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A radiofrequency/microwave heating method for thermal heavy oil

recovery based on a novel tight-shell conceptual design

Matteo Bientinesi a,n, Luigi Petarca b, Alessandro Cerutti c, Mauro Bandinelli c,Michela De Simoni d, Matteo Manotti d, Giuseppe Maddinelli d

a Consorzio Polo Tecnologico Magona, via Magona, Cecina (LI) 57023, Italyb Dipartimento di Ingegneria civile e industriale, Universit di Pisa, Largo Lucio Lazzarino, Pisa 56124, Italyc Ingegneria dei Sistemi, via Enrica Calabresi 24, Pisa 56121, Italyd Eni, Exploration and Production Division, via Emilia 1, San Donato Milanese (MI) 20097, Italy

a r t i c l e i n f o

Article history:

Received 10 October 2011

Received in revised form

8 February 2013

Accepted 19 February 2013Available online 30 April 2013

Keywords:

enhanced oil recovery

thermal heavy oil recovery

electromagnetic heating

radiating antenna

oil sand

modeling

a b s t r a c t

The ongoing depletion of light oil resources and the increasing global energy demand is driving oil&gas

companies towards the exploitation of unconventional oil resources. In order to extract crude oil from

these resources, a sufciently low oil viscosity must be achieved, for instance through temperature

increase. Electromagnetic irradiation through downhole antennae can be a suitable method for in situ

heating of reservoirs. Potential problems for this technique are the extremely high temperatures that can

be reached at the well containing the radiating element and the strong dependence of temperature

proles on local variation of reservoir material properties. These problems can be solved to a large extent

by inserting around the radiating well a tight shell made of a low loss dielectric material, and by selecting

the proper irradiation frequency.

The experimental work described in this paper aims to verify the effectiveness of a similar structure

during the electromagnetic heating of over 2000 kg of oil sand in a sandbox up to 200 1C, using a dipolar

antenna. Oil sand was irradiated at 2.45 GHz frequency with variable power (12 kW). The temperature

in the oil sand mass and on the boundary were recorded throughout the test in several speci c points, in

order to estimate temperature pro

les along the distance from the antenna.Experimental results conrmed that the presence of the low lossy material shell realized around the

antenna is extremely efcient in lowering the temperature in this critical zone and in better distributing

the irradiated energy in the oil sand mass.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The ongoing depletion of light oil resources and the increasing

global energy demand is driving oil&gas companies towards the

exploitation of unconventional oil resources. These include viscous

crude oil reserves (heavy and extra-heavy oils), oil sand and oil

shales. The International Energy Agency (IEA) estimates that there

are about 6 trillion barrels of such resources in place worldwide(Clark, 2007).

Due to high viscosity, heavy oils generally require enhanced oil

recovery (EOR) techniques to be produced economically. Thermal

recovery methods, which introduce heat into the reservoir to

reduce oil viscosity, are effective techniques to enhance heavy oil

productivity. Steam injection processes namely cyclic steam

stimulation (CSS), steam ooding (SF) and steam assisted gravity

drainage (SAGD) are the main thermal methods currently

employed commercially. However, steam assisted oil recovery

requires large amounts of fresh water, cannot be used for deep

or very shallow reservoirs, and its effectiveness depends on

reservoir geological properties. The economical protability or

even the technological feasibility of these methods can be greatly

reduced in thin payzones, in low permeability formations, or in

presence of a weak sealing cap rock (Clark, 2007).Electromagnetic (EM) irradiation, at radiofrequency (RF) or micro-

waves (MW) frequencies, can be a sound alternative for in situ

heating of unconventional reserves. A RF/MW heating method, based

on a downhole radiating antenna, is less affected by formation

geology and is capable to distribute heat over a large reservoir

volume thanks to the propagation of electromagnetic energy through

the medium. Other advantages are equipment compactness (suitable

for off-shore elds), high efciency in the energy generation-

radiation process and the possibility to focus the energy on oil

bearing strata, reducing heat losses through the overburden.

Many industrial patents have been registered about this topic by

several companies in the last 60 years. Some examples are: Ritchey

Contents lists available atSciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/petrol

Journal of Petroleum Science and Engineering

0920-4105/$ - see front matter & 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.petrol.2013.02.014

n Corresponding author. Tel.:+39 586 632142; fax: +39 586 635445.

E-mail addresses: [email protected] (M. Bientinesi),

[email protected] (L. Petarca),[email protected] (A. Cerutti),

[email protected] (M. De Simoni).

Journal of Petroleum Science and Engineering 107 (2013) 1830
http://www.elsevier.com/locate/petrolhttp://www.elsevier.com/locate/petrolhttp://dx.doi.org/10.1016/j.petrol.2013.02.014mailto:[email protected]:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.petrol.2013.02.014http://dx.doi.org/10.1016/j.petrol.2013.02.014http://dx.doi.org/10.1016/j.petrol.2013.02.014http://dx.doi.org/10.1016/j.petrol.2013.02.014mailto:[email protected]:[email protected]:[email protected]:[email protected]://crossmark.dyndns.org/dialog/?doi=10.1016/j.petrol.2013.02.014&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.petrol.2013.02.014&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.petrol.2013.02.014&domain=pdfhttp://dx.doi.org/10.1016/j.petrol.2013.02.014http://dx.doi.org/10.1016/j.petrol.2013.02.014http://dx.doi.org/10.1016/j.petrol.2013.02.014http://www.elsevier.com/locate/petrolhttp://www.elsevier.com/locate/petrol


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(1956), Haagensen (1965), Kasevich et al. (1979), Bridges et al.

