WO1993011337A1 - Method and apparatus for heating a hot-setting substance injected in a borehole - Google Patents

Abstract

Method for heating of substance being injected in a borehole (2) where the heating is undertaken to harden at least parts of the substance to obtain a dense or a somewhat permeable barrier at a predetermined place in the borehole. An electrical heating means to carry out such a method is also described. First a settable substance (a liquid or a suspension) which is hardening completely or partly at a predetermined temperature, is pumped down to the desired area in the well hole, and thereafter the substance is heated as a current is supplied to the heating means being situated within the pumped-down substance. The substance (18) is preferably electrically and thermally highly conductive. The object with the barrier which is built up, is to establish a zone-isolating barrier or to obtain a sand consolidation within the well.

Description

METHOD AND APPARATUS FOR HEATING A HOT-SETTING SUBSTANCE INJECTED ■TN A BOREHOLE.

The present invention relates to a method and an appara¬ tus to provide a complete or partial densifying of confined regions in and/or about a bore hole penetrating the crust of the earth.

The invention in particular relates to a method and an apparatus which may be used to provide a completely stable and impermeable, solid layer at a required place down in the borehole (blocking) , or a partly permeable layer, but all the same a layer having a stable configuration with a firm consolidation down in a borehole in a sub-structure (consolidating) . When such dense or permeable layers are wanted down in a borehole this is mainly due to the following reasons.

On the first hand it may be desirable to bar penetration of unwanted fluids into the production fluid resulting in a an undesired mixture. Such separation of different fluids being present in one well is below referred to as zone isola- tion. To obtain such zone isolation, dense, blocking barri¬ ers have to be generated to separate or isolate the different fluids from each other.

On the second hand it may be desirable to prevent disin¬ tegration of solid matter from the sub-stratum so that particles may follow the production fluid to the surface.

Such a stabilizing of the formation to retain particles will below be called sand consolidation.

The present invention may be used both in connection with zone isolation and sand consolidation, and may also be used to form relatively dense layers of many different geome¬ tric shapes. For instance vertical and also horizontal separation may be obtained by using this invention. In a similar manner dense walls, dense plugs or dense sections of annular shape may be provided if desired. Below the different zones are denoted as a water zone and an oil zone. The reason for this is that the invention most often will be used in connection with wells producing oil, but where the risk exists that undesired water can easily enter the oil. Oil and water are, however, only mentioned as practical examples, as the invention may be used for isolating any undesired fluid from any different, desired fluid. Thus, the invention may be used for isolation of all kinds of fluids in wells, including liquids and gases as well as mixtures of such fluids. Further, the invention may be used to consolidate formations consisting of all types of materials and all sizes of particles as well as different combinations of such elements.

There are several previously known conventional methods of accomplishing a dense or a somewhat penetrable barrier down in a borehole.

From US patent Nos. 3.297.088, 3.800.975, 3.525.398, and 4.155.405 different previously known examples of zone isola¬ tion, and sand consolidation are known.

However, the above solutions are, as far as we know, only known from the litterature. In practical use all the known solutions have shown considerable disadvantages, and they also lead to rather expensive implementations. The proposed solutions, therefore, have not found broad practical application.

All conventional methods have entailed problems related to the arrangement of the stabilizing material into the correct position down in the ground, and an equaly difficult problem in obtaining the setting process at the correct site in the formation. When stabilizing materials comprised of two components which set when they touch each other are used, the problem will be based substantially on the contradictory desires that the two components have to be thoroughly mingl¬ ed, but at the same time should not harden until brought into correct position.

In a similar manner it is difficult to obtain control of the extension of the stabilizing material and the degree of permeability obtained. Similar problems also arise in con¬ nection with conventional solutions where a heating process has been used, in particular as the heating is undertaken according to methods which are hardly controlable, for ins¬ tance using back-burning of a fluid in the channel. Neither the degree of heating nor the exact place to be heated will be easily controlled using previously proposed heating methods, which have been suggested only for consolidation and not for zonal isolation. Today only the two following methods are still in use:

Cement is pumped down into the water zone, and as it hardens the water-producing perforations will be closed.

A pipe section is arranged within the casing. The pipe section has a length corresponding to the depth of the water-production zone. By arranging tight gaskets against the wall of the casing, both at the upper and the lower part of the pipe section, the produced water will be confined.

