WO2024018237A1

WO2024018237A1 - Modular downhole heaters for use with plugging and sealing alloys

Info

Publication number: WO2024018237A1
Application number: PCT/GB2023/051942
Authority: WO
Inventors: Paul Carragher
Original assignee: Bisn Tec Ltd
Priority date: 2022-07-22
Filing date: 2023-07-24
Publication date: 2024-01-25

Abstract

The present invention provides a modular downhole heating tool assembly that can be used in downhole operations in oil/gas wells, such as the deployment and removal of alloy seals/plugs. The present invention also provides self-contained heating modules that are configured to combine in various combinations to form modular downhole heating tool assemblies that suit the operational requirements of a downhole operation. The heating modules are configured to combine directly with other heating modules via the engagement of module engagement means, such as complementary threads or twist and lock components, or indirectly via intermediate components with equivalent module engagement means.

Description

MODULAR DOWNHOLE HEATERS FOR USE WITH PLUGGING AND SEALING ALLOYS

Field of the Invention

The present invention relates to using modular, exothermic, chemical heaters to melt metal alloy which will solidify and form a downhole plug or seal in oil and gas wells.

Background of the Invention

Temporarily or permanently plugging oil and gas wells is a necessary task that can be accomplished with downhole tools, cement, and/or alternative materials such as resins, thermoset plastics or low melting temperature metal alloys.

Since the early days of the oil field, squeezing cement through perforations has been an acceptable method to seal annular leaks between tubing strings, between the tubing and the casing or between the casing and the borehole. Unfortunately for a variety of reasons cement solutions do not always hold pressure forever. The same results have been experienced with columns of cement set inside the tubing.

Newer methods and techniques are being used to temporarily or permanently plug a well utilizing downhole heaters which melt metal alloys that cool, solidify and form plugs or annular seals inside the tubing or casing. These systems have also been used to form annular seals placed between the tubing and the casing or between the casing and the borehole. Early patents and patent applications can be found describing many of these techniques by inventors such as: Marcel Schlumberger, Walter Wells, Carroll Irons, Walter Evans, Gerard Bosma, Louis Wardlaw, Homer Spencer, Robert Eden, Ronald Bass, Harold Vinegar, Manuel Gonzalez, and William Lowry.

It has been proven that eutectic alloys or bismuth-based alloys are quite effective at forming plugs and seals in oil and gas wells by being run in the well as small diameter beads or cast on the outer diameter (OD) of a heater tube, melted at the target depth, and allowed to cool to a solid state.

These alloy plugs can be set in a variety of different geometries such as inside the casing, inside the tubing, inside the open hole, or even in the annular space between tubing strings. Setting inside the open hole is often referred to as a “rock to rock” seal and is commonly used to plug and abandon the well after the tubing and casing have been removed.

Summary of the Invention

The present invention provides a heating module for a modular downhole heating tool assembly in accordance with claim 1. In particular, the present invention provides a heating module for a modular downhole heating tool, said module comprising: a metal tubular main body with a first open end and a second open end, at least one of which is provided with module engagement means that is configured to connect with the module engagement means of another heating module such that, in use, multiple heating modules can be combined to form the modular downhole heating tool; and a seal provided adjacent to each of the open ends such that the main body and said seals combine to define a cavity within which an exothermic chemical reaction heat source is housed.

The invention utilizes a way to make modular heaters which contain module engagement means, which may take the form of threads, between the sections (i.e. , modules) for the ease of transport, handling, and assembly. Modular designs will also help the business with lower inventory costs, higher component part number use, fewer assemblies, and so forth.

Preferably said module engagement means may comprise a threaded region located in the proximity of the first and/or second open end of the tubular main body. In the way the heating modules can be quickly and easily connected to other components (e.g., other heating modules, well deployment means/running tools, ignitions means/starting tool) to form a complete module downhole heating tool assembly.

Preferably the threads provided on the first and second open ends of the main body may be complementary to one another such that multiple heating modules can be screwed together to form the modular downhole heating tool.

For example, consider that a 30 ft downhole heating tool assembly can be manufactured in modular three parts, each of which is 10 feet long. These can be easily shipped anywhere by truck, boat, or airplane. If the design threads together it can be assembled at the wellsite utilizing a drilling rig, work over rig, crane or snubbing unit.

