WO2022194656A1

WO2022194656A1 - Device and method for forming a permanent barrier in a well

Info

Publication number: WO2022194656A1
Application number: PCT/EP2022/056117
Authority: WO
Inventors: Stian TØNDEL; RUØ Ingrid LOTHE
Original assignee: Interwell P&A As
Priority date: 2021-03-19
Filing date: 2022-03-10
Publication date: 2022-09-22
Also published as: NO20210353A1

Abstract

The present invention relates to a well tool device (10) for forming a permanent barrier in a well (WE). The well tool device (10) comprises a pyrotechnic mixture (40) comprising a first constituent and a second constituent, wherein the first constituent is a metal oxide and the second constituent is a metal. An ignition device (50) is provided within or adjacent to the pyrotechnic mixture (40). The pyrotechnic mixture (40) is forming a self-supported pyrotechnic structure (20).

Description

DEVICE AND METHOD FOR FORMING A PERMANENT BARRIER IN A WELL

FIELD OF THE INVENTION

The present invention relates to a well tool device for forming a permanent barrier in a well. The present invention also relates to a method for manufacturing of a well tool device for forming a permanent barrier in a well.

BACKGROUND OF THE INVENTION

Plugging and abandonment operations, often referred to as P&A operations, are performed to permanently close oil and/or gas wells. Typically, this is performed by providing a permanent well barrier above the oil and/or gas producing rock types, typically in the cap rock in which the well has been drilled through.

There are several technical and regulatory requirements for such permanent well barriers, some of which are a) impermeability of oil and/or gas through the permanent well barrier, b) long term integrity, c) non shrinking of the permanent well barrier, d) ductility (non brittle) - the permanent well barrier must be able to withstand mechanical loads or impact, e) resistance to different chemicals/ substances (H2S, C02 and hydrocarbons) and f) wetting - to ensure bonding to steel.

In WO2013/135583 (Interwell P&A AS), it is disclosed a method for performing a P&A operation wherein a first step, it was provided an amount of a heat generating mixture (for example thermite) at a desired location in the well and thereafter to ignite the heat generating mixture to start a heat generation process. It is also disclosed a tool for transporting the heat generating mixture into the well before ignition. Such a heat generating mixture may also be referred to as a pyrotechnic mixture. In short, the above prior art will be described with reference to fig. la and lb. In fig. la, a well WE is shown to be provided through a section of a cap rock CR. The inner surface of a well bore WB is provided by an inner casing IC, where cement CM is provided in the annulus between the inner casing IC and the cap rock CR. It should be noted that some wells have several casings provided radially outside of each other, where cement or fluids are provided in the respective annuli(es). In fig. la, it is shown that a lower barrier LB has been provided in the well bore WB. A well tool 110 has been lowered into the well above the heat insulating material HI by means of a wireline 102. The well tool 110 comprises a housing 120 with a compartment 130 which contains a heat generating mixture 140 (for example thermite). An ignition device 150 is also provided in the compartment 130. The ignition device 150 starts the heat generating process of the heat generating mixture 140. The ignition device 150 may be time actuated or pressure actuated. Alternatively, the ignition device 150 may be actuated by means of a topside signal transferred via wire to the ignition device.

The result after the ignition is shown in fig. lb. Here it is shown that the elements of the well, i.e. inner casing IC, cement CE and cap rock CR have melted and thereafter hardened into one solid permanent well barrier PB containing constituents of rock, cement, steel and other elements being present in the well. Such other elements are the end product of the heat generation process, remains of the tool used to transport the heat generating mixture into the well, the ignition system etc.

This technology has been tested in test centers and in field trials, in order to verify that the permanent well barrier fulfills technical and regulatory requirements.

One object of the present invention is to provide a more efficient method for providing a permanent barrier in a well.

A further object of the present invention is to reduce the amount of impurities in a permanent barrier in a well.

SUMMARY OF THE INVENTION

The present invention relates to a well tool device for melting surroundings of a well at a location of the well tool device in the well, thereby forming a cap rock to cap rock permanent barrier in the well, wherein the well tool device comprises:

- a pyrotechnic mixture comprising a first constituent and a second constituent, wherein the first constituent is a metal oxide and the second constituent is a metal;

- an ignition device provided within or adjacent to the pyrotechnic mixture; wherein the pyrotechnic mixture is forming a self-supported structure.

