GB2611583A

GB2611583A - Retrievable downhole heater

Info

Publication number: GB2611583A
Application number: GB2114521.4A
Authority: GB
Inventors: Carragher Paul; Moody Mike; Chancellor Bobby; Dearmon Jake
Original assignee: Bisn Tec Ltd
Current assignee: Bisn Tec Ltd
Priority date: 2021-10-11
Filing date: 2021-10-11
Publication date: 2023-04-12
Also published as: WO2023062335A1; GB202114521D0

Abstract

The present invention provides a retrievable downhole heater that can readily be retrieved from a downhole location once it has been used. The heater 1 comprises an elongate tubular body 3 with a heat source housed therein. The tubular body is formed from a material that does not melt during the downhole operation of the heater. The heater also is provided with at least one reduced friction region 7 that has a lower co-efficient of friction than the material from which the tubular body is formed. The reduced friction regions of the heater are configured to retain the lower co-efficient of friction following the downhole operation of the heater. The reduced friction region may be fitted with bearings 8 or a coating.

Description

RETRIEVABLE DOWNHOLE HEATER

Field of the Invention

The present invention relates to heaters for use in downhole operations, such as in oil/gas wells, and in particular to heaters that are retrieved from a well bore once a heating operation has been completed.

Background of the Invention

A wide range of operations conducted downhole in oil/gas wells can require that heat is delivered to a downhole target region within an oil/gas well. The heating of the target region can be achieved by employing a downhole tool assembly that has a heater, which can be operating to heat the target region to achieve a variety of tasks.

One task carried out using downhole tool assemblies with heating means is the deployment of alloy plugs or seals within a target region of an oil/gas well.

International patent applications W02011/151271 and W02014/096858 represent examples of assemblies used in the downhole deployment of alloy plugs/seals. Typically when setting an alloy plug or seal downhole, which in most cases form a metal on metal bond with existing well structures (e.g. well tubing or casing), a heater is provided in the downhole target region at the same time as a quantity of alloy.

The heat generated by the heater is used to melt the alloy, after which the alloy is allowed to cool and re-solidify to form an alloy plug or seal within the target region of the oil/gas well.

The most common types of downhole heaters used are electrical heaters, which receive power from above ground via a wireline connection, and chemical heaters, which use an on-board chemical reaction heat source that undergoes an exothermic reaction to generate heat in the target region of the oil/gas well.

Thermite and thermite mixtures are an example of a chemical reaction heat source typically employed in the chemical heaters of assemblies used in the formation of alloy plugs and seals in oil/gas wells.

Another task carried out using downhole tool assemblies with heating means is the clearance or removal of well structures, such as pre-existing alloy seals or portions of well tubing/casing, from within a target region of an oil/gas well. International patent application W02015/150828 relates to assemblies used in the clearance of downhole target regions.

On many occasions it is desirable to retrieve a heater from a downhole location once the heating operation has been completed (e.g. an alloy has been melted). However this retrieval process can be challenging, especially in situations where the wellbore is deviated or obstructed in any way.

It has been discovered that the retrieval of used heaters can be further impeded in situations where the heater becomes warped and/or bent. This can occur under the influence of the high temperatures generated by the heater's heat source, which can be a particular problem at the higher temperatures that are achievable using chemical heat sources.

Summary of the Invention

In order to address the difficulties faced when attempting to retrieve used downhole heaters from within downhole environments, such as oil/gas wells, the present invention provides a retrievable downhole heater in accordance with claim 1. In this regard the present invention provides a retrievable downhole heater comprising an elongate tubular body with a heat source housed therein, said tubular 20 body being formed from a material that does not melt during the downhole operation of the heater; and wherein the heater is provided with at least one reduced friction region having a lower co-efficient of friction than the material from which the tubular body is formed and whereby each reduced friction region is configured to retain the lower co-efficient of friction following the downhole operation of the heater.

