EP3085883A1

EP3085883A1 - Downhole tool string for plug and abandonment by corrosive agent

Info

Publication number: EP3085883A1
Application number: EP15164698.1A
Authority: EP
Inventors: Lars Mangal
Original assignee: Welltec AS
Current assignee: Welltec AS
Priority date: 2015-04-22
Filing date: 2015-04-22
Publication date: 2016-10-26

Abstract

The present invention relates to a downhole tool string configured to be submerged into a well, comprising at least one tool section comprising a first compartment having inner faces, the first compartment containing a corrosive or tempering agent during the submersion of the downhole tool into the well, and the tool section having an outlet for ejecting the corrosive agent contained in the first compartment into the well. The present invention furthermore relates to a downhole plug and abandonment system and to a downhole plug and abandonment method.

Description

Field of the invention

The present invention relates to a downhole tool string configured to be submerged into a well. The present invention furthermore relates to a downhole plug and abandonment system and to a downhole plug and abandonment method.

Background art

When a well becomes less productive, and all attempts to improve the production of hydrocarbons from a reservoir have failed, the unproductive part of the well, if not the whole well, is plugged and abandoned. In some cased wells having parts where the casing or production tubing is surrounded by an annulus which has not been filled with cement during completion, the plug and abandonment becomes more complicated, since when the casing is filled with cement, the cement cannot gain formation contact and there is a risk of a blowout through that annulus.

Summary of the invention

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, it is an object to provide an improved plug and abandonment system capable of plugging also cased wells having an annulus between the casing/production tubing and the formation and/or the annular space between the intermediate casing and the production casing.
The above objects, together with numerous other objects, advantages and features, which will become evident from the below description, are accomplished by a solution in accordance with the present invention by a downhole tool string configured to be submerged into a well, comprising at least one tool section comprising a first compartment having inner faces, the first compartment containing a corrosive or tempering fluid or agent during the submersion of the downhole tool into the well, and the tool section having an outlet for ejecting the corrosive agent contained in the first compartment into the well.
In an embodiment, the inner faces of the first compartment may be made of a ceramic material or are fully covered by a ceramic material, such as S_iO.
Also, the agent may comprise hydrogen sulphide, hydrosulfuric acid or sulfhydric acid, or nitric acid, sulphuric acid or any mixtures thereof.
In addition, a plug or shear disc may be arranged in the outlet in order to control the flow of corrosive fluid out of the compartment.
Furthermore, the tool section may comprise a second compartment containing a foam generating agent, such as a gas or liquid.
Also, the tool section may comprise a foam generating unit having a mixing chamber which is in fluid communication with the first compartment and the second compartment so that the foam generating agent from the second compartment is mixed with the corrosive fluid or agent in the mixing chamber to provide a corrosive foam to be ejected into the well.
In addition, the tool section may comprise an ejecting device for ejecting the corrosive fluid or agent or corrosive foam out through the outlet.
The ejecting device may comprise a displacement element configured to displace the corrosive fluid or agent.
In an embodiment, the displacement element may be made of a ceramic material.
Furthermore, the first compartment may be an insertable compartment configured to be inserted into the tool section.
Also, the corrosive fluid may be nitric acid, sulphuric acid or any mixtures thereof.
Moreover, the tool string may comprise a milling section or a cutting section.
The tool section may comprise inner faces and outer faces, the inner or outer faces being exposed to a corrosive fluid or agent when being in the well, and at least one of the faces exposed to the corrosive fluid or agent may be partly or fully covered with a ceramic material, such as S_iO.
In an embodiment, the tool section may comprise a tool part having at least the inner faces or outer faces partly covered with the ceramic material.
Furthermore, the tool part may be a projectable element, such as an anchoring element, a key element, a wheel arm having a wheel, a cutting arm, a milling arm or a centraliser arm.
The present invention furthermore relates to a downhole plug and abandonment system for a well, comprising:

a well tubular metal structure having an inside and a wall and being arranged in a borehole, the borehole and the well tubular metal structure defining an annulus or annular space, and
a downhole tool string according to any of the preceding claims, being submerged and arranged in the well tubular metal structure,

