The invention relates to a method for permanently plugging and abandoning a well and a permanent barrier formed by the method.

Background of the invention

To meet governmental requirements during plugging and abandonment (P&A) operations in a well, a deep set barrier must be installed as close to the potential source of inflow as possible, covering all leak paths. A permanent well barrier shall extend across the full cross section area of the well, including all annuli, and seal both vertically and horizontally in the well. This requires removal of tubing mechanically, or perforating tubulars followed by washing behind the tubulars. This will lead to that swarf and debris from for example mechanical milling, need to be cleaned out of all flowlines, including the BOP system, to the rig, either provided onshore or offshore. Normally cement is used for the purpose of P&A operations. However, the well barrier has to comply with all of the following requirements for a P&A plug; a) impermeability, b) long term integrity, c) non shrinking, d) ductility (non brittle) - 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 WO 2013/135583 A1 (Interwell P&A AS) it is described a method of performing P&A operations, using a heat generating mixture (also denoted pyrotechnic mixture), e.g. a thermite mixture. Thermite is normally known as a pyrotechnic composition of a metal powder and a metal oxide. The metal powder and the metal oxide produce an exothermic oxidation -reduction reaction known as a thermite reaction. A number of metals can be the reducing agent, e.g. aluminium. If aluminium is the reducing agent, the reaction is called an aluminothermic reaction. Most of the varieties are not explosive, but may create short bursts of extremely high temperatures focused on a very small area for a short period of time. The temperatures may reach as high as 3000°C. In a first step in the method of

WO2013/13558, it is provided an amount of a heat generating mixture (for example thermite) at a desired location in the well which heat generating mixture is thereafter ignited in order 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.

During the last years, this technology has tested in test centers and in field trials, in order to verify that the permanent well barrier fulfills technical and regulatory requirements. An objective of the invention is to provide a method for permanent plugging and abandonment of a well providing barriers which seals against any inflow or influx in the well.

Summary of the invention

The invention is set forth and characterized in the independent claims, while the independent claims describe other characteristics of the invention.

The invention is applicable for use in permanent plugging and abandonment (P&A) of wells (e.g. hydrocarbon wells), for permanently sealing a storage of nuclear waste (i.e. radioactive material), C02, water wells, geothermal wells etc. The cap rock may be at different depths at different locations.

For reservoirs to form, at least two criteriums have to be in place (definitions from Schlumberger Oilfield Glossary, https://www.glossary.oilfield.slb.com/en):

A reservoir. The reservoir, or reservoir rock, is subsurface body of rock having sufficient and to store and transmit fluids. Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and form under temperature conditions at which hydrocarbons can be preserved.

A cap rock. The cap rock is a relatively impermeable rock, commonly shale, anhydrite or salt, that forms a barrier or seal above and around the reservoir rock so that fluids cannot migrate beyond the reservoir. It is often found atop a salt dome.

It has proven advantageous to set the cap rock sealing barrier at the depth of the cap rock. As indicated above, the cap rock is the natural geological sealing rock and seals vertically in the well. However, surprisingly it has proven the cap rock is not necessarily fluid-tight, i.e. impervious, sideways (i.e. in the horizontal direction). Porosity and permeability are not equal in the vertical direction and the horizontal direction, i.e. the cap rock may have none, or minimum (typically the vertical permeability of a cap rock capable of retaining fluids through geologic time is ~ 10 ⁶ -10 ⁸ darcies (re. Schlumberger Oilfield Glossary, October 2018)), vertical porosity and permeability whereas the horizontal porosity and permeability within the same cap rock can be higher. A typical setup of a hydrocarbon well is disclosed in Fig.

1 A, starting from the top:

- a water-filled reservoir (WFR) is shown,

- a cap rock (CR) immediately below the water-filled reservoir,

- a reservoir rock below the cap rock (CR), the reservoir rock being divided in three areas based on density of the fluids therein, and includes:

- an area with gas (G) trapped below the dome-shape of the cap rock (CR), - an area with oil (O) immediately below the area with gas, the area with oil is also trapped below the dome-shaped cap rock (CR),

- an area with water-filled reservoir (WFR) below the oil (O) and trapped below the dome-shaped cap rock (CR).

