The present invention concerns a heating device to be introduced in a well bored in an underground formation containing a solid hydrocarbonaceous layer, comprising at least a first heating sleeve having an outer surface intended to be at least partially immersed in a heating fluid in contact with the formation.

Such a heating device is intended in particular for heating a solid material comprising oil shale or consisting of oil shale. Liquid and gas oil are advantageously produced from the hydrocarbonaceous solid material located in the ground.

More generally, the heating device applies to the in situ heating of conventional, heavy or unconventional oils, mobile or stationary oils, directly in the ground, for example in a cavity or in a well bored in the ground.

Oil shale is an organic rich fine-grained sedimentary rock. Oil shale contains immature kerogen in a mineral matrix. A retorting process can be carried out to heat the kerogen and produce liquid oil and gas under an oxygen-free environment.

As a consequence, oil shale can be a source of oil which could be a substitute for conventional liquid crude oil. Deposits of oil shale are located around the world, with major deposits in the United States of America. Estimates of global deposits are about 4 trillion barrels of equivalent crude oil.

In order to extract liquid oil with a good quality, the oil shale is preferably heated for long times at relatively low temperatures to obtain a higher hydrogen to carbon ratio and a higher API gravity degree. A preferred temperature for the treatment is around 320°C to 400°C.

An example of operation which can be carried out to extract liquid oil from oil shale solid material in an underground formation is known as "Conduction, Convection, Reflux" retorting process and is disclosed in WO2010132704.

Such an operation includes introducing a heating device in a well which is bored through a formation comprising oil shale. A pool of liquid shale oil is maintained at the bottom of the well. The heating device is activated to heat the liquid, which evaporates and heats up the neighboring formation, to a temperature corresponding to the retort of the oil shale. Liquid oil is collected at the bottom of the well and is then extracted through a production well.

The known heating device comprises at least one electrical resistance able to produce heat by Joule effect. The electrical resistance is powered from the surface through a high tension electrical cable. Such a heating device is not entirely satisfactory. In particular, the electrical resistance of the heating device induces high thermal and electrical losses along the well, when the electrical power is supplied to the heating device at the bottom of the well. The power consumption is high, leading to increased operational costs.

Moreover, the temperature of the heating device at the bottom of the well is quite difficult to control in the harsh environment of a well. Indeed, the fluids which are collected contain hydrocarbons, water, acids such as hydrogen sulfur which can easily degrade the components of the heating device.

The precise location at which the heat produced by the heating device applies to the formation is hard to manage. The heating device may also be subjected to overheating, with potentially hazardous consequences on the well exploitation.

One aim of the invention is to obtain a heating device which can be used very efficiently and with a precise control to recover liquid oil and gas from a solid hydrocarbonaceous material directly in the ground.

To this aim, the subject-matter of the invention is a heating device of the preceding type, characterized in that the first heating sleeve delimits an inner central passage able to receive the heating fluid in contact with the formation,

the first heating sleeve comprising at least an inner inducer, in thermal communication and/or delimiting the central passage, an inner thermal thermally insulating layer and an inner electrically conductive coil able to be electrically powered to generate heat in the inner inducer by induction.

The heating device according to the invention comprises one or more of the following technical features, taken solely or according to any possible technical combination:

- the first heating sleeve comprises, outside of the inner inducer, an outer inducer, in thermal communication and/or delimiting the outer surface, an outer thermal thermally insulating layer, and an outer electrically conductive coil able to be electrically powered to generate heat in the outer inducer by induction.

- the inner electrically conductive coil is electrically connected to the outer electrically conductive coil in the heating device.

- the inner electrically conductive coil has a first helix shape wound in a first direction along the heating sleeve axis, the outer electrically conductive coil having a second helix shape wound in a second direction opposite to the first direction along the heating sleeve axis. - the heating device comprises at least a second heating sleeve, and a connecting section connecting the first heating sleeve and the second heating sleeve, the connecting section defining an inner lumen connecting the central passage of the first heating sleeve with the central passage of the second heating sleeve.

- the connecting section defines at least one radial passage opening inwardly in the central lumen and outwardly outside of the heating device.