There are many problems in connection with these earlier attempts to solve the problem. On the first hand a really tight connection is not obtained, and on the other hand, even when a tight connection is present, it is not quite sure where the packing is obtained . In addition these solutions are expensive, as the boring platform often has to be placed just above the borehole. This fact leads to very expensive solutions.

The main object of the present invention is to provide a method and an apparatus to produce a tight or somewhat perme¬ able barrier at a desired place down in a subterranean well. In addition the object of the present invention is to provide a method and an apparatus to obtain stable structures consolidating the formation, and exhibiting a suitable perme¬ ability at a predetermined place in the substructure, especi¬ ally within a well which already has been bored, and where the above disadvantages are not pronounced.

A further object is that a predetermined site within or around the well may be heated to a certain extent when a thermal setting substance has been pumped down to obtain a denser formation at this place. A further object is to provide a so called zonal isola¬ tion and/or a so called formation consolidation without bringing the boring rigg to the borehole. Still a further object is that it should be possible to make a zonal isolation and/or a formation consolidtion also where a gravel packing already is made.

A further object is to provide a simple method for controlling that densifying is obtained at the desired site. A further object is to obtain a densifying process which may be undertaken by relatively simple and inexpensive meas¬ ures and with a shortest possible time consumption.

Especially important is that the present invention leads to a completely certain determination of the depth and the extension of the site where the densifying is obtained, and also that the time at which the densifying occurs is exactly known.

Earlier on it has not been known exactly where the densifying process was fulfilled. The reason for this pri¬ marily was that one did not know exactly where the thermal setting substance pumped down stopped, and secondly one did not know exactly where the conditions for thermosetting existed. The present invention cannot give full control over where the setting substance is located either, but on the other hand it gives a high degree of control over where conditions will be such that setting will take place.

All the objects above are met by using a method and an apparatus according to the claims below. To give a better understanding of the present invention reference is made to the embodiments mentioned below and to the accompanying drawings where:

Fig. 1 shows a well having a casing and a tubing install¬ ed, and where a heating device according to the present invention is in use,

Fig. 2 shows a similar well as that shown in fig. l, how¬ ever this well is gravel-packed and therefore the lower part of the tubing is finished by a screen or a filter having minor perforations allowing a hori- zontal flow of liquids through the screen.

Fig. 3 shows the borehole with gravel packing and inserted screen, seen from above, Fig. 4 shows in a similar way as fig. 3, the borehole including the gravel packing, but now comprising an inserted plug, and Fig. 5 shows a specific embodiment of one electrode in which one or several sections of the casing or the tubing make up the electrode(s) . All the figures are simplified and only the details required to illustrate the present invention are included, as further details which are not relevant to the invention itself are omitted. It should also be mentioned that the same reference numbers are used for similar components and structure details where possible.

In fig. 1 there is shown a borehole arranged in the crust of the earth. The hole is delimited by the outer, mainly cylindrical wall 2. At the outer wall of the borehole a casing 3 is often arranged adjacent to and partly in cont¬ act with the side wall in the borehole. The casing 3 is, at the productive depth, provided with perforations 4 through which the well product flows in from the surrounding struc- ture 5 and into the well proper.

The figure only illustrates the zone close to the upper part of the production depth.

Down in the casing 3 a tubing 1 protrudes, sealed to the inner surface of the casing 3 and above the perforated zone of same. The sealing is obtained by means of an annular gasket 6.

In the formation or in the structure 5 there will often be found certain amounts of oil 0 floating on the top of existing water V. The problem arises that both oil and water are forced through the perforations 4 and reach the lower part 7 of the tubing 1. Special measures have to be taken to prevent undesired fluid (water) penetrating the desired fluid (oil) contaminating the latter.

In fig. 1 a solution to this problem is suggested accor- ding to which a method or a device according to the present invention is used. To begin with it is assumed that the formation 5 is solid and properly consolidated and also has a high, internal formation pressure, and that fluid having escaped the formation and entered the well will therefore not be able to flow back into the formation.

According to the present invention a cetain amount of substance 18 having a proper consistency, specific gravity, and quality is injected into the water zone 11 through a supply tube 19 shown in fig. 1. This substance has such qualities that it has a low viscosity at the normal tempera¬ ture of the formation while it sets into a solid at a defined and higher temperature. As suggested on the figure the injected substance 18 will then, under pressure, fill up the complete cross-section of the casing 3 on the top of the water-level 11, and make up a confined plug of approximately spherical shape as shown by the dotted line in fig. 1. The substance 18 may preferrably have a low viscosity before heating, and will therefore flow out through the perforations 4 and make a dense layer towards the formation 5 in the wall of the borehole. When a larger amount of the substance is pumped down, this amount will fill a spherical room as sugge¬ sted by a dotted line on the figure. However, as the chemic- als 18 have a low viscosity they will so far not constitute any kind of a blocking zone.