The well tubular metal structure may be a first well tubular metal structure, and the downhole plug and abandonment system may comprise the first well tubular metal structure and a second well tubular metal structure arranged within the first well tubular metal structure, and a packer may be arranged between the first and the second well tubular metal structure defining an annular space above the packer. The plug may be arranged in the second well tubular metal structure.
In an embodiment, the plug may be made at least partly of a ceramic material.
Also, the plug may comprise a layer of noble metal.
Moreover, the plug may have a plug thickness, the plug thickness being larger than a wall thickness of the well tubular metal structure.
Furthermore, the first part of the well tubular metal structure may have a structure section above the plug, which structure section is to be removed.
In an embodiment, the first compartment may comprise the corrosive fluid or agent, or the foam generating unit may be arranged in the first part of the well tubular metal structure adjacent the structure section so that the corrosive fluid or agent or corrosive foam is ejected from the first compartment or the foam generating unit to partly or fully corrode the wall of the structure section by means of the corrosive fluid or agent or corrosive foam.
The system may further comprise a pump device configured to pressurise the first part.
Furthermore, the tool string may comprise a stroking tool section for displacing the milling/cutting section along the longitudinal extension of the well tubular metal structure.
In addition, the projectable parts of the cutting section may be moved in a reprocicating movement to scratch in the surface and increase the surface area of the structure section so that the corrosive agent contacts a larger surface area when chemically reacting with the metal surface.
Also, the system may comprise a second cutting string for cutting the well tubular metal structures near the seabed or surface to release the well head from the well.
Furthermore, the tool string may comprise a logging section configured to detect if a part of the structure section has been corroded.
In addition, the system may comprise a cement section configured to provide cement in the first part above the plug.
Also, the tool string may be submerged and powered through a wireline.
Moreover, the tool string may comprise a driving section, such as a downhole tractor, for driving the tool string forward in the well.
The present invention further relates to a downhole plug and abandonment method comprising the steps of:

arranging the first compartment of the downhole tool string described above in the first part of the well tubular metal structure adjacent the structure section,
ejecting the corrosive, tempering or modifying fluid or agent or corrosive foam into the structure section from the tool section, and
corroding or modifying the metal wall of the structure section partly or fully from the inside of the structure section to provide a decreased wall thickness of the structure section or an opening in the structure section.

The method may further comprise the step of milling or cutting the structure section being partly corroded to provide an opening to the annulus in the structure section.
In addition, the method may comprise the step of setting a plug of the downhole plug and abandonment system described above inside the well tubular metal structure.
Moreover, the method may comprise the steps of ejecting cement into the well tubular metal structure and into the annulus via the opening, and abandoning the well.
Also, the method may comprise the step of pressurising the first part after the step of corroding the metal wall of the structure section partly or fully in order to detect if the corrosive fluid or agent or corrosive foam has corroded the wall sufficiently to provide the opening.
Furthermore, the method may comprise the step of ejecting a second portion of corrosive fluid or agent or corrosive foam into the structure section if the previously ejected corrosive fluid or agent or corrosive foam was insufficient to provide the opening.
Before the step of ejecting a second portion of corrosive, tempering or modifying fluid or agent or corrosive foam into the well tubular metal structure, a layer of protective material, such as cement, may be ejected on top of the plug and/or the packer.
In addition, the method may comprise the steps of taking a sample of a well fluid in the well tubular metal structure at least before the step of ejecting the corrosive fluid or agent or corrosive foam, and detecting a content of the well fluid.
The present invention further relates to downhole tool string configured to be submerged into a well, comprising:

at least one tool section comprising inner faces and outer faces, the inner or outer faces being exposed to a corrosive fluid or agent when being in the well, wherein at least one of the faces being exposed to the corrosive fluid or agent is partly or fully covered with a ceramic material, such as S_iO.