- the connecting section comprises an upper connecting end, mechanically connected to the first heating sleeve, a lower connecting end, mechanically connected to the second heating sleeve, and a plurality of axial ribs extending between the upper connecting end and the lower connecting end, the ribs defining between them radial passages.

- the first heating sleeve comprises at least at one connecting end comprising an inner annular terminal electrically connected to the inner electrically conductive coil.

- the connecting end of the first heating sleeve comprises an outer annular terminal electrically connected to the outer electrically conductive coil, the outer annular terminal being located around the inner annular terminal.

- the connecting end comprises a sealing and electrical contact assembly able to ensure a seal and electrical contact upon connection of the first heating sleeve with a connecting end of another element.

- the sealing and electric contact assembly comprises at least an annular expandable sealing joint, the expandable sealing joint comprising an outer membrane and an inner elastic member received in the outer membrane.

- the sealing and contact assembly comprises at least a conductive elastic element, able to be inserted in compression between the inner annular terminal and a facing annular terminal of the other element.

- at least one connecting end of the first heating sleeve comprises a threaded connection to another element.

The subject-matter of the invention is also an installation for producing a hydrocarbon fluid from an underground formation containing a solid hydrocarbonaceous layer, comprising:

- at least a heating well, containing a heating fluid placed in contact with the underground formation containing a hydrocarbonaceous layer;

- a heating device as defined above at least partially immersed in the heating fluid, the heating fluid in contact with the underground formation being able to circulate in the central passage of the first heating sleeve. The invention also concerns a method of heating an underground formation containing a solid hydrocarbonaceous layer, comprising the following steps:

- providing a heating device as defined above into a well bored in the formation;

- providing a heating fluid in the well, the heating device being at least partially immersed into the heating fluid;

- providing electrical power to the inner conductive coil to generate heat in the inner inducer by induction;

- transferring heat generated by the inner inducer to the heating fluid contained into the central passage.

- circulating the heating fluid out of the central passage to an annular space between the outer surface and the underground formation to heat the underground formation.

The method according to the invention comprises one or more of the following technical features, taken solely or according to any possible technical combination:

- the first heating sleeve comprises, outside of the inner inducer, an outer inducer, in thermal communication and/or delimiting the outer surface, an outer thermal thermally insulating layer, and an outer electrically conductive coil able to be electrically powered to generate heat in the outer inducer by induction, the method further comprising :

- providing electrical power to the outer conductive coil to generate heat in the outer inducer by induction;

- transferring heat generated by the outer inducer to the heating fluid contained into the annular space between the outer surface and the underground formation to heat the underground formation.

The invention will be better understood, upon reading of the following description, given only as an example, and made in reference to the appended drawings, in which:

- figure 1 is a schematic view of a first fluid production installation comprising a heating device according to the invention;

- figure 2 is a partial perspective view of the heating device of the installation of figure 1 ;

- figure 3 is a cross-sectional view, taken along a median axial plane of the heating device of claim 1 , illustrating the different layers of a heating sleeve of the heating device;

- figure 4 is a perspective view illustrating the configuration of the inner coil and of the outer coil of the heating device of figure 3;

- figure 5 is a perspective view illustrating a connecting section of the heating device; - figure 6 is a partial view, taken in cross-section along an axial median plane of the heating device, illustrating the mechanical and electrical connections between a heating sleeve and a connecting section of the heating device of figure 2, at an upper connecting end of the heating sleeve;

- figure 7 is a view of a connector for the hot power cable to a connecting section and/or to a heating sleeve;

- figure 8 is an end terminal of the connector of figure 7;

- figure 9 is a perspective view of an end piece of the heating device, connecting the inner coil to the outer coil;

- figure 10 is a view similar to figure 6, at a lower connecting end of the heating sleeve.

A first installation 10 for producing a hydrocarbonaceous fluid from an underground formation 12 containing a solid hydrocarbonaceous material layer 14 is illustrated in figure 1 .

The solid material is preferably oil shale, as defined above. The hydrocarbonaceous material preferentially contains kerogen, which is able to be heated to produce liquid oil and/or natural gas, generally defined as hydrocarbon fluids.

The layer 14 is located underground, at a depth generally comprised between 100 m and 2000 m.

