The invention relates to an arrangement for plugging a wellbore in an underground formation, in which a non-rotatable tool string extends from a well-plugging module arranged over a wellhead and down to a plugging zone in the wellbore.

Hydrocarbon wells that are being abandoned must be plugged before the ordinary barriers, which prevent leakage from the hydrocarbon-containing structures to the surface, are removed and the field is cleared. Plugging is often strictly regulated by the regulatory authorities for the field activity. For example, Norwegian authorities demand that, above a hydrocarbon-bearing structure, two plugs of cement or an equivalent material be set, each with a length of 50 metres and in direct contact with the surrounding formation structure, which implies that the casing of the well must be removed in this area. The work of setting such plugs is demanding, both technically and financially, and, as a rule, plugging subsea fields requires the use of large and costly drilling rigs. The removal of casing is carried out with rotating milling tools on the end of a pipe string far from the surface. Often, the milling and cementing equipment is connected to an ordinary, sectioned, rotatable pipe string. It is difficult to achieve sufficient control both of the removal of the casings and of the subsequent cementation, as, with today's techniques, it is difficult to equip the applied tools with instruments (sensors and so on) for monitoring the processes carried out.

From US2010326665 A1 , methods and apparatus for intervention operations in subsea wells, including plugging, are known. To a pipe stem, which can be connected to a cable or the like, a tool is attached and the cable may be connected to a hoisting crane or the like. Energy is supplied through an umbilical.

WO2013006208 A2 discloses a system for plugging a well by means of cable- or coiled-tubing- operated equipment without the use of a drilling rig.

US3294185 A discloses the use of an underwater vehicle, which can be connected to a blowout preventer placed on the seabed to pull up and disassemble a sectioned drill string.

US8261828 B2 discloses a milling tool for removing well casings. The tool may be connected to a cable, a coiled tubing or other known means for positioning a tool in a well. The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.

The object is achieved through the features, which are specified in the description below and in the claims that follow.

An arrangement including a sectioned, non-rotatable, axially displaceable and extendable tool string, extending from a feeding apparatus on a surface installation and into a wellbore, is provided. The tool string is lengthened or shortened by successively connecting or disconnecting, respectively, string sections at an upper tool-string end projecting above the feeding apparatus. At a lower end portion of the tool string, a tool module is detachably arranged. The tool module communicates with the surface installation via one or more lines extending along the tool string via respective line reels arranged on the surface installation to suitable energy- or fluid-supply units and control units. The lines are successively paid out from, or reeled onto, the respective line reels as the tool string is, respectively, extended during insertion into the wellbore or pulled out of the wellbore. The lines are arranged in grooves extending along the tool string and are thereby protected in relation to elements that come into contact with the tool string, for example the feeding apparatus and the casing of the wellbore. In its simplest form, the tool string may be formed with a cross section like an H- or I-beam, wherein the lines are laid against the web portion connecting the flanges. The connections between the statically arranged energy-/fluid-supply units and control units and the rotatable reels are provided by means of swivels, slip rings and so on in a manner known per se.

The lower end portion of the tool string is adapted for connection to various types of tool modules.

The feeding apparatus and the tool string may be configured as described in the patent application NO 10131 157.

For the removal of casing and surrounding concrete a milling tool is used, which includes a rotating, radially expandable milling head connected to a drive unit attached to the tool string. The drive unit may be operated by means of electric power or hydraulic fluid supplied from an energy source on the surface installation via suitable lines extending along the tool string. Preferably, the milling tool includes elements for radial centring in the wellbore to stabilize the milling tool. Axial displacement may be provided by means of actuator devices connected to the centring elements or by the tool string being displaced by means of the feeding apparatus on the surface installation.

To delimit a cement plug downwards in the wellbore, it may be necessary to set a boundary plug. A plug-setting tool may be attached to the tool string for this purpose and be lowered into the wellbore. For cement to be distributed in a milled-out portion of the wellbore, a cementing head attached to the tool string is used.

In association with the different tools (the milling head, the plug-setting tool and the cementing head) there are suitable sensors connected to control units on the surface installation via signal lines extended along the tool string, for recording relevant process data, for example image data for quality checks on the work performed, position data for tool elements, temperature and pressure data, speed data and so on. The transmission of such data can be done reliably, efficiently and continuously by means of the line connections between the tool and the control units on the surface installations.

Two or more tools may possibly be combined so that a tool change will not be necessary during the plugging operation. The plug-setting tool may, for example, be detachably connected to the cementing head and be left behind in the wellbore after the boundary plug has been placed below the area where the cement plug is to be established. The milling tool may include one or more cement- outlet openings, which can be connected to a cement-supply line extending along the tool string from a cement-mixing plant on the surface installation. During cementation, the milling head may possibly rotate to work as a distributor.

The surface installation may be placed over a subsea wellhead, that is to say on a seabed over a subsea well, on a vessel floating on a sea surface above a subsea well, or it may be placed over a wellhead on land.

