CA2257523C - Centralizer - Google Patents

Abstract

A centralizes (1) comprises two annular bands (2, 3) which are spaced apart by a plurality of members (4, 5, 6) extending therebetween. The members (4, 5, 6) are inclined to the longitudinal axis of the centralizer by an angle a of typically 30° to 45°.

Description

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CENTRAI~I ZER
This invention relates to centralizers for use in the construction of oil and gas wells.
During the construction of oil and gas wells a borehole is drilled into the ground. A string of casing then is lowered down the borehole and the annular space between the casing and the borehole filled with cement.
It is important to ensure that the casing is held centrally in the borehole during cementation, and it is usual to provide the casing with a plurality of centralizers which act between the casing and the borehole.
Various types of centralizes are known. One type comprises two annular bands which are spaced apart by a plurality of longitudinally extending members. These members can comprise spring bows or rigid members. They share the common feature that, in use, the members extend parallel to the longitudinal axis of the casing on which they are mounted. However, WO 91/08374 and U.S. 2,312,600 both disclose centralizers where the members are inclined to the longitudinal axis of their respective centralizers.
As can be seen from the above-identified prior art the members have a generally planar outer surface as maximum wall contact is considered desirable to inhibit the centralizes damaging the casing through which it is passed.
However, this feature does lead to difficulties in long horizontal wells where friction between the centralizes and the surface on which it bears becomes a major issue.
Another type of centralizes, often referred to as a "solid centralizes", comprises a casting having a solid central section on which are formed a plurality of fins. In some embodiments the fins are disposed so that, in use, they extend parallel to the longitudinal axis of the casing on which they are mounted. In other embodiments the fins n I i ii.4 ~.Irvi~ll i a 11 .Ildi~u~ArcV-."ei ~A. li

2 are disposed at an angle so that, in use, when circulating fluid is pumped through the annular space between the casing and the borehole prior to introducing the cement, the circulating fluid is encouraged to swirl around the casing. In many situations this also enhances the distribution of cement when it is subsequently injected into the annular space.
The present invention provides a centralizer which comprises a pair of annular bands which are spaced apart by a plurality of members extending therebetween, and are inclined to the longitudinal axis of the centralizer. The outer surface of each member is curved when viewed in a plane perpendicular to the longitudinal axis of the centralizer.
By combining the features of members which are both curved and inclined, the frictional resistance of the centralizer can be reduced whilst the load can be more uniformly distributed over the internal surface of the casing through which it is being passed.
Preferably, the members have a radially inner surface which is substantially flush with the radially inner surface of the annular bands. This feature, whilst not absolutely essential, is desirable since it encourages the desired swirling action.
In a preferred embodiment of the invention, the centralizer has a longitudinal axis and comprises a pair of annular bands which have a radially-inner surface and are longitudinally-spaced apart by a plurality of hollow curved members extending therebetween. Each curved member has two spaced-apart ends, and the curved members are of curved cross-section and inclined to the longitudinal axis of the centralizer at an angle between about 20° to 60° to the longitudinal axis. Each curved member extends radially ' 3 inwardly and has an inner surface flush with radial inner surfaces of the annular bands, each curved member is curved along its length to its entire extent, and the curved members are tapered both radially and circumferentially towards each of their two spaced-apart ends to facilitate movement of the centralizes.
The members themselves may be solid or hollow, and may be of any convenient cross-section. In a particularly preferred embodiment the members are of curved cross-section. Advantageously, the members are tapered towards each end.
Advantageously, the members may have a strength so that, in use, if the centralizes is subjected to a lateral load of from 5 to 20 tonnes, and more preferably from 5 to 15 tonnes, they will substantially irreversibly collapse against the casing. This concept is more fully disclosed in Applicant's PCT Published Application WO 96/09459.
If desired, the annular bands may be formed in two separate parts which may be bolted together around a casing or which may be hinged together.
For a better understanding of the present invention reference will now be made, by way of example, to accompanying Figure 1, which shows a perspective view of one embodiment of a centralizes in accordance with the invention mounted on a length of casing.
Referring to Figure 1, there is shown a centralizes which is generally identified by the reference numeral 1.
The centralizes 1 comprises two annular bands 2 and 3 which are spaced apart by eight members, three of which 4, 5 and 6, are clearly visible.
Each member 4, 5, 6 is inclined at an angle a of from between 20° to 60° and preferably from about 30° to 45° to the longitudinal axis of the centralizes 1.

