WO2013135297A1

WO2013135297A1 - Centralizer for expandable casing

Info

Publication number: WO2013135297A1
Application number: PCT/EP2012/054606
Authority: WO
Inventors: Steinar Wasa Tverlid
Original assignee: Statoil As
Priority date: 2012-03-15
Filing date: 2012-03-15
Publication date: 2013-09-19

Abstract

A centralizer (5) for an expandable casing (6), the centralizer comprising at least one elastically deformed element (7) held in a non-expanded position relative to an expandable casing wall by a weak region (8). The weak region is arranged to break during expansion of the expandable casing, causing the elastically deformed element to revert to an expanded position in order to centralize the expandable casing.

Description

CENTRALIZER FOR EXPANDABLE CASING

TECHNICAL FIELD

The present invention relates to a centralizer for expandable casing.

BACKGROUND

Wellbores for producing hydrocarbons are provided with pipes made of tubulars functioning as casings. Casings are formed by jointing a large number of tubular sections. A casing is located in a wellbore, and is usually cemented into place to provide mechanical strength, isolate the casing from the wellbore and prevent unwanted fluid flow between the geological formations of the wellbore.

Referring to Figures 1a and 1 b, a casing 1 is located in a wellbore 2. Cement 3 is pumped through the casing 1 to the bottom of the wellbore 2 and allowed to enter the annular space 4 between the casing and the sides of the wellbore. A consistent and predictable level of cement is required to ensure that the casing 1 is adequately isolated from the sides of the wellbore 2.

If the casing 1 is offset within the wellbore 2, the cement 3 is unlikely to evenly fill the annulus. In Figures 1a and 1 b, the casing 1 is shown to be offset towards the left side of the wellbore 2. The narrower gap on the left side of the annulus 4 makes it more difficult for the cement 3 to fill the annulus 4, so it will tend to fill more easily on the right side of the annulus 4, leading to an uneven cement job. Furthermore, if the cement job is not uniform then hydrocarbons may migrate freely through the uncemented narrow passage from the bottom to the top and cause a blow out. It is therefore important to ensure that the casing 1 is located in the wellbore 2 such that the annular gap 4 between the casing 1 and the wellbore 2 is substantially uniform along the length of casing 1 to be cemented. This ensures that the cement 3 is more likely to fill the annular gap 4 evenly and uniformly, leading to a more reliable cement job.

It is known to use one or more centralizers disposed along the length of the casing 1 to hold the casing 1 away from the wellbore 2 walls and keep the casing 1 as close as possible to the centre of the wellbore. However, most centralizers are not designed for use with expandable casing. Expandable casing is relatively thin wall casing that is, prior to expansion, located in the wellbore 2. It is subsequently expanded to a larger diameter using a cold working process by driving a body having a larger diameter than the diameter of the unexpanded casing down into or up through the casing 1. Such bodies are typically substantially conical and may have a surface treatment to provide hardness and reduce the risk of spalling on the interior wall of the casing 1. The tubular may be lubricated inside, also to address the issue of spalling and reduce friction. They may be made from a single element or several assembled smaller elements. Alternatively, the body may comprise cylindrical rollers having skewed or slanted axes with respect to the longitudinal axis of the pipe. A problem with expandable casing is that most centralizers cannot be used, as the expansion process damages the centralizers, typically by plastic deformation. This can lead to the casing 1 being offset within the wellbore 2.

There is a gap between the casing to be expanded and a previously set casing, through which the expandable casing has to be run. The gap is sufficiently small for enough drilling fluid to pass without causing trouble. This is to avoid expanding the tubular more than strictly necessary, as the degree of expansion must be a trade off against the final strength of the expanded casing. The centralizer occupies the space in this gap, and so needs to be as slim as possible in run-into-well state.

US 2008/0283253 describes a centralizer for an expandable casing. This is similar to a conventional centralizer, except the centralizer collar that extends around the outer diameter of the casing has sliding elements that allow the centralizer collar to expand when the casing is expanded.

US 2003/0230411 describes a centralizer for expandable casing. This is also similar to a conventional centralizer, but is a flexible non-metallic material such as polyethylene. The flexibility of the material allows it to withstand expansion without breaking, but non- metallic flexible materials may not have the required stability at the sort of temperatures that may be encountered within a wellbore, and may not be sufficiently rigid to ensure adequate centralizing of the casing 1. SUMMARY

The present invention provides a centralizer for an expandable casing. According to a first aspect, there is provided a centralizer for an expandable casing. The centralizer comprises at least one elastically deformed element held in a non-expanded position relative to an expandable casing wall by a weak region. The weak region is arranged to break during expansion of the expandable casing, causing the elastically deformed element to revert to an expanded position in order to centralize the casing. As an option, the elastically deformed element comprises a plate and the weak region comprises a groove in the plate, the groove being parallel to a main axis of the expandable casing. This ensures that, during expansion, the groove breaks preferentially and the plate "springs back" to its original form to centralize the casing. As a further option, the plate is provided with three grooves, each groove being spaced substantially 120° radially apart from the other grooves.