It is described a method of performing a permanent plugging and abandonment operation of a well by forming at least two barriers, the method comprising the steps of:

- providing an amount of a heat generating mixture,

- positioning the heat generating mixture at a first position in the well, the first position being at a cap rock of the well,

- igniting the heat generating mixture thereby melting surrounding materials at the first position,

- waiting a period of time, thereby allowing the melted materials to solidify into a reservoir sealing barrier which seals against a reservoir in the well,

- forming a cap rock sealing barrier at a second position in the well by positioning a cap rock sealing material at the second position thereby sealing against the cap rock.

The second position may be at or near an upper part of the reservoir sealing barrier. The second position is at a different location than the first location. The second position is above, i.e. closed to the top of the well than the first position.

The second position may be at or near a cap rock in the well, and the method may further comprise the step of:

- positioning the cap rock sealing material to form a cap rock sealing barrier which seals radially against any leaks in a casing in the well. The cap rock sealing barrier is thus preferably formed by a cap rock sealing material which is positioned at the second position in order to seal against sideways, radial or horizontal cap rock influx.

The cap rock sealing material may comprise a second heat generating mixture with a reduced reaction temperature relative the heat generating mixture, and the method may comprise the step of:

- igniting the second heat generating mixture such as to form the cap rock sealing barrier.

The second heat generating mixture, which may also be denoted a pyrotechnic mixture, may be a thermite mixture or any of the other examples given below.

As used herein, the term“pyrotechnic mixture” or“heat generating mixture” is a particulate mixture of a first metal and an oxide of a second material which, when heated to an ignition temperature, will react spontaneously in an exothermic and self-sustained chemical reaction where the first metal is oxidized to a metal oxide and the second metal is reduced to elementary metal. I.e. the pyrotechnic mixture can be defined as any substance or mixture of substances designed to produce an effect by heat, light, sound, gas/smoke or a combination of these, as a result of non- detonative self-sustaining exothermic chemical reactions. Pyrotechnic substances do not rely on oxygen from external sources to sustain the reaction.

An example of a possible reaction may be the the reaction between particulate ferric oxide and particulate aluminium:

f ? ₂ 0 ₃ T 2 Al — * 2 F + AI O

Other examples are presented in the detailed description below.

As used herein, the term“in an over-stochiometric amount” means that the metal oxide of the second metal is present in excess such that when the exothermic and self-sustained red-ox reaction has oxidized all of the particulate first metal of the pyrotechnic mixture to an oxide, there is still left an amount of the oxide of the second metal which will react with and consume metal of the wall of the housing since the wall is made of the first metal.

As used herein, the term“the first metal is more reactive than the second metal” means that the first metal of the pyrotechnic mixture has a higher reactivity than the second metal of the metal oxide. The reactivity of metals is determined empirically and given in reactivity series well known to the person skilled in the art. An example of a reactivity series of metals is found in e.g. Wikipedia:

https://en.wikipedia.org/wiki/Reactivity_series

After ignition of the heat generating mixture at the first position at the depth of the cap rock, the heat generating mixture will burn with a temperature of up to 3000°C and melt a great part of the proximate surrounding materials, with or without the addition of any additional metal or other meltable materials to the well. Such a heat generating mixture may also be referred to as a pyrotechnic mixture. The

surrounding materials may include any material normally present in the well, and can be selected from a group comprising, but not limited to: tubulars, e.g. casing, tubing and liner, cement, formation sand, cap rock etc. The heat from the ignited mixture will melt a sufficient amount of said materials. When the heat generating mixture has burnt out, the melted materials will solidify forming the reservoir sealing barrier at the first position. If the first position is at the cap rock, the reservoir sealing barrier melts and bonds in a transition area with the cap rock forming a continuous cap rock - to - cap rock barrier. This reservoir sealing barrier seals from inflow from any reservoir(s) below the reservoir sealing barrier. The operation is particularly suitable in vertical sections of the well, but may also be suitable in deviating or diverging sections such as horizontal sections or sections differing from a vertical section.