In the next step of the process heat is supplied in a controlled manner via an electrical cable 20, which at its distal end is provided with at least one heating means 21, comprising at least one electrode or at least one heating element being in direct thermal contact with the liquid substance 18 pumped down into the well.

The expression "electrode" is defined as an uninsulated electrical contact element being in electrical connection with its surroundings. An electrode of this kind will trans¬ mit an electrical current out into its surroundings, and will heat the surroundings due to directly resistive losses in the surrounding formations as well as by thermal convection. A heating element is on the contrary an electrical insulated resistance element heated by a passing electrical current.

While an electrode only needs supply of electricity from one conductor a heating element has to be supplied from at least two conductors. However, both these solutions will give the highest heating effect close to the heating means and gives an even heating effect in all directions.

Both the substances 18 and the heating means 21 may preferrably be guided down into the well within a so-called coiled tubing of a per se known type (not specified here) but may also be guided down into the well by means of jointed shorter tube sections or by means of a conventional cable when possible.

According to the present invention the substance 18 being pumped down into the well has such characteristics that it will set and change into a solid body when heated. Both cellulose-based and epoxy-based solutions exist on the mar¬ ket, having such qualities that heated to 60-70°C or higher, they will set into a solid structure. There are also many useable chemicals which do not harden until approximately 150°C has been reached.

Considering all the different substances to be injected into a well they must meet many different requirements as the substance must set at a temperature above the formation temperature,

- the substance must not have any detrimental inf¬ luence on any contacting installations,

- the substance must be compatible with, and not have any detrimental effect on, the formation itself, for instance by reducing the permeability of same in an undesired manner. surplus amounts of the substance i.e. substance which does not set and therefore flow back into the system together with the product, must not have any detrimental effect on the product itself (the oil or the gas produced) .

Therefore the injected substance will, when ideal condi¬ tions exist, not introduce any change either in structure, plant or product, and the superfluous residue of the substa- nee, i.e. residue which is not sufficiently heated and there¬ fore do not set, and/or components of the substance being of a non-hardening nature, have to be removed e.g. as they follow the product back to the surface.

As a rule it is required that more substance or liquid has to be pumped down into the well than required according to the desired establishing of a blocking area or to estab¬ lish the desired consolidation. The remaining not-hardened residue of the liquid will then return to the surface as soon as production restarts.

If the formation around the well is not firm and stable, a further problem may arise. This is not only dependent on the original condition of the formation, as the conditions will change during the working time of the well, for instance as the formation pressure often will be reduced during oil production. Both when a new well is considered, but still more in a well where production has gone on for some period, small particles and disintigrated mass may be released from the formation to be transported together with the produced fluid back into the well.

Filling the space between the tubing and the casing with gravel 10 as shown in figure 4, is an earlier known method to avoid or reduce such sand production. Further a supporting screen 9 with perforations has been arranged at the distal end of the tubing 1. The effect is that the gravel then acts as a filter removing the undesired particles keeping them back in the annular space 15 so that they do not follow the production fluid into the tubing, while the screen primarily is used to support the gravel. The particle size of the gravel has to be selected according to the size of the parti¬ cles loosening from the formation. However, a gravel-packing according to the above is very time-comsuming and very expensive, in particular as it only may be obtained having the boring rigg arranged at the well. Traditional gravel packing to stop sand production may be totally omitted when methods according to the present invention are used. It should again be stated that two different tipes of substances may be used according to the present invention:

- The substance may be homogenous and will then har¬ den to a dense, quite impermeable layer as a result of heating. This substance is to be used when zone isolation is desired.

The substance may instead comprise a suspension of solid particles in a liquid carrier. In this case the single particles may be coated by a thermo- setting material. The object then is to obtain a substance which in a not heated condition will behave as a liquid, while the solid particles du¬ ring heating adhere to each other and make up a solid strucuture having a pre-determined porosity.

However, the liquid carrier does not harden and will be removed together with the product. When the present invention is to be used for consolida¬ tion of a formation a substance comprising a suspension of coated particles is preferred. By pumping such a suspension of pre-coated gravel down into a well which has started sand production, the particles will be forced, due to the pres¬ sure, out into the formation 5 and through the perforations 7 in the casing 3. This may be obtained both for the solution according to fig. 1, 2, 3, and 4.