(1979), Kiamanesh (1992), Kasevich (2008). Such patents describe

set-ups based on the introduction of an electromagnetic power

emitting apparatus in a well and differ from each other for

characteristics such as operating frequency, apparatus design and

well conguration.

In spite of the high number of patents, really few data were

published demonstrating the applicability of this technique and

assessing its efciency. In particular, three main lab-scale tests werepresented:Sresty et al. (1986)veried the heating up to temperature

between 150 1C and 350 1C of a 300 kg Utah oil sand sample,

irradiated through a triplate line system; Kasevich et al. (1994)

irradiated a small sample of heavy oil through an antenna, while Hu

et al. (1999)used two electrodes placed into a poly(methyl methacry-

late) (PMMA) box to heat with radiofrequency (RF) a reconstructed oil

sand sample. However, none of these authors show a temperature

prole arising from electromagnetic irradiation, and little attention has

been paid to the temperature reached in the vicinity of the wellbore

during the irradiation process. This is one of the key issues of a RF/MW

reservoir heating technique. High reservoir heating rates require high

EM power irradiation by the downhole antenna. This can turn out in

an extremely high EM eld density located in the volume surrounding

the radiating element. Reliable RF/MW heating processes must there-

fore take into account the EM energy distribution through the

reservoir and must be designed to achieve a volume heating as

uniform as possible. This is a key factor to prevent the exposure of

well completion components to extreme temperatures, while irradiat-

ing high EM power rates into the reservoir.

In order to meet the requirements addressed above, in the

present work a new RF/MW method, which combines a downhole

antenna with an interface structure realized between radiating

well and reservoir (calledtight shell), is proposed. A theoretical and

experimental study has been conducted to evaluate the potenti-

ality of the proposed solution and to design a RF/MW method

capable to operate a high power, long term irradiation process,

required to heat a considerable volume of reservoir.

In this paper we describe the novel tight shell conceptual

design, assessed by means of a simple numerical model and an

experimental test. The objectives are to assess the effectiveness of

the novel tight shell conceptual design and to study the different

phenomena taking place in a reservoir during electromagnetic

irradiation and their effects on the application.

2. Design of the system and preliminary modeling

The new system design, described in what follows, was rst

analyzed with preliminary numerical simulations.

In order to achieve short calculation times and a device as

exible as possible, a simple numerical model was developed (as

described in Appendix A) assuming static conditions (no ow is

described) and spherical symmetry. This allows us to perform awide spectrum of simulations in order to analyze:

(1) the optimal operating frequencies to achieve homogeneous

reservoir heating;

(2) the impact of reservoir dielectric and thermal properties and

of operating conditions;

(3) the effectiveness of the tight shell solution and its design

parameters.

2.1. Novel tight shell system design

In order to optimize the electromagnetic heating process, a

novel system was designed, with two main objectives: heating the

reservoir up to temperatures high enough to mobilize hydrocar-

bons at relatively long distances from the well and, in order to

simplify the thermal resistance requirements for well completion

materials, limiting near wellbore temperature. In another way, the

system aims to obtain temperature proles versus the distance

from the radiating element as uniform as possible.

Fig.1 shows one possible embodiment of the conceptual design

of the new RF/MW method, in which a radiating antenna is

coupled with a well-reservoir interface structure (tight shell)whose scope is to limit temperature increase at the well. The

system is composed by:

a production well (whose completion scheme is speciallydesigned in order to host the RF/MW components and to allow

EM irradiation); a high power RF/MW energy applicator (composed by a surface

unit with a high power RF/MW energy source, a downhole

transmission line and a bottom hole antenna); a tight shell (a spherical or more likely cylindrical structure

interposed between oil well and reservoir, realized at the

antenna installation depth through drilling and completion

operations; the tight shell is made of a low loss dielectric

material and is impermeable to reservoir uids).

The same conceptual design can be applied also in congura-

tions where the RF/MW irradiation well is separated from the

producer.

2.2. Preliminary numerical simulations

In order to be feasible, the electromagnetic irradiation techni-

que must allow to heat large volumes of reservoir up to a

temperature at which oil viscosity is low enough to be produced.

Different heavy oils require different temperatures, varying from

70 1C to over 200 1C; in the preliminary study, we considered the

limit of 150 1C, over which the oil from the analyzed oil sand

samples has a viscosity lower than 100 mPa s. At the same time,

maximum temperature at the radiating well must be kept at a

level tolerated by well completion materials; we considered 350 1C

as the maximum temperature at the well for the present study.

Numerical simulations, performed varying a number of settings

such as reservoir material properties, irradiation frequency and

power, tight shell diameter, show that the operating system para-

meters have important effect on the thermal process, and the desired

uniform heating can be achieved only with a proper system design.

In particular, the following general observation can be stated.