The installation 10 comprises a heating well 16, bored from the surface 18 through the solid hydrocarbonaceous layer 14 and a heating fluid 20 received at the bottom of the heating well 16, at the location of the solid hydrocabonaceous layer 14.

The installation 10 further comprises a heating device 22 according to the invention, at least partially immersed in the heating fluid 20 to heat the heating fluid 20 and the solid hydrocarbonaceous layer 14 around the heating device 22, by conduction, convection and radiation.

The installation 10 further comprises a production well 24 connected to the bottom of the heating well 16, in order to recover the hydrocarbonaceous fluid produced from the retort of the solid hydrocarbonaceous layer 14, and a surface installation 26 located at the top of the wells 16, 24.

The heating well 16 comprises at least an inclined or horizontal bottom section 28 bored through the solid hydrocarbonaceous layer 14. It comprises a upper section 30 connecting the bottom section 28 to the surface 18.

The upper section 30 is preferentially equipped with a casing 32 and with at least a lower packer 34 providing a sealing between the hot bottom section 28 intended to be heated by the heating device 22 and the upper section 30 intended to stay relatively cold. The upper section 30 further comprises a gas evacuation tubing 36, connecting the surface 18 to the packer 34. The gas evacuation tubing runs through the packer 34 to the bottom section 28, in order to collect gases accumulating in the bottom section 28.

The bottom section 28 is preferentially an open hole, i.e., without casing but with a cradle or a screen to maintain the formation.

The solid hydrocarbonaceous layer 14 is directly in contact with the heating fluid 20 located in the bottom section 28.

The production well 24 reaches the bottom of the heating well 16, which emerges laterally in the production well 24.

The production well 24 is equipped with a lower packer 38 located above the bottom of the heating well 16 and a production tubing 40. The production tubing 40 connects the bottom of the production well 24 to the surface 18 to collect the fluid located below the packer 38.

The surface installation 26 comprises an oil and gas recovery unit 42, located at the top of the production well 24. It comprises a gas recovery unit 44, located at the top of the gas evacuation tubing 36 and an electric power unit 46 able to power the heating device 22 from the surface.

The heating fluid 20 contains a hydrocarbonaceous liquid introduced in the well from the surface 18 before the start of production. After the production has started, the heating fluid also contains the liquid collected from the retort of the hydrocarbonaceous solid material 14.

The level of heating fluid 20 in the production well 24 and in the heating well 16 is controlled to maintain the heating device 22 at least partially immersed into the liquid 20. The control is made by sampling some liquid though the production well 24 using the production tubing 40 when the level of liquid increases.

The heating fluid 20 is in direct contact with the heating device 22 and with the formation 12, in the layer 14. The annular space between the heating device 22 and the formation 12 contains heating fluid 20.

In reference to figures 1 and 2, the heating device 22 according to the invention comprises a downhole heating apparatus 50 intended to be inserted in the bottom section 28 of the heating well 16 to heat the heating fluid 20, and an electric power line 52 electrically connecting the downhole heating apparatus 50 to the electric power unit 46 at the surface 18.

The electric power line 52 comprises a cold upper electric cable 54, extending into the upper section 30 of the heating well 16 to the packer 34. The electric power line 52 also includes a hot lower electric cable 56 connecting the upper cable 54 to the downhole heating apparatus 50 in the bottom section 28 of the heating well 16.

The cold electric cable 54 is a classical insulated cable comprising at least two electric leads.

The hot electric cable 56 comprises at least a supplementary thermal insulation sheath 57 (visible in figure 2) surrounding at least two leads 58A, 58B (see figure 7). The insulation sheath 57 is for example a ceramic-based sheath, advantageously made of mica or magnesium oxide.

The two leads 58A, 58B end in a connector 60, shown in figure 7, connected to the downhole apparatus 50. The connector 60 has a lower cylindrical connecting area 61 .

The connector 60 comprises, for each lead 58A, 58B, an annular contact terminal 62A, 62B connected to the downhole apparatus 50, as shown in figure 8. The annular contact terminals 62A, 62B are electrically insulated from one another by an annular insulating ring 64.

In reference to figure 2, the downhole heating apparatus 50 comprises at least a heating sleeve 66A, preferentially a plurality of heating sleeves 66A to 66C, which are connected together in series directly or through an open connecting section 68E to 68G.