As the particle size is adapted to the density and structure of the formation it is obtained that a very large amount of the coated particles are forced into the formation at its less stable areas as the formation is most injective at those places. The final consolidation is due to a subse¬ quent heating process.

Finally it should be mentioned that earlier on it has not been practically viable to undertake zone isolation in gravel-packed wells. However, this is now made possible when the present invention is used. A more complete explanation of this is given below.

In fig. 4 it is suggested that in a gravel-packed oil well after a certain production time there will be found some water 11 at the bottom of the well with a floating layer 12 of oil on the top, thus forming a borderline 13 between oil and water as shown in fig. 2. To avoid water mingled into the produced oil in the tubing, it has earlier been arranged a plug 14 at the borderline 13 between water and oil. How¬ ever, a dense plug at this place will only represent a par- tial solution to this problem, as water which has been forced into the screen 9 and into the gravel packing 10 at still further depth, also will flow in an upward direction between the gravel layer 10 in the annular space 15 between the casing 3 and the screen 9, and again penetrate the casing towards the center of the tubing together with produced oil. In fig. 2 this situation is illustrated by the arrows 16 and 17. A plug 14, for instance made of cement, allowed to harden within the screen 9, will not give a complete separat- ion of all water in the product. All the same the cement does not have a sufficiently low viscosity and particle size to give a complete densifying in the gravel packing.

As already mentioned these problems may be solved using the present invention, as the invention does not only produce a dense blocking within the screen 9, but the blocking layer will, when the present invention is used, continue outwards into the gravel layer 10 in the annular space 15. How this is obtained is more clearly shown below, in particular in connection with fig. 2. In this connection two different conditions are impor¬ tant:

- when cement has been used it is slow-flowing and filled up of coarse particles which are not allowed to pass through the screen to fill the annular space. - when cement was used, it was never quite certain where the dense layer would be formed.

By means of the present invention, however, there may be used a quite homogenous liquid which easily penetrates out into the annular space, or when it should be used in connec- tion with consolidtion of the formation, the suspension having microscopic particles (fines) , which cannot be stopped either by a screen or by gravel layers, suspended in a liquid carrier. As the hardening does not take place until a speci¬ fic temperature has been obtained during heating by means of specific heating means, it will clearly be defined where the plugs are formed. And because neither a liquid nor the heat is prevented from passing out into the annular space 15, this space will be blocked also in between the gravel elements. According to this the produced water will, when zone isola- tion by means of a homogenous liquid is used, be completely separated from the tubing. When an additional consolidation of the formation by means of a suspension is used, a corres¬ ponding stabilizing of formation beyond and about the well is obtained as well as an additional stabilizing and densifying of the gravel packing.

To obtain the highest possible density of the formation it may also be important to have a liquid including particles of different sizes. This can enhance the densifying process in certain structures comprising voids of different dimen¬ sions.

Especially when suspensions comprising pre-coated par¬ ticles are used, very good results may be obtained if the pumping down process goes on for a relatively long time before the heating begins. Then the concentration of pre- coated particles will reach its highest value just where the densifying is most needed before the heating process starts.

According to the present invention the necessary heat will be generated by means of an electrical current, and according to a certain embodiment only one electrode 21 is used to apply the current to the ground through water exist¬ ing in the formation. The situation then is that the higher the saturation of water, the higher the obtained heating. It has been found possible to heat a formation up to 150-200°C using one single electrode lowered in the cable. When the chemicals in the substance 18 have filled the central space in the well and also has flowed through the gravel packing and out into the radially directed perforations 6 and further out in the formation, then the part of this chemical layer being close to the heating means may be heated sufficiently for hardening to take place by means of one single (or plu¬ ral) heating device(s) at the center of the tubing.

When one or more electrodes are used, it is necessary to obtain a current also flowing through the area outside the tubing 7, 8, 9 and possibly also outside the casing 3 depend¬ ing on the location of the electrode. However, this process has to be planned and prepared. This may be done by dividing the casing and/or the tubing into electrically insulated sections already before mounting. This is shown in fig. 5. In fig. 5 it is shown that the casing 3 (or possibly the tubing 1) consists of several metalic sections as 22, 23, 24 and 25, each being completely electrically insulated relative to the adjacent tube sections by means of insulating distance elements or insulating rings 26, 27, 28, 29.