Job flexibility is a key attribute to drive a modular heater design. If last minute changes are recognized before the heater is run in the well which causes changes to the heater string, such as adding a pre-heat section, this can be accomplished by easily adding a heater module or two to the existing string prior to running the tools into the well to perform the plugging/sealing intervention.

Utilizing metal to metal, sealing, threads the heater can be designed in short sections (i.e., heating modules) and assembled to make downhole heating tool assemblies of varied lengths. Each module/section can have upper and lower threads, as well as at least one running tool and starter module.

Each heating module is pre-filled with the chemical compounds necessary for the exothermic reaction. Each end of the tubular main body is be sealed to keep the chemical compounds safe, dry, and secure insuring consistent results in the well bore.

Different sections/modules of the downhole heating tool assembly can have slightly different characteristics such as start time, burn speed, heat output. This will allow each tool assembly to better fit the type of task being performed. If a large amount of heat is needed for long periods of time such as for extremely large casing like 30-inch diameters which will be plugged, then the downhole heating tool assembly can be optimized for such a job. If the well bore is near the sea floor, extremely cool, then a pre-heater heater and a melt heater might be needed. The modular downhole heating tool assembly of the present invention can be optimized to support those criteria.

The chemical makeup of the heater contents required to provide heat to melt the alloys have been disclosed in the applicant’s earlier patent filings, examples of which include EP3179030 and WO2014/096857.

The exothermic chemicals are preferably a modified thermite mix. Successful field runs have proven the reaction will generate enough heat for over a long enough time period to safely and effectively melt the preferred metal alloys in the well. As disclosed in WO2017/203247, refractory lining inside the heater tube tempers the heat transfer from the heater to the alloy.

The tubular main body of the heating module of the present invention can be threaded at each end with machined features that might include metal to metal seals, elastomeric seals, non-elastomeric, seals, or a combination of any sealing means. Alternatively, the module engagement means may comprise a twist and lock component selected from either a plurality of outwardly projecting pins or a plurality of complementary slots configured to receive said pins. It will be appreciated that twist and lock components allow the formation of a bayonet-type connection between the heating module and other components (e.g., other heating modules, well deployment means/running tools, ignitions means/starting tool) to form a complete module downhole heating tool assembly. Further preferably, the complementary slots may comprise J-shaped slots.

Once the tubular main bodies are loaded with the chemicals that will produce the exothermic chemical reaction the ends of the tubular main bodies can be sealed.

These seals (also referred to herein as bulkheads) can be simple stamped sheet metal parts that are press fit into the internal diameter (ID) of the tubular main body or more elaborate machined components with elastomeric O-ring seals.

In view of this, preferably the heating module of the present invention may have at least one seal that comprises a metal sheet secured to the main body to occlude the cavity. Preferably said metal sheet may take the form of a freeze plug or a core plug.

Additionally or alternatively, the heating module of the present invention may preferably have at least one seal that comprises a metal ring with at least one bore that is occluded by a metal sheet, said metal ring having an annular channel running around the exterior thereof within which an O-ring sits.

If simple, thin, sheet metal parts (e.g., freeze/core plugs) are used they can be produced from materials that will be melted or consumed during the exothermic chemical reaction. If the bulkhead seals contain machined parts, they may contain one or more holes for the exothermic chemical reaction to move from one tubular main body to another. These holes may then be covered by a sheet metal part (e.g., a smaller diameter freeze/core plug).

As such, preferably the melting point of said metal sheet is lower than the temperature generated by the exothermic chemical reaction heat source such that said metal sheet is melted by the heat generated during the exothermic chemical reaction.

Another option is to glue, braze or weld the bulkhead seals to the heating modules tubular main body to seal and protect the chemical compounds contained inside. Another option is to arrange the spacing of the seals/bulkheads with very little air gap so the flame front devours the sheet metal plugs and moves to the next volume of chemicals in the adjacent heating module without much, if any, disruption to speed or heat output. To this end, preferably the exothermic chemical reaction heat source may fill the entire cavity formed by the main body and the seals.

