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This Application is the U.S. National Phase Application of PCT International Application No PCT/GB03/01022 filed Mar. 11, 2003. 
   This Application relates to a centraliser for an oil well tubular. 
   DESCRIPTION OF THE RELATED ART 
   Expandable centralisers are known, such as the bow-spring centraliser, which employs resilient bow-springs that are biased into an expanded configuration, and forced into a narrower bore so that the springs deform between the body of the centraliser and the borehole to space the centraliser body apart from the borehole. 
   BRIEF SUMMART OF THE INVENTION 
   According to the present invention there is provided a slotted expandable centralizer. 
   Typically the centraliser has a body with a bore to accept a tubular, and is radially expandable to an expanded configuration on application of a force in a radial direction. 
   Preferably, the centraliser has blades that can project radially outward from the body of the centraliser in a non-expanded configuration. 
   Preferably, the blades and the centraliser are made from a metal such as steel, and can be of the same thickness. 
   Optionally, the blades can project outwardly from the body of the centraliser in the expanded configuration. Alternatively, the blades can change configuration during expansion of the centraliser so that the expanded configuration can have a more uniform radius. 
   Preferably, the centraliser has at least two slots. Preferably, the slots are longitudinal in the non-expanded configuration, and open to generally diamond-shaped apertures in the expanded configuration. Typically, slots are arranged in longitudinally aligned rows with slots in adjacent rows being axially offset with respect to one another, so that the ends of circumferentially adjacent slots overlap. The rows and the slots themselves need not be axially aligned; this is merely a preferred option. 
   Alternatively, the slots are C-shaped in the non-expanded configuration. Other shapes of slots are possible, such as Z-shapes. 
   Preferably, the slots are of uniform dimension, but this is not necessary. 
   Optionally, slots are uniformly distributed over the body and the blades. Alternatively, the centraliser has slotted portions circumferentially adjacent to non-slotted portions. 
   Optionally, the non-slotted portions include at least one blade. 
   Optionally, all of the blades are located in non-slotted portions. 
   Typically, the centraliser is made from a material which is capable of plastic and/or elastic deformation. 
   Typically the centraliser is adapted to receive an expandable tubular within its bore and is adapted to deform radially with the expandable tubular during expansion. 
   According to another aspect of the present invention, there is provided a centraliser assembly comprising a slotted expandable centraliser which has a body with a bore to accept a tubular, and is radially expandable on application of a force in a radial direction to an expanded configuration; and an expandable tubular, located in the bore of the centraliser. 
   The tubular can comprise production tubing, casing, liner, drill pipe, screen, perforation guns or any other kind of downhole tubular. 
   Preferably, the force to expand the centraliser is provided by an expander device such as an expansion cone being pushed or pulled through the tubular. 
   The slots can have a typical length of between 1 and 5 cm, but this is only optional, and other lengths of slot can be used. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
       FIG. 1A  shows a perspective view of a centraliser in an initial, non-expanded configuration; 
       FIG. 1B  shows the centraliser of  FIG. 1A  in an expanded configuration; 
       FIG. 2A  shows an alternative embodiment of a centraliser in a non-expanded configuration; and 
       FIG. 2B  shows the centraliser of  FIG. 2A  in an expanded configuration. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings,  FIG. 1A  shows a steel centraliser  10  in a non-expanded configuration, attached to a slotted expandable steel tubular  12 . The slotted expandable steel tubular  12  is well known in the art. Both the centraliser  10  and the tubular  12  have many slots  18 , distributed approximately uniformly over the surface. 
   The centraliser  10  comprises a body  14  and blades  16  which project radially outwards from the body  14  in the non-expanded configuration shown in  FIG. 1A . In this embodiment the blades  16  are hollow projections formed by pressing the blade shape from the body  14 , and are of the same thickness and material as the body of the centraliser  10 . The blades  16  each comprise an outer face  16 A, side walls  16 B and end walls  16 C. 
   The slots  18  are typically between 1–5 cm in length and are arranged in parallel rows that are aligned with the axis of the tubular  12  and the centraliser  10 . Slots in circumferentially adjacent rows are axially offset with respect to one another, so that the ends of the circumferentially adjacent slots overlap, leaving a web of metal between the ends of axially adjacent slots, and their circumferentially adjacent neighbours. Each slot  18  has a much shorter length than the axial length of the centraliser  10 . The slots  18  cover both the body  14  and the blades  16 . 
   All of the slots  18  may be of uniform size and shape, or alternatively, the slots on the blades  16  could be differently shaped to the slots on the body  14 . 