The sufficient amount of heat generating mixture or pyrotechnic mixture, e.g.

thermite mixture, varies dependent on which operation that is to be performed as well as the design well path. As an example, NORSOK standard D-010, which relates to well integrity in drilling and well operations, defines that a cement plug shall be at least 50 meters and in some operations up to 200 meters when used in abandonment operations. For example, one may fill whole of the inner volume of the pipe. In the embodiment regarding permanent well abandonment, a pipe having an inner diameter of 0,2286 m (9 5/8”) has a capacity of 0,037 m ³ per meter pipe. In order to provide a 50 meter plug by means of the method according to the invention, one would need 1,85 m ³ heat generating mixture comprising thermite. Similarly, if a cement plug of 200 meters is required, the amount of heat generating mixture needed would be 7,4 m ³ . It should though be understood that other plug dimensions may be used, as the plug provided by means of the invention will have other properties than cement and the NORSOK standard may not be relevant for all applications and operations. Any amount of heat generating mixture may be used, dependent on the desired operation, the properties of the heat generating mixture and the materials.

The cap rock sealing material may comprise a solidifiable fluid, and the method may comprise positioning the solidifiable fluid at the second position.

The solidifiable fluid may be chosen from a group consisting of: cement, resin, quick clay, clay, epoxy, or combinations thereof.

In other words, the cap rock sealing material can be a curable pumpable material which can change phase, i.e. from fluid phase to solid phase.

The solidifiable fluid can be any material used to permanently seal any damages in the casing resulting from melting during the creation of the reservoir sealing barrier. The solidifiable fluid is thus used to seal formations to prevent inflow of cap rock fluids. In other words, the cap rock sealing material can be a curable material which can change phase, i.e. from fluid phase to solid phase.

The solidifiable fluid may be a cement, where the most common type is API Oilwell Cement, known informally as Portland cement. In general, oilfield cement is thinner and exhibits far less strength than cement or concrete used for construction due to the requirement that it be highly pumpable in relatively narrow annulus over long distances. Various additives are used to control density, setting time, strength and flow properties. The cement-slurry, commonly formed by mixing cement, water and assorted dry and liquid additives, is pumped into place and allowed to solidify .

More advanced oilfield cements achieve higher set-cement compressive strengths by blending a variety of particle types and sizes with less water than conventional mixtures of Portland cement, water and chemical additives.

If clay is used, liquified clay, i.e. quick clay (without salt), can be inserted into the well to the second position. Once at the second position, salt may be added to the quick clay. When salt is added to quick clay, the quick clay solidifies and a cap rock sealing barrier in the form of a clay plug is formed.

The method may further comprise the step of arranging an igniting head in connection with the heat generating mixture. The igniting head may be suitable for igniting the heat generating mixture.

In an embodiment the method comprises the step of positioning at least one high temperature resistant element at or close to the first position. The high temperature resistant element serves to protect parts of the well or well elements that lies above, below and / or contiguous to the first position. The high temperature resistant element may be made of high temperature resistant materials such as a ceramic element or a glass element. There may be arranged one or more high temperature resistant elements in the well.

In another embodiment the method comprises the steps of positioning the heat generating mixture in a well tool device and lowering the well tool device to the first position, i.e. the depth of the cap rock, by the use of a lowering tool such as e.g. wire line, e-line, drill pipe, a deployment tool, a dedicated running tool, a snubbing tool or coiled tubing. The well tool device may then form a container for the heat generating mixture, where the heat generating mixture may be arranged in a compartment formed in the housing of the well tool device.

The method may comprise the steps of:

- preparing a well tool device, the well tool device comprising both the heat generation mixture and the cap rock sealing material,

- providing the well tool device at the cap rock of the well.

Possibly, both the heat generating mixture for forming the reservoir sealing barrier and the cap rock sealing material for forming the cap rock sealing barrier can be provided within the compartment of the well tool device. This renders possible lowering of the heat generating mixture and the cap rock sealing material to the first position in a single run which may save time in terms of operation time, and also reduce the size of the equipment such that less/Smaller equipment can be

transported to rig site. The heat generating mixture and the cap rock sealing material can be separated by a heat resistant element in order to protect the cap rock sealing material upon heat from ignition of the heat generating mixture.