In an embodiment, the tool section may comprise a tool part having at least the inner faces or outer faces partly covered with the ceramic material.
Furthermore, the tool part may be a compartment arranged within the tool section, the compartment having the inner faces fully covered by the ceramic material and exposed to the corrosive fluid or agent carried by the compartment.
In addition, the tool part may comprise an outlet for ejecting the corrosive fluid or agent carried in the compartment into the well.
Also, the compartment comprising the corrosive fluid or agent may be arranged in the first part of the well tubular metal structure adjacent the structure section so that the corrosive fluid or agent is ejected from the compartment to provide at least one opening in the wall of the structure section by corroding part of the structure section by the corrosive fluid or agent.
Also, the system may comprise a cement section configured to provide cement in the first part above the plug.
The present invention furthermore relates to a downhole plug and abandonment method comprising the steps of:

setting a plug of the downhole plug and abandonment system described above inside the well tubular metal structure,
arranging the compartment of the downhole tool string in the first part of the well tubular metal structure adjacent the structure section above the plug,
ejecting the corrosive fluid or agent into the structure section,
providing at least one opening in the wall of the structure section by corroding the wall of metal,
ejecting cement into the well tubular metal structure and into the annulus via the opening, and
abandoning the well.

The method may further comprise the step of:

pressurising the first part after the step of providing at least one opening in order to detect if the corrosive fluid or agent has corroded the wall sufficiently to provide the opening.

In addition, the method may further comprise the step of:

ejecting a second portion of corrosive fluid or agent into the structure section if the previously ejected corrosive fluid or agent was insufficient to provide the opening.

The method may further comprise the step of circulating cement down the second well tubular metal structure and up through the annular space between the first and the second well tubular metal structure, or vice versa.
Finally, the method may further comprise the steps of:

taking a sample of a well fluid in the well tubular metal structure before the step of setting the plug, and
detecting a content of the well fluid.

Brief description of the drawings

The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments and in which

Fig. 1 shows a cross-sectional view of a downhole plug and abandonment system having a tool string,
Fig. 2 shows a cross-sectional view of part of a tool string having a compartment,
Fig. 3 shows a cross-sectional view of part of another tool string having a compartment with displacement element,
Fig. 4 shows a cross-sectional view of part of another tool string having two compartments,
Fig. 5 shows another downhole plug and abandonment system partly covered by ceramic material,
Fig. 6 shows yet another downhole plug and abandonment system ejecting foam,
Fig. 7 shows another downhole plug and abandonment system with a plug,
Fig. 8 shows another downhole plug and abandonment system with an intermediate casing and a plug in the production casing,
Fig. 9 shows the downhole plug and abandonment system of Fig. 8 in which corrosive agent is ejected by the tool,
Fig. 10 shows the downhole plug and abandonment system of Fig. 8 in which part of the production casing has been removed,
Fig. 11 shows the downhole plug and abandonment system of Fig. 8 in which a cement plug has been set above the plug and packers,
Fig. 12 shows the downhole plug and abandonment system of Fig. 8 in which further corrosive agent is ejected,
Fig. 13 shows the downhole plug and abandonment system of Fig. 8 in which part of the intermediate casing has been removed, and
Fig. 14 shows a section of the well tubular structure in which part of the wall has been eroded by corrosive agent and the cutter section of the tool string removes the rest of the wall.

All the figures are highly schematic and not necessarily to scale, and they show only those parts which are necessary in order to elucidate the invention, other parts being omitted or merely suggested.