Even changing reservoir material properties and system con-guration, best irradiation frequencies turn out to be those in

the 1020 MHz range. Higher frequencies are as well capable to

heat deep into the reservoir, but temperature at wellbore

results too high. This is shown clearly by the sensitivity analysisin Fig. 2, which describe the temperature reached at two

different distances from the antenna (0.15 m and 10 m) after

1000 and 2000 days of irradiation at 200 kW for frequencies in

the range 103000 MHz. Two temperatures are outlined in

Fig. 2: 150 1C is indicatively the minimal temperature to be

reached at 10 m of distance from the antenna in order to

mobilize the oil, 350 1C is a limit for the temperature near the

wellbore in order to avoid structural problems. These limits can

change for different scenarios, in particular, several kinds of

heavy oil are mobile at much lower temperatures. Anyway, the

fact that lower frequencies give a better energy distribution in

the reservoir can be generalized. The transient process of connate water evaporation in a volume

surrounding the radiating antenna can remarkably reduce

M. Bientinesi et al. / Journal of Petroleum Science and Engineering 107 (2013) 1830 19


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energy dissipation close to the well. This is due to the change in

dielectric properties of reservoir material, once the water has

been removed, as shown as a way of example in Fig. 3for an oil

sand sample with an original water and oil content respectively

of 4.2% and 10% by weight. Fig. 3 shows how, after a slight

initial increase both of the real and imaginary part of the

dielectric constant with increasing temperature, led by the

increase of salt water electrical conductivity (Hayashi, 2003),

there is a fall of both quantities once the boiling point of

connate water (about 100 1C at atmospheric pressure for the

analyzed sample) is reached and water is evacuated. This

situation is representative of reservoirs in which both the oil

and the inorganic matrix are non-dispersive. Anyway, several

oil sand, oil shale or heavy oil dry samples show higher values

1

10

100

1000

10000

10 100 1000

Temperature[C]

Frequency [MHz]

r = 0.15 m; t = 200 day r = 10 m; t = 200 day

r = 0.15 m; t = 600 day r = 10 m; t = 600 day

r = 0.15 m; t = 1000 day r = 10 m; t = 1000 day

350C

150C

100

150

200

250

300

350

400

10 15 20 25 30 35 40 45 50

Fig. 2. Frequencytemperature analysis for 200 kW irradiated power, 3 m radius tight-shell (tg() 103). The diagram shows the temperatures reached at 10 m distance

from the radiating well (solid lines) and near the wellbore (circle spotted lines) for different irradiation times, as a function of the transmitted frequency (in the range

103000 MHz). In the window, the range 1050 MHz is zoomed.

Fig. 1. Conceptual design of a production/radiating well with a spherical tight shell.

M. Bientinesi et al. / Journal of Petroleum Science and Engineering 107 (2013) 183020


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of imaginary dielectric constant, depending on oil composition(presence of polar compounds) or crystallographic composition

of the matrix (Friso et al., 1998; Kovalyova and Khaydar, 2004;

Fuji et al., 1999; Saraev et al., 2005, Epov et al., 2009). Thus, we

analyzed two different scenarios (see Fig. 4): for a non-lossy

reservoir (0.01), the temperature near the wellbore is

slightly above the boiling temperature of water in the reservoir

(about 160 1C for the analyzed scenario), since a volume of

reservoir where water has been evaporated and electromag-

netic energy is poorly dissipated soon forms and increase its

radius during the irradiation operation; for lossy reservoir (

0.1), water evaporation is not sufcient to keep down the

temperature, since even in the dry volume dissipation takes

place, even if at a lower extent. We can then conclude that

water evaporation contribute to limit the temperature rise

nearby the antenna, but the extent of this reduction dependson the reservoir pressure, which in turn determines the boiling

temperature of connate water, and on the dielectric loss of the

water dried reservoir material, which can vary signicantly for

different reservoirs. The heating process, realized without a tight shell interface, is

then very sensitive to possible local variation of the reservoir

dielectric properties. In favorable conditions (low lossy solid

matrix and heavy oil, and shallow reservoirs with depth lower

than 1000 m) water vaporization can be an effective natural

method to produce a uniform heating. Nevertheless the robust-

ness of a RF/MW method that relies only on connate water

vaporization appears to be dependent on dielectric properties

of materials, which, in real eld operation, can be unknown or

affected by reservoir heterogeneity.

0

2

4

6

8

10

12

14

16

18

20

10 100 1000

'

Frequency [MHz]

T = 20C

T = 50C

T = 100C

T = 125C

0

10

20

30

40

50

60

10 100 1000

''

Frequency [MHz]

Fig. 3. Measurements of the real (left) and the imaginary (right) parts of dielectric permittivity of an oil sand sample with composition: 10% oil, 4.2% water, 85.8% quartz sand

by weight. The measurement is taken in the 101000 MHz range at 20 1C, 50 1C, 100 1C and 125 1 C, according to the methodology described inSarri et al. (2012).

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20 25 30

Tem

perature[C]

Distance from antenna [m]

'' (dry) = 0.1, 2000 d

'' (dry) = 0.1, 1000 d

'' (dry) = 0.01, 2000 d

'' (dry) = 0.01, 1000 d

Tb = 160C

0

100

200

300

400

500

600

700

800

0 10 20 30 40 50

Fig. 4. Simulation results: comparison between thermal proles obtained varying the dielectric properties of reservoir material after water evaporation, after 1000 and 20 00

days of irradiation, without the use of the tight shell. Irradiation frequency is 10 MHz and emitted power is 200 kW. The water dried reservoir material has imaginary

dielectric constant 0.1 or 0.01. In the window, the full range 050 m is shown.