Advantageously, the downhole heating assembly 50 comprises at least a connecting section 68E, preferably a plurality of connecting sections 68E to 68G, connecting together a heating sleeve 66A with another heating sleeves 66B to 66C or a heating sleeve 66A with the hot power cable 56.

Accordingly, the number of heating sleeves 66A to 66C may vary from 1 to 200 depending on the length of the bottom section 28 of the heating well 16 which has to be heated.

The number of connecting sections 68E to 68G and their individual length also vary to adapt to the configuration of the bottom section 28 and to the heating power to deliver.

In this example, the downhole heating apparatus 50 extends linearly along an axis A-A'. In a variation, the connecting sections 68E to 68G allow at least a degree of freedom in rotation between two adjacent heating sleeves 66A, 66B.

The heating sleeve 66A to 66C here have the same length, taken along axis A-A'. In a variation, they have different lengths, adapted to the location of the formation 12 which has to be heated.

The length of each heating sleeve 66A to 66C is generally comprised between 0.5 m and 2 m, preferably 1 m. Generally, in the example of figure 2, the connecting sections 68E to 68G have each a length which is less than the length of the heating sleeves 66A to 66C.

The heating sleeves 66A to 66C have similar structures. Only the heating sleeve 66A will be described in details in the following passage.

The heating sleeve 66A extends longitudinally along the axis A-A' between an upper connecting end 70A and a lower connecting end 72A.

It defines a cylindrical outer surface 74, which is directly into contact with the heating fluid 20 in which it is immersed.

The sleeve 66A is tubular. It defines an inner central passage 76, visible in figure 3, which is filled with the same heating fluid 20 which contacts the formation 12.

The central passage 76 emerges axially upwards through the upper connecting end 70A and downwards through the lower connecting end 72A.

As shown in figure 3, the heating sleeve 66A comprises a plurality of concentric layers assembled together.

From the inside to the outside, the heating sleeve 66A comprises an inner inducer

78, delimiting inwardly the central passage 76, an inner thermally insulating layer 80, and an inner electrically conductive coil 82.

The heating sleeve 66A further comprises an intermediate thermally insulating layer 84, an outer electrically conductive coil 86, an outer thermally insulating layer 88, and an outer inducer layer 90.

The inner inducer 78 and the outer inducer 90 are tubular. They are made of metal, for example of steel such as stainless steel 316 L.

Such a metal is extremely impervious to liquids. It is resistant to corrosion for extensive periods of time, allowing the downhole apparatus to remain immersed in the heating fluid 20 without substantial degradation.

The inner inducer 78 and the outer inducer 90 may have different compositions to exhibit different heating characteristics. Similarly, the compositions of the inducers 78, 90 may vary from one heating sleeve 66A to 66C to another to provide locally different heating power to the heating fluid 20 and/or to the formation 12.

The Curie temperature of the metal is lower than 500 °C, and is advantageously comprised between 450°C and 470 °C. The metal is ferromagnetic below the Curie temperature, so that inductive currents are generated in the inducers 78, 90 when the respective coils 82, 88 are excited with an alternative electric current. Above the Curie temperature, the metal becomes paramagnetic, greatly diminishing the generation of inductive currents. The inducers 78, 90 are therefore able to heat up the heating fluid 20 respectively inside the central passage 76 and outside the outer surface 74 by direct contact. The heating fluid 20 can then pass the heat to the formation 12 in the solid hydrocarbonaceous layer 14 by generation of a convective flow and contact with the formation 12.

The thermally insulating layers 80, 84, 88 are made of at least a ceramic tube, for example of a tube made of mica. They extend over the whole length of the heating sleeve 66A. They prevent electrical contact between the coils 82, 86 and their respective inducers 78, 90 or between the coils 82, 86.

The thickness of each thermally insulating layer 80, 88, 84 is for example more than 5 mm.

According to the invention, the inner coil 82 and the outer coil 86 are made of a spiral band of conductive material. The coils 82, 86 are for example made of an alloy of copper, chrome and zirconium, which presents high electric conductivity even at temperatures higher than 600°C.

Each coil 82, 86 has a helix shape, of axis B-B', as shown in figure 4.