Preferably at least one seal may be secured within the cavity at a location spaced away from the ends of the main body. Further preferably the location of said at least one seal ensures the creation of gap between the seals of adjacent heating modules when the modules are connected together.

In this way an open area can be created between the seals of two adjacent heating modules that are screwed together. This open area can be left empty or filled with the same chemical compounds needed to keep the exothermic reaction moving from one heating module to another.

Alternate embodiments might include higher or lower heat output chemical blends being inserted in these areas at the time the module engagement means of adjacent heating modules are brought together (e.g., threads are torqued up) and the heating modules are connected together.

It is envisaged that higher energy blends might be needed if, rather than connecting (e.g., screwing) heating modules together directly, couplings are provided between adjacent heating modules tubes to increase the heat output in the thickwalled areas.

It is envisaged that machined rings with 0-rings or other types of seals suitable for downhole oil and gas use might be ideal solutions to serve as secondary seals to the metal to metal, premium type, casing or tubing threads used on the tubular main bodies of the heating modules. Many, premium, high integrity, metal to metal seal threads are available from vendors such as VAM®, Tenaris, Hunting and others.

It is also envisaged that there are many thread styles, geometries, materials, thread compounds, and torque specs to choose from. Thread styles that could be suitably employed in the heating modules of the present invention include: tool joint threads, semi flush, near flush, and threaded and coupled, (see references from manufacturers and well as API 5CT a bulletin published by the American Petroleum Institute).

Elastomeric plugs with interference fits might be good choices for the design of the machined seals/bulkheads, especially if there is more than one hole in each machined ring. Some fluoropolymers are used with specialized thermite reactions. Reference the many scientific papers published concerning PTFE, magnesium, aluminium and thermite.

It is envisaged that the heating modules might contain areas that are melted allowing heating chemical and sealing alloys to exit the tubular main body of the heater and enter the well bore coming into contact with tubing, casing, or the bore hole rock itself.

As such, preferably, the tubular main body may comprise one or more weakened areas that are configured to melt before the rest of the main body so as to create exit ports through with the contents of the cavity can escape the heating module.

The low melt temperature alloys have been used for a couple of decades and are well known. These may include lead, tin, bismuth and many other metals. See references US20060144591 (GONZALEZ), US2381929 (SCHLUMBERGER), and US6828531 (WARDLAW).

The present invention also provides a modular downhole heating tool assembly comprising a plurality of heating modules according to the present invention; and at least one starter module configured to trigger the exothermic reaction of the chemical reaction heat source housed within the cavity of said heating module, wherein each starter module comprises module engagement means that engage with the module engagement means of one of said heating modules.

It is envisaged that the assembly is provided with a least one starter module. Having modular heaters allows any combination of heater starting. The exothermic chemical reactions are normally started by a running tool I starter module. Since there are various ways to communicate with a running tool I starter module such as electrical, radio wave, pressure change or timer delay or other methods it is possible to start the heaters in any order: bottom to top, top to bottom, all at once or another pattern.

Preferably at least some of the plurality of heating modules may be connected together directly in the formation of said modular downhole heating tool.

Additionally or alternatively, the assembly further comprises at least one coupling that is configured to engage with the module engagement means of adjacent heating modules in the formation of said modular downhole heating tool assembly. Preferably the positioning of the seals of adjacent heating modules is such that a gap is formed between said seals within the modular downhole heating tool assembly. Further preferably, an exothermic chemical reaction heat source is housed within the gap.

Preferably a eutectic, bismuth based and/or low melting point alloy that melts at temperatures of 300°C and below may be provided on the outside of the tubular main bodies of the combined heating modules. It will be appreciated that mounting the alloy on the outside of the heating tool assembly facilitates the formation of downhole alloy seals/plugs in a single downhole deployment.

Further preferably the alloy may be provided on the outside of the tubular main bodies in the form of a plurality of annular rings. In addition, preferably the alloy annular rings may interlock with one another. This helps to retain the alloy rings in position once they have been stacked onto the heating tool assembly. Further, a plurality of locking rings may preferably be securely located on the outside of the tubular main bodies so that the alloy annular rings are retained in position on the outside of heating tool.

Preferably a gas-tight seal may be provided at each junction between adjacent heating modules and also between heating modules and couplings. In this way the chemical reaction of the chemical heat source can be retained inside the heating tool assembly.