The cap rock sealing material can be arranged such in the well tool device that it is influenced from the heat from the heat generating mixture. This may e.g. be done by making or manufacturing the housing of the well tool device of a material or material thickness allowing the part of the housing where the cap rock sealing material is arranged to at least partly melt such that the cap rock sealing material may flow out to an outside of the well tool device into contact with the casing above the reservoir sealing barrier and prevent any radial influx/inflow from the cap rock.

In case using a second heat generating mixture as cap rock sealing material, a first and second timer may be arranged in the well tool device or well tool assembly to activate first and second igniting heads connected to the heat generating mixture and the second heat generating mixture, respectively, at predefined time intervals.

In an alternative embodiment where several wells are to be abandoned, the method comprises the step of arranging a timer in connection with the igniting head at each heat generating mixture. A timer function might be favorable for example in situations where a number of wells are to be abandoned at nearby locations, e.g. from the same template. If used offshore, the timer in each well may be set to ignite at the same time, or at different times, subsequent to that the operation vessel has left the location. This reduces the risk of personal injury.

The method may further comprise, after forming the cap rock sealing barrier, a verification process, the verification process may comprise the steps of:

- providing at least one pressure sensor in communication with a closed volume formed between the cap rock sealing barrier and a top of the well,

- monitoring any pressure changes in the closed volume using the at least one pressure sensor for a predetermined time such as to verify that a combination of the reservoir sealing barrier and the cap rock sealing barrier seal against both the reservoir and the cap rock.

The predetermined time can sufficient to detect pressure changes, and can be seconds, minutes, hours, days, months, years, etc. utilizing wireless sensor(s) or wired sensors. For example, the predetermined time can be dictated by the expected potential pressure build-up after forming the reservoir sealing barrier and the cap rock sealing barrier.

The method may further comprise the steps of:

- installing a temporary plug above the cap rock sealing barrier to form the closed volume, which closed volume is formed between the cap rock sealing barrier and the temporary plug, and

- installing the pressure sensor at a position at the cap rock sealing barrier, at the temporary plug or at any position in the closed volume between the cap rock sealing barrier and the temporary plug.

In an alternative embodiment, the closed volume may be formed between the cap rock sealing barrier and a wellhead or a Xmas tree on top of the well, and the method may comprise: - connecting the pressure sensor to the wellhead or the Xmas tree.

It is further described a permanent plugging and abandonment barrier comprising a reservoir sealing barrier and a cap rock sealing barrier, wherein the permanent plugging and abandonment barrier is formed by the method as described above.

The desired amount of heat generating mixture is prepared at the surface and positioned in the well tool device. The mixture may for example be a granular or powder mixture. The well tool device may be any well tool device suitable for lowering into a well, and may preferably have a circular cross section of the same, or smaller size as the inner diameter of any present tubing or other pipes in the well bore. Dependent on the desired operation, the well tool device, or a set of a number of well tool devices, may be a short or a long well tool device. In a P&A operation, where the need of a large melting area is desired, the set of well tool devices may be several meters, ranging from 1 meter to 1000 meters.

In an embodiment the method comprises the step of circulating the heat generating mixture to the first position in the well. The heat generating mixture may be mixed with a fluid, forming a fluid mixture.

Although various denotations have been used throughout the description, tubing, liner, casing etc. should be understood as pipe or tubular of steel or other metals normally used in well operations.

The relative terms“upper”,“lower”,“below”,“above”,“high er” etc. shall be understood in their normal sense and as seen in a cartesian coordinate system. When mentioned in relation to a well,“upper” or“above” shall be understood as a position closer to the surface of the well (relative to another component), contrary to the terms“lower” or“below” which shall be understood as a position further away from the surface of the well (relative another component).

By the use of the described invention, all operations can be performed onshore and, if in water, from a light well intervention vessel or similar, and the need for a costly rig is eliminated. Prior to the ignition of the heat generating mixture, the well may be pressure tested to check if the seal is tight. This might be performed by using pressure sensors or other methods of pressure testing known to the person skilled in the art.