Detailed description of the invention

Fig. 1 shows a downhole tool string 1 which is submerged into a well tubular metal structure 101 having an inside 102 and a wall 103, and the well tubular structure is arranged in a borehole 104. The borehole and the well tubular metal structure define an annulus 105 therebetween. The downhole tool string 1 comprises at least one tool section 2 comprising a first compartment 19 having inner faces 4. The first compartment 19 contains a corrosive fluid or agent 6 during the submersion of the downhole tool string 1 in the well. The tool section has an outlet 3 for ejecting the corrosive fluid or agent contained in the first compartment into the well.
The corrosive fluid is ejected into the well to provide an opening in the well tubular metal structure 101 so that cement can enter the opening and plug the well, as shown in Fig. 14. The tool string 1 is powered and submerged via a wireline 314, as shown in Fig. 1. Thus, the well can be plugged and abandoned as a simple wireline intervention which no longer requires a large rig for pulling the well tubular metal structure 101 out of the well. In known plug and abandonment operations, a large derrick is installed on top of the well to pull the production casing, i.e. the well tubular structure, out of the well. By providing an opening in the well tubular structure, cementing the well can be performed without pulling the casing, meaning that no large, expensive equipment is necessary to perform for the plug and abandonment operation.
As shown in Fig. 2, the inner faces 4 of the first compartment 19 are covered by a ceramic material 16, such as S_iO, also known as glass. The inner faces 4 may thus be coated or otherwise be provided with a layer of ceramic material 16. The compartment 19 has a wall 5 of metal forming a tool housing. By having the layer of ceramic material 16 covering the inner faces of the metal wall, the metal wall is thus protected from being eroded by the corrosive fluid or agent when carrying the corrosive fluid or agent down the well.
A plug or shear disc 7 may be arranged in the outlet 3 in order to control the flow of corrosive fluid out of the compartment. The plug or shear disc may be destroyed when applying a certain pressure to the corrosive fluid, or the plug may be destroyed by an electric field activated around the plug, or by similar arrangement.
The corrosive fluid or agent may be pressurised when kept in the compartment 19, so that once the outlet is open, the corrosive fluid 6 flows automatically out of the compartment and into the well tubular metal structure.
As shown in Fig. 3, the tool section of the downhole tool string 1 comprises an ejecting device 8 for ejecting the corrosive fluid or agent 6 out through the outlet 3. The ejecting device 8 comprises a displacement element 9, such as a piston, configured to displace the corrosive fluid or agent. The displacement element 9 may be made of a ceramic material or a disc-shaped plug 10 of ceramic material 16 arranged in front of the displacement element 9 as shown in Fig. 3. The disc-shaped plug 10 of ceramic material protects the displacement element 9 when carrying the corrosive fluid in the compartment 19. When the corrosive fluid or agent 6 is to be ejected out of the outlet 3, the displacement element 9 presses onto the disc-shaped plug 10, a shear disc 11 shears and the displacement element 9 moves towards the outlet, pressurising the corrosive fluid or agent 6 in the compartment 19 until the pressure exceeds a certain level and the plug or shear disc 7 in the outlet brakes, and the corrosive fluid or agent 6 is allowed to flow out of the outlet. When moving further, the displacement element 9 displaces almost all of the corrosive fluid or agent 6 out into the well tubular structure.
The first compartment 19 in Fig. 3 is an insertable compartment configured to be inserted into the tool section and fastened by means of a lid 12. By having an insertable compartment, the corrosive fluid can be safely provided in the compartment in a safe environment and then properly closed before being inserted into the tool section 2.
In Fig. 4, the tool section 2 comprises a first compartment 19 and a second compartment 20, at least divided by a metal wall 27 covered by ceramic material 16. The first compartment comprises the corrosive fluid or agent 6, the second compartment contains a foam generating agent 24 in the form of a gas or liquid which, when mixed with the corrosive fluid or agent 6, forms foam. The foam may be formed by oxygen and acid to create a highly corrosive foam.
Thus, the tool section 2 comprises a foam generating unit 18 having a mixing chamber 23 which is in fluid communication with the first compartment 19 and the second compartment 20, so that the foam generating agent 24 from the second compartment is mixed with the corrosive fluid or agent 6 in the mixing chamber in order to provide a corrosive foam to be ejected into the well. As the foam forms, the pressure inside the mixing chamber 23 increases and the foam pressure brakes the plug or disc 7 and flows out of the chamber into a structure section 322 being a section of the well tubular metal structure 101 shown in Fig. 6. When forming foam opposite the wall of the structure section 322 which is to be partly or fully eroded, there is no longer a need for a plug, even though one could be used for securing that the foam remains in the same position opposite the part of the wall of the structure which is to be removed or for the subsequent cement operation.