Further preferably the gas-tight seal may be is formed by an O-ring of a malleable material, such as copper, that is trapped between the open ends of adjacent heating modules when they are connected together. Alternatively the gas-tight seal may be achieved by employing a tapered thread for the module engagement means of each heating module.

Brief Description of the Drawings

The present invention will now be described in more detail with reference to the preferred embodiments shown in the drawings, wherein:

Figure 1 shows a short modular downhole heating tool assembly in accordance with the present invention;

Figure 2 shows a preferred embodiment of the modular downhole heating tool assembly of the present invention; Figure 3 shows a further preferred embodiment of the modular downhole heating tool assembly of the present invention;

Figure 4 shows another preferred embodiment of the modular downhole heating tool assembly of the present invention;

Figure 5 shows a preferred embodiment of a seal/bulkhead employed in the heating module of the present invention; and

Figure 6 shows another preferred embodiment of the modular downhole heating tool assembly of the present invention with alloy mounted.

Detailed Description of the Preferred Embodiments

The individual heating modules of the present invention and the modular downhole heating tool assemblies that they form will now be described in more detail with reference to the Figures.

Figure 1 shows a short modular downhole heating tool assembly which is formed from two heating modules, an upper heating module and a lower heating module.

The upper heating module includes a top end cap (1 ) mounted adjacent to the upper opening of a metal tubular main body (2), also referred to here as the top heater tube. A pin thread (2a) is provided on the tubular main body (2) at a location adjacent the lower opening.

The lower heating module includes a second metal tubular main body (3) with a box thread (3a) adjacent the opening at the top of metal tubular main body (also referred to herein as the lower heater tube) and a bottom end cap (4), which is mounted adjacent to the lower opening of the second metal tubular main body (3).

The end caps (1 ) and (4) serve to protect the ends of the heater modules during storage and transit.

Although not shown in, both modules have an internal cavity, that is defined by a pair of seals, within which a chemical reaction heat source material (e.g., a thermite blend) within each heating module. Although not shown, it should be appreciated that each heating module is provided with a pair of seals located adjacent the threaded ends of each module to ensure the thermite is securely retained within each heating module.

It is envisaged that the tubular main bodies can suitably be made from steel. However, the skilled person will appreciate that any metals/alloys that are typically employed in downhole heating tools can be suitably employed in the heating modules/assemblies of the present invention. The end caps can be made of any material that provides suitable protection to the threaded ends of the heating modules that would otherwise be exposed.

The pin thread (2a) of the upper heating module and the box thread (3a) of the lower heating module engage directly with one another to secure the two modules together and form the modular downhole heating tool assembly.

For the sake of clarity, only adjoining threaded regions (2a) and (3a) and the end caps (1 ) and (4) are shown in Figure 1. It should be appreciated that each heating module would have threaded regions at both ends thereof. Also, as noted above, each heating module would also have a pair of seals, that work together to define the internal cavity in which the chemical reaction heat source material of each heating module is housed. These arrangements will be better appreciated with reference to Figure 4.

Figure 2 shows a modular downhole heating tool assembly (A) that comprises three heating modules (6) connected in series. Unlike the assembly of Figure 1 , where the heating modules are connected directly to one another, the heating modules (6) in assembly (A) are indirectly connected together via intermediate components in the form of a running tool/starting tool (5). Each starting tool is configured to initiate the chemical reaction heat source material housed within the adjacent heating module. As noted above, using this arrangement of starting tools it is possible to start the heating modules in any order: bottom to top, top to bottom, all at once or another pattern.

Although not shown, it is envisaged that the heating modules (6) and the running tool/starting tools (5) may be provided with complementary threads so that they can be screwed together to form the complete assembly.

It is also envisaged that although threads represent a preferred form of module engagement means, other types of module engagement means may be employed to connect the heating modules and intermediate components together. One preferred alternative is a twist and lock connection system, which are also referred to a bayonet type connectors.