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Simulation then focused on evaluating the effectiveness of the

new tight shell conceptual design in achieving a more uniform

volume heating, limiting extreme temperatures nearby the

antenna, and to make the RF/MW method less dependent on

reservoir material properties.

InFig. 5, we compare temperature proles obtained with and

without the use of the tight shell (with 3 m radius) in the case of a

lossy reservoir (0.1 for the dried material). The irradiation

frequency is 10 MHz and the emitted power 200 kW. It is evidentthat a few meters radius shell would be sufcient to keep down

well temperatures, also in presence of a lossy reservoir medium

(0.1), in which case the temperature reduction caused by the

shell is dramatic.

Several simulations were performed, varying the loss tangent

(tg()) of the tight shell material between 104 and 103; in any

case, the tight shell turns out to be extremely successful in

lowering well temperature. Therefore, an effective tight shell

completion does not required a very transparent material (for

instance, quartz sand and several proppants used in oil well

applications, measured bySarri et al. (2012), have tg() 103).

2.3. Design of the experimental test

According to preliminary numerical results, the tight shell

turned out to be extremely effective in lowering the temperature

at the radiating well and in allowing to heat larger volumes of

reservoir. This conclusion led us to project and perform an

experimental test, with the major aim of evaluating the effective

decrease of the temperature near the wellbore arising from the

insertion of a low lossy material (tight shell) and the effect of

water evaporation on the development of temperature proles.

Even though the optimal irradiation frequency was located

between 10 MHz and 20 MHz, an experimentation at these fre-

quencies would be hard to be performed with an antenna-like

emitting apparatus, if not directly on eld, in a pilot well. As a

consequence, we chose to operate in the microwave range, at

2.45 GHz. The smaller wave length and penetration depth allowed

us to scale down the volume of interest, while still observing the

dielectric heating process and the temperature proles developed

in the oil sand mass. The numerical model was adapted to the

radiation pattern of the experimental antenna and to the geometry

of the experimental set-up, as described in Appendix A, and was

used to interpolate the data.

3. Experimental methodology and material

The lab test was performed irradiating a sand box, properly

lled with oil sand, by an antenna and recording temperature data

along predened sections. In what follows, the experimental set-up is described.

3.1. Lab-scale experimental set-up

The experiment was performed with a 2.45 GHz radiation. This

experimental choice, having a shorter wavelength and a smaller

penetration depth with respect to lower frequencies, allowed to

reduce the oil sand amount needed, however providing reliable

experimental results.

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20 25 30

Temperatu

re(C)

Distance from antenna (r)

Tb= 160C

0

200

400

600

800

1000

0 10 20 30 40 50

No tight shell, 1000 d

No tight shell, 2000 d

3 m tight shell, 1000 d

3 m tight shell, 2000 d

r = 3 m

Fig. 5. Simulation results: comparison between thermal proles obtained without the tight shell or using a 3 m radius spherical shell (constituted by a material with tg( )

103) after 1000 and 2000 days of irradiation. Irradiation frequency is 10 MHz and emitted power is 200 kW. The water dried reservoir material has imaginary dielectric

constant0.1. In the window, the full range 050 m is shown.

Fig. 6. Steel containment tank: the inner side of the walls are covered with radar-

absorbent material sheets.



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The mass of oil sand used in the experiment has a base

dimension 1.25 1.25 m and a height of 0.75 m. The choice of

the dimensions depended on preliminary simulations results and

on the antenna design; the latter has in fact a dipole-like, non-

spherical radiation pattern which extends more in the horizontal

direction than in the vertical one. To properly ll the space, the

samples were shaped in cubic blocks pressed into a mold of proper

shape. In order to press the blocks, oil sand material was heated up

to 70/80 1C, turning it malleable; each produced block weighedbetween 28.5 kg and 32 kg, resulting in a void grade of 210%, with

an average value of 3%; 73 blocks were used, for a total amount of

over 2200 kg of oil sand material. The oil sand assembly was

conned by a steel containment tank (Fig. 6).

On the base of the tank, a 100 mm thick layer of quartz sand

was poured. On this layer, oil sand blocks were then assembled,

leaving between each block a 10 mm thick gap, lled again with

quartz sand. A vacant position was left in the center for the

insertion of the antenna (Fig. 7) and a cylindrical hole was realized,

in which the antenna was placed (Fig. 8). In the cylindrical hole,

two concentric polytetrauoroethylene (PTFE) tubes, closed at the

bottom, were installed. Outside the outer tube, a low lossy quartz

sand was poured, realizing a cylindrical shell. The gap between

outer and inner tube was lled with the silicone oil Rhodorsil Oil

47 V20 (Bluestar silicones). The inner tube contained a rigid

coaxial line, terminating with the antenna (Fig. 9).

The internal face and the base of the containment tank were

covered with sheets of radar-absorbent material (RAM, Eccosorb

SF2.5), glued to the metal with a room temperature vulcanizing

(RTV) silicone adhesive. The RAM absorbs all the electromagnetic

radiation reaching the tank walls, converting it into thermal

energy and avoiding reection of microwaves by the steel walls.