The invention will now be described in non-limiting embodiments and with reference to the attached drawings, wherein;

Brief description of the drawings

Fig. 1A is a typical setup of a hydrocarbon well; Fig. IB is an overview of a setup of a prior art well tool device 10 as disclosed in the Applicant’s own publication WO 2013/135583 A prior to the ignition of the heat generating mixture;

Fig. 2A shows a similar situation as in Fig. IB, again prior to the ignition of the heat generating mixture, where the well tool device is arranged at the depth of the cap rock in the well;

Fig. 2B shows the situation of Fig. 2 A after the ignition of the heat generating mixture;

Fig. 2C shows that a cap rock sealing material has been provided to a second position, wherein the second position is directly above the reservoir sealing barrier;

Fig. 2D shows a cap rock sealing barrier formed at a second position in the well bore WB, the second position being directly above the reservoir sealing barrier;

Fig. 2E shows a well tool device which can be used together with the present invention;

Fig. 2F shows an example of a setup which can be used in a verification process to verify that the reservoir sealing barrier and the cap rock sealing barrier provide sufficient barriers towards the reservoir and inflow from the cap rock, utilizing a temporary plug to form a closed volume where a pressure sensor is arranged between the cap rock sealing barrier and the temporary plug;

Fig. 2G shows an example of a setup which can be used in a verification process to verify that the reservoir sealing barrier and the cap rock sealing barrier provide sufficient barriers towards the reservoir and inflow from the cap rock, utilizing a pressure sensor arranged in closed volume between the cap rock sealing barrier and a wellhead or Xmas tree;

Detailed description of a preferential embodiment

Fig. 1A is discussed above and discloses a typical setup of a hydrocarbon well.

Fig. IB shows an overview of a setup of a prior art system disclosed in the

Applicant’s own publication WO 2013/135583 A. Fig. IB indicates the situation prior to the ignition of the (first) heat generating mixture 40. A vertical well bore WB has been drilled in a formation F. The wellbore WB is provided with casing CA cemented to the formation wall (not shown), and a tubing or liner TBG in the lowermost part of the well bore WB. In a lower part of the well bore WB a first permanent plug 4 has been set. A first high temperature resistant element 5, such as ceramic element or glass element, is arranged above the first permanent plug 4 to protect the first permanent plug 4. A heat generating mixture 40, e.g. a thermite mixture, is arranged above the first high temperature resistant element 5. Similarly, there may be arranged a second high temperature resistant element 7 as well as a second permanent plug element 8 above the heat generating mixture 40. In addition, an igniting head 1 1, for ignition of the heat generating mixture 40, is arranged in connection with the heat generating mixture 40. A timer element 9 may be arranged to time set the detonation of the igniting head 11, and thus the heat generating mixture 40. It is shown a wellhead 70 at the top of the well W and a Xmas tree 80 connected to the wellhead 70.

As an alternative to a timer element, a physical connection such as an electric wire or similar providing signal transmission between the surface and the igniting head 1 1, may be established such that the ignition can be activated from surface.

Fig. 2A shows a similar situation as in Fig. IB, again prior to the ignition of the heat generating mixture 40. However, as shown in Fig. 2 A, the well tool device 10 is lowered in the well bore WB to the depth of the cap rock CR e.g. by using a lowering tool LT. As shown in Fig. IB, a vertical well bore WB has been drilled in a formation F through the cap rock CR and into the reservoir R. the reservoir R may be any reservoir R holding a natural resource such as hydrocarbons

(oil/gas/condensates), water, geothermal (heat etc.) or serve as an injection reservoir for nuclear waste, C02 capture, radioactive materials etc.

The well bore is provided with casing CA cemented with cement CE to the formation wall, and a tubing or liner TBG in the lowermost part of the well bore WB. In a lower part of the well a first permanent plug 4 has been set. A first high temperature resistant element 5, such as ceramic element or glass element or a plug formed of a heat generating mixture with a lower reaction temperature such that it may solidify inside the tubing or liner TBG, is arranged above the first permanent plug 4 in order to protect the first permanent plug 4. However, the presence of the first permanent plug 4 and the first high temperature resistant element 5 is optional as there may be other means or material which provide the same effect as these elements. For example, a heat resistant element, possibly in combination with a permanent plug, may form an integral part of the well tool device 10. Then the need of an additional high temperature resistant element 5 below the well tool device 10 may be superfluous.

Further referring to Fig. 2A, the well tool device 10 may further comprise a housing 20. The housing 20 may enclose a fluid tight compartment 30, in which

compartment 30 the heat generating mixture 40 can be arranged. An igniting head 1 1 can be arranged in connection with the heat generating mixture 40.