The provision of one or more reduced friction regions on the heater helps to minimise the frictional forces that can be generated when attempting to retrieve a used heater from a downhole location within, for example, an oil/gas well It is envisaged that without the reduced friction regions, any interactions between the outer surface of the heater and the surrounding inner surfaces of well structures (such as well tubing and well casing) can place a frictional drag on the heater that, at best, can increase the force required to extract the heater and, at worst, can wedge the heater in place in a downhole location.

By providing one or more reduced friction regions, that are much better suited to slide past obstructions and the surrounding inner surfaces of well structures, it is possible to ensure that the heaters of the present invention have a much greater level of retrievability.

It is envisaged that the reduced friction regions provided on the heaters of present invention can be achieved in a variety of ways.

Preferably at least one reduced friction region on the heater may comprise one or more rotatable bearings that project beyond the outer surface of the elongate tubular body.

It is appreciated that the rotatable bearings, examples of which include ball bearings, needle bearings, barrel bearings, rollers and wheels, enable the heater to effectively roll along the inner walls of the surrounding well structures. This greatly reduces the frictional resistance created between the heater and the surrounding well structures during the retrieval of the heater.

Preferably one or more rotatable bearings may projects from the outer surface of the elongate tubular body. Alternatively or additionally, one or more rotatable bearings may be mounted on a sub that is attached to the leading and/or trailing end of the elongate tubular body.

Further preferably the sub may be provided in the form of an open-ended skirt portion that extends from the leading end of the heater.

The skirt portion, which essentially comprises a cylindrical body that is open at zo one end, allows downhole fluids to enter it, which has a cooling effect on the skirt that helps in cooling down molten alloy as it flows passed the skirt. The use of a skirt portion is considered in more detail in the applicant's International patent application W02011/151271.

Preferably at least one reduced friction region on the heater may comprise one or more contact surfaces formed from a material that has a lower co-efficient of friction than the material from which the tubular body is formed and whereby each contact surface projects outwards beyond the outer surface of the elongate tubular body.

It is envisaged that as an alternative approach to providing means that roll across the inner walls of the surrounding well structures, the heater may be provided with low friction contact surfaces that are configured to effectively slide along the inner walls of the surrounding well structures.

It is appreciated that in the broadest sense of the present invention the materials used to provide the low friction contact surfaces merely need to have a lower co-efficient of friction than the material used to manufacture the main body of the heater to improve the recovery of the heater. Typically steel alloys, such as low carbon steel, are used to form the main components of the heater.

However the low friction contact surfaces are preferably formed a low friction material selected from the group consisting of Aluminium, Bronze, Aluminium Bronze and Brass; Polytetrafluoroethylene (PTFE), Teflon®, Teflon® based materials and other low-friction polymers such as polyimide, polyether ether ketone (PEEK), polyphenylene sulfide (PPS), nylon, acetal and polyester might also be employed. In addition, graphite impregnated polymers can also be used.

Preferably one or more contact surfaces may be provided by a low friction component that is mounted on the outer surface of the elongate tubular body project from the outer surface of the elongate tubular body. Alternatively or additionally, one or more contact surfaces may be provided by a low friction component that is mounted on a sub that is attached to the leading and/or trailing end of the elongate tubular body.

Preferably the low friction component may be annular-shaped. Alternatively, the low friction component may be a pad that does not extend around the entire circumference of the heater. Multiple pads may be arranged around the heater's circumference. The pads can be retained in suitable shaped grooves that are formed in the outer surface of the heater's tubular body.

Preferably at least one reduced friction region on the heater may comprise a friction reducing coating applied to the outer surface of the elongate tubular body. In this way the entire surface of the heater's tubular body has a greater ability to slide along the inner walls of the surrounding well structures.

Examples of suitable friction reducing agents that can be coated onto the heater include: polytetrafluoroethylene (PTFE), Xylane; molybdenum disulfide, tungsten disulfide, TiCN, TiSiCN and TiNbN; metals such as aluminium, copper and alloys thereof.

Further preferably the friction reducing agents may be heat activated, such that they only act to reduce the co-efficient of friction for the reduced friction region after the heater has been operated. Examples of friction reducing agents that would work in this way include wax and paraffin; high-temp, high viscosity greases/oils; and oil embedded polymers that only release the oils at higher temperatures.

