lashburn, Saskatchewan, Canada

3.18 tonnes/day

– 637 K for Avintaquin Canyon and more than 473 K for Asphalt Ridge Bridges et al. [88]

30–35%

Heavy oil (20 Pa.s @ 20 °C Heavy oil with 11.4°API





Reservoir rock Oil shale and tar sand 200 °C –

Radiofrequency (13.56 MHz, 25 kW) Radiofrequency (13.56 MHz, 5–20 kW for Avintaquin Canyon and 13.56 MHz, 200 kW for Asphalt Ridge) EM heating Kasevich et al. [73]



Permeability

0–0.183 micm2 (Yultimirovskoye)

Rock type

Porosity

Ishimbayskoye oil (20  106 m2/s at 20 °C)

Oil type Temperature

313 K after 5 day heating

Oil recovery



Method of heating

Radiofrequency (13.56 MHz, 60 kW)

Field name

Ishimbayskoye oil field in Bashkortostan and Yultimirovskoye bitumen field, Russia Bakersfield, California, United States Avintaquin Canyon and Asphalt Ridge, Utah, USA Sayakhov et al. [85–87]

223

14. Conclusions and future development in EM heating

Author (s)

Table 8 Tabulated from of literatures on field applications of electrical/EM Heating.

Reservoir rock

25% (Yultimirovskoye)

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A detailed critical review of EM heating was provided in this paper. Most of the simulation and laboratory studies showed that the process is economically feasible and is a promising alternative technology for the future of EOR by thermal method. A limited number of pilot scale field applications were also reported showing the efficiency of the method. Tables 6–8 provide a summarized survey of experimental, numerical, and field application of EM heating for heavy oil recovery, respectively. We hope this attempt helps researchers to gain quick and concise knowledge on EM heating method for heavy oil recovery. It is, however, obvious that this method still requires optimal engineering design for well stimulation to increase the efficiency of the process for the improvement of the production of heavy oil, oil sands, and oil shales. Microwaves are very much effective to produce heat by getting sufficiently absorbed by the materials. Since crude oil is not a good absorber of microwaves, microwave receptors like activated carbon, nano-metal oxides, and polar solvents should be used to make the microwave process faster. A limited number of laboratory studies do not properly reflect the field scale performance. Hence, further research is needed to implement the enhanced metal-nanoparticle incorporating electromagnetic heating (EMNIEH) at the field scale. In this case, the main question is how to inject nanoparticles into reservoir through the wellbore hole during EM heating. Greff and Babadagli [10] proposed that nanofluids can be injected at different stages of EM heating after viscosity reduction of the oil by applying EM heating for a certain time. Research on the recovery of heavy oil by EMNIEH is very much important in the present context and still requires further efforts. The behavior of EM heating is different in tight/shale environment containing light or heavy oil compared to sands. This type of reservoir requires fracking and heating fractures shales would be a complex problem. This is, however, a critical task as shaly structure do not permit the use of steam/water even if they are fractured and EM heating (with help of solvents possibly) is the only option for heavy-oil recovery from this type of geological structures. Acknowledgements This research was conducted under Tayfun Babadagli’s Natural Sciences and Engineering Research Council of Canada (NSERC) Industrial Research Chair in Unconventional Oil Recovery (the industrial partners are Canadian Natural Resources Limited (CNRL), Suncor Energy Inc., Touchstone Exp. Inc., Sherritt Oil, Apex Engineering Inc., Statoil, Husky Energy, and Pemex) and an NSERC Discovery Grant (RES0011227). References [1] Meyer RF, Attansi ED, Freeman PA. Heavy oil and natural bitumen resources in geological basins of the world. U.S. Geological Survey Open-File Report 2007-1084; 2007. . [2] Ritchey HW. Radiation heating. USA Patent Application, Serial No. 2,757,738, filed on 20 September, 1948; 1956.

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