The cap rock to cap rock permanent barrier is forming a barrier across the entire cross-sectional area of the well at the location of the well tool device in the well.

In one aspect, the well tool device is a cap-rock to cap-rock permanent barrier extending across the whole cross-section of a wellbore.

The pyrotechnic mixture will, when ignited by the ignition device, start a heat generating exothermic reduction-oxidation process. The heat generating process is configured to melt the surroundings of the well at the location of the well tool device in the well.

As used herein, the term “self-supported” means that the pyrotechnic structure can be handled as, and will behave as, one single mechanically assembled structure both topside and when lowered into a desired position in the well. Hence, the self- supported pyrotechnic structure may be handled as and will behave as prior art well tool devices.

In one aspect, only pyrotechnic mixture is forming the self-supported structure.

Alternatively, the pyrotechnic mixture and a binding agent are forming the self- supported structure.

In one aspect, the first constituent is bismuth oxide and second constituent is aluminum.

In one aspect, a sintered pyrotechnic mixture is forming the self-supported structure.

As used herein, the term “sintering” is defined as the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.

Alternatively, a baked pyrotechnic mixture is forming the self-supported structure. As used herein, the term “baking” is defined as the process of dehydration of the mixture, where particles become bound to each other due to surface friction. Also here, the mixture may be compacted, however not to the same extent as during sintering.

In one aspect, the pyrotechnic structure comprises voids. The voids may be a result of the sintering or dehydration/baking process.

Alternatively, there are no voids in the pyrotechnic structure. This can be achieved by melting and subsequent solidification of the pyrotechnic mixture. Alternatively, a void filling material may be used.

In one aspect, the well tool device comprises a wireline connection interface provided in an upper end of the pyrotechnic structure.

In one aspect, an outer surface of the pyrotechnic structure is provided with a coating.

In one aspect, the coating is a fluid ingress preventing coating. In one aspect, the coating is a non-metallic coating. In one aspect, the coating is an aluminum coating, a bismuth coating or a copper coating. In one aspect, the coating is a thermo-plastic coating or a shrink-wrap plastic coating. In one aspect, the coating is an epoxy or a thermosetting polymer.

In one aspect, the coating is a liquid, which is sprayed, painted or in other ways smeared onto the outer surface of the pyrotechnic structure and then subsequently the coating is allowed to harden and/or dry into a solidified coating. The pyrotechnic structure may also be dipped into a coating bath before hardening/drying. Alternatively, the coating may be a thin sheet of plastic or a foil rolled onto the outer surface of the pyrotechnic structure. Glue may or may not be used. In case of a shrink-wrap type of coating, the coating is subsequently exposed to heat to allow the coating to shrink.

In one aspect, the pyrotechnic mixture is forming a plurality of self-supported pyrotechnic substructures assembled into a self-supported pyrotechnic structure.

The present invention also relates to a method for manufacturing of a well tool device for melting surroundings of a well (WE) at a location of the well tool device in the well, thereby forming a cap rock to cap rock permanent barrier in the well, wherein the method comprises the steps of:

- mixing a first constituent and a second constituent into a pyrotechnic mixture, wherein the first constituent is a metal oxide and the second constituent is a metal;

- providing the pyrotechnic mixture into a casting mould;

- sintering the pyrotechnic mixture into a self-supported pyrotechnic structure in the casting mould;

- providing an ignition device within or adjacent to the pyrotechnic structure.

In one aspect, the step of sintering comprises:

- pressurizing the pyrotechnic mixture in the casting mold.

In one aspect, the step of sintering comprises:

- heating of the pyrotechnic mixture in the casting mold.

DETAILED DESCRIPTION

Embodiments of the present invention will be described in detail below with reference to the enclosed drawings, wherein:

Fig. la and lb illustrates a prior art well tool for performing a P&A operation;

Fig. 2 illustrates a first embodiment of a well tool device;

Fig. 3 illustrates a second embodiment of the well tool device;

Fig. 4 illustrates a third embodiment of the well tool device;

Fig. 5a, 5b, 5c and 5d illustrate steps of a method for manufacturing of a well tool device 1;

Fig. 6a illustrate a self-supporting disc of pyrotechnic mixture;

Fig. 6b illustrates a first embodiment of a well tool device assembled by means of a number of self-supporting discs;

Fig. 6c illustrates a second embodiment of a well tool device assembled by means of a number of self-supporting substructures;

Fig. 6d illustrates a carrying element secured to the self-supporting disc.