Once activated the heated friction reducing agents act to counter frictional interactions between the heater and the surrounding well structures that would otherwise impede the retrieval of the heater.

The skilled person will appreciate that the application of the friction reducing 5 agents onto the heater's tubular body can be achieved via a range of coating methods, examples of which include painting, spray coating and thermal spray coating.

Preferably said reduced friction region is provided at: a) a leading end of the heater, which, in use, is deployed downhole first; b) a trailing end of the heater; and/or c) a location on the elongate tubular body between the leading and trailing ends of the heater.

It is envisaged that the reduced friction region(s) may be provided in specific locations on the heater that are particularly vulnerable to catching on the inner walls of the surrounding well structures. This may be most appropriate when employing the bearings, for example, which it might otherwise be uneconomical to cover an entire heater with.

It is also envisaged that a combination of the various reduced friction regions described above might be employed on a single heater.

Preferably said reduced friction region may extend over the entire outer zo surface of the elongate tubular body. This arrangement may be best achieved by using a low friction coating, for example.

Although the heater of the present invention may have any type of suitable heat source, preferably the heater comprises a chemical reaction heat source. Thermite and therm ite based mixtures are considered a particularly suitable chemical reaction heat source for heaters of the present invention.

It is envisaged that the retrievable heater of the present invention can be used in a variety of downhole operations where the heating of a target region is required. However, the retrievable heater described herein is considered to be particularly suitable for use in the formation of alloy plugs and seals within downhole environments, such as oil/gas wells.

As such, according to further aspect of the present invention there is provided a downhole well plugging or sealing tool, said tool comprising: a heater in accordance with the present invention; and an alloy, which can be melted downhole by the heater in order to form a plug or seal within a target region of a downhole environment.

It is envisaged that the tool of the present invention can be used to deploy metal on metal plugs/seals downhole that are formed from a range of alloys.

However, eutectic alloys such as bismuth based alloys are considered particularly preferable because of their tendency to contract on melting and expand again on cooling, which can facilitate the formation of tight seals.

Preferably the tool may further comprise a dump bailer and the pre-melted alloy is delivered downhole within the dump bailer in the form of alloy beads or shot.

In an alternative arrangement the pre-melted alloy is mounted on the outer surface of the elongate tubular body. This arrangement can be achieved by providing melted alloy within a mold around the heater and then allowing it to cool and set on the outer surface of the elongate tubular body.

Further preferably, the pre-melted alloy may cover said at least one reduced friction region.

Alternatively the pre-melted alloy and said at least one reduced friction region may be arranged on the outer surface of the elongate tubular body so that they do not overlap with one another.

In one preferred example of this non-overlapping configuration the alloy and the reduced friction regions could be arranged in stripes around the outer surface of the elongate tubular body. The stripes could be arranged in a plane perpendicular to the main longitudinal axis of the heater or alternatively they could be arranged helically (i.e. in a corkscrew shape around the outer surface of the heater.

In a further aspect of the present invention there is provided a method of manufacturing a retrievable downhole heater, said method comprising: providing an elongate tubular body formed from a material that does not melt during the operation of the heater; and installing a heat source within the tubular body and providing at least one reduced friction region on heater; wherein said reduced friction region has a lower co-efficient of friction than the material from which the tubular body is formed and is configured to retain said lower co-efficient of friction following the downhole operation of the heater.

Preferably the heat source installed within the tubular body of the heater is a chemical reaction heat source.

Preferably at least one reduced friction region may be provided by mounting one or more rotatable bearings on the heater such that said bearings project beyond the outer surface of the elongate tubular body.

Preferably at least one reduced friction region may be provided by applying a coating of a friction reducing agent to the outer surface of the elongate tubular body.

As noted above, examples of suitable friction reducing agents that can be applied as a coating on the heater include: polytetrafluoroethylene (PTFE), Xylane; molybdenum disulfide, tungsten disulfide, TiCN, TiSiCN and TiNbN; metals such as aluminium, copper and alloys thereof.