The inner coil 82 has an helix shape, along axis B-B', which is wound in a first direction D1 , opposed to the direction D2 of the winding of the helix shape of the outer coil 86. Moreover, the diameter of the helix of the inner coil 82 is less than the diameter of the helix of the outer coil 86.

The helixes of the inner coil 82 and of the outer coil 86 are thus inverted, with a minimal overlap of the respective turns of each coil 82, 86. Electromagnetic disturbances are then minimized, which increases the electrical yield of the heating sleeve 66A.

The inner coils 82 of successive heating sleeves 66A to 66C are connected together in series, through the connecting sections 68E to 68G when present. They are connected upwardly to a first lead 58A of the power cable 52. The outer coils 86 of successive heating sleeves 66A to 66C are connected together in series, through the connecting sections 68E to 68G when present. They are connected upwardly to a second lead 58A of the power cable 52.

The inner coils 82 and the outer coils 86 are connected together in series at the bottom of the downhole apparatus 50, in a connecting end piece 92 shown in figure 2 and in figure 9.

This avoids the use of an electric cable to ensure the return of the current towards the upper part of the downhole apparatus 50, and limits the bulkiness of the downhole apparatus 50.

Advantageously, the longitudinal spaces between the turns of each coil are filled with an insulating band 93 (visible in figure 3), for example made of glass. As illustrated in figure 6, the upper connecting end 70A is here a female connecting end.

It comprises an outer sealing skirt 100 protruding upwards from the outer inducer 90 and an inner sealing extension 102, extending upwards from the inner inducer 78.

The upper connecting end 70A also comprises an insulating annular base 104 interposed in the annular space between the inner sealing extension 102 and the outer sealing skirt 100, an inner annular terminal 106 connected to the inner coil 82 and an outer annular terminal 108, connected to the outer coil 82 and electrically insulated from the inner terminal 106 by the insulating base 104.

The outer skirt 100 has an inner thread 1 10 able to receive a corresponding outer thread of a male lower connecting end 72E, here of a connecting section 68E. In variant, the male connecting end, may be of another element, such as the lower connecting end of another heating sleeve.

The upper connecting end 70A further comprises an assembly 1 12 for sealing and electrical contact.

The sealing and electrical contact assembly 1 12 comprises a plurality of expandable ring shaped sealing joints 1 14 placed in corresponding annular housings 1 1 1 of the outer sealing skirt 100 and of the inner sealing extension 102, to be constricted in the housing 1 1 1 when the lower connecting end 72E is connected to the upper connecting end 70A.

Each expandable joint 1 14 is for example a Helicoflex joint comprising an helical inner spring surrounded by a tubular sheath.

The assembly 1 12 comprises, for each annular terminal 106, 108, at least an elastic element 1 16 able to axially expand to make contact between the annular terminals 108, 106 and a corresponding terminal 108, 106 on the lower connecting end 72E connected to the upper connecting end 70A.

As illustrated in figure 10, the lower connecting end 72A has a shape complementary to the shape of the upper connecting end 70A. It has an outer male sealing skirt 120 and an inner sealing extension 102, an insulating base 104, an inner annular terminal 106 and an outer annular terminal 108 which are of shape complementary to the shapes of the corresponding pieces at the upper connecting end 70A.

The male outer sealing skirt 120 comprises an outer thread 122 threaded with the inner thread 1 10 of an upper connecting end 70F, which is here of a connecting section 68F. The connecting sections 68E to 68G are all of similar structures. Only the connecting section 68E will be described in detail in the following passage.

A shown in figure 5, the connecting section 68E comprises an upper connecting end 70E of annular shape, a lower connecting end 72E of annular shape, and longitudinal ribs 130 connecting the upper connecting end 70E to the lower connecting end 72E. The ribs 130 define between them a central lumen 132 emerging upwardly and downwardly through the connecting ends 70E, 72E to be in fluid communication with a central passage 76 of a heating sleeve 66A to 66C.

The ribs 130 delimit radial passages 134 emerging out of the connecting section 68E, facing the formation 12, connecting the outside of the connecting section 68E with the lumen 132 and accordingly with the central passage 76 of each heating sleeve 66A to 66C. The passages 134 allow the heating fluid 20 in contact with the formation 12 and surrounding the outer surface 74 to enter in the central passage 76 of each heating sleeve 66A to 66C.