Fig. 10 shows the schematic of the experimental set-up. The

microwaves are generated by a water cooled magnetron with a

maximum output of 2 kW, fed by a switching power generator,

commanded via PC. Downstream the magnetron, an isolator

avoids the reected microwaves to come back to the magnetron;

the reected power is measured through a linear power sensor.

Microwaves travel through a waveguide from the control room

to the experiment room. Through an adapter, microwaves are

transferred into a rigid coaxial line, terminating in the antenna

situated in the exact center of the oil sand mass (Fig. 11).

The experiment room is thoroughly ventilated in order to

remove any gas possibly exhaled by the heated oil sand. However

operators were not allowed into the room throughout the

experiment.

Several ber optic temperature sensors were inserted in

selected oil sand blocks, in the quartz sand shell and in the silicone

oil between the two PTFE tubes. In addition, several K-type

thermocouples were used to monitor the temperature of the outer

surface of the steel walls.

The position of the various sensors is illustrated in Fig. 12,

where a section of the experimental set-up, at the height of the

center of the antenna, is depicted. Fiber optic sensors (FO##,

where ## is a number) and thermocouples (TC##) recorded the

temperature continuously. TCx (x being a letter) indicates the

thermocouples located in the gap between different oil sand

blocks, which periodically measured the temperature at different

depths, after temporarily stopping the irradiation.

The position of the ber optic sensors was dened in order to

record the temperature proles versus the distance from the

antenna, at the height of the antenna itself, where the radiating

power is at its maximum. The ber optic sensors provided the

temperature prole in the main direction A and allowed to

conrm the individuated trend on two other directions (B and C,

as dened inFig. 12).

In order to obtain a good contact between the probes and the

oil sand material, once a sensor had been inserted in the pre-

drilled block, oil (originating from the same oil sand samples) was

poured (at a temperature of 80 1C) in the hole in order to assure

the immobilization of the probe and the thermal equilibrium.

The thermocouples attached to the outer side of the tank walls

were used to qualitatively evaluate the thermal energy dissipated

by natural convection towards the surrounding environment.

Table 1 denes the position of each temperature probe,

assuming (Fig. 12) an xyz coordinate system with the origin at

the center of the antenna.

3.2. Antenna design and testing

The project of the antenna was developed through four stages:

requirements analysis, antenna type selection, antenna simulation

and individuation of mechanical devices for impedance adjust-

ment and return loss minimization.

Requirements analysis lead to the individuation of the follow-

ing specications:

geometrical specications (maximum diameter 60 mm); operating frequency 2.45 GHz; return loss lower than 10 dB;

radiation diagram optimized for power radial distribution.

Fig. 7. Assembling of the oil sand cubic blocks. Noteworthy, the bottom layer is

complete while the central layer, as well as the not shown top layer, has a vacant

position in the center for the introduction of the antenna.

Fig. 8. Realization of the cylindrical hole for the introduction of the antenna and

opticalber positioning.



7/13

The antenna was designed through simulations with a modeling

tool developed by IDS (Ingegneria dei Sistemi), considering the

environment in which the antenna is inserted for the experiment

(PTFE tubes, silicone oil, low lossy sand shell, oil sand). The

projected antenna (Fig. 13) is constituted by:

a rigid coaxial line with the inner conductor opportunely longerthan the external conductor;

several circular elements terminating the inner conductor,which are useful for impedance adjustment;

a mobile choke, which is used to decrease return currents onthe outer conductor and to adjust antenna impedance.

Once the antenna was realized, the return loss was measured

using a Network Analyzer (Agilents PNA-X-N5242A) in severalcongurations, and specically in air, sand, wet sand, as well as in

the experimental set-up before and after lling the tank with oil

sand. The return loss at 2.45 GHz, in the denitive set-up, turned

out to be29 dB. The antenna is moreover extremely adaptable to

different parameters of the surrounding environment.

3.3. Reservoir and tight shell materials

Two different bulk materials were used to ll the sand box in

the experimentation, namely:

oil sand samples, furnished by Eni; low lossy quartz sand.

Fig. 9. Insertion of the PTFE tubes and of the radiating element in the low lossy

sand shell.

Fig. 10. Schematic of the experimental set-up and identication of the main equipment.

Fig. 11. Experimental set-up.



8/13

These materials were characterized through a series of techni-

ques described in Sarri et al. (2012). The results of interest are

reported inTable 2for oil sand and in Table 3for quartz sand.

About the dielectric properties of oil sand samples, they remain

practically stable with increasing temperature as long as connate

water evaporation is limited while, after water evaporation,

imaginary permittivity decrease signicantly (Table 2).

The type of quartz sand used was selected for its particularlylow imaginary permittivity, which makes it a really low lossy

material, thus particularly suitable for the tight shell simulation

(though the shell is not actually tight in this specic case).

3.4. Electromagnetic heating experiment

A long term experiment was conducted in order to:

(1) evaluate the time evolution of temperature proles at different

radiating power levels;

(2) compare experimental data with the numerical model devel-

oped and described in Appendix A;

(3) evaluate the degree of mobilization of the bitumen in the

temperature range 150200 1C.

Temperature data from ber optic sensors and thermocouples

were recorded with a frequency of 60 records per hour. Periodi-

cally, power was briey switched off in order to measure the

temperature at different depths.