Twist and lock connection systems employ a plurality of pins and a plurality of complementary slots (e.g., J-shaped slots) that are provided on the ends of adjacent heating modules/intermediate components that are to be connected together. Although not shown, it is envisaged that the connections between adjacent heating modules and/or intermediate components are preferably gas-tight to ensure the chemical reaction of the chemical reaction heat source is retained with the tool assembly. The gas-tight seal can be achieved, for example, by employing a suitable tapered thread or by trapping and squeezing an O-ring of malleable metal (e.g. copper) between adjacent heating modules and/or intermediate components.

The assembly shown in Figure 2 is capped off by a heater end cap (4) mounted on the end heating module, which serves to protect the module engagement means located on the end of the heating tool assembly during transit.

It should be again appreciated that each heating module is provided with seals that define a cavity that houses the chemical reaction heat source material (e.g., thermite blend). However, for clarity the seals of each heating module are once again not visible in Figure 2.

Figure 3 shows an alternative preferred embodiment of a modular downhole heating tool assembly (B) which comprises four heating modules (6) serviced by a single one running tool/starting tool (5). As in the arrangement of Figure 2, the heating modules (6) are indirectly connected together in series. However, in the embodiment shown in Figure 3, adjacent heating modules are instead connected via couplings (7).

Once again the running tool/starting tool (5), the heating modules (6) and couplings (7) are screwed together using complementary threads to form the complete assembly. The assembly (B) is capped off by a heater end cap (4) mounted on the end heating module.

As noted above, it is envisaged that the couplings (7) may also be provided with an amount of a suitable chemical reaction heat source material to enable the heating reaction to be transmitted from one heating module to the next via said coupling (7). In this way, a single starter tool (5) can be used to initiate all of the heating modules in the assembly.

Although Figures 2 and 3 show assembly comprising three and four heating modules (6) respectively, it is envisaged that downhole heating assemblies of various lengths and different heating characteristics can be achieved by varying the number of heating modules and the specific nature of the chemical reaction heat source material housed within the respective heating modules. The provision of a sealed cavity in each heating module enables the heating characteristics of each module to be controlled by adjusting the specific nature of the chemical reaction heat source material in that module.

It should be appreciated that although the Figures show the heating modules and the other components (e.g., running tool (5)) as having different diameters (the running tools (5) are shown with a diameter that is less than that of the heating modules (6). One skilled in the art would note that these diameters might also be the same. Indeed, this might in be an optimized design as it would enable the same thread connections with metal to metal seals to be used in all the connections of the assembly.

The internal seals of the heating modules of the present invention will now be described in more detail with reference to Figures 4 and 5.

Figure 4 shows the details inside a mid-section of a modular downhole heating assembly that shows a pair of heating modules (8) and (9) (also referred to as heater tubes) connected directly to one another using module engagement means in the form of complementary screw threads. It is envisaged that in one alternate geometry that falls within the scope of the present invention each of the heating modules could be indirectly connected via intermediate components such as running tools/starter tools (see Figure 2) or couplings (see Figure 3).

Figure 4 shows the mid-section of an assembly (C) where the pin thread (21 ) and box thread (22) of adjacent heating modules (8) and (9) are mated together.

Each heating module (8/9) is provided with seals (10) and (11 ), also referred to herein as bulkhead seals, which combine with the metal tubular main body of their respective heating module to define a cavity within which the chemical heat source (19) is housed. It should be noted that the thickness of the seals (10) and (11 ) shown in Figure 4 has been exaggerated so that they can be readily identified. In practise, the seals would be considerably thinner that the tubular main bodies of the heating modules.

During the manufacture of each heating module, the cavity in the tubular main body is filed with an exothermic chemical compound, like a modified thermite (19) and then sealed with the seals/bulkhead seals (10,11 ). Preferably the cavity is filled entirely with the chemical to minimise the presence of air gaps that might prevent the transmission of the chemical reaction through the heating module.

Preferably the seals/bulkhead (10,11 ) are stamped sheet metal parts that are press fit inside the tubular main body of the heating module (8, 9), such as freeze plugs or core plugs. Alternatively the seals could be steel rings welded, brazed, glued, or press fit in place (see Figure 5).

It is envisaged that the seals used within a heating module could be the same at both ends of the tubular main body. Alternatively, different seal types could be used at the pin end (21 ) and box end (20) of the tubular main body.