A contact device 33 equipped with sliding contacts 29, 30, 31, 32 on its sides is lowered down into the tube in such a manner that the sliding contacts are forced towards the inner surface of the casing. Contact device 33 is connected to a current supply 36 arranged at the surface of the well. A DC current supply producing pulsed current at a relatively low frequency, for instance in the range 10-35 Hz may be used.

As shown in the figure it will then be obtained that one section 22 of the casing functions as an anode while another (possible adjacent) section 24 of the casing functions as a cathode in a circuit completed as the current passes directly through the formation (and the water within the well) , as assumed by the arrows 34 and 35 in fig. 5. Heat will be generated through directly resistive heat¬ ing of the formation and will also be conducted to the sur¬ rounding regions by means of thermal convection.

This method of heating is in particular advantageous as the length of the lowered cable and/or the arrangement of the anodes and cathodes in the tube walls exactly defines where the mamimum heat is generated, and in addition at least one temperature sensor may be used to detect the actual temper¬ ature reached at the surface.

According to certain embodiments the heating means 21, the outer parts of same, or possibly the contact device 33, may be retained down in the sub-surface included by the hardened compound, but in other embodiments the heating means or at least substantial parts of same, may be pulled back to the surface after the fulfilment of the hardening process. To avoid draining of the current to the tubing or to the casing, and to ensure that the current really continues in the formation on the outside of the tubes and thus also generates heat in that location, the complete tube or parts of same may be produced of insulating material or of material being insulated at its surface.

However, the required heat will also be distributed outside the tubes by means of a direct convection of heat through the moist ground. Normally there will be a combina¬ tion so that the resulting formation heat outside the tube partly is due to the current's passage through this zone and partly to the fact that heat energy is conducted outwardly from the hottest central zone within the well hole. In specific embodiments two different electrodes may be lowered in the well in such a manner that the heating by and large is obtained between the electrodes and not spherically round one single one as shown in fig. 3. It should also be mentioned that AC as well as DC may be used for heating. If it is required that the heating means shall be used again, they may be equipped with an enveloping and protecting cover arranged outside the heating means with a conductive outer portion, and possibly an insulating inner portion. The electrical connection between the outer cover and the inner portion of the heating means, may be arranged in a simple manner by means of a plug-in contact or a sliding contact so that the internal part of the heating means may be brought back to the surface by simply pulling it up with the cable, which in this case preferrably should be provided with a tension release to endure such stresses. The only part remaining down in the well should then be the cover encompas- ing the heating means.

It should also be mentioned that the heating means before mounting may be located at the surface or some of the other structures which are to be arranged in the borehole.

In all these cases the embodiment may have such a design that beforehand there are installed a plurality of heating means at different depths in the different tubes, as each single heating means during use has one separate, insulated connec- tion up to the surface. By selecting the correct terminal at the surface the correct heating means may be activated at the correct depth.

Many different combinations of heating means may be used, including one simple centrally arranged heating means, one centrally arranged heating means combined with different surrounding heating means at the outer surface of the casing, and also comprising a plurality of periferally arranged heating means at the tubing combined with corresponding heating means at the perifery of the casing. Finally it has to be mentioned that the heating means not exclusively has to be arranged in on single horizontal plane, but also may be arranged at different depths to gain a heating volume having a certain vertical extension. Close at or adjacent to one or more of the heating means there may also be arranged a thermal sensor having connec¬ tions to the surface, to supervise the obtained temperature at the correct site, and possibly also detectors sensing other parameters as for instance permeability. By combining heating means arranged at the correct place and injection of one or more substances having proper harden¬ ing temperatures, a very exact control may be obtained consi¬ dering the generation of permeable or dense layers down in the borehole and in its surroundings. When an oil-well is considered the level representing the borderline between water and oil in the well will slowly rise within the well. Accordingly new zone isolating layers may be generated at the desired level. Therefore it may be necessary to repeat the injection and thermosetting process at consecutively lower depths within the well as the production continues.

Many different embodiments of the invention may be used within the scope of this invention. Thus, the chemicals or the substance may be injected through a tube being integrated in or fastened to the inner or outer surface of the screen, the casing or the tubing instead of using a separate coiling tube. Additional sensors may be used to register the perm¬ eability obtained or the composition of the material and/or its characteristics at different depths. Measurements of the volume of the injected compound, and control of the specific gravity of the compound to keep it stable may also be used, and a pulsed current or a current having an increasing or decreasing current value may be used.