In Table 4, the journal of the experiment is reported. Note-

worthy, radiating power was rst set at 1000 W and then

increased to 1500 W and 2000 W once a quasi-stationary tem-

perature prole was achieved. Finally, in order to further increase

the temperature in the oil sand, the outer side of the metallic walls

and the top surface were covered with 5 cm thick rockwool

insulating panels. The experiment lasted over 13 days.

4. Results and discussion

4.1. Experimental results

Fig. 14shows temperature data from all the sensors placed inthe A direction (see Fig. 12), for the entire duration of the

experiment. In particular, seven ber optic sensors (FO6, placed

in the silicone oil between the two concentric PTFE tubes, FO7 and

FO10, inserted in the quartz sand shell, and FO1, FO2, FO3, FO5, in

the oil sand) and one thermocouple (TC7) are considered. These

data have to be analyzed taking into account the actions occurred

during the experiment, as reported in Table 4.

Figs. 15 and 16report the time evolution of the temperature

prole along the x coordinate (seeFig. 12), during the rst part of

the experiment and after the thermal insulation of the set-up

respectively.

Fig. 17 shows the time evolution of the temperature prole

along thezcoordinate in the point TCa (see Fig. 8), at a distance of

390 mm from the antenna.

Fig. 12. Position of ber optic sensors and of thermocouples for temperature

measurements.

Table 1

Position of temperature sensors during oil sand electromagnetic heating experi-

mental testing. The coordinate system used has origin in the center of the antenna,

and is depicted in Fig. 8.

FO## x

[mm]

y

[mm]

z

[mm]

TC## x

[mm]

y

[mm]

z

[mm]

TCx x

[mm]

y

[mm]

FO1 200 0 0 TC2 0 650 0 TCa 390 0

FO2 320 0 0 TC3 0 650 0 TCb 390 0

FO3 460 0 0 TC4 650 0 0 TCe 0 390

FO4 0

260 0 TC5 650

319 0 TCf 0

390FO5 580 0 0 TC7 650 0 0 TCt 85 0

FO6 40 0 0 TC9 650 319 0

FO7 60 0 0 TC14 650 0 250

FO8 520 0 0 TC16 650 0 250

FO9 260 0 0

FO10 120 0 0

Fig. 13. MW radiating antenna.



9/13

Some main observations can be made based on these results:

oil sand temperature increases monotonically during electro-magnetic irradiation due to the dielectric heating phenomenon

(Figs. 1417); notwithstanding the higher electromagnetic eld present as a

consequence of the smaller distance from the antenna, the

temperature of the quartz sand shell (FO7 and FO10 sensors in

Fig. 14and xo200 mm inFigs. 15and 16) is always lower thanthe maximum temperature achieved in the oil sand material

(FO1 sensor in Fig. 14), thus conrming the efciency of the

shell design in decreasing the temperature around the well and

the antenna; once the temperature of the oil sand material overcomes

100 1C, a sensible decrease in the heating rate can be observed

(Fig. 15); this arises from the evaporation of the small amount

of connate water present and the subsequent decrease of oil

sand loss tangent. The water evaporation clearly contributes to

Table 2

Average properties of oil sand material used in the experimentation.

Property Value

Sand void grade 40%

Water content 0.25 wt%

Oil content 13 wt%Dielectric constant 3.8

(@ 2.45 GHz, 25 1C) 0.1

Dielectric constant 2.5

(@ 2.45 GHz, 4100 1C) 0.01

Density 2100 kg/m3

Specic heat (@ 25 1C) 930 J/kg K

Thermal conductivity (@ 25 1C) 0.93 W/mK

Table 3

Properties of quartz sand used in the experimentation.

Property Value

Dielectric constant

2.9(@ 2.45 GHz, 25 1C) 0.001

Bulk density 1620 kg/m3

Void grade 39.1%

Specic heat (@ 25 1C) 800 J/kg K

Thermal conductivity (@ 25 1C) 0.24 W/mK

Table 4

Journal of the experiment.

Time Action

0 Magnetron switched on, power 1000 W. Start recording temperature data

17 h 15 min Magnetron switched off, measure with thermocouples (MTC1)

17 h 22 min Magnetron switched on, power 1000 W


28 h 56 min Magnetron switched on, power 1500 W36 h 16 min Magnetron switched off, measure with thermocouples (MTC3)




68 h 59 min Magnetron switched off






183 h 21 min Magnetron switched off, steel wall and top surface covered with berglass insulation






326 h 06 min Mag netr on swit ch ed off, measur e with th ermoc ouples (MTC6), in sul ati on r emoved

0

20

40

60

80

100

120

140

160

180

200

220

0 24 48 72 96 120 144 168 192 216 240 264 288 312 336

Temperature[C]

Time [h]

FO6, 40 mm

FO7, 60 mm

FO10, 120 mm

FO1, 200 mm

FO2, 320 mm

FO3, 460 mm

FO5, 580 mm

TC7, 650 mm

Wall

insulation

Fig.14. Temperature measured by the sensors placed along direction A (seeFig.12)

for the entire duration of the experiment. In the legend, the distance of each probe

from the antenna is reported.