Preferably at least one reduced friction region may be provided by mounting a component formed from a material with a lower co-efficient of friction than the material used to form the elongate tubular body on the heater such that a contact surface of the component sits proud of the outer surface of the tubular body. Preferably the low friction component may be annular-shaped. Alternatively, 15 the low friction component may be a pad that does not extend around the entire circumference of the heater. Multiple pads may be arranged around the heater's circumference and could be retained within suitable shaped grooves formed in the outer surface of the heater's tubular body.

Preferably said reduced friction region may be provided at: a) a leading end of the heater, which, in use, is deployed downhole first; b) a trailing end of the heater; and/or c) a location on the elongate tubular body between the leading and trailing ends of the heater.

Alternatively or additionally said reduced friction region may extend over the entire outer surface of the elongate tubular body.

Brief Description of the Drawings

The present invention will now be described with reference to the drawings, wherein: Figure 1 shows a preferred embodiment of the downhole heater of the present invention being retrieved from a wellbore; Figure 2 shows the lower portion of a downhole heater that is in accordance with the present invention; Figure 3 shows the lower portion of a preferred embodiment of the downhole heater of the present invention; and Figure 4 shows the lower portion of a further preferred embodiment of the downhole heater of the present invention.

Detailed Description of the Preferred Embodiments

Although it is envisaged that the downhole heaters of the present invention can be suitably deployed within a range of conduits and boreholes formed within the ground (e.g. such as water conduits), the main applications considered by the inventors were the use of downhole heaters in oil and gas wellbores.

As noted above, downhole heaters of both the chemical type and the electric type can be used in a range of downhole operations in which an operator wishes to deploy heat within a target region of an oil/gas wellbore. However, in the examples shown herein the heaters will be of the chemical type. Despite this it is envisaged that the technical benefits described could also be employed in downhole electric heaters.

In its broadest sense the present invention provides improvements to downhole heaters that help reduce the heater's tendency to drag against the inner walls of the structures such that they can be more readily retrieved once a downhole heating operation has been completed.

It has been found that on occasions the high temperatures generated by downhole heaters, particularly heaters that employ chemical reaction heat sources such a thermite and thermite mixtures, can cause the heater to warp and set in a bent or helical (i.e. corkscrew like) shape downhole.

This can present problems when the operator comes to retrieve the heater at the end of the heating operation because the clearance between the heater and the internal diameter of the surrounding well structure (e.g. well tubing, well casing, formation) is effectively reduced.

As a result, when the clearance between the heater and the internal diameter of the surround well structure was already tight and/or there are other obstructions downhole (i.e. deviations in the well tubing/casing or partial blockages), parts of the heater can be brought into contact with the surrounding well structure and/or the obstructions. At those points where the heater makes physical contact with a surround surface friction can be created when the operator attempts to retrieve the heater from down hole.

This friction can impede the retrieval of the heater and necessitate the use of additional pulling force to free the heater.

In some situations, the deliver means (e.g. wireline, slick line, coiled tubing) that was originally employed to deliver the heater down hole may not have the required tensile strength to take the additional strain placed on it when the operator attempts to pull the heater free.

Although the extraction of heaters that have become warped during their operation downhole is the main focus of the present invention, it is envisaged that the present invention can also be beneficial in situations where a used heater, which is otherwise unbent, is impeded by obstructions within the retrieval path of the heater.

It is envisaged that providing the retrievable heaters of the present invention with reduced friction regions makes it possible for the heater to move passed those 'pinch' points that might in the past have impeded the progress of the heater's retrieval due to the frictional build up between the heater and the surrounding well structures.

Having discovered the above problem, the inventors have devised a range of approaches to reducing the frictional interactions that take place between the heater and the surfaces of surrounding well structures. A range of examples of how the zo heaters of the present invention can be provided with low friction regions will now be described with reference to the figures.

However, it should be appreciated that the examples shown are not intended to be limiting and other approaches to rendering the heater less prone to gripping the walls of surrounding well structures may be employed without departing from the general concept of the present invention.