The tool section 2 of Fig. 1 comprises both inner faces 4 and outer faces 34. When being in the well, both the inner or outer faces may be exposed to corrosive fluid or agent. Therefore, more than one of the faces of a tool part being exposed to the corrosive fluid or agent may be partly or fully covered with a ceramic material, such as S_iO, in order to protect that part from eroding. Thus, the tool section comprises a tool part, such as the first compartment 19 or a wheel arm 309, shown in Fig. 6, having at least the inner faces or outer faces partly or fully covered with the ceramic material. Other kinds of tool parts may be a projectable element, such as an anchoring element, a key element, a wheel arm having a wheel, a cutting arm, a milling arm, a centraliser arm.
Fig. 5 shows a downhole tool string 300 which is submerged in a well tubular metal structure 301 arranged in a borehole 302 in a well 303. The downhole tool string 300 comprises a tool section 304 comprising outer faces 305 which are exposed to a corrosive fluid 306 or agent when being in the well. In order to protect the tool string and its primary functions as well as expensive parts of the tool string, a main part of the outer faces being exposed to the corrosive fluid or agent is partly or fully covered with a ceramic material 307, such as S_iO. The tool section 304 comprises a tool part 308 whose outer face is partly covered with the ceramic material. The tool part 308 is shown as a wheel arm 309 having a wheel 310 of one of the driving sections 311 for propelling the tool string forward in the well. The tool part 308 is also shown as an operational tool 312 in front of the tool string 300. The tool string 300 is powered and submerged via a wireline 314. The tool string 300 comprises an electric section 315, an electric motor 316 and a pump 317 for driving the driving section 311. The tool string 300 comprises a housing 318 mounted from several housing sections for covering each tool section. Some housing sections may be made of metal and may not be covered by the ceramic material since these housing sections are very easy to replace when the tool string is withdrawn from the well and redressed for another operation. The process of applying a layer of ceramic material on one or more of the outer faces 305 is expensive, and therefore mainly the vital parts of the tool string are provided with such a layer. Vital parts may be the wheel arms and part of the wheels.
As shown in Fig. 6, the tool part 308 may also be a compartment 319 having inner faces 320 exposed to corrosive fluid or agent 306 kept within the compartment 319 to be carried down the well to a structure section 322, which is a section of the well tubular metal structure which is corroded with the corrosive fluid 306 in the compartment, in order to remove at least part of the wall of the structure section so that the wall of structure section 322 becomes thinner.
Part of the tool section comprising the compartment 319 also comprises a foam generator 323 for turning the corrosive fluid or agent 306 into a foam 324 before or as the corrosive agent 306 is ejected through the outlet 321 of the compartment.
As shown, the foam generator may be a separate compartment comprising the foam generating agent 325 which, when mixed with the corrosive fluid or agent 306, forms foam 324.
In Fig. 6, the downhole tool string 300 further comprises a tool section which is an anchoring section 326 having a projectable element 327 being an anchoring element 328. The downhole tool string 300 further comprises a tool section, such as a milling/cutter section 330, having a projectable element 327 being a cutting or milling arm 329.
The corrosive fluid or agent may be nitric acid, sulphuric acid or any mixtures thereof. The corrosive fluid or agent is highly corrosive and has a concentration sufficient to erode metal tubing or casing when ejected into well fluid in a well downhole. The agent or fluid may comprise hydrogen sulphide, hydrosulfuric acid or sulfhydric acid, or any mixtures thereof. The agent or fluid may temper or tamper with the metal wall of the structure section so that the wall becomes brittle, which makes it much easier to cut into and pulverise to provide the circumferential opening. Thus, the agent or fluid in the compartment of the tool may be a chemical changing the material properties of the metal in the structure section of the well tubular structure.
Fig. 7 discloses a downhole plug and abandonment system 100 for a well which is to be partly or fully abandoned. The system comprises a well tubular metal structure 101 having an inside 102 and a wall 103, and the well tubular structure is arranged in a borehole 104. The borehole and the well tubular metal structure define an annulus 105 therebetween. The system further comprises a downhole tool string 1 being submerged and arranged in the well tubular metal structure 101. The downhole tool string 1 comprises a compartment 19 comprising a corrosive fluid or agent 6 for corroding part of the well tubular metal structure 101. The compartment 19 has an outlet for letting the corrosive fluid into the well tubular metal structure. The downhole plug and abandonment system further comprises a plug 106 arranged in the well tubular metal structure, dividing the well tubular metal structure into a first part 107 and a second part 108. The first part 107 is arranged closest to a top 112 of the well. Above the plug in the first part 107 of the well tubular metal structure 101, the well tubular metal structure has a structure section 22 which must be partly removed in order to fill this part of the well with cement or a similar material to be able to plug and abandon this part of the well. By removing part of the structure section, the cement can be ejected into the annulus and the cement plug formed by the ejected cement gets formation 110 contact.