The invention is defined by the independent claim. The dependent claims define advantageous embodiments of the invention.

The invention relates, more specifically, to an arrangement for plugging a wellbore in an underground formation, in which a non-rotatable tool string extends from a well-plugging module arranged over a wellhead down to a plugging zone in the wellbore, characterized by

- the well-plugging module including

a feeding apparatus arranged for the axial displacement of the tool string, and

at least one line reel; and

- the tool-string being formed from

string sections which are provided with asymmetric string couplings at their respective end portions, and

the tool-string including at least one line groove arranged to receive one or more lines extending from an energy-supply unit via a first line reel to a tool module arranged in a lower end portion of the tool string; and - the tool module including

at least one of a tool taken from a group including a milling tool with a radially expandable, rotatable milling head, a tool for setting a boundary plug in or near the plugging zone, and a cementing head, and

at least one sensor which is connected in a signal-communicating manner to a control unit via a signal line which is reelable onto a second line reel on the well-plugging module.

The tool module may be formed of a combination of two or more of said tools. Thereby the advantage of allowing more plugging operations to be carried out in a reduced number of trips of the tool string into the wellbore is achieved.

The tool module may be formed from a combination of the radially expandable, rotatable milling head, the tool for setting the boundary plug, and the cementing head, wherein the tool for setting the boundary plug is arranged to be detachable from the milling head, and the milling head includes one or more cement-outlet openings. The advantage of allowing all plugging operations to be carried out in just one trip of the tool string into the wellbore is thereby achieved.

The at least one sensor may be taken from the group comprising an image sensor, a position sensor, a temperature sensor, a pressure sensor and a speed sensor. The plugging operations can thereby be carried out with better quality than before, because a better impression of the condition in the wellbore and the work being carried out in the wellbore is formed.

The well-plugging module may be arranged on a subsea wellhead, and the energy-supply unit and the control unit may be arranged on a surface installation. Thereby the plugging operations may be remote-controlled via flexible connections between the surface installation and the wellbore, for example umbilical connections for the transfer of energy, fluids and signals.

The well-plugging module may be arranged on a subsea wellhead and may comprise an injector head adapted for pressure-sealing insertion of the tool string into the wellhead. Thereby the insertion of the tool string can be carried out with a greater degree of safety in a wellbore, which exhibits an overpressure.

The milling tool may include several radially displaceable centring elements. The milling of the plugging zone can thereby be carried out with a greater degree of precision.

The milling tool may include a drive unit, which is fixable relative to the wellbore by means of several radially displaceable centring elements, and the milling head may be displaceable relative to the drive unit by means of one or more axial-displacement actuators. Alternatively, the milling tool may be radially fixable relative to the wellbore by means of several radially displaceable centring elements, and the milling head may be axially displaceable by means of the feeding apparatus.

In what follows, examples of preferred embodiments are described, which are visualized in the accompanying drawings, in which:

Figure 1 shows a side view of a lower portion of a tool string with a milling tool;

Figure 2 shows a cross section ll-ll according to figure 1 through the tool string, on a larger scale;

Figure 3 shows a side view, on a smaller scale, of a feeding apparatus together with line reels and a tool-string-section manipulator;

Figure 4 shows a side view of the feeding apparatus together with the line reels and the tool-string-section manipulator during lowering from a surface installation along guides to a subsea wellhead;

Figure 5 shows a vertical section on a larger scale through a wellbore in which a cement plug is being established by means of a cementing head (shown in a side view) over a boundary plug in a portion from which a casing has been milled away; and

Figure 6 shows a vertical section through a wellbore, in which a tool module (formed as a combination of a milling tool, a setting tool for a boundary plug and a cementing head) is shown in a side view, and in which the setting tool (shown in a side view) has been left in a lower portion of the plugging zone together with the boundary plug.

A wellbore 1 1 (see figures 5 and 6) extends downwards in a formation 1 and is bounded in a manner known per se towards the formation 1 by means of a well lining 1 1 1 which is shown here as one casing 1 1 1 a which is fixed to the formation 1 by the annulus between the casing 1 1 1 a and the formation 1 having been filled with cement 11 1 b. A plugging zone 1 12 in which a plug 12 is to be established for the permanent closure of the well bore 1 1 is formed by the well lining being removed from a prescribed extent of the wellbore 1 1 to ensure that the plug 12 will be sufficiently bonded to the surrounding formation 1 .

A well-plugging module 5 (see figures 3 and 4) includes a frame 51 carrying a feeding apparatus 52 of the kind that is described in the patent application NO 20131 157 for the controlled, axial, non- rotatable displacement of a tool string 6. The well-plugging module 5 includes a tool-string-section manipulator 54 arranged to handle string sections 61 when the tool string 6 is being lengthened or shortened. First and second line reels 53, 53a are arranged at an injection head 58, which at least forms guides for lines 55, 55a that have been reeled up on their respective line reels 53, 53a and can be paid out and pulled in together with the tool string 6. The first line 55 is an energy-supply line, which is connected via a swivel device known per se (not shown) in the line reel 53 to an energy-supply unit 56. By the term "energy supply" are to be understood both a supply of electrical energy and a supply of hydraulic energy, that is to say fluid under pressure, for example hydraulic oil, cooling liquid and cement. The second line 55a is shown here as a signal line which is connected via a swivel device, not shown, to a control unit 57.