The retrievable downhole heater of the present invention will now be described with reference to the examples shown in the Figures 1 to 4.

Figure 1 shows a retrievable downhole heater 1 of the present invention being retrieved from within a well tubing 2.

The heater 1 comprises a main elongate tubular body 3 within which the heater's heat source (e.g. a chemical heat source such as therm ite or a therm ite mixture) is housed. The tubular body 3 is typically formed from a steel alloy, such as low carbon steel, such that the body 3 is structurally strong and does not melt during the operation of the heat source.

The top end of the heater, also referred to herein as the trailing end because it is the end of the heater that enters the wellbore last, is provided with a cap 4.

The cap 4 provided the point at which the heater 1 is connected to a suitable delivery support 5 (e.g. wireline, slick line, coiled tubing) so that the heater 1 can be 5 delivered downhole and then subsequently retrieved once the heating operation has been completed.

The bottom end of the heater, also referred to herein as the leading end because it is the end of the heater that goes down the wellbore first, is provided with a cap 6.

The top and bottom caps 4, 6, which also tend to be formed from a steel alloy, can be connected to the tubular body 3 by way of a threaded relationship, a welded joint or a combination thereof. The skilled person will appreciate that other connection means may be employed without departing from the general scope of the present invention.

A ball bearing assembly 7, which comprises an array of ball bearings 8, is mounted on the tubular body 3 of the heater 1. Although not shown it is envisaged that the assembly 7 may be welded in position or retained in position by the action of retaining rings provided either side of the assembly 7.

The size of the ball bearings will to a certain extent be determined by the size zo of the heater tool.

The skilled person will appreciate that ball bearings can be made from a range of materials. With that said, it is envisaged that because the ball bearings are present throughout the heating operation they must be formed from a material with a melting point that is above the temperatures generated downhole by the heater.

In view of this, examples of preferred materials for the ball bearings include metal alloys such as Bronze alloys, Aluminium alloys, Copper alloys, Brass alloys, high strength steel and chrome steel. Aluminium Bronze is considered particularly suitable because ball bearings made from this material have good shock and wear resistance, whilst at the same time retaining high strength at temperatures of above 260°C (500T).

Alternatively, suitable polymers such as PTFE, glass filed PTFE, Teflon ®, polyimide, polyether ether ketone (PEEK), polyphenylene sulfide (PPS), nylon, acetal and polyester might also be employed. Polymers impregnated with graphite can also be used. The ball bearings may alternatively be formed from ceramic.

Although ball bearings are used in the shown embodiment, it is envisaged that other rotating bearings, such as needle bearings or barrel bearings, or even rollers could be employed without departing from the present invention provided they are capable of retaining their functionality despite being subjected to high temperatures.

It is envisaged that, whichever rotating bearings are used, they can be arranged in a variety of array configurations on the assembly 7. The bearings can be arranged in parallel to one another or in an off-set arrangement. As such they can be arranged in straight lines or in spiral patterns around the outside of the heater.

In those arrangements where the assembly is held in place by retaining rings, it is envisaged that the assembly may also be free to rotate or even pivot slightly about the outside of the tubular body 3. It is appreciated that this additional freedom of movement further reduces the build-up of friction between the heater 1 and the surrounding well tubing 2.

As will be appreciated from Figure 1, the elongate tubular body 3 of the heater 1 has become bent during the heating operation, which has resulted in the heater bowing towards the surrounding well tubing 2 at both the middle of the heater and at the trailing end thereof.

The presence of the ball bearing assembly 7 in the middle region of the heater 1 ensures that when the heater comes into contact with the well tubing 2, the ball zo bearing are able to roll along the inner surface of the well tubing rather than the heater's tubular body 3 simply scraping along it and creating drag.

Although the retrievable heater of the present invention may be provided with a single reduced friction region, it is considered beneficial to provide more than one reduced friction region on the heater. These reduced friction regions can be formed using the same low friction solution or using a range of alternative solutions In the heater 1 shown in Figure 1 two reduced friction regions are provided, one in the middle region of the heater (as described above) and one at the leading end of the heater.