Well tool device It is now referred to fig. 2. Here, a well tool device 10 is shown, oriented along its vertical center axis I-I. The well tool device 10 comprises a pyrotechnic mixture 40 comprising a first constituent and a second constituent. The well tool device 10 further comprises an ignition device 50 provided enclosed within the pyrotechnic mixture 40. The well tool device 10 further comprises a wireline connection interface 12 provided in an upper end 20a of the pyrotechnic structure 20. The igniter 50 may start ignition of the pyrotechnic mixture by means of a signal received from a timer, a pressure sensor etc. The igniter 50 may alternatively start ignition of the pyrotechnic mixture by means of a signal received from the topside of the well, either via a signal wire or wirelessly.

The well tool device 10 may have a cylindrical shape, i.e. having a circular cross sectional shape perpendicular to the longitudinal axis I-I. It should be noted that the well tool device 10 alternately may have a triangular, square or even polygonal circular cross sectional shape.

The first constituent is a metal oxide. In the present embodiment, the metal oxide is bismuth oxide, also referred to as bismuth(III) oxide or Bi203. The second constituent is a metal. In the present embodiment, the metal is aluminum A1 or an aluminum alloy.

The pyrotechnic mixture 40 will, when ignited by the ignition device 50, start a heat generating exothermic reduction-oxidation process:

Bi203 + 2A1 -> A1203 + 2Bi + heat

This type of pyrotechnic mixture 40 is often referred to as thermite, and the heat generating reaction is often referred to as a thermite reaction.

The heat will melt the surroundings at the location of the well tool device, such as casing, cement, and possibly also parts of the formation radially outside of the casing and cement. It should be noted that there may be two or more casings outside of each other. The annulus between the casings may be fluid-filled, filled with cement, gravel or other materials. After cooling, a cap-rock to cap-rock permanent barrier extending across the whole cross-section of a wellbore may be the result. Hence, the result may be similar to the result shown in fig. lb.

In the present embodiment, the pyrotechnic mixture 40 is forming a self-supported pyrotechnic structure 20. When comparing fig. 2 with the prior art of fig. la, it is apparent that the pyrotechnic mixture 40 in fig. 2 is not transported into the well inside a compartment of a housing, as in fig. 1. The self-supported pyrotechnic structure 20 can be handled as, and will behave as, one single mechanically assembled structure both topside and when lowered into a desired position in the well. One advantage of this is that the housing, which is typically used in prior art to contain the pyrotechnic mixture, can be omitted. Typically, the housing is made of steel, and hence, impurities i.e. steel remnants in the permanent barrier is reduced. The prior art housing may also be made of aluminum, which may take part in the heat generation process. However, in some applications it is considered an advantage that the metal is mixed together with the metal oxide in a stochiometric ratio. An aluminum housing will add more metal than needed, and it may be difficult to predict if the metal oxide will react with the metal in the housing instead of the metal in the pyrotechnic mixture.

In addition, the heat generating process will be more efficient, as no heat is required to melt the housing.

In a first embodiment, only pyrotechnic mixture 40 is forming the self-supported pyrotechnic structure 20.

In a second embodiment, the pyrotechnic mixture 40 is mixed with a binding agent to form the self-supported pyrotechnic structure 20.

In the embodiment shown in fig. 3, it is shown that the pyrotechnic mixture 40 comprises voids 43, i.e. self-supported pyrotechnic structure 20 is a porous structure. These voids 43 are present due to the particulate matter of the metal oxide and the metal used in the pyrotechnic mixture 40. The voids 43 may be reduced in size and number by compacting the pyrotechnic mixture 40, by reducing the particle size of the metal oxide and the metal used in the pyrotechnic mixture 40 or by using a so-called void-filler. This will be described further in detail below.

In the embodiment shown in fig. 4, it is shown that an outer surface of the pyrotechnic structure 20 is provided with a coating 24. It should be noted that the properties of the coating are different from the properties of the prior art housing, as the purpose of the coating is not to provide structural integrity of the well tool device. One purpose of the coating 24 may be to prevent fluid ingress of the fluid in the well (and seawater in the sea in case of a subsea well) into the pyrotechnic mixture 40 and hence to form a barrier to keep the pyrotechnic mixture 40 on the inside of the coating 24 protected from interacting with the well-fluids. The coating 24 may be a precaution to reduce the risk of well fluids reducing the structural integrity of the self-supported pyrotechnic structure 20.