' 4 Each member 4, 5, 6 has skirt portions 7, 8 which extend radially inwardly and finish flush with the radially inner surface of the two annular bands 2 and 3.
In use, the centralizer 1 is slid over a tubular 9.
The centralizer 1 may be secured by stop collars (not shown) placed above and below the centralizer or by bolts (not shown), which pass through the annular bands 2 and 3 and bear against the casing 9. Alternatively, the centralizer 1 may be glued to the casing 9.
Each member 4, 5, 6 comprises a thin sheet of steel which is about 3 mm in thickness, and is shaped into a curve as viewed in a plane perpendicular to the longitudinal axis of the centralizer 1. Accordingly, each member 4, 5, 6 is hollow. Each member 4, 5, 6 also is tapered both radially and circumferentially towards each end, to facilitate movement of the centralizer 1 in the borehole.
In use, a plurality of centralizers similar to centralizer 1 are secured to a string of casing which is then lowered down a borehole. When the casing is in place circulating fluid is pumped down the inside of the casing and travels upwardly in the annular space between the casing 9 and the borehole. As the circulating fluid passes upwardly over the annular band 3 between the members it is diverted sideways . As it passes over the upper edge of the annular band 3 a turbulent rolling action is initiated which is amplified as the circulating fluid passes over the annular band 2. The combined swirling and rolling action provides an effective cleaning and scouring action which is highly desirable.
After the annular space between the casing 9 and the borehole has been prepared, cement is pumped down the n .I~, ,~*,~ilr~r.,r rr.ll.uli~,.,l,~k...r..e~ ..6.".

casing and up into the annular space and allowed to set in the conventional manner.
Occasionally, part of the borehole will collapse when running casing. The usual procedure when this occurs is to 5 withdraw the casing, make good the problem as necessary, and reinstate the casing. Although the casing can usually be withdrawn without too much difficulty, the forces imposed on traditional centralizers often result in their disintegration with the result that broken parts of centralizers remain in the borehole. This is most undesirable. Applicant's PCT Published Application WO
96/09459 addresses this problem by providing a centralizer with members which have sufficient strength to centralize the casing, but which will collapse if withdrawn through a relatively rigid constriction. The underlying principle is that it is better to replace a permanently deformed centralizer at the surface, rather than to leave parts of a disintegrated centralizer in the borehole. Typically, the members should substantially completely collapse when subjected to a lateral load of from 5 to 15 tonnes, with 11 tonnes being currently used for design purposes for most occasions.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. ~A centralizer for centralizing a pipe in a casing, the centralizer comprising a pair of annular bands which are spaced apart by a plurality of members which extend therebetween and are inclined to the longitudinal axis of the centralizer, the outer surface of each of the members being curved when viewed in a plane perpendicular to a longitudinal axis of the centralizer.

2. ~A centralizer as defined in claim 1, wherein said members have a radial-inner surface which is substantially flush with a radially-inner surface of said annular bands.

3. ~A centralizer as defined in claim 1 or 2, wherein said members are hollow.

4. ~A centralizer as defined in claim 1, 2 or 3, wherein said members are tapered towards each end.

5. ~A centralizer as defined in any one of claims 1 to 4, wherein said members, in use, will substantially irreversibly collapse against said casing when subjected to a lateral load of from 5 to 15 tonnes.

6. ~A centralizer as defined in any one of claims 1 to 5, wherein said annular bands are formed in two separate parts which can be mounted circumjacent said casing.

7. ~A centralizer for centralizing a pipe in a casing, the centralizer having a longitudinal axis and comprising a pair of annular bands which have a radially-inner surface and are longitudinally spaced apart by a plurality of hollow curved members extending therebetween, each curved member having two spaced-apart ends, wherein the curved members are of curved cross-section and inclined to the longitudinal axis of the centralizer at an angle between about 20° to 60° to the longitudinal axis, each curved member extends radially inwardly and has an inner surface flush with radial inner surfaces of the annular bands, each curved member has a length and is curved along that length to its entire extent, and wherein the curved members are tapered both radially and circumferentially towards each of their two spaced-apart ends to facilitate movement of the centralizer.

8. ~A centralizer as defined in claim 7, wherein said members are made of thin steel about 3 mm. thick.

9. ~A centralizer as defined in claim 7 or 8, wherein said curved members, in use, will substantially irreversibly collapse against said casing when subjected to a lateral load of from 5 to 15 tonnes.

10. ~A centralizer as defined in claim 7, 8 or 9, wherein said annular bands are formed in two separate parts which can be mounted circumjacent said casings.