The lowering tool LT may be used for lowering at least one of the first permanent plug 4, the first high temperature resistant element 5, the well tool device 10 with heat generating mixture 40 and/or the igniting head 11 and/or any timer.

Fig. 2B shows the embodiment of Fig. 2A after the ignition of the heat generating mixture 40 such that proximate surrounding materials present at the position of the heat generating mixture 40 have melted, e.g. tubing or liner TBG, cement CE, cap rock CR, well tool device 10, well tool device housing 20, igniting head 11, other tubulars etc. After waiting a period of time, the melted surrounding materials have solidified into a reservoir sealing barrier RSB which seals against the reservoir R in the well bore WB. The sketched area formed in the well bore WB and extending radially into the cap rock CR indicates the melted surrounding materials (i.e. the reservoir sealing barrier RSB which has been formed). The transition areas between non-affected cap rock CR and complete melted materials now forming part of the reservoir sealing barrier RSB is denoted transition zone TZ. In order for a successful reservoir sealing barrier RSB to form, it is advantageous that the bonding between the cap rock CR and the reservoir sealing barrier is satisfactory. Whether the reservoir sealing barrier seals against the reservoir, including in the transition zone, verification test such as pressure tests or sample test(s) of substances not naturally occurring above reservoir sealing barrier RSB can be performed. Such sample tests may be e.g. H2S or other gases. The pressure tests may monitor whether the pressure above the reservoir sealing barrier increases or not.

As indicated by arrows CRI (cap rock influx) in Fig. 2B, cap rock influx CRI may occur directly above the reservoir sealing barrier RSB due to holes formed in the casing CA. The holes can be formed as a result of the high temperature occurred from the reaction in the heat generating mixture 40 which also melt parts of the casing CA close to the reservoir sealing barrier RSB. One would normally not consider holes/cracks in the casing at the depth of the cap rock to be an issue as the cap rock is expectedly fluid tight (see definition of cap rock above). However, as discussed above, surprisingly, it has proven that the cap rock is not necessarily fluid-tight, i.e. impervious, sideways/ radially (i.e. in the horizontal direction).

In Fig. 2C a cap rock sealing material CRSM has been provided to a second position. The second position is disclosed as being directly above the reservoir sealing barrier RSB, but may also be at other locations higher up in the well bore WB relative the reservoir sealing barrier RSB.

Referring to Fig. 2D, a cap rock sealing barrier CRSB is formed at a second position in the well bore WB, at the second position being directly above the reservoir sealing barrier RSB. The cap rock sealing barrier CRSB is formed by a cap rock sealing material CRSM which is positioned at the second position in order to seal against sideways, radial or horizontal cap rock influx CRI. The cap rock sealing barrier CRSB may be formed within, and seal against an inner surface of the casing CA. The cap rock sealing material CRSM can be a cement, resin, quick clay etc.

The casing CA is normally made of steel and the cap rock sealing material CRSM shall preferably provide a proper bonding to steel (i.e. the inner surface of the casing CA) after the cap rock sealing barrier CRSB has formed. Alternatively, according to another aspect, the cap rock sealing barrier CRSB can be a second heat generating mixture with a lower reaction temperature than the heat generating mixture used in forming the reservoir sealing barrier RSB such that the damage to the casing CA is minimized upon ignition of the second heat generating mixture and that only parts of the casing CA may melt to form improved bonding with the second heat generating mixture after ignition.

Fig. 2E shows a well tool device 10 comprising a housing 20 and a compartment 30. A heat generating mixture 40 for forming the reservoir sealing barrier RSB and a cap rock sealing material CRSM for forming the cap rock sealing barrier CRSB are provided within the compartment 30. A heat resistant element 60 may arranged between the heat generating mixture 40 and the cap rock sealing material CRSM for protecting the cap rock sealing material CRSM upon ignition of the heat generating mixture 40. The setup of the well tool device 10 in Fig. 2E renders possible lowering of the heat generating mixture 40 and the cap rock sealing material CRSM to the first position in a single run using e.g. a lowering tool LT.

