NO20190949A1 - Tool and metod of reducing friction of casing or disintegrating cement - Google Patents

Description

Technical Field

[0001] The present invention relates to the technical field of well intervention. In particular, the invention relates to removal of sections of casing in a petroleum well. The well may be a subsea well or a land well. Even more particularly the invention relates to a tool for reducing friction of cement or other bonding agent outside a casing or liner in a subterranean well, the tool being attachable to a lower end of a drill string, as defined in the preamble of claim 1, as well as a method of reducing friction of cement or other bonding agent outside a casing or a liner in a subterranean well, as defined in the preamble of claim 6.

Background Art

Petroleum wells are lined with casing pipes extending from the surface, and liner pipes extending along parts of the well such as from a casing pipe's lower end. Several casings may be arranged with decreasing diameter within previously installed casing pipes along with the sinking of a well into the ground. A casing may be cemented to the surrounding borehole wall at critical parts along the casing, such as in the upper portion, in the lower portion, and selected points along its length. A subsequent casing or liner may be cemented to the surrounding casing (or to the borehole wall) also at critical points in order to avoid leakage, or to stabilize the casing, or to isolate oil or gas producing zones, or to isolate from water-bearing zones. The cement in the casing annulus, whether in the first or subsequent annuli, is designed to be leak-proof and to have excellent bonding to steel and/or the surrounding geological formation.

In this context, not only cement may hold the casing. Alternatives, such as resin and polymers (possibly mixed with cement) may also be used. The term "cement" in the context of the present application is intended to cover all such materials that are used to hold casings and liners in the well, and which does not disintegrate or dissolve by liquid flow alone. These may also comprise, in addition to homogeneous cement, partly cracked cement, barite, salt-based deposits, calcium carbonate-based deposits and the like, as well as scaling and hard packed particulate matter in the annulus.

It may be required to remove a given section of the casing for several different reasons. It may be done in order to permanently abandoning the well or increasing the available annulus flow area enabling placement of a new barrier. The annulus about a casing to be cemented may already comprise a cement fill, which may be imperfect or simply too short. Current official regulations for plugging and abandonment of a zone of a well may require 50 m of plug, while an existing plug in the annulus may have shorter extension and thus not be acceptable for the P&A (Plugging and Abandonment) purpose. Such an annulus plug will then pose an obstacle to cementing the annulus over a desired, required new length, and should thus be removed in order to circulate in new cement slurry to form a new annulus barrier.

There have been a few attempts to provide devices and methods to perform work on casings, but these are all from some time back and involves relatively complicated special equipment. The most relevant prior art examples are the following:

US1695749 is directed towards at tool for cleaning of a casing. It can also be used to dislodge a casing from the formation. The tool has an eccentrically mounted rotor. The rotor is driven by fluid flow through a pipe to which the tool in coupled. When the rotor rotates it will jar against the casing and loosen this from the formation.

This tool requires a fluid flow through it to work. Moreover, the efficiency will highly depend of the fluid flow and there is a great potential for the tool to fail due to the hard impacts with the casing.

US 2730176 shows a tool for loosening casings in a wellbore. The tool comprises a disk with a smaller diameter than the casing. The disk is attached to a drill rod by a universal joint. When the drill rod is set into rotation, the disk will roll against the inner surface of the casing. This sets the casing into vibrations or oscillations. The frequency of the oscillations depends on the ratio between the diameter of the casing and the diameter of the disk. The oscillations are supposed to loosen the casing from the formation.

This tool has serious drawbacks. Firstly, the diameter of the disk has to be adapted to the diameter of the casing, which means that there has to be a selection of disks available. It also means that the tool has to be pulled back to the surface for the disk to be changed if another section of casing is to be loosened. This is very time consuming. Secondly, the universal joint is a vulnerable element, the oscillations of the casing will inevitably be transferred to the universal joint, which in turn will be subjected to great wear. There is a significant risk that the joint will fail.

GB541057 describes a device which in constructive features is similar to US 2730176, but which is used to accelerate placing and consolidation of a concrete mix by imparting vibrations to the mix.