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Temperature[C]

x [mm]

0 h

5 h

10 h

15 h

20 h

25 h

30 h

35 h

40 h

Fig. 15. Temperature proles in the direction A (see Fig. 12) versus the distance

from the antenna, before set-up thermal insulation.



10/13

limit temperature rising near the well and favors the penetra-

tion of electromagnetic energy; the silicone oil temperature (FO6 inFig.14) increases initially up

to 70 1C; once this temperature is reached, convective motions

establish, favoring the dissipation of heat towards the environ-

ment through the upper surface of the uid; this heat dissipated

is the cause of the qualitative difference between the curves for

the tight shell scenario in Fig. 5, where the temperature

maximum is always at the antenna, and the curves inFig. 15; the temperature of the external metallic walls (curve TC7 inFig. 14) is determined by two concurrent phenomena: on one

side, the RAM sheets absorb practically all the residual electro-

magnetic energy reaching the borders, on the other side, heat is

removed from the walls through natural convection of the

surrounding air. This leads to avoid heat accumulation into the

oil sand thus providing boundary conditions that can be

roughly representative of a larger oil sand mass (obviously this

holds as long as the set-up is not thermally insulated); the measured vertical temperature proles show a maximum

for z0 (Fig. 17), i.e. on the horizontal plane intersecting the

center of the antenna, decreasing quasi-symmetrically upwards

(zo0) and downwards (z40), in agreement with the predictedpattern of the antenna;

the nal phase of the experiment, after the insulation (seeFig. 16), is used exclusively in order to increase the temperature

over to 100 1C in the whole mass of oil sand, allowing to evaluate

the possible mobilization of the oil at higher temperatures. Once

the set-up was dismounted, oil sand blocks were visually

analyzed and several points, where the oil partially had drained,

were detected (Fig. 18). It is clear that for this specic oil sand

sample gravity drainage is not sufcient, even at high tempera-

ture, to obtain a signicant oil mobilization.

4.2. Comparison with the numerical model

The numerical model was adapted, as described in Appendix A,to the geometry and radiation pattern of the experimental set-up,

in order to compare simulation results with experimental results.

In the model, material properties are time-independent, thus

the model is adequate to experimental data description only for

temperature below the boiling point of water (100 1C). Above this

temperature, water vaporizes and escapes from the oil sand mass,

dielectric permittivity decreases and electromagnetic energy dis-

sipation decreases as well.

InFigs. 19 and 20, experimental temperature proles in the r

andzdirections are compared with simulation results. Noteworthy

the agreement is excellent as long as the temperature of 100 1C is

not overcome; once the water contained in the oil sand is

evaporated, the model is no longer valid and thus the prediction

of temperature proles becomes largely inaccurate.

5. Conclusion

This paper presents a novel design for the downhole electro-

magnetic heating of oil sand/heavy oil reservoir, including a tight

shell made of low lossy material inserted in the surrounding of the

well containing the radiating antenna. Preliminary numerical

simulation performed with a simplied model showed that the

presence of a tight shell could help to reach uniform heating of

large volumes of reservoir avoiding the risk of high temperatures

at the wellbore, and making the recovery method much less

sensitive to local dielectric properties of the reservoir materials.

An experimental test was set up to conrm these claims.

Experimental results showed that electromagnetic irradiation is

0

20

40

60

80

100

120

140

160

180

200

220

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Tem

perature[C]

x [mm]

40 h

180 h

200 h

230 h

260 h

320 h

Fig. 16. Temperature proles in the direction A (see Fig. 12) versus the distance

from the antenna, after set-up thermal insulation.

0

20

40

60

80

100

120

140

160

180

200

-350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350

Temperature[C]

z [mm]

17 h

29 h

36 h

62 h166 h326 h

Fig. 17. Temperature proles along the vertical coordinate in the point TCa (at a

distance of 390 mm from the antenna along the direction A, see Fig. 12).

Fig. 18. Oil draining from oil sand at the end of the test.



11/13

capable of heating oil sand, even above the boiling temperature of

connate water. The low lossy material shell realized around the

antenna turned out to be efcient in lowering the temperature in

this critical zone, as demonstrated by the fact that the maximum

temperature is reached outside the shell.

In a real oil well, from a technical point of view a 3 m radius

tight shell is hardly feasible with current technologies but future

studies will be performed in order to asses if less stringent

temperature limits and the effect of convective cooling due to

uids ow could lead to signicant shell radius reduction.

It was moreover shown that water vaporization has a signi-

cant impact on temperature proles and contributes to limit the

temperature rise near the wellbore region and to better distributethe irradiated energy in the reservoir.

Current ongoing activities are aiming to develop a more

realistic reservoir model that includes a 3D geometry, the con-

vective term in the thermal equation, as well as oil and water ow,

in order to assess the effective benets of the designed system in

terms of oil recovery and productivity index improvement.

Acknowledgements

Eni, e&p division, is gratefully acknowledged for funding this

research activity and providing reservoir materials for the experi-

mental activities.

Appendix A

The numerical model describes the coupled electromagnetic

and thermal problem related to RF/MW irradiation of a heavy oil

reservoir through an antenna installed inside a wellbore, at an oil

bearing level. Fluid ow due to production is not considered.