The upper connecting end 70E and the lower connecting end 72E of the connecting section 68E are of shape similar to the upper connecting end 70A and to the lower connecting end 72A of the heating sleeve 66A described above.

As a consequence, each connecting end of any heating sleeve 66A to 66C can mate with a corresponding connecting end of any other element, such as a connecting section 68E to 68G or heating sleeve 66A to 66C. The connection between the heating sleeve 66A and the other elements can be carried out very simply by screwing a lower connecting end 72A of the heating sleeve 66A into the upper connecting end of the other element or/and by screwing the lower connecting end of the other element in the upper connecting end 70A of the heating sleeve 66A.

The mounting of the downhole apparatus 50, and its adaptation to the particular formation to be heated is therefore very simple to handle. The components of the downhole apparatus 50 can therefore be manufactured in a very standardized manner, reducing the costs, and improving the sealing and electrical contact properties.

As opposed to the heating sleeves 66A to 66C, the connecting section 68E does not comprise an electric coil.

As shown in figure 6, the connecting section 68E comprises an inner longitudinal lead 136 providing an electrical connection between the inner annular terminal 106 of the upper connecting end 70E and the inner annular terminal 106 of the lower connecting end 72E. It also comprises an outer longitudinal lead 138, providing an electrical connection between the outer annular terminal 108 of the upper connecting end 70E and the outer annular terminal 108 of the lower connecting end 72E. The inner longitudinal lead 136 and the outer longitudinal lead 138 for example extend through a rib 130. The leads 136, 138 are electrically insulated from one another for example by a layer of ceramic insulator.

In reference to figure 9, the lower end piece 92 has a cup shape. It defines a central through hole 140 which emerges out of the device 22. The central through hole emerges upwardly into the central passage 76.

The lower end piece 92 has a shape similar to a female end connector described above, with an outer sealing skirt 100, an inner sealing extension 102, an insulating base 104, an inner annular terminal 106 and an outer annular terminal 108.

It differs from other female connecting ends by a lower annular bottom wall 142 sealingly closing the end piece between the outer sealing skirt 100 and the inner sealing extension 102 and by at least one electrical connection lead 144 (visible in figure 4) connecting the inner annular terminal 106 and the outer annular terminal 108.

As a consequence, the inner electrically conductive coil 82 of the heating sleeve 66C is successively electrically connected to the inner annular terminal 106 of the end piece 92, to the outer annular terminal 108 of the end piece 92 and then, to the outer electrically conductive coil 86 of the heating sleeve 66C.

The set-up and the operation of the heating device 22 according to the invention will be now described.

Initially, a set of heating sleeves 66A to 66C and a set of connecting sections 68E to 68G are provided, along with and end piece 92.

Depending on the configuration which is chosen by the user, a heating sleeve 66A can be connected to the connector 60 of a hot power cable 56, to a connecting section 68, or to another heating sleeve 66B to 66C.

In the example shown in figure 2, a connecting section 68E is first connected to the hot power cable 56 through the connector 60, by screwing the connector 60 into the upper connecting end 70E of the connecting section 68E.

Thereafter, the lower connecting end 72E of the connecting section 68E is inserted into the upper connecting end 70A of heating sleeve 66A and is mechanically connected to it by screwing.

Then, the connecting section 68F, the heating sleeve 66B, the connecting section 68B, and the heating sleeve 66C are successively mounted one on another, and the end piece 92 is screwed to the lower connecting end of heating sleeve 66C.

At each connection between a connecting section 68E to 68G and a heating element 66A to 66C, the inner annular terminals 106 facing one another are in electrical contact, advantageously with the help of the sealing and contacting assembly 1 12, providing a continuous electric path from the inner electrical lead 58A of the power line 52 to the successive inner electrically conductive coils 82, passing through the inner longitudinal leads 136.

The inner electrically conductive coils 82 are connected electrically to the outer electrically conductive coils 86 through the end piece 92.