Each seal (10, 11 ) is located adjacent to an opening of the tubular main body to maximise the chemical storage capacity of each heating module whilst at the same time accommodating the threaded engagement of the heating module with other heating modules or intermediate components.

Preferably the seals are positioned so as to minimise the gap between adjacent modules/components and thereby ensure progression of heat through the assembly.

Whilst the gap (13) could be minimized and empty, it is appreciated that gap itself could be filled with a can, bag, or other container filled with chemicals for the exothermic heater. This volume may also contain the chemical compound for the exothermic heater and the low melt temperature alloy.

As noted above, although the seals used to retain the chemical reaction heat source material can take the form of simple stamped sheet metal (e.g., a freeze plug of core plug), it is envisaged that the seals/bulkhead seals could be more sophisticated.

Figure 5 shows a cross section of an alternative design for bulkhead seals (10a, 11 a). The seal (10a, 11 a) comprises a machined metal ring (16), an O-ring (15), a bore (17) that is sealed by a stamped sheet metal plug (18). This seal (10a, 11a) could be welded or torch brazed inside the end of the heater tube. It could replace item (10) and/or (11 ) in Figure 4.

As will be appreciated, the O-ring, which is preferably formed from an elastomer, will enable the seal (10a, 11 a) to be pushed into position within the internal diameter of the tubular main body of a heating module. Once in position, the O-ring will help retain the seal in place such that the chemical reaction heat source material is securely housed in its respective heating module.

As noted above, preferably the stamped sheet metal (10, 11 , 18) is configured to be melted/consumed during the operation of the heating module. In this way the heat and the molten chemical reaction heat source material can pass from one heating module to its neighbour. In the case of the seal shown in Figure 5, it will be appreciated that once the stamped sheet metal has been destroyed the molten contents of the heating module can pass through the bore (17).

Figure 6 shows the mid-section of an assembly (D) in which two heating modules (30) and (31 ) according to a further preferred embodiment of the present invention are about to be connected together.

As with the embodiments shown in Figures 2 and 3, although the internal seals, cavity and chemical reaction heat source are not visible it should be appreciated that they would be present within the tubular main bodies of the respective heating modules (30) and (31 ) shown in Figure 6.

In contrast to the heating modules shown in Figure 4, the heating modules shown in Figure 6 have module engagement means that take the form of a twist and lock connection system (i.e. , bayonet type connection). Each heating module (30) and (31 ) is provided with a twist and lock component adjacent each open end of the tubular main bodies.

The first open end of heating module (30) is provided with a pair of outwardly projecting pins (32). It is envisaged that more than two pins could also be employed in the module engagement means. The second open end of the heating module (31 ) is provided with a pair of corresponding J-slots (33) into which the pins (32) of an adjacent heating module can be received and then twisted into place.

Each of the heating modules (30) and (31 ) is provided with a plurality of annular alloy rings (34) stacked on their outer surface. Although the rings (34) are shown spaced apart it is envisaged that when the heating modules are connected together, the alloy rings (34) may stack together to provide a continuous layer of alloy on the outside of the complete heating tool assembly.

Although not shown, it is envisaged that the lowermost and uppermost heating modules in the constructed heating tool assembly may be provided with locking end rings that prevent the annular alloy rings from sliding off the assembly.

In order to achieve a gas-tight seal between the adjacent heating modules (30) and (31 ), a malleable metal O-ring (35), preferable made of copper, is trapped and squeezed between the two modules.

Clauses

The present invention will be described with reference to the following clauses:

1 . A modular heater used to melt low temperature material to be used a plugs and I or annular seals in well bores.

2. A modular heater can be made of one or more tubes which are sealed at or near the connections.

3. The modular heater can be pre-loaded with chemicals which will create the exothermic chemical reaction.

4. The chemical compounds inside the heater tubes can be sealed with bulkhead seals.

5. The bulkhead seals can be press fit in place.

6. The bulkhead seals can be made of elastomers, non elastomers, or metal.

7. The bulkhead seals can be made of several component parts.

8. The heater assembly components might be sealed with metal to metal sealing threads

Claims

1 . A heating module for a modular downhole heating tool, said module comprising: a metal tubular main body with a first open end and a second open end, at least one of which is provided with module engagement means that is configured to connect with the module engagement means of another heating module such that, in use, multiple heating modules can be combined to form the modular downhole heating tool; and a seal provided adjacent to each of the open ends such that the main body and said seals combine to define a cavity within which an exothermic chemical reaction heat source is housed.

