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CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of co-pending U.S. patent application Ser. No. 10/320,187, filed Dec. 16, 2002, which claims benefit of United States patent U.S. application Ser. No. 09/469,681 filed on Dec. 12, 1999, now U.S. Pat. No. 6,527,049. This application further claims benefit of GB 9828234.6 dated Dec. 22, 1998, GB 9900835.1 dated Jan. 15, 1999, GB 9923783.6 dated Oct. 8, 1999, and GB 9924189.5 dated Oct. 13, 1999. Each of the aforementioned related patent applications is herein incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to a straddle, and in particular a straddle for use in selectively isolating a section of tubing. The invention also relates to a method of isolating a section of tubing.  
         [0004]     2. Description of the Related Art  
         [0005]     In the oil and gas exploration and production industries, subsurface hydrocarbon-bearing formations are accessed via casing-lined wellbores. The lower section of a bore, which intersects the hydrocarbon-bearing formation, is typically lined with perforated “liner”, oil and gas flowing into the bore through the perforations. The location of the perforations is predetermined on the basis of surveys, to ensure that only selected formations are in fluid communication with the bore. Over the life of a well it may occur that the properties of particular formations change, for example the pressure in a formation may fall, or a formation may begin to produce any unacceptably high volume of water. In these circumstances it is known to run straddles into the liner, these straddles being sections of tubing with sealing arrangements at either end. A straddle may be located within the section of liner intersecting the problem formation, and the seals then set to isolate the section of liner between the seals. However, existing straddles are problematic to set, and the requirement to accommodate the seals and a seal setting mechanism result in a significant loss in bore cross section, which reduces the production capacity of the well and also makes it more difficult to access the section of well beyond the straddle.  
       SUMMARY OF THE INVENTION  
       [0006]     It is among the objectives of embodiments of the present invention to provide an improved straddle which obviates or mitigates these difficulties.  
         [0007]     According to the present invention there is provided a method of isolating a section of downhole tubing, the method comprising: 
        running a length of expandable tubing into a tubing-lined borehole and positioning the expandable tubing across a section of tubing to be isolated; and     deforming the expandable tubing by increasing the diameter of at least portions thereof to sealingly engage the tubing and to isolate said section.        
 
         [0010]     According to another aspect of the present invention there is provided apparatus for use in isolating a section of tubing-lined borehole, the apparatus comprising: a length of expandable tubing; and an expander device including a radially extendable member for deforming at least portions of the expandable tubing to increase the diameter of said portions to sealingly engage a section of tubing to be isolated.  
         [0011]     Preferably, the expandable tubing is deformed by compressive plastic deformation or yield of the tubing and a localised reduction in tubing wall thickness with a subsequent increase in tubing diameter. Conveniently this is achieved by rolling expansion, that is the expander device is rotated within the expandable tubing with an expander member in rolling contact with an inner face of the expandable tubing.  
         [0012]     The deformation of the expandable tubing preferably creates an annular extension. This annular extension may extend over all or a substantial portion of the expandable tubing, or may be restricted to a selected portions of the expandable tubing on either side of the section of tubing to be isolated. The former arrangement will be more secure, but would be more difficult to remove from the tubing.  
         [0013]     The tubing lining the bore may be casing or liner, or may be secondary tubing, such as production tubing itself positioned within a section of casing or liner.  
         [0014]     The expandable tubing may include relative ductile portions corresponding to the portions of the tubing to be expanded. These portions may be welded or otherwise secured to portions of less ductile tubing.  
         [0015]     The expandable tubing is preferably initially cylindrical.  
         [0016]     Preferably the expander device  28  as shown in  FIGS. 1 and 4  comprises a body  30  carrying a plurality of expander roller member  32 . Most preferably, a plurality of the expander members  32  are radially extendable. Preferably, the expander members  32  are fluid activated, for example the members  32  may be operatively associated with a piston. In one embodiment illustrated in  FIG. 4 , the members  32  may be mounted on respective radially movable pistons  33  and in other embodiments the members may have tapered ends for engaging cones or wedges coupled to an axially movable piston.  
         [0017]     The expandable tubing may carry seal bands on an outer surface thereof. The seal bands may comprise at least one of an elastomeric seal and a band of relatively ductile metal, such as copper or a tin/lead alloy.  
         [0018]     The expandable tubing may carry grip bands on an outer surface thereof. The grip bands may comprise relatively hard elements, such as balls, chips or grains, held in a matrix, whereby the elements bite into the relatively soft material of the tubing and the expandable tubing on deformation of the expandable tubing. In other embodiments the relatively hard elements may be in a form other than bands. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0020]      FIGS. 1 and 2  are schematic sectional views of a straddle setting operation in accordance with an embodiment of an aspect of the present invention; and  
         [0021]      FIG. 3  is a schematic sectional view of a straddle in accordance with another embodiment of the present invention.  