US 5515922 describes a tool for recovering downhole equipment, such as a drill bit, from a well by imparting vibrations to the equipment. The tool has an unbalanced weight arranged within a housing. Rotation of the weight is done by fluid circulation. When the weight is rotated it will impart vibrations to the drill string, which in turn will be transferred to the equipment. These vibrations will assist to recover the stuck equipment.

This tool has many of the same drawbacks as the tool of US1695749, as it also depends on fluid power. In addition, it is not designed to impact the casing and will therefore not be able to fulfil the aims of the present invention.

It is therefore a need for a more robust and efficient tool to assist in reducing the grip of the cement on the casing or to disintegrate existing cement in order to inject new cement.

Summary of invention

A main object of the present invention is to disclose a device and a method for reducing the external casing friction in order to retrieve casings from the well. This is achieved by the use of a novel tool to disintegrate the cement previously present in the annulus and thereby reducing the friction between the casing and the walls of the well or an outside casing, which in turn will enable easier casing retrieval, or disintegrating old and inadequate cement so that new cement can be injected. This is achieved by a tool that forms a unitary element with an eccentric mass distribution and is fixedly attachable to the drill string.

In the method of the invention the friction is reduced by lowering the tool string into the well and activating the tool by either rotating the entire string using rig surface rotary equipment or rotating specific down hole tools using a down hole motor and move the string upwards and downwards over a pre-determined distance covering the interval of interest. Annulus cement/barite will be thereby be disintegrated and annulus voids are created. This allows for an annulus flow as well as reduction of tubular friction and enables retrieval of longer sections of casing. This is achieved by a method comprising the steps of:

- fixedly attaching a cement crushing tool to the lower end of a drill string, said crushing tool having an eccentric mass distribution,

- running the drill string with said crushing tool into said well,

- positioning said crushing tool within a selected section of the casing,

- providing rotation to said crushing tool to make the tool impact the inside of the casing or liner and disintegrate the cement or bonding agent outside the casing,

- bringing a casing retrieval tool into the selected section,

- retrieving said selected section of the casing by pulling the drill string with the casing retrieval tool upwards.

Brief description of drawings

[0002]

Figure 1 shows a first embodiment of the tool of the invention, and

Figure 2 shows a second embodiment of the tool of the invention.

Detailed description of the invention

[0003] Figure 1 shows a well section 1 having a casing 2 that is cemented in place in the well by cement 3. A tool 4 of the invention is attached to a drill string 5. The drill string 5 and the tool 4 has a centre axis 7. In a first embodiment, the tool 4 comprises a head 8 and a weight 6 arranged on the head 8 outside of the centre axis 7. This provides eccentricity or imbalance to the tool. The weight 6 may be welded or bolted to the head 8 or formed integrally with the head 8.

[0004] When the tool 4 is rotated, either by rotating the drill string 5 or by a downhole motor (not shown), the eccentricity or imbalance will make the tool 4 wobble within the casing 2 and as the speed of rotation increases it will start impacting the inside of the casing 2. These impacts will be transferred to the cement behind the casing 2 and tend to break up the cement.

[0005] Figure 2 shows an alternative embodiment of the invention. It is similar to the embodiment of figure 1 but for the weight 6 on the outside of the head 8. Instead the tool 4 has been shaped into an S-shape by having a first section 9 concentric with the centre axis 7 of the drill string 5, a second section 10 adjoining the first section 9 but being arranged eccentric to the centre axis 7 and a third section 11 adjoining the second section, which is arranged concentric with the centre axis 7. The sections 9, 10, 11 are preferably made in one integrated piece, such as by forging or casting.

[0006] This provided imbalance to the tool, and upon rotation of the tool, either by rotating the drill string 5 or by a downhole motor (not shown) the tool 4 will wobble and impact the casing 2.

[0007] The greater the total weight of the tool is, and the larger the imbalance of the weight is, the slower rotational speed is sufficient to achieve wobbling and impacting of the casing 2.