The leading end of the heater shown in Figure 1 is provided with a ring component 9 that is formed from a suitable low friction material. It is envisaged that any material that has a lower co-efficient of friction that the main body of the heater (e.g. steel alloy) provides a beneficial effect.

However, preferably the ring component 9 is formed from a material such as Bronze alloys, Aluminium alloys, Copper alloys and Brass alloys. Once again, for the reasons noted above, Aluminium Bronze is considered particularly suitable.

Alternatively, suitable polymers such as PTFE, glass filed PTFE, Teflon (g), polyimide, polyether ether ketone (PEEK), polyphenylene sulfide (PPS), nylon, acetal and polyester might also be employed. Polymers impregnated with graphite can also be used.

The ring component 9 is trapped in position on the outside of the heater main tubular body 3 between the end cap 6, which presents a flange for the ring component 9 to abut against because it has a slightly greater diameter than the tubular body 3, and a retaining ring 10, which is secured to the tubular body by way of suitable connection means (such as a welded joint or axially oriented screw/bolts). It will be appreciated that because the ring component 9 projects beyond the tubular body 3, the end cap 6 and the retaining ring 10 its surface is first to come into contact with the inner surface of the surrounding well tubing 2.

As a result the trailing end of the heater 1 is more inclined to slide along the well tubing that would have otherwise been the case if it were the heater's steel end cap that made contact with the well tubing 2.

It is envisaged that as an alternative to using retaining rings to secure the low friction ring component on the outside of the heater's tubular body, the ring component may be seated within a groove or channel formed in the outer walls of the tubular body.

An alternative arrangement of a reduced friction region that is provided at the trailing end of a heater of the present invention is shown in Figure 2, which provides a view of the lower portion only of the heater 11.

The heater 11 is provided with a tubular body 12, which is again preferably made from a steel alloy such as low carbon steel. Although not shown it will be appreciated that the tubular body 12 retains a heat source (e.g. chemical heat source) within it central cavity.

In contrast to the reduced friction region shown at the leading end of the heater of Figure 1, the reduced friction region of the heater 12 shown in Figure 2 comprises two distinct low friction ring components 13 and 14.

In the preferred embodiment shown the lower ring component 13 is formed from a polymer material, such as Teflon 0, and the upper ring component 14 is formed from a metal alloy, such as Bronze.

The ring components are trapped in position on the tubular body 12 by the clamping action of the bottom end cap 15 and the retaining ring 16, which is held in position on the tubular body by the action of one or more screws 17 that engage the tubular body 12 of the heater.

As before, the bottom end cap can be connected to the tubular body by way of a threaded connection, a welded joint or a combination of both.

Both ring components are configured to project beyond the other components of the heater (e.g. the tubular body 12, end cap 15 and retaining ring 16) so that they make contact with a surrounding well structure first.

However, in the shown embodiment, the polymer ring component 13 is configured to project beyond the metal ring component 14. In this way the metal ring 14 serves as a back-up in the event that the polymer ring 13 is worn down by its interaction with the surround well structures.

In the embodiments of the present invention shown in Figures 1 and 2 the reduced friction regions are provided on the tubular body of the heater. However it is envisaged that, within the general scope of the present invention, the reduced friction regions may be added to the heater in the form of an additional component, such as a sub, that can be secured to the top (trailing) or bottom (leading) ends of the retrievable downhole heater to achieve the same benefits.

Figure 3 shows a preferred embodiment of a heater that has a sub, in the form of a skirt portion, which provides the heater with a reduced friction region. As detailed in W02011/151271, a skirt portion can be provided on the leading end of heaters that are used in the deployment of alloy plugs and seals within oil/gas wells. The skirt portion 24 comprises a tubular body that is preferably formed from a suitable steel alloy (e.g. low carbon steel) with a plurality of holes therein (not shown) and is also open-ended at its leading end. During downhole operations, well fluids can enter into the interior of the skirt portion and apply a cooling effect. This in turn helps to cool the temperature of the molten alloy that is formed by the action of the heater on a reservoir of alloy within the target region and in so doing cause the alloy to solidify more quickly.