Fig. 2F shows an example of a setup which can be used in a verification process to verify that the reservoir sealing barrier RSB and the cap rock sealing barrier CRSB provide sufficient barriers towards the reservoir R and inflow or influx CRI from the cap rock CR. In the example of Fig. 2F a temporary plug 90 is installed to form a closed volume CV between the cap rock sealing barrier CRSB and the temporary plug 90. A pressure sensor 45 is arranged inside the closed volume CV. The pressure sensor 45 may be used in order to monitor any pressure changes in the closed volume CV to identify any leaks from the reservoir or the cap rock, via the reservoir sealing barrier RSB or the cap rock sealing barrier CRSB, respectively.

Fig. 2G shows another example of a setup which can be used in a verification process to verify that the reservoir sealing barrier RSB and the cap rock sealing barrier CRSB provide sufficient barriers towards the reservoir R and inflow or influx CRI from the cap rock CR. In the example of Fig. 2G a pressure sensor 45 is arranged in the closed volume CV formed between the cap rock sealing barrier CRSB and a wellhead or Xmas tree 70,80 on top of the well W. The pressure sensor 45 may be used in order to monitor any pressure changes in the closed volume CV to identify any leaks from the reservoir or the cap rock, via the reservoir sealing barrier RSB or the cap rock sealing barrier CRSB, respectively.

The embodiments disclosed in the figures provide a proposed solution to the object of the invention, which is to provide a method for permanent plugging and abandonment of a well by forming a reservoir sealing barrier and a cap rock sealing barrier. The pyrotechnic process

The heat generating mixture (pyrotechnic mixture) 40 comprises a particulate of a first metal and a particulate metal oxide of a second metal in an over-stoichiometric amount relative to a red-ox reaction.

The first metal is oxidized to a metal oxide and the second metal is reduced to elementary metal where the first metal is a different metal than the second metal. Heat is a result of this reaction.

One example of such a pyrotechnic mixture is the following:

Fe ₂ 0 ₃ + 2 A1— > 2 Fe + AI2O3 + heat (1)

Here, the first metal is aluminum (Al) and the second metal is iron oxide (Fe203). The first metal is oxidized to the metal oxide aluminum oxide (A1203) and the second metal is reduced to the elementary metal iron (Fe). Heat is produced during this process, which often is referred to as a thermite process.

In the above example, the first metal is more reactive than the second metal as defined in a reactivity series of metals.

In alternative embodiments for such a reaction, the first metal in the heat generating mixture or pyrotechnic mixture may be of the following metals: Mg, Al, Ti, Mn, V, Zn, Cr, Mo, Fe, Co, Ni, Sn, Pb, Cu, or B and the metal oxide of the second metal is one of: copperll oxide, chromiumlll oxide, ironll, III oxide, manganeselV oxide, silicon dioxide, boron trioxide, or leadll, IV oxide. When combining the above, the first metal is more reactive than the second metal as defined in a reactivity series of metals.

Some examples of alternative processes, in which the first metal is aluminum, are disclosed below:

Fe ₂ 0 ₃ + 2 Al ® 2 Fe + Al ₂ 0 ₃ + heat (2)

3 Fe ₃ 0 ₄ + 8 Al ® 9 Fe + 4 Al ₂ 0 ₃ + heat (3)

3 Mn0 ₂ + 4 Al ® 3 Mn + 2 Al ₂ 0 ₃ + heat (4)

Mn ₂ 0 ₃ + 2 Al ® Mn + Al ₂ 0 ₃ + heat (5)

3 Si0 ₂ + A Al ® 3 Si + 2 Al ₂ 0 ₃ + heat (6)

It should be noted that the heat produced in the above processes will vary from process to process. In addition, the speed of the reaction will vary from process to process. As mentioned above, it is also possible to use manganese as the first metal, as disclosed below:

Fe ₂ 0 ₃ + 3 Mg ® 2 Fe + 3 MgO + heat (7) The invention is herein described in non-limiting embodiments. It should though be understood that the embodiments may be envisaged with a lower or higher number of permanent plugs and high temperature resistant elements. The skilled person will understand if it is desirable to set none, one, two or several permanent plugs dependent on the desired operation. Similarly, the number of high temperature resistant elements positioned in the well may vary from zero, one, two or several, dependent on the operation.

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