There are other types of weak region that may be used, examples of which include any of a series of perforations in the elastically deformed element, a thinned section of the elastically deformed element, a weld between two elastically deformed elements, a forged weak region, and a region having a region with different mechanical properties.

The elastically deformed element may be attached to at least one ductile end part, the ductile end part configured to be attached to the expandable casing by any of welding, gluing, bolting and clamping. Furthermore, the elastically deformed element may additionally be attached to a further ductile end part at an opposite end of the elastically deformed element to the at least one ductile end part. This allows the centralizer to be attached to the expandable casing. According to a second aspect, there is provided an expandable casing joint that comprises at least one centralizer as described above in the first aspect.

According to a third aspect, there is provided a method of expanding an expandable casing in a wellbore. The method comprises providing the casing with a centralizer, the centralizer comprising at least one elastically deformed element held in a non- expanded position relative to an expandable casing wall by a weak region. The casing is then located in a wellbore, and an expansion cone is driven through the expandable casing to increase the diameter of the casing. The increase in diameter causes the centralizer weak region to break, thereby causing the elastically deformed element to revert to an expanded position in order to centralize the expandable casing within the wellbore.

As an option, the elastically deformed element comprises a plate and the weak region comprises a groove in the plate, the groove being parallel to a main axis of the expandable casing, wherein the expansion of the cone causes the groove to break.

As an alternative option, a weak region is provided by providing any of a series of perforations in the elastically deformed element, a thinned section of the elastically deformed element, a weld between two elastically deformed elements, and a forged weak region.

The method may further comprise providing the expandable casing with a centralizer by attaching the elastically deformed element to at least one ductile end part, the ductile end part configured to be attached to the expandable casing by any of welding, gluing, bolting and clamping. As an additional option, the method may include attaching the elastically deformed element to a further ductile end part at an opposite end of the elastically deformed element to the at least one ductile end part.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1a illustrates schematically a side elevation cross section view of a wellbore with an offset casing;

Figurel b illustrates schematically a plan elevation cross section view of the wellbore of Figure 1a;

Figure 2a is a perspective view of a centralizer according to an embodiment of the invention before expansion of the casing; Figure 2b illustrates schematically a plan elevation cross section view of the centralizer and casing illustrated in Figure 2a located offset in a wellbore before expansion of the casing; Figure 3a is a perspective view of a portion of a centralizer according to an embodiment of the invention after expansion of the casing;

Figure 3b illustrates schematically a plan elevation cross section view of the centralizer and casing illustrated in Figure 3a located substantially centrally in a wellbore after expansion of the casing;

Figure 4 is a side elevation schematic view of a casing comprising at least two tubular sections; and Figure 5 is a flow chart showing the steps of the deployment of a centralizer according to an embodiment of the invention.

DETAILED DESCRIPTION An expandable casing is provided with a centralizer that has elastically deformed elements disposed around its circumference. The elastically deformed elements are held in place at a weak region. When the casing is expanded, the weak region is broken in preference to any other part of the centralizer and the deformed elements can revert to their original shape. The elements are arranged such that their original shape causes them to project radially outwards from the casing, thereby centralizing the casing within the wellbore.

Figures 2a and 2b illustrate a centralizer 5 according to a first embodiment of the invention before expansion. In this embodiment, the casing 6 has an elastically deformed plate 7 with a high yield stress disposed around the circumference of the casing 6. This plate 7, when unstressed, forms a planar plate. However, it is elastically deformed so as to form a tube that has an inner diameter that corresponds with the outer diameter of the unexpanded expandable casing 6. The plate 7 is placed in a condition of stress, but has not exceeded its yield stress. Therefore the plate is only deformed elastically, and no plastic deformation has occurred. Three weak regions 8 in the form of sharp grooves are milled into the pre-stressed plate 7, 120° apart from one another. Each weak region 8 has a high stress concentration when deformed, and so can be subsequently used as a controlled weak region. It is important that the weak regions are sufficiently strong to avoid breaking during installation of the casing 6, but sufficiently weak to ensure that they fail uniformly as the casing 6 is expanded.

The pre-stressed plate 7 may be fixed to two more ductile pipe parts 9 at each end of the plate 7. Any suitable fixing method may be used, and welding is thought to be particularly suitable. The welding is performed at three fixing regions 10 midway between each weak region 8. Each fixing region 10 is disposed 120° from the other fixing regions 10 and 60° from the surrounding weak regions 8. The ductile pipe parts 9 at each end of the deformed plate ensure that the deformed plate 7 is held in its elastically deformed tubular form.