Another purpose of the coating 24 may be to protect the outer surface of the pyrotechnic structure 20 against mechanical damage (for example scratches and physical impacts) during transportation, topside handling, etc..

Coating examples In a first example, the coating 24 is a non-metallic coating, for example a thermo plastic coating, a shrink-wrap plastic coating, an epoxy, a thermosetting polymer , a wax, etc. These non-metallic coatings may prevent fluid ingress into the pyrotechnic mixture 40 forming the self-supported pyrotechnic structure 20. In a second example, the coating 24 is a metal or metal alloy, for example an aluminum coating, a bismuth coating , a copper coating or a tin coating. The aluminum may be a part of the heat generating process. A bismuth coating (not bismuth oxide) will melt due the heat, and will solidify as part of the barrier resulting from the heat generating process. A bismuth coating may contribute to a more impervious permanent barrier, as bismuth expands when it solidifies. A copper coating may contribute to the mechanical strength of the permanent barrier. A combination of bismuth and copper, referred to as a bismuth bronze or bismuth brass, also has desired properties with respect to corrosion resistance and mechanical strength for the permanent barrier.

Method for manufacturing

The method for manufacturing of a well tool device 10 will now be described with reference to fig. 5a, 5b, 5c. In fig. 5a, a casting mould 100 is shown schematically, comprising a lower part

100a and an upper part 100b. Initially, the metal oxide and the metal are mixed in a stochiometric ratio. As is known, the particle size of the metal oxide and the metal may influence of the speed, temperature and other factors of the heat generating process. Depending on the desired process, the metal oxide and the metal may be in powder form, in granulate form, or even larger particles, or any of the beforementioned in combination.

Fig. 5a shows the pyrotechnic mixture 40 in the casting mould. As an example, the pyrotechnic mixture 40 here has a porosity of ca 60%, the density is considered relative low (~3 g/cm3) and the tensile strength is low. The ignition device 50 and a signal conductor for transferring a signal to the ignition device 50 may also provided in the casting mould in fig. 5a.

In fig. 5b, the pyrotechnic mixture 40 is compressed in the casting mould by a predetermined compression force. During prototype testing, a force of 70 tons has been used with promising results. The porosity now becomes ca 30%, the density is is relatively higher (~6 g/cm3) and the tensile strength is considered higher. In fig. 5c, the casting mould 100 containing the pyrotechnic mixture 40 is heated to a temperature less than the melting temperature of the metal oxide and the melting temperature for the metal. In this example, the melting temperature of bismuth oxide is 817°C, while the melting temperature of aluminum is 660°C. Hence, in this example the pyrotechnic mixture 40 is heated to a temperature of below 800°C. The same or a different compression force may be applied during this heating step.

The porosity is now 15%, the density is relatively higher (~8 g/cm3) and the tensile strength is considered sufficiently high. Hence, for the purpose of being handled topside and for the purpose of being lowered into the well, with or without coating, the pyrotechnic mixture 40 is considered to have been formed into a self-supported pyrotechnic structure 20.

The above steps pressurizing the pyrotechnic mixture 40 in the casting mold and heating of the pyrotechnic mixture 40 in the casting mold are referred to as a sintering process. It should be noted that the sintering may comprise a step of using a relatively low compacting force, for example sufficient compacting force to shape the pyrotechnic mixture 40 into its desired force, and then use a relatively high temperature to form the solid mass of the material. Alternatively, the sintering may comprise a step of using a relatively high compacting force, and then use a relatively low temperature to form the solid mass of the material.

The self-supported pyrotechnic structure 20 may be further processed. In case the igniting device 50 was not inserted into the casting mould 100, it is possible to drill a hole into the structure 20 and insert the igniting device 50 into this hole. The wire to the igniting device 50 may be provided in a recess formed in the surface of the pyrotechnic structure 20. The igniting device 50 and wire may alternatively be fastened to the outer surface of the structure 20.

As described above, the pyrotechnic mixture 40 may comprises additives before the compression step and the heating step. It should be noted that the pyrotechnic mixture 40 may be considered to be self-supported by the above heating step alone, or by the above compressing step alone, in particular when a binding agent is used in the pyrotechnic mixture 40 together with the metal and metal oxide.