The plug 106 is made partly of a ceramic material 16 and partly of a noble metal in the form of a layer of noble metal 28. By having the plug made of a ceramic material, the plug is able to withstand the corrosive fluid 6 ejected from the compartment of the tool string so that the corrosive fluid 6 corrodes the metal wall of the structure section and not the plug.
The plug 106 may also be made of cement, and as shown in Fig. 14, the plug may be an extension along the longitudinal extension of the well tubular structure of several 100 metres and even kilometres.
In another embodiment, the plug has a plug thickness t_p which is larger than a wall thickness t_s of the well tubular metal structure, meaning that even though the corrosive fluid corrodes part of the plug, the plug is still capable of providing the seal.
When the corrosive fluid or agent is ejected from the compartment, the corrosive fluid erode part of the structure section and thus one or more opening(s) in the wall of the structure section. The openings provide access for the cement to gain contact with the inner wall 109 of the borehole and thus formation contact. In another embodiment, the corrosive fluid or agent 6 is used to remove part of the wall of the well tubular metal structure and subsequently, the tool string comprising also a milling/cutter section 330, as shown in Fig. 6, is arranged so that the cutter arm 329 cuts through the remaining part of the well tubular metal structure. In this way, the corrosive fluid is used to reduce the wall thickness t_s of the well tubular metal structure so that the subsequent milling or cutting process becomes easier and the success rate of the milling or cutting operation is increased.
In Fig. 7, the system 100 further comprises a pump device 111 configured to pressurise the first part after the corrosive fluid has been ejected and after the corrosion process has ended. By pressurising the first part 107 of the well tubular metal structure, the corrosion process can be tested so that if the first part can be pressurised, then the corrosion process has not provided any openings. In that event, an additional portion of corrosive fluid or agent can be ejected into the structure section to erode the remaining part of the structure section as well, or a milling or cutting tool section can be arranged opposite the structure section to provide a proper opening or fully cut the well tubular metal structure in half. Subsequently, the first part of the well tubular metal structure can be withdrawn from the well and/or cement can be ejected above the plug to plug the well from above, so that the cement contacts the inner wall of the borehole and surrounds the plug. To eject cement, the system may comprise a cement section 114 configured to provide cement in the first part above the plug.
The tool string may comprise a logging section 115, as shown in Fig. 6, configured to detect if a part of the structure section has been corroded and to what extent. The tool string may comprise a sampling section for taking a sample of the well fluid present in the well before ejecting the corrosive fluid into the well. The sample is tested for hydrofluoric acid which has to be removed before ejecting the highly corrosive fluid or agent into the well to remove some of the wall of the well tubular metal structure.
As shown in Fig. 8, the downhole plug and abandonment system comprises a first well tubular metal structure 101a and a second well tubular metal structure 101b arranged inside the first well tubular metal structure. A packer 350 is arranged in a sealing manner between the first and the second well tubular metal structure defining an annular space 351 therebetween above the packer. The plug 106 has been set opposite the packer 350, closing the second part 108 of the second well tubular metal structure 101b. Often, part of the well above the packer 350 is plugged and abandoned. The plug 106 may be cement poured down the second well tubular metal structure and may thus have a much longer extension downwards, as illustrated in Fig. 13. In Fig. 9, the tool string 1 has been submerged and the corrosive agent 6 has been ejected above the plug to erode at least part of the structure section 22. In Fig. 10, most of the structure section has been removed along the entire circumference of the well tubular metal structure by means of the corrosive agent. Subsequently, a layer 352 of cement has been arranged above the plug and packer, as shown in Fig. 11, and this process is continued until a sufficient amount of cement to plug the well is provided above the packer. Cement may also be circulated down the second well tubular metal structure and up through the annular space to provide the cement plugging above the packer.
Before plugging the well with cement, a second portion of corrosive agent 6 may be ejected into the first well tubular metal structure above the plug and the packer to also erode part of the first well tubular metal structure, as shown in Fig. 12. A first layer of cement or another protective layer may be arranged on top of the packer and plug before the second portion of corrosive agent is ejected to prevent the packer from being eroded together with the metal wall of the well tubular metal structure. Erosion of the first well tubular metal structure is done to obtain contact with cement adhered to the formation and thus obtain formation contact when cementing above the packer and the plug, as shown in Fig. 13. Subsequently, cement is arranged in a layer on top of the packer and plug, or cement is circulated as earlier described.