[0008] The tool may also be shaped and formed otherwise than described above, as long as the tool forms a unitary element with an eccentric mass distribution.

[0009] The drill string 5 may also be equipped with a casing retrieval tool (not shown), a perforating gun, a casing cutting tool, washing tool, packers and circulation equipment or other tools to be used in connection with the tool of the invention.

Alternatively, these tools may be used separately in subsequent downhole trips.

[0010] When it is desired to break up the cement along a particular section of the well, the tool 4 of the invention is attached to the lower end of a drill string 5. Then the drill string 5 is lowered into the well until the tool 4 reaches the section to be treated. Then the drill string is rotated so that the tool impacts or hammers the casing. If the section to be treated is short, it may be sufficient to keep the tool 4 at the same depth in the well, but to treat a longer section, the drill string 5 is conveniently moved up and down in the well along the section to be treated.

[0011] It may be difficult to know how long duration the treatment has to have to sufficiently crush the cement. One way of detecting this is to have a casing retrieval tool attached to the drill string. This can be intermittently brought to grip the casing so that a pull can be exerted on the casing. If the casing is loose, it will be possible to move the casing upwards in the well.

[0012] Another way of detecting sufficient crushing of the cement is to perforate the casing and applying fluid pressure within the casing. If fluid escapes, this will be because it migrates through the crushed cement. The pressure and rate at which the fluid escapes provide a good measure of the penetrability of the cement. The drill string is conveniently equipped with packers to seal off a section of the well so that fluid does not escape elsewhere.

[0013] When the cement has been found to be sufficiently crushed, the casing may be retrieved or, if the well is to be plugged and abandoned, new cement may be injected into the well.

Claims (1)

1. Claims

   1.

   A tool for reducing friction of cement or other bonding agent outside a casing or liner in a subterranean well, the tool being attachable to a lower end of a drill string, characterised in that the tool forms a unitary element with an eccentric mass distribution and is fixedly attachable to the drill string.

   2.

   The tool of claim 1, characterised in that the tool comprises a head that is arranged to be concentric with a centre axis of the drill string and a weight arranged on the head outside the centre axis, said weight providing an eccentric mass distribution relative to the centre axis.

   3.

   The tool of claim 1, characterised in that the tool comprises a generally S-shaped element.

   4.

   The tool of claim 3, characterised in that the tool comprises a first section attachable to the drill string and which is concentric with the centre axis, a second section adjoining the first section, said second section being eccentric relative to the centre axis, and a third section, adjoining the second section, said third section being concentric with the centre axis.

   5.

   The tool of any of the preceding claims, characterised in that it is adapted to be run into the well on a drill string together with other tools, such as a casing retrieval tool, a perforating gun, a casing cutting tool, washing tool, packers and circulation equipment.

   A method of reducing friction of or disintegrating cement or other bonding agent outside a casing or a liner in a subterranean well, comprising the steps of:

   - fixedly attaching a cement crushing tool to the lower end of a drill string, said crushing tool having an eccentric mass distribution,

   - running the drill string with said crushing tool into said well,

   - positioning said crushing tool within a selected section of the casing,

   - providing rotation to said crushing tool to make the tool impact the inside of the casing or liner and disintegrate the cement or bonding agent outside the casing.

   7.

   The method of claim 6, wherein it further comprises the steps of:

   - bringing a casing retrieval tool into the selected section,

   - retrieving said selected section of the casing by pulling the drill string with the casing retrieval tool upwards.

   8.

   The method of claim 6 or 7, wherein it further comprises the steps of running a perforating gun into said casing section, perforating said casing section, running a washing tool into said casing section and washing out the disintegrated cement or bonding agent by injecting washing fluid through the perforated casing.

   9.

   The method of claims 7 and 8, wherein it further comprises injecting cement or bonding agent into the annulus between the casing and the well bore formation.

   10.

   The method of claim 7, 8 or 9, wherein the casing retrieval tool, the perforating gun or the washing tool are run on the drill string together with the crushing tool.