The use of ductile pipe parts 9 also means that the centralizer 5 can be fitted to an existing casing joint, without the need to further modify the casing joint. However, it will be appreciated that the elastically deformed plate 7 of the centralizer 5 could be fitted directly to the casing 6.

The centralizer 5 at this stage consists of an elastically deformed plate 7 having weak regions 8 and held in a tubular configuration by the ductile pipe parts 9 at either end of the elastically deformed plate 7.

The centralizer 5 is then placed around the casing 6. The ductile pipe parts 9 are fixed to the expandable casing by any suitable means, such as gluing, clamping or similar.

Before expansion, the inner diameter of the centralizer 5 is flush with the outer circumference of the expandable casing 6. The overall thickness of the casing 6 including the centralizer 5 is only increased by the wall thickness of the elastically deformed plate 7 and the ductile pipe parts 9. As the casing is lowered into the wellbore 2, the centralizer 5 protects the casing 6 against scores and scratches which potentially could cause stress concentration spots that would open the expandable casing 6 during the expansion process. The centralizer 5 does not extend along the entire length of the casing 6. For example, a casing joint may be 12 metres in length and have one or two centralizers 5 disposed around it. Each centralizer 5 may be only 30 cm in length.

Figure 2b shows that, before expansion of the casing 6, the casing 6 is not centralized in the wellbore 2. Once the unexpanded casing 6 has been located in position within the wellbore 2, cement is pumped through the casing 6 to the bottom of the wellbore 2 and allowed to rise up to a desired height in the annulus between the outside of the casing 6 and the wellbore. At this point the casing 6 is not centralized, and so the cement may be unevenly distributed, as shown in Figure 1a.

Before the cement sets, the expansion operation is carried out. As described above, this involves driving an expansion cone through the casing 6. When the expansion cone passes through the casing 6, it cold works the casing and expands it to a larger diameter. As the expansion cone passes a section of the casing 6 having a centralizer

5, the ductile pipe parts 9 also expand along with the casing 6. However, the expansion cone causes the weak regions 8 in the elastically deformed plate 7 to exceed their yield stress and break, as illustrated in Figure 3a. The pre-stressed plate breaks into effectively three separate plates 1 1 , 12, 13, which revert to their original shape and lift the casing 6 away from the wellbore 2 walls. This centralizes the casing

6, as illustrated in Figure 3b, even in horizontal sections of the wellbore 2.

Before expansion, the plate 7 is under a great deal of stress. When the weak regions 8 break and the plate springs back to its undeformed shape, a large amount of energy is released. The casing 6 quickly moves to a centralized position in the wellbore 2. This squeezes the liquid cement to a more even distribution around the entire casing 6. It is therefore a requirement that the cement does not set until after the expansion operation.

Figure 4 shows a side elevation schematic view of a casing comprising at least two casing sections. It can be seen that each casing section 15, 16 in this example has two centralizers 5 fixed to it. As the centralizers 5 do not extend along the full length of the casing 6, when the casing 6 is expanded, the cement can flow through the gaps formed between the three separate plates 1 1 , 12, 13 of each centralizer 5. The centralizers 5 therefore do not impede the flow of cement and allow a uniform cement job to be achieved.

Steps of an embodiment of the invention are shown in Figure 5. The following numbering corresponds to that of Figure 5:

51. The unexpanded casing 6 fitted with centralizers 5 is installed in the wellbore 2.

52. The cement job is performed. This involves pumping cement through the casing to the bottom of the wellbore and allowing the cement to fill the annulus between the expandable casing 6 and the wellbore 2.

53. Before the cement sets, the expansion cone is driven through the casing 6, increasing the diameter of the casing 6.

54. As the expansion cone passes through the casing 6 at a point adjacent to a centralizer 5, the weak regions 8 in the centralizer break, causing the elastically deformed plate 7 to break into three plates 11 , 12, 13 that spring back. S5. The action of the three plates 11 , 12, 13 springing back causes the casing 6 to adopt a centralized position in the wellbore, and redistributes the cement to give a more even cement job. The cement can then be allowed to set.

It will be appreciated that many configurations of centralizer can be designed using the principle of having one or more weak regions that, when broken by the expansion operation, cause an elastically deformed element to spring back and centralize the casing 6.