In the event that the corrosive fluid or agent merely partly removes the wall of the well tubular metal structure or the tempering or tampering agent has changed the properties of the metal, a milling section or cutting section 330 having projectable parts 331 with bits 332 is used to mechanically remove the wall of the well tubular metal structure 101, as shown in Fig. 14. The tool string therefore comprises a milling section or cutting section 330 and a stroking tool section 340 for displacing the cutting section along the longitudinal extension of the well tubular structure. As the cutting section is moved upwards by the stroking tool section, the bits pulverise or grind the metal wall, and the structure section 22 is removed so that cement can enter the annulus/annular space surrounding the well tubular metal structure. The tool string further comprises an anchoring section 326 for providing a fixed support when stroking and moving the cutting section and when cutting.
The present invention further relates to a downhole plug and abandonment method comprising the following steps. First, a plug of the downhole plug and abandonment system is set inside the well tubular metal structure. Second, the compartment of the downhole tool string is arranged in the first part of the well tubular metal structure adjacent the structure section above the plug. Then, the corrosive fluid or agent is ejected into the structure section, after which at least one opening is provided in the wall of the structure section by corroding the wall of metal. Subsequently, cement is ejected into the well tubular metal structure and into the annulus via the opening, and finally the well is abandoned.
The metal properties of the well tubular metal structure may also be changed by the agent which is a tempering, tampering or modifying agent, e.g. so that the metal becomes brittle and easier to cut through to provide the opening.
The method may further comprise the step of pressurising the first part after the step of providing at least one opening in order to detect if the corrosive fluid or agent has corroded the wall sufficiently to provide the opening. In addition, the method may involve ejecting a second portion of corrosive fluid or agent to the structure section if previous ejected corrosive fluid or agent was insufficient to provide the opening, and finally the method may further comprise the steps of taking a sample of a well fluid in the well tubular metal structure before the step of setting the plug and detecting a content of the well fluid.
The method may involve the step of ejecting a second portion of corrosive fluid or agent on top of the packer and the plug to also erode the first well tubular metal structure. The method may also involve circulating cement down the second well tubular metal structure and up the annular space between the first and the second well tubular metal structure.
The downhole system 100 shown in Figs. 5, 6 and 7 may have merely inner faces or outer faces fully or partly covered with ceramic material. The system may comprise a cement section arranged in another tool string being submerged in a second run after retracting the previous tool string from the well.
The tool string may further comprise a stroking tool section which is a tool providing an axial force. The stroking tool section comprises an electrical motor for driving a pump. The pump pumps fluid into a piston housing to move a piston acting therein. The piston is arranged on the stroker shaft. The pump may pump fluid into the piston housing on one side and simultaneously suck fluid out on the other side of the piston.
By well fluid is meant any kind of fluid that may be present in oil or gas wells downhole, such as natural gas, oil, oil mud, crude oil, water, etc. By gas is meant any kind of gas composition present in a well, completion, or open hole, and by oil is meant any kind of oil composition, such as crude oil, an oil-containing fluid, etc. Gas, oil, and water fluids may thus all comprise other elements or substances than gas, oil, and/or water, respectively.
By a casing or production tubing is meant any kind of pipe, tubing, tubular, liner, string etc. used downhole in relation to oil or natural gas production. The first well tubular metal structure may thus be an intermediate or conductor casing, and the second well tubular metal structure may be the production casing or tubing.
In the event that the tool string is not submergible all the way into the casing, a downhole tractor can be used to push the tool all the way into position in the well. The downhole tractor may have projectable arms having wheels, wherein the wheels contact the inner surface of the casing for propelling the tractor and the tool forward in the casing. A downhole tractor is any kind of driving tool capable of pushing or pulling tools in a well downhole, such as a Well Tractor®.
Although the invention has been described in the above in connection with preferred embodiments of the invention, it will be evident for a person skilled in the art that several modifications are conceivable without departing from the invention as defined by the following claims.