The problem is governed by the equation of thermal energy

conservation

effCeffTt KeffT q A1

where effCeffand Keff are respectively the effective heat capacity

and the effective thermal conductivity of the reservoir material,

and q is the heat source term which accounts for the energy

released per unit time and unit volume by the electromagnetic

eld into the reservoir.

In order to calculate the q term, the following equation, which

governs the attenuation of the irradiated energy ux (Von Hippel,

1966), is rst solved

dPr

dr 2Pr A2

where P is the electromagnetic wave power per unit solid angleirradiated by the antenna, r is the coordinate along the wave

propagation direction, and is the electromagnetic wave attenua-

tion coefcient.

The attenuation coefcient is a function of the wave

frequency and of the complex dielectric permittivity of the

material

f; mixn

2f

c

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffimix

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

mix

mix

2s 1

0@

1A

vuuut A3where f is the wave frequency, c is the speed of light in vacuum,

mix and mix are the real and imaginary part of the complex

dielectric permittivity of the reservoir medium, which is a mixture

of different components (namely solid matrix, oil and water).

Hence,is not actually constant but varies with time and with the

spatial coordinate, as a consequence of material compositional

changes (water vaporization).

Once provided the solution of the EM power eldP(r), the heat

source term can be calculated applying the following equations

q !

F!

A4

Fr Pr

r2 A5

where F(r) is the electromagnetic energy ux (i.e. the energy

carried by the electromagnetic wave crossing the unit area per

unit time).

In order to study the effect on the reservoir heating process, themodel was congured to take into account water vaporization.

A strong assumption we make is that the produced steam can

escape the reservoir and does not cause a pressure increase. This

assumption is hardly realistic, but it signicantly simplify the

model. The reservoir is ideally divided in three zones, with

different dielectric and thermal properties:

water saturation zone (with ToTboil): dielectric and thermalproperties of the reservoir are those of the rock-heavy oil

water system (dielectric properties are assumed equal to the

values measured at 20 1C and shown inFig. 3); boiling layer zone (T Tboil): transition phase for a little interval

aroundTboil; the reservoir thermal capacity is calculated so as

to include the water evaporation latent heat;

0

20

40

60

80

100

120

140

160

180

-350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350

Temperature[C]

z [mm]

17 h 15 min

28 h 46 min

36 h 13 min62 h 00 min

Fig. 20. Comparison between experimental (symbols) temperature prole alongz

in the point TCa (r390 mm, seeFig. 12) and simulation results (continuum lines).

0

20

40

60

80

100

120

140

160

180

0 100 200 300 400 500 600

Tem

perature[C]

x [mm]

1 h

3 h

5 h

10 h

15 h

20 h

25 h

28 h

Fig. 19. Comparison between experimental (symbols) temperature prole along x

and simulation results (continuum lines).



12/13

dried zone (for T4Tboil): dielectric and thermal properties ofthe reservoir are those of the rock-heavy oil system, from which

water has been removed (dielectric properties are assumed

equal to the values measured over 100 1C and shown inFig. 3).

These three zones evolve with time, determining the material

properties to be used in Eq. (A1)and Eq.(A2)at a certain time for a

given value of the spatial coordinate.

The described physical model is implemented using Comsol

Multiphysics, in a 1D spherical geometry. Initial and boundary

conditions are:

thermal equation: uniform temperature (25 1C) for t0 through the entire

domain; adiabatic boundary condition at the well, imposed by the

symmetry of the problem; constant temperature at the outer boundary, equal to the initial

temperature (this condition is justied by the fact that the

considered domain is much larger compared with the heating

radius; electromagnetic wave attenuation equation (since this is a rst

order, stationary equation, it requires a single boundary

condition): boundary condition at the well, where P0 is the total power

irradiated by the antenna.

The setting of reservoir material parameters in the model is

based on dielectric and physical characterization, carried out on oilsand samples. Detailed description of the experimental laboratory

set-up and of the methods for the RF/MW dielectric characteriza-

tion and for the measure of thermal properties of reservoir

materials is provided by Sarri et al. (2012).

Other relevant settings used in the preliminary numerical

simulations are:

reservoir average depth 80 m; reservoir pressure 6 bar; boiling temperature of connate water 160 1C; sand porosity 25%; initial water saturation 23%; initial oil saturation 77%;

latent heat of vaporization of water 2080 kJ/kg.

In order to compare simulation results with experimental

results, the model was adapted to the geometry and to the

radiation pattern of the experimental set-up. Two major modica-

tions were made:

a 2D axisymmetric geometry was used in substitution of thespherical geometry;

material properties dependence on temperature was neglectedfor simplicity.

The choice of a 2D geometry is determined by the fact that the

radiation pattern of the experimental antenna is not isotropic but

dipole-like, so focused in the horizontal plane and symmetric with

respect to the vertical axis, with a 3 dB gain in the direction of

maximum irradiation. In order to simulate the distribution of

energy in the material, a 2D axisymmetric geometry was adopted

and an analytical expression was used for the approximate

calculation of the radiation diagram:

fpattern 2 sin4

A6

whereis the polar angle.

Considering material properties independent of temperature, it

was possible to write the analytical solution of the heat source

term in the thermal balance

r0orrshellqr; 2shellP0fpattern

4r2 exp 2shellrr0

rshellororcontqr; 2OSP0fpattern

4r2 exp 2shellrshellr0 exp 2OSrrshell

8

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