   In use, an unexpanded centraliser  10  is fitted onto a string of expandable tubulars  12 , with the tubular  12  received within the bore of the centraliser as shown in  FIG. 1A . The string is lowered into a borehole to the depth where expansion of the tubular  12  is desired. An expander device (not shown) is then pulled or pushed through the tubular  12 . A possible expander device is an expander cone, which is typically pulled/pushed by a hydraulic ram or by fluid pressure. The expander device expands the tubular  12  as it passes through it, and as the tubular expands this expands the centraliser  10  located on the outer surface of the tubular  12 . 
   The largest end of the cone has a greater cross-sectional area than that of the non-expanded centraliser, so as the cone passes the centraliser  10 , the centraliser  10  experiences a radial expansion force from the expander cone (transmitted via the expandable tubular  12 ). The two sides of each slot on the centraliser  10  are pushed apart from each other, which widens the slot to the extent permitted by the web of metal between adjacent slots. Thus, the slots change shape; from being long and thin, they become shorter, fatter diamond-shaped apertures. The centraliser radially expands to the size of the widest part of the expander cone. The shape of the final aperture in the expanded centraliser  10  is determined by the size, shape and strength of the web between the slots. 
   The blades  16  do not need to expand as much as the body  14  of the centraliser  10  in order to accommodate the expander cone, as they have already been pressed out of the body of the centraliser  10 . Thus, the slots of the outer faces  16 A may adopt a different shape (e.g. narrower) on expansion as compared with the slots on the body of the centraliser  10 . Likewise, parts of the side walls  16 B and end walls  16 C need to expand more than other parts, so there can optionally be a non-uniform pattern of apertures on the expanded centraliser, which can be used to influence the shape and strength characteristics of the expanded centraliser  10 . After the cone has passed the centraliser  10 , the whole centraliser  10  adopts approximately the same inner diameter as the outer diameter of the tubular  12 . 
     FIG. 1B  shows the centraliser  10  of  FIG. 1A  in an expanded configuration. The outer faces  16 A of the arms  16  have expanded less than the body of the centraliser  10 , so that the expanded centraliser  10  has a generally uniform radius. 
   This embodiment is useful for inserting expandable tubulars such as screens into a borehole, where the blades  16  of the centraliser  10  are required to ease entry of the string into the hole but are not required after expansion of the screen against the borehole wall. With slotted blades as in this embodiment, the centraliser can ease the passage of the string into the hole, reducing friction between the screen and the hole, and spacing the screen from the wall to enhance insertion, and after expansion of the string can virtually disappear against the borehole wall. 
   In this embodiment the pattern of the slots on the blades and the body are substantially the same and this can give rise to a non-uniform pattern of apertures on the expanded centraliser. In other embodiments, the pattern or shape of the slots on the blades  16  can differ from the pattern or shape of the slots on the body of the centraliser  10 , so as to adopt a more uniform pattern of apertures after expansion of the centraliser  10 . 
     FIG. 2A  shows an alternative embodiment of a centraliser  10 A. The centraliser  10 A has a body  24  and longitudinal strips  20 , which are not slotted. Blades  25  are positioned on the longitudinal non-slotted strips  20 . The rest of the centraliser  10 A is slotted, as in the embodiment of  FIGS. 1A and 1B . 
   Slots  28  are aligned axially in rows, as in the embodiment of  FIGS. 1A and 1B . Slots  28  in adjacent rows are axially offset with respect to one another. Each slot  28  has a much shorter length than the axial length of the centraliser  10 A. 
   In use, the centraliser  10 A is attached to a portion of slotted pipe and expanded in the same way as the centraliser  10  of  FIGS. 1A and 1B , i.e. by means of an expander cone. The slotted parts of the centraliser  10 A expand in the way described above: the two sides of each slot are pushed apart from each other, which widens the slot. The long thin slots become shorter, fatter diamond-shaped apertures. 
   The non-slotted strips  20  do not substantially expand (apart from possibly some plastic/elastic deformation). Thus, the non-slotted strips  20  do not change their shape substantially, and the blades  25  remain protruding from the expanded body  24 . They may become further circumferentially spaced apart from each other, due to the expansion of the slotted parts of the body  24  between the blades  25 .  FIG. 2B  shows the centraliser  10 A of  FIG. 2A  in an expanded configuration. 
   This embodiment is suitable for expandable casing strings that still require a centraliser function after expansion, for example to provide an annulus for cement, or to wash out debris or other material from the well after insertion of the casing. 
   It should be noted that it is possible to provide some embodiments with intermediate properties, for example a slotted body and blades with comparatively fewer slots, so that the blades can expand less than the body, and a small blade structure is left after expansion. 
   Modifications and improvements can be incorporated without departing from the scope of the invention.

Summary:
This invention relates to a slotted expandable centraliser. In preferred embodiments, the centraliser is adapted to be used in conjunction with slotted casing, and can expand with the casing when an expander cone is propelled through the casing.