Claims

A downhole tool string (1, 300) configured to be submerged into a well, comprising at least one tool section (2, 304) comprising a first compartment (19, 319) having inner faces (4), the first compartment containing a corrosive or tempering agent (6, 306) during the submersion of the downhole tool into the well, and the tool section having an outlet (3) for ejecting the corrosive agent contained in the first compartment into the well.
A downhole tool string according to claim 1, wherein the inner faces of the first compartment are made of a ceramic material (16, 307) or are fully covered by a ceramic material (16, 307), such as S_iO.
A downhole tool string according to claim 1 or 2, wherein the agent comprises hydrogen sulphide, hydrosulfuric acid or sulfhydric acid, or nitric acid, sulphuric acid or any mixtures thereof.
A downhole tool string according to any of claims 1-3, wherein the tool section comprises a second compartment (20) containing a foam generating agent (24), such as a gas or liquid.
A downhole tool string according to claim 4, wherein the tool section comprises a foam generating unit (18) having a mixing chamber (23) which is in fluid communication with the first compartment and the second compartment so that the foam generating agent from the second compartment is mixed with the corrosive fluid or agent in the mixing chamber to provide a corrosive foam (324) to be ejected into the well.
A downhole tool string according to any of claims 1-5, wherein the tool section comprises an ejecting device (8) for ejecting the corrosive fluid or agent or corrosive foam out through the outlet.
A downhole tool string according to any of the claims 1-6, wherein the tool string comprises a milling section or a cutting section (330).
A downhole tool string according to any of the claims 1-7, wherein the tool section comprises inner faces (4) and outer faces (34), the inner or outer faces being exposed to corrosive fluid or agent when being in the well, and wherein at least one of the faces exposed to the corrosive fluid or agent is partly or fully covered with a ceramic material (16, 307), such as S_iO.
A downhole plug and abandonment system, comprising:
- a well tubular metal structure (101, 301) having an inside (102) and a wall (103) and being arranged in a borehole (104, 302), the borehole and the well tubular metal structure defining an annulus (105) or annular space (351), and

- a downhole tool string (1) according to any of the preceding claims, being submerged and arranged in the well tubular metal structure,
the system further comprising a plug (106) arranged in the well tubular metal structure, thereby dividing the well tubular metal structure into a first part (107) and a second part (108), the first part being closest to a top (112) of the well.
A downhole plug and abandonment system according to claim 9, wherein the first part of the well tubular metal structure has a structure section (22, 322) above the plug, which structure section is to be removed.
A downhole plug and abandonment system according to claim 10, wherein the first compartment comprises the corrosive fluid or agent or the foam generating unit is arranged in the first part of the well tubular metal structure adjacent the structure section so that the corrosive fluid or agent or corrosive foam is ejected from the first compartment or the foam generating unit to partly or fully corrode the wall of the structure section by means of the corrosive fluid or agent or corrosive foam.
A downhole plug and abandonment method comprising the steps of:
- arranging the first compartment of the downhole tool string according to any of the claims 1-8 in the first part of the well tubular metal structure adjacent the structure section,

- ejecting the corrosive fluid or agent or corrosive foam into the structure section from the tool section, and

- corroding the metal wall of the structure section partly or fully from the inside of the structure section to provide a decreased wall thickness of the structure section or an opening in the structure section.
A downhole plug and abandonment method according to claim 12, further comprising the step of:
- milling or cutting the structure section being partly corroded to provide an opening to the annulus in the structure section.
A downhole plug and abandonment method according to any of the claims 12-13, further comprising the step of:
- setting a plug of the downhole plug and abandonment system according to any of the claims 9-11 inside the well tubular metal structure.
A downhole plug and abandonment method according to any of the claims 12-14, further comprising the steps of:
- ejecting cement into the well tubular metal structure and into the annulus via the opening, and

- abandoning the well.