The skirt portion 24 is provided with a threaded portion 25 on its closed, top end. The tubular body 21 of the heater 20 is provided with a corresponding threaded portion 23 at its leading end so as to securely engage the skirt portion 24 and form the complete heating assembly.

It will be appreciated from Figure 3 that, although not shown, the internal cavity 22 of the heater tubular body 21 (of which only a portion is shown) would house the heater's heat source (e.g. chemical or electrical heat source).

A low friction ring component 26 is provided on the threaded top end 25 of the skirt portion 24 and it retained in position when the tubular body 21 and the skirt portion are screwed together.

The low friction ring component 26 can be made for any suitable low friction material, and in particular one of the materials identified in relation to the ring components of the heaters shown in Figures 1 and 2.

Although only a single ring component is shown in Figure 3, it is envisaged that multiple low friction ring components (possibly made from different materials) can be clamped between the tubular body 21 and the skirt portion. Also, although not essential, if the ring is made from a suitable material it too may be provided with a screw thread to enable it to engage with the threaded top end of the skirt portion 24.

Figure 4 shows an alternative arrangement of a sub-mounted reduced friction region, wherein, rather than employing a low friction ring component, the skirt portion 34 is provided with rotating bearings 36 of a type similar to those discussed above.

The rotating bearing 36 are shown as being off-set from one another, however this is not necessarily always the case.

As in Figure 3, the skirt portion 34 is provided with a threaded top end 35 that is received within a corresponding screw thread 33 on the leading end of the heating tubular body 31.

Although not shown in the Figures it is envisaged that the heater of the present invention may also be provided with one or more reduced friction regions that are formed by the presence of a low friction material coating that is applied to the outer surfaces of the heater.

It is envisaged that the low friction coating can be applied directly to the tubular body and end caps of the heater either by painting it on or by way of a spray coating.

In some instances the reliability of the low friction coating may be further improved by pre-treating the outer surface of the heater, for example by roughening up the outer surface using suitable approaches such as grinding or sand blasting, before applying the low friction coating.

It is appreciated that low friction coatings can be particularly useful when the reduced friction region of the heater needs to extend over large portions (and perhaps even all) of the heater, not least due to the cost savings when compared with other approaches of providing a reduced friction region (i.e. rotating bearings, low friction rings components).

It is envisaged that low friction coatings can be used in combination with other forms of reduced friction regions to afford the retrievable downhole heater the maximum low friction characteristics.

Figures 1 to 4 all show examples of downhole heaters that are in accordance with the present invention. However, it is appreciated that the heaters of the present invention may usefully be employed in combination with suitable alloys, such as bismuth based alloys, low melt alloys and suitable eutectic alloys, in order to provide downhole well plugging or sealing tools that can be operated within oil/gas wells, for example.

In such tools, it is envisaged that the alloy can be delivered downhole with the heater in the same run. To this end, alloy can be either mounted on the outside of the tubular body of the heater or retained as alloy beads/shot within an in-line dump bailer that is positioned above the heater.

Once downhole, the tool can be operated either to melt the alloy directly off the heater tubular body or once the alloy has been deployed from the dump bailer.

Either way the heat generated by the heater caused the alloy to melt and flow before it cools and re-solidifies to form a metal on metal seal within the target region of the oil/gas well.

Once the alloy has been melted the heater can be retrieved using the same delivery means that we used to deliver the tool downhole. It is at this time that the reduced friction regions play their part in ensuring the heater can be readily retrieved.

In those embodiments where the alloy is mounted on the outside of the tubular body it is envisaged that the alloy may or may not overlap with the reduced friction regions of the heater.