For example, in an alternative embodiment, three separates plates may be welded to the casing 6 along the axis of the casing at the same points 10 as the plate 7 in the first embodiment are attached to the ductile pipe parts 9. The plates are then bent to curve around the outer diameter of the casing 6 connected to one another at the same point as the weak regions 8 in the above-described first embodiment. The connections of the plates to one another form the weak regions, rather than milling a notched groove. This may be done by any suitable means, such as spot welding, bolting, gluing etc. In this way, the same effect of three plates 11 , 12, 13 springing out and centralizing the casing 6 when it is expanded is achieved, and the casing 6 is centralized in the wellbore 2. In a further embodiment, otherwise compatible with the first embodiment, the weak region is not provided by milling sharp grooves into the elastically deformed plate 7, but by providing an elastically deformed plate 7 that has perforations along the axis of the casing. The perforated section forms a weak region that preferentially breaks when the expansion cone is driven through the casing 6.

In a further embodiment, otherwise compatible with the first embodiment, the weak region is provided by forging.

In a further embodiment, otherwise compatible with the first embodiment, the weak region is provided by using a region with different mechanical properties from the rest of the centralizer. For example, a more brittle structure can be achieved at the weak region by heat treating the weak region using a precise heat treatment technique such as induction heating. A further embodiment uses more than three weak regions. However, it is thought that three is the minimum number of weak regions 8 required to ensure that the casing 6 is centralized in the wellbore 2 after expansion of the casing 6.

In a further embodiment, the elastically deformed element is in the form of an elastically deformed ribbon wrapped more than one time around the casing and held in place by a single weak region. When the weak region breaks during the expansion process, the ribbon 'unwinds' and attempts to revert to its undeformed state, thereby centralizing the casing. An advantage of a centralizer for an expandable casing of the sort described above is that it does not significantly increase the diameter of the casing, making it no more difficult to install in a wellbore than a corresponding casing that has no centralizer. The design is robust and protects the casing as it is installed in the wellbore. The lack of moving parts also improves the robustness of the design, reducing the risk of damage to the centralizer and the casing during installation of the casing into the wellbore and expansion of the casing after the annulus between the casing and the wellbore has been filled with cement.

The design also allows a centralizer to be manufactured and retrofitted to existing casing joints prior to installation of a casing.

The above discussion describes specific embodiments of the invention, but it will be appreciated that other specific embodiments may be developed that use the same principle of providing an elastically deformed element held by a weak region, and ensuring that the weak region breaks in preference to any other parts of the centralizer during the expansion process to allow the elastically deformed element to revert to its original shape, thereby centralizing the casing in the wellbore.

Claims

CLAIMS:

1. A centralizer for an expandable casing, the centralizer comprising:

at least one elastically deformed element held in a non-expanded position relative to the expandable casing wall by a weak region;

wherein the weak region is arranged to break during expansion of the expandable casing, causing the elastically deformed element to revert to an expanded position in order to centralize the casing.

2. The centralizer according to claim 1 , wherein the elastically deformed element comprises a plate and the weak region comprises a groove in the plate, the groove being parallel to a main axis of the expandable casing.

3. The centralizer according to claim 2, comprising three grooves in the plate, each groove being spaced substantially 120° radially apart from the other grooves.

4. The centralizer according to claim 1 , wherein the weak region comprises any of a series of perforations in the elastically deformed element, a thinned section of the elastically deformed element, a weld between two elastically deformed elements, a forged weak region, and a region having a region with different mechanical properties.

5. The centralizer according to any of claims 1 to 4, wherein the elastically deformed element is attached to at least one ductile end part, the ductile end part configured to be attached to the expandable casing by any of welding, gluing, bolting and clamping.

6. The centralizer according to claim 5, wherein the elastically deformed element is attached to a further ductile end part at an opposite end of the elastically deformed element to the at least one ductile end part.

7. An expandable casing joint comprising at least one centralizer according to any of claims 1 to 6.

8. A method of expanding an expandable casing in a wellbore, the method comprising: providing the casing with a centralizer, the centralizer comprising at least one elastically deformed element held in a non-expanded position relative to an expandable casing wall by a weak region;

locating the casing in a wellbore;

driving an expansion cone through the expandable casing to increase the diameter of the casing, wherein the increase in diameter causes the centralizer weak region to break, thereby causing the elastically deformed element to revert to an expanded position in order to centralize the expandable casing within the wellbore.

9. The method according to claim 8, wherein the elastically deformed element comprises a plate and the weak region comprises a groove in the plate, the groove being parallel to a main axis of the expandable casing, wherein the expansion of the cone causes the groove to break.

10. The method according to claim 8, comprising providing a weak region by providing any of a series of perforations in the elastically deformed element, a thinned section of the elastically deformed element, a weld between two elastically deformed elements, and a forged weak region.

1 1. The method according to any of claims 8 to 10, comprising providing the expandable casing with a centralizer by attaching the elastically deformed element to at least one ductile end part, the ductile end part configured to be attached to the expandable casing by any of welding, gluing, bolting and clamping.

12. The method according to claim 11 , comprising further attaching the elastically deformed element to a further ductile end part at an opposite end of the elastically deformed element to the at least one ductile end part.