Further it may be mentioned that the densifying or stabilizing layer may be arranged horizontally within or outside the well, or even may build up a vertical seal within the well streching out also through a possible gravel layer on the outside of the casing. Finally a relatively thin, cylindric filtre or a so-called screen may be provided down in the well by means of a method according to the present invention.

To reduce corrosion on the structure it may further be of importance whether AC or DC voltage are used, and it may also be desireable to use a slowly increasing leading voltage edge and a corresponding slowly reducing trailing edge when the current is connected respectively disconnected. This last measure may reduce the risk for overheating and arc formations when combustible fluids are present in the well. The examples above may also be combined in different manners easily understood by skilled people. For instance different substances having different hardening termperatures may be pumped down to different depths to obtain zone isola¬ tion layers, and/or to enhancing the consolidation at corres¬ ponding depths. When the different examples above are combined, for instance when more than one heating device is to be used, some of these, or parts of some, may be maintained in the depth while other heating means may be brought back to the surface as complete units to be used again. The substance used is preferrably highly conductive so that it represents an excellent conductor for electric curr¬ ent and also for flows of heat energy.

Claims

Claims :

1. Method for heating a thermosetting substance injected in a borehole (2) , where the heating aims at hardening at least portions of the substance (18) to a dense or partly permeable barrier in at least one pre-determined location in the boreh¬ ole (2) , c h a r a c t e r i z e d by that at least one thermosetting substance (18) is pumped down into the borehole to a desired depth, and that the heating thereafter is obtained by means of at least one electrical heating means (21,22,24) arranged down in the area(s)where the densifying is desired to take place.

2. Method according to claim 1, c h a r a c t e r i z e d in that the heating means (21) comprises at least one uninsulated electrode receiving energy via a single insulated conductor from the surface, as the return path for the current(s) completely or partly passes through the ground around said electrode(s) .

3. Method according to one of the claims 1-2, c h a r a c t e r i z e d in that the heating means (21) at least comprises one resistance element being electrically but not thermally insulated from the surroundings.

4. Method according to one of the claims 1-3, c h a r a c t e r i z e d in that at least one of the heating means (21-24) or parts of same remain(s) on the site when the heating and setting processes are fulfilled.

5. Method according to claim 1, 2, 3, or 4, c h a r a c t e r i z e d in that at least one of the heating means (21) is(are) removed when the desired setting is ob- tained.

6. Method according to claim 1, 2, 3, 4, or 5, c h a r a c t e r i z e d in that the substance (18) which is used where a dense, and impermeable barrier is required, is a homogenous, thermosetting compound with low viscosity, while the substance (18) used where a partly permeable barrier is required comprises a suspension including particles having a suitable size being pre-coated by a thermally setting coating having a pre-determined hardening temperature above the formation temperature, suspended in a liquid carrier which is not at all setable or only is setable at a substantially higher temperature.

7. Heating device for heating of a substance injected into a borehole for wholely or partly hardening of said substance

(18) to a dense or partly permeable compound at the desired place and depth in the borehole (2) , c h a r a c t e r i z e d in that the device comprises at least one electrical heating means (21,22,24) arranged at desired depth in the borehole close to the injecting point, and at least one insulated connector (19) leading from the surface right down to the heating means (21,22,24).

8. Heating device according to claim 7, c h a r a c t e r i z e d in that the heating means co pri- se(s) one or more uninsulated electrodes (21, ,22,23,24) which either is(are) mounted in or is(are) lowered down to a pre¬ determined depth in the borehole and is(are) arranged on an electrical cable (20) and/or one of the tubes or other insta¬ llation structures arranged in the borehole.

9. Heating device according to claim 7 or 8, c h a r a c t e r i z e d in that one or several heating means (22,23,24,25) is(are) pre-arranged in the wall of the tubing (1) and/or the casing (3) at the same or different depths, and that each heating means or each set of heating means separately is fed from the surface via at least one insulated conductor fastened to or representing an integrated part of the tubing or casing in question.

10. Heating device according to claim 9, c h a r a c t e r i z e d in that one or several heating means (22,23,24,25) are fixedly mounted at desired depth is and is(are) supplied with energy via a contacting device (33) which is connected to a current supply (36) on the surface, being equipped with at least one sliding contact (29,30,31- ,32) which makes contact with the heating means (22,24) when the contacting device (33) is situated at desired depth in the borehole (2) .

11. Heating device according to one of the claims 7-10, c h a r a c t e r i z e d in that at least one detector is (are) arranged close to the heating means (22,23,24,25) to sense the real temperature obtained and/or the resulting permeability.