As an alternative to sintering, a baked pyrotechnic mixture 40 may form the self- supported pyrotechnic structure 20. As used herein, the term “baking” is defined as the process of dehydration of the mixture, where particles become bound to each other due to surface friction. Here, the metal and metal oxide are first mixed with a fluid and possibly also a binding agent and put in a casting mould. The mixture is then allowed to dry by heating the pyrotechnic mixture 40 to a temperature of 80 - 200°C. In the present embodiment, the drying process starts at 80°C and are gradually increased to 200°C to allow water to evaporate slowly, as this may reduce cracks in the self-supported pyrotechnic structure 20 resulting from the process. In fig. 5d, the self-supported pyrotechnic structure 20 has been taken out from the casting mould 100. Now, a step of applying the coating 24 is performed.

The coating may be applied as a liquid, for example by a spraying device 121 whereafter the coating is allowed to dry.

The coating may be applied as a metal foil, a plastic foil etc. The overlapping parts of the foil may optionally be adhered to each other.

The structure 20 may be rotating during this step, or it may be stationary.

As described above, a binding agent may be added to the pyrotechnic mixture 40 in addition to the metal and the metal oxide, before the sintering and/or baking process. The main purpose of the binding agent is to improve the self-supporting properties of the structure 20.

It is further possible that a void-filler may be added to the pyrotechnic mixture 40 in addition to the metal and the metal oxide, before the sintering and/or baking process. The main purpose of the void-filler is to reduce or eliminate the number of and/or the size of voids in the structure 20 and hence to reduce or eliminate any pressure difference between the inside of the structure 20 and the outside of the structure 20 as the structure 20 is lowered into the well.

The binding agent and the void-filler may be the same material or they may be different materials. Examples of materials having both properties are solder alloys or fusing alloys. One example is a eutectic alloy of bismuth (60%) and cadmium (40%) with a melting temperature of 60C.

The alloy is heated and mixed with the pyrotechnic mixture 40 to form a solid cast of thermite with the voids filled with the fusible alloy. Once the structure 20 is deployed in the well the alloy will melt and become a low viscosity heavy liquid.

Once ignited there will be an excess oxygen available from the bismuth oxide to oxidize the cadmium to form cadmium oxide and energy. The cadmium in the fusible alloy will then contribute to the energy as a fuel in the thermite reaction.

The bismuth metal in the alloy is less reactive and will be passive in the reaction and become separated from the cadmium and mix with the bismuth formed from the thermite reaction. Since the melting temperature of the bismuth is higher than for the alloy it will form a solid once the melt cools down and improve the properties of the permanent barrier as discussed above. The cadmium oxide will mix with the other oxides from the thermite reaction and solidify as part of the permanent barrier.

Alternatives to cadmium or additives to the alloy is indium and tin which are commonly alloyed with bismuth in fusible alloys. If the well tool device has applied a void filler with sufficient strength, this could be done during production process and thereby allowing machining by mechanical equipment to customize geometry and shape.

It is now referred to fig. 6a. Here it is shown that pyrotechnic mixture 40 has been formed into a self-supported pyrotechnic substructure 21 by means of the method described above. The substructure 21 is shaped as a cylinder or disc.

In fig. 6b, a through bore has been provided in each disc, and a plurality of such discs 21 has been inserted onto central carrying structure 60, which may be a rod, a wire etc. The upper end of the central carrying structure 60 is connected to the wireline 2a.

In fig. 6c, the pyrotechnic substructure 21 is adhered above each other and a coating 24 has been provided outside of assembled pyrotechnic substructure 21. It should be noted that each pyrotechnic substructure 21 must be self-supported. If they are not self-supported, they would fall apart or be separated when handled topside or when lowered into the well.

In fig. 6d, it is shown that a carrying element 61 of the mechanical carrying structure 60 is secured to the self-supporting substructure 21 during the sintering process. The carrying element 61 is then used to assemble the carrying elements 60 and the self-supporting substructure 21 into the pyrotechnic substructure 21.

Operation of the well tool device

As mentioned above, the well tool device 10 may be handled as a prior art well tool device 10 due to the self-supporting properties of the structure 20.

When lowered into the well, the fluid pressure outside of the structure 20 will increase.