At each connection between a connecting section 68E to 68G and a heating element 66A to 66C, the outer annular terminals 108 facing one another are in electrical contact, advantageously with the help of the sealing and contacting assembly 1 12, providing a continuous electric path to the outer electrical lead 58A of the power line 52 from the successive outer electrically conductive coils 86, passing through the outer longitudinal leads 138.

Moreover, at each connection between a connecting section 68E to 68G and a heating element 66A to 66C, the screwing of the inner thread 1 10 of the outer sealing skirt 100 with the outer thread 122 of the inner sealing skirt 120, provides mechanical sealing between the skirts 100, 120 facing each other, as well as between the inner sealing extensions 102.

The expandable joints 1 14 constricted respectively between the skirts 100, 120 and the inner extensions 102 ensures that the annular space defined between the inner inducer 78 and the outer inducer 90, containing the coils 82, 86, is totally fluid tight. This feature prevents contamination of the annular space by the heating fluid 20 located in the central passage 76 and around the outer surface 74.

The downhole apparatus 50 is then lowered into the well until the bottom section 28 is at least partially immersed into the heating fluid 20. The packer 34 is set in place to provide a sealing between the hot bottom section 28 containing the heating fluid 20 and the downhole apparatus 50 and a colder upper section 30.

Then, the electrical power unit 46 is activated to generate an alternative electrical current, for example at a frequency comprised between 100 Hz and 500 Hz, preferably between 150Hz and 450Hz, most preferably between 350 Hz and 450 Hz. The electrical current is transmitted to the downhole apparatus 50 through the cold power cable 54 and the hot power cable 56.

The successive variations of the current circulating in the inner electrically conductive coils 82 produce induced currents in the inner inducer 78, which generate heat by Joule effect. The heat is transmitted to the heating fluid 20 received in the central passages 76.

Similarly, the successive variations of current circulating in the outer electrically conductive coils 86 produce induced currents into the outer inducer 90, which generate heat by Joule effect. The heat is transmitted to the heating fluid 20 located in the annular space between the formation 12 and the downhole apparatus 50.

The heating in the central passage 76 advantageously generates a circulation of the heating fluid from bottom to top in the successive central passages 76 of the heating sleeves 66A to 66C.

At least part of the heating fluid 20 is expelled radially from the central passages 76 through the radial passages 134 made in the connecting sections 68E, 68F, 68G, and at the top of downhole apparatus 50, which generates a convection loop.

The heating fluid 20 recirculates downwardly along the formation 12 at the wall of the well 16, creating not only a conductive heating to the formation 12 but also a convective heating which is extremely efficient to heat up the formation 12 containing the solid hydrocarbonaceous layer 14 at a temperature generally comprised between 320°C to 400°C.

The heating of the formation 12 produces a retort of the hydrocarbonaceous solid, producing liquid oil and gas. The oil flows down into the bottom section 28 in the heating fluid 20 and is recovered to the surface 18 through the production well 24.

The heating method is extremely efficient. Indeed, induction heating is immediate, and can be carried out at very high temperatures, without significant degradation of the yield.

The heating is localized at the places where it is needed, by the selective use of heaters sleeves 66A to 66C separated by appropriate connecting sections 68E to 68G.

The heating is very safe, since the temperature of the heating sleeves 66A to 66C cannot raise above the Curie temperature of the materials constituting the inner inducer 78 and the outer inducer 90. This avoids a local overheating of the downhole apparatus 50.

The heating is therefore perfectly controlled, which allows a continuous use for a long period of time, ensuring a significant production of oil and gas from the layer 14, without having to lift the downhole apparatus 50 at the surface.

The provision of connecting sections 68E to 68G with radial passages 134 enhances the convective effect of the heating, by creating turbulences along the annular space between the formation 12 and the downhole apparatus 50.

The use of a heating fluid 20 directly in contact with the formation 12 also enhances the heat transfer to the formation 12, making the production of oil much more economical. The shape of the rib 130 of the connecting section 68E can be optimized to create more turbulence and enhance the convective effects along the annular space between the formation 12 and the downhole apparatus 50.

Also, fins can be installed on the inner inducer 78 and/or outer inducer 90 to increase turbulence and convection (the Nusselt number is then increased).

In another variation, the inner inducer 78 and the outer inducer 90 are covered by a protective coating in order to protect against external aggression, for instance corrosion or mechanical damage.