         [0022]      FIG. 4  is a cross-sectional perspective view of one embodiment of an expander device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]     Reference is first made to  FIG. 1  of the drawings, which illustrates a straddle  10  in accordance with an embodiment of the present invention located in a section of a drilled bore  12  lined with perforated steel liner  14 . The straddle  10  has been run into the bore  12  and will be utilised to isolate a section of the bore  12 , in particular a particular formation  16  which is in fluid communication with the bore via perforations  18  in a section of the liner  14 .  
         [0024]     The straddle  10  comprises a section of expandable tubing  20  carrying seal bands  22  of relatively ductile metal at each end, and also grip bands  23  comprising small elements of relatively hard material in a relatively ductile matrix. The tubing  20  defines a solid wall and is of slightly smaller outside diameter than the liner  14 . Initially, the tubing  20  is of substantially constant diameter along its length. The ends of the tubing  20   a,    20   b  and formed of relatively ductile metal and are welded to a central tubing section  20   c.    
         [0025]     The straddle is run into the bore  12  on a tool string  26 , and is mounted to the string  26  via an expander device  28  mounted to the lower end of the string  26 . The expander device  28  comprises a body  30  carrying three radially movable rollers  32 . The body  30  also contains an axially movable piston which is coupled to a loading cone which cooperates with the tapered ends of the rollers  32 . Application of elevated fluid pressure, via the tool string  26 , thus urges the rollers  32  radially outwardly. Shear pins  34  couple the straddle  10  to the expander body  30 .  
         [0026]     In use, the straddle is run into the bore  12  on the tool string  26  and positioned across the group of perforations  18  to be closed off from the bore. Pressure is then applied to the expander  28  to activate the rollers  32 ; an initial application of elevated pressure causes the rollers  32  to extend radially, and deforms the tubing  20 , towards a triangular form, such that the areas of tubing  20  adjacent the rollers  32  are pushed into contact with the inner surface of the liner  14 . This initial contact is sufficient to prevent relative rotation between the straddle  10  and the liner  14 , such that when the string  26  and the expander  28  are rotated from surface the straddle  10  is held relative to the liner  14  and the pins  34  shear. The expander  28  then rotates with the straddle  10  with the rollers  32  in rolling contact with the inner wall of the tubing  20 . The rollers  32  are urged outwardly and progressively compress the tubing wall to create a localised reduction in wall thickness, and a corresponding increase in wall diameter. There is thus created a annular section of increased tubing diameter  36  at the tubing end section  20   a,  as shown in  FIG. 2 , which provides an interference fit with the surrounding liner  14 , the sealing bands  22  being deformed to form a fluid-tight seal between the expanded tubing  36  and the liner  14 . The hard material in the grip bands  23  also assists in keying the tubing section  36  to the liner  14 . There may be a degree of elastic and even plastic deformation of the liner  14 , which will serve to provide a more secure location for the straddle  10 .  
         [0027]     Following creation of the annular extension  36 , the pressure in the tool string  26  is reduced such that the rollers  32  may retract. The expander  28  is then advanced towards the lower end of the straddle  10 , and engages a stop  38  provided on the lower end of the tubing  20 . The pressure in the tool string is then increased once more to actuate the rollers  32 , and the expander  28  is rotated to create a second annular section of increased diameter  40 .  
         [0028]     The expander  28  may then be deactivated and retrieved from the bore, leaving the straddle  10  locked in place in the bore, and serving to isolate the formation  16  from the bore.  
         [0029]     To remove the straddle  10 , the locking and sealing sections  36 ,  40  are milled out, and the remaining section of tubing then removed.  
         [0030]     In other embodiments, the increased diameter sections  36 ,  40  may be formed simultaneously, by provision of two expanders located one at either end of the straddle.  
         [0031]     Reference is now made to  FIG. 3  of the drawings, which illustrates a permanent straddle  50  in accordance with another embodiment of the invention locked and sealed in a bore  52 . The straddle  50  is located in a substantially similar manner to the straddle  10  described above, however the straddle tubing  54  has been deformed along it whole length, such that there is a much larger area of contact between the tubing  54  and the surrounding liner  56 , and a smaller loss in cross-section in the liner  56  from the provision of the straddle  50 .  
         [0032]     Those of skill in the art will recognise that the above described embodiments of the present invention provide straddles which are relatively simple in construction and installation and which avoid many of the problems associated with prior art straddles featuring slips and energisable elastomer seals.  
         [0033]     Those of skill in the art will also recognise that the embodiments described herein are merely exemplary and that various modifications and improvements may be made thereto without departing from the scope of the present invention. For example, the above described embodiments are shown isolating sections of formation from a bore lined with perforated liner. In other embodiments, the straddle may be utilised to repair damaged tubing, including risers, casing, liner or production tubing. The straddle may be run in on any suitable form of tool string, including reeled supports such as coiled tubing, when the straddle will be provided in combination with a downhole motor for rotating the expander  28 .

Summary:
A method of isolating a section of downhole tubing comprises: running a length of expandable tubing ( 20 ) into a tubing-lined borehole ( 12, 14 ) and positioning the expandable tubing ( 20 ) across a section of tubing to be isolated; deforming at least portions of the expandable tubing ( 36, 40 ) to increase the diameter of the portions to sealingly engage the tubing ( 14 ) and to isolate the tubing section.