In case the well tool device comprises a self-supporting structure 20 with voids and coating, the coating will prevent against water/well fluid ingress into the voids. Hence, the mechanical strength of the structure 20 will still be sufficient to maintain the self-supporting properties of the structure 20. As discussed below, the particle size should be relatively smaller to reduce the size of and/or number of voids. During the ignition and start of the heat generating process, the coating will protect the heat generating process. After a while, the heat will create an opening in the coating, causing water/well fluid ingress, which may or may not affect the heat generation process negatively depending on the number of voids and the size of the voids. In case the well tool device comprises a self-supporting structure 20 with void- filler, there will be no water/well fluid ingress. Hence, the mechanical strength of the structure 20 will not be affected by the fluid pressure and the self-supporting properties of the structure 20 is maintained. In case the well tool device comprises a self-supporting structure 20 with void-filler and coating, the result will be as in the above two cases, the self-supporting properties of the structure 20 is maintained.

In case the well tool device comprises a self-supporting structure 20 with voids and no coating, water/well fluid will ingress into the self-supporting structure. The structure itself will maintain its self-supporting properties. However, in this case, the particle size should be relatively smaller to reduce the size of and/or number of voids. If the voids are too large and/or too many, there is a risk that the heat generating process will not be a success. The temperature and duration of the heat generating process may be too low to melt the surroundings, or the heat generation process may stop or it may be difficult to start the heat generation process.

Other alternative embodiments

It should be noted that the pyrotechnic mixture 40 may comprise other metal oxides and metals than the abovementioned bismuth oxide and aluminum. One alternative embodiment is iron oxide and aluminum, but there are various other metal oxides and metals.

Claims

1. Well tool device (10) for melting surroundings of a well (WE) at a location of the well tool device (10) in the well (WE), thereby forming a cap rock to cap rock permanent barrier in the well (WE), wherein the well tool device (10) comprises: - a pyrotechnic mixture (40) comprising a first constituent and a second constituent, wherein the first constituent is a metal oxide and the second constituent is a metal;

- an ignition device (50) provided within or adjacent to the pyrotechnic mixture (40); wherein the pyrotechnic mixture (40) is forming a self-supported pyrotechnic structure (20).

2. Well tool device (10) according to claim 1, wherein only pyrotechnic mixture (40) is forming the self-supported pyrotechnic structure (20).

3. Well tool device (10) according to claim 1, wherein the pyrotechnic mixture (40) comprises a binding agent and/or a void-filler. 4. Well tool device (10) according to any one of the above claims, wherein the first constituent is bismuth oxide (Bi203) and second constituent is aluminum (Al).

5. Well tool device (10) according to any one of the above claims, wherein a sintered pyrotechnic mixture (40) is forming the self-supported pyrotechnic structure (20). 6. Well tool device (10) according to any one of the above claims, wherein the self- supported pyrotechnic structure (20) comprises voids (43).

7. Well tool device (10) according to any one of the above claims, wherein the well tool device (10) comprises a wireline connection interface (12) provided in an upper end (20a) of the self-supported pyrotechnic structure (20). 8. Well tool device (10) according to any one of the above claims, wherein an outer surface of the self-supported pyrotechnic structure (20) is provided with a coating (24).

9. Well tool device (10) according to any one of the above claims, wherein the wherein the pyrotechnic mixture (40) is forming a plurality of self-supported pyrotechnic substructures (21) assembled into a self-supported pyrotechnic structure

(20).

10. Method for manufacturing of a well tool device (10) for melting surroundings of a well (WE) at a location of the well tool device (10) in the well (WE), thereby forming a cap rock to cap rock permanent barrier in the well (WE), wherein the method comprises the steps of: - mixing a first constituent and a second constituent into a pyrotechnic mixture (40), wherein the first constituent is a metal oxide and the second constituent is a metal;

- providing the pyrotechnic mixture (40) into a casting mould;

- sintering the pyrotechnic mixture (40) into a self-supported pyrotechnic structure (20) in the casting mould;

- providing an ignition device (50) within or adjacent to the self-supported pyrotechnic structure (20).

11. Method according to claim 10, wherein the step of sintering comprises:

- pressurizing the pyrotechnic mixture (40) in the casting mold. 12. Method for according to claim 10 or 11, wherein the step of sintering comprises:

- heating of the pyrotechnic mixture (40) in the casting mold.

13. Method according to any one of claims 10 - 12, wherein the method further comprises the step of:

- applying a coating (24) to an outer surface of the self-supported pyrotechnic structure (20).