Reference is now made to figures 1 and 2, in which a lower portion 62 of the tool string 6 is shown in greater detail. The tool string 6 is composed of several string sections 61 which are connected to each other via a string coupling 61 1 , 612 of the kind that is described in the patent application NO 20131 157, that is to say an asymmetric tool-string coupling which ensures that the string sections 61 will have the same orientation relative to each other to ensure that longitudinal grooves 613, 614 on the periphery of the string sections 61 (see figure 2 in particular) extend uninterruptedly along the entire tool string 6. The grooves 613, 614 form protective recesses for the lines 55, 55a extending along the tool string 6 from the line reels 53, 53a to the tool module 7. As shown in figure 2, the grooves 613, 614 may have different cross sections in order to be precisely adapted to the line 55, 55a that each groove 613, 614 is going to accommodate. Even though the exemplary embodiments show and describe two lines and two grooves, the invention is not restricted to this. If necessary, a groove may also be designed for the accommodation of several lines.

In its simplest embodiment, the tool module 7 includes one tool, for example a milling tool 71 as is shown in figure 1 . When one operation has been carried out, the tool string 6 is pulled up for a change of tool before it is run down the wellbore 1 1 again for the next operation to be carried out. Other relevant tools are a plug-setting tool 72 (see figure 6) and a cementing head 73 (see figure 5). Several tools can be combined as is suggested in figure 6 and further described below.

The milling tool 71 (see figures 1 and 2 in particular) includes a drive unit 712, which is attached in a rotationally rigid manner to the tool string 6. The drive unit 712 is typically an electromotor or a hydraulic motor, which is supplied with energy from the energy-supply unit 56 via a power line 55. Connected to the drive unit 712, there is a rotatable milling head 71 1 , which is provided with radially displaceable cutting edges 71 1 a. To stabilize the milling tool 71 when in operation, it is provided with centring elements 713, which can be brought into abutment against the well lining 1 1 1 or the formation 1 . In the embodiment shown in figure 1 , the milling head 71 1 is axially displaceable relative to the drive unit 712, and thereby relative to the tool string 6, by means of an axial-displacement actuator 714. Alternatively, the milling head 71 1 may be displaced by means of the tool string 6. In this embodiment (not shown), it is obvious to arranged the centring elements 713 in such a way that the milling tool 71 can be displaced axially relative to the centring elements 713 when these have been clamped against the well lining 1 1 1 or the formation 1 .

The plug-setting tool 72 is arranged to carry a boundary plug 8 and set it in or near the lower end of the plugging zone 1 12 to form a boundary for the material of the plug 12 (see figure 5) when this is pumped into the plugging zone 1 12. The plug-setting tool 72 may be of the kind that is left behind in the wellbore 1 1 together with the boundary plug 8, as is shown in figure 6.

The cementing head 73 (see figure 5), which may be supplied with a pumpable plug material, typically a cement mixture, from the surface installation 9 via a fluid line 55, is provided with one or more outlet openings 731 for distributing the plug material in the plugging zone 1 12.

Two or more tools can be combined in one and the same tool module 7 as is indicated in figure 6, the plug-setting tool 72 being shown here as released from a tool connector 721 on the cementing head 73 which in turn forms a lower part of the milling tool 71 .

One or more of the tools are provided with one or more sensors 74, 75, 76, shown on the milling tool 61 in figure 1 and shown on the cementing head 73 in figure 5. The sensors 74, 75, 76 communicate with the control unit 57, typically via one or more signal lines 55a extending along the tool string 6. The sensors 74, 75, 76 are selected from a group comprising an image sensor, a position sensor, a temperature sensor, a pressure sensor and a speed sensor, so that all the processes that are to be carried out may be monitored by an operator.

The well-plugging module 5 may be arranged on a surface installation 9 located above a wellhead 3, for example on a vessel floating on a water mass 2, or directly on the wellhead 3. The well- plugging module 5 may, as is indicated in figure 4, be lowered onto a subsea wellhead 3 and be remotely controlled via an umbilical 34. The well-plugging module 5 may be guided down onto the wellhead 3 by means of several guide wires 33, which are extended in a manner known per se between the wellhead 3 and the surface installation 9.

It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative embodiments without departing from the scope of the attached claims. In the claims, reference numbers in brackets are not to be regarded as restrictive. The use of the verb "to comprise" and its different forms does not exclude the presence of elements or steps that are not mentioned in the claims. The indefinite article "an" or "a" before an element does not exclude the presence of several such elements.

The fact that some features are stated in mutually different, dependent claims, does not indicate that a combination of these features cannot be used with advantage.