Claims

Claims 1. A retrievable downhole heater comprising an elongate tubular body with a heat source housed therein, said tubular body being formed from a material that does not melt during the downhole operation of the heater; and wherein the heater is provided with at least one reduced friction region having a lower co-efficient of friction than the material from which the tubular body is formed and whereby each reduced friction region is configured to retain the lower coefficient of friction following the downhole operation of the heater.
2. The heater of claim 1, wherein at least one reduced friction region on the heater comprises one or more rotatable bearings that project beyond the outer surface of the elongate tubular body.
3. The heater of claim 2, comprising one or more rotatable bearings projecting from the outer surface of the elongate tubular body.
4. The heater of claim 2, wherein one or more rotatable bearings are mounted on a sub that is attached to the leading and/or trailing end of the elongate tubular body.
5. The heater of claim 4, wherein the sub is provided in the form of an open-ended skirt portion that extends from the leading end of the heater.
6. The heater of any one of the preceding claims, wherein at least one reduced friction region on the heater comprises one or more contact surfaces formed from a material that has a lower co-efficient of friction than the material from which the tubular body is formed and whereby each contact surface projects outwards beyond the outer surface of the elongate tubular body.
7. The heater of claim 6, wherein at least one contact surface is provided by a low friction component that is mounted on the outer surface of the elongate tubular body.
8. The heater of claim 6 or 7, wherein at least one contact surface is provided by a low friction component that is mounted on a sub that is attached to the leading and/or trailing end of the elongate tubular body
9. The heater of claim 7 or 8, wherein said component is annular-shaped.
10. The heater of any of the preceding claims, wherein at least one reduced friction region on the heater comprises a friction reducing coating applied to the outer surface of the elongate tubular body.
11. The heater of any one of claims 1 to 10, wherein said reduced friction region is provided at: a) a leading end of the heater, which, in use, is deployed downhole first; b) a trailing end of the heater; and/or c) a location on the elongate tubular body between the leading and trailing ends of the heater.
12. The heater of any one of claims 1 to 11, wherein said reduced friction region extends over the entire outer surface of the elongate tubular body.
13. The heater of any of the preceding claims, wherein the heat source comprises a chemical reaction heat source.
14. A downhole well plugging or sealing tool, said tool comprising: a heater in accordance with any of the preceding claims; and an alloy, which can be melted downhole by the heater in order to form a plug or seal within a target region of a downhole environment.
15. The tool of claim 14, wherein the tool further comprises a dump bailer and the pre-melted alloy is delivered downhole within the dump bailer in the form of alloy beads or shot.
16. The tool of claim 14, wherein the pre-melted alloy is mounted on the outer surface of the elongate tubular body.
17. The tool of claim 16, wherein the pre-melted alloy covers said at least one reduced friction region.
18. The tool of claim 16, wherein the pre-melted alloy and said at least one reduced friction region are arranged on the outer surface of the elongate tubular body so that they do not overlap with one another.
19. A method of manufacturing a retrievable downhole heater, said method comprising: providing an elongate tubular body formed from a material that does not melt during the operation of the heater; and installing a heat source within the tubular body and providing at least one reduced friction region on heater; wherein said reduced friction region has a lower co-efficient of friction than the material from which the tubular body is formed and is configured to retain said lower co-efficient of friction following the downhole operation of the heater.
20. The method of claim 19, wherein the heat source installed within the tubular body of the heater is a chemical reaction heat source.
21. The method of claim 19 or 20, wherein at least one reduced friction region is provided by mounting one or more rotatable bearings on the heater such that said bearings project beyond the outer surface of the elongate tubular body.
22. The method of any one of claims 19, 20 or 21, wherein at least one reduced friction region is provided by applying a coating of a friction reducing agent to the outer surface of the elongate tubular body.
23. The method of any one of claims 19 to 22, wherein at least one reduced friction region is provided by mounting a component formed from a material with a lower co-efficient of friction than the material used to form the elongate tubular body on the heater such that a contact surface of the component sits proud of the outer surface of the tubular body.
24. The method of claim 23, wherein the component is annular-shaped.
25. The method of any one of claims 19 to 24, wherein said reduced friction region is provided at: a) a leading end of the heater, which, in use, is deployed downhole first; b) a trailing end of the heater; and/or c) a location on the elongate tubular body between the leading and trailing ends of the heater. 10
26. The method of any one of claims 19 to 25, wherein said reduced friction region extends over the entire outer surface of the elongate tubular body.