2. The heating module of claim 1 , wherein said module engagement means comprise a threaded region located in the proximity of the first and/or second open end of the tubular main body.

3. The heating module of claim 2, wherein the threads provided on the first and second open ends of the main body are complementary to one another such that multiple heating modules can be screwed together to form the modular downhole heating tool.

4. The heating module of claim 1 , wherein said module engagement means comprise a twist and lock component selected from either a plurality of outwardly projecting pins or a plurality of complementary slots configured to receive said pins.

5. The heating module of claim 4, wherein the plurality of outwardly projecting pins is provided in the proximity of the first open end and the plurality of complementary slots is provided in the proximity of the second open end such that multiple heating modules can be twisted and locked together to form the modular downhole heating tool.

6. The heating module of any one of the preceding claims, having at least one seal that comprises a metal sheet secured to the main body to occlude the cavity.

7. The heating module of any one of claims 1 to 5, having at least one seal that comprises a metal ring with at least one bore that is occluded by a metal sheet, said metal ring having an annular channel running around the exterior thereof within which an O-ring sits.

8. The heating module of claim 6 or 7, wherein the melting point of said metal sheet is lower than the temperature generated by the exothermic chemical reaction heat source such that said metal sheet is melted by the heat generated during the exothermic chemical reaction.

9. The heating module of any one of the preceding claims, wherein the exothermic chemical reaction heat source fills the entire cavity formed by the main body and the seals.

10. The heating module of any one of the preceding claims, wherein at least one seal is secured within the cavity at a location spaced away from the ends of the main body.

11 . The heating module of claim 10, wherein the location of said at least one seal ensures the creation of gap between the seals of adjacent heating modules when the modules are connected together.

12. The heating module of any one of the preceding claims, wherein the tubular main body comprises one or more weakened areas that are configured to melt before the rest of the main body so as to create exit ports through with the contents of the cavity can escape the heating module.

13. A modular downhole heating tool comprising a plurality of heating modules that are in accordance with any one of claims 1 to 12; and at least one starter module configured to trigger the exothermic reaction of the chemical reaction heat source housed within the cavity of said heating module, wherein each starter module comprises module engagement means that connect to the module engagement means of one of said heating modules.

14. The modular downhole heating tool of claim 13, wherein at least some of the plurality of heating modules are connected together directly in the formation of said modular downhole heating tool.

15. The modular downhole heating tool of claim 13 or 14, further comprising at least one coupling that is configured to engage with the module engagement means of adjacent heating modules in the formation of in the formation of said modular downhole heating tool.

16. The modular downhole heating tool of claim 13, 14, or 15, wherein the positioning of the seals of adjacent heating modules is such that a gap is formed between said seals within the module downhole heating tool.

17. The modular downhole heating tool of claim 16, wherein an exothermic chemical reaction heat source is housed within the gap.

18. The modular downhole heating tool of any one of claims 13 to 17, wherein a eutectic, bismuth based and/or low melting point alloy that melts at temperatures of 300°C and below is provided on the outside of the tubular main bodies of the combined heating modules.

19. The modular downhole heating tool of claim 18, wherein the alloy is provided on the outside of the tubular main bodies in the form of a plurality of annular rings.

20. The modular downhole heating tool of claim 19, wherein the alloy annular rings interlock with one another.

21 . The modular downhole heating tool of claim 19 or 20, further comprising a plurality of locking rings that are securely located on the outside of the tubular main bodies so that the alloy annular rings are retained in position on the outside of heating tool.

22. The modular downhole heating tool of any one of claims 13 to 21 , wherein a gas-tight seal is provided at each junction between adjacent heating modules and also between heating modules and couplings.

23. The modular downhole heating tool of claim 22, wherein the gas-tight seal is formed by an O-ring of a malleable material, such as copper, that is trapped between the open ends of adjacent heating modules when they are connected together.

24. The modular downhole heating tool of claim 22, wherein the gas-tight seal is achieved by employing a tapered thread for the module engagement means of each heating module.