Patent Publication Number: US-7708064-B2

Title: Wellbore pipe centralizer having increased restoring force and self-sealing capability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to the field of pipe centralizers used to position pipes within wellbores drilled into the Earth. More specifically, the invention relates to structures for pipe centralizers that have increased restoring force, without corresponding increase in running force. The invention also relates to structures for centralizers that are self-sealing to enable moving through a wellbore pipe string sealing device. 
   2. Background Art 
   Centralizers are used to laterally or radially position a pipe or pipe “string” within a wellbore drilled into the Earth. A common type of centralizer is the so-called “bow spring” centralizer. A bow spring centralizer includes a plurality of circumferentially spaced apart single-leaf springs coupled to one or more devices arranged to affix the springs to the exterior of the pipe to be positioned in the wellbore. The leaf springs provide a force known as “restoring force” to laterally urge the pipe away from the wall of the wellbore. At the same time, the bow springs are laterally compressible so that the pipe may be moved along the interior of the wellbore notwithstanding the presence in the wellbore of small diameter restrictions and other obstacles to longitudinal movement of the pipe along the wellbore. 
   Examples of bow spring centralizers are described in U.S. Pat. No. 7,159,668 issued to Herrera and U.S. Pat. No. 6,457,519 issued to Buytaert. The centralizer described in the &#39;519 patent is intended to address a particular problem associated with bow spring centralizers, namely how to minimize the “staring force”, which is the force required to insert the centralizer into interior of the wellbore and the “running force”, which is the amount of force required to move the pipe longitudinally along the wellbore with such centralizers affixed to its exterior, while maximizing the restoring force. Specifications for the amount of restoring force, and proper use of centralizers are described in a document entitled, Specifications for Bow-Spring Centralizers, API Specification 10D, fifth edition, American Petroleum Institute, Washington, D.C. (1994). Generally speaking, casing centralizers are made to center a particular outside diameter (OD) pipe within a particular nominal diameter wellbore or outer pipe. The pipe OD is selected by the wellbore operator to closely match, for example, the wellbore diameter, which primarily related to the diameter of the drill bit used to drill a particular segment of the wellbore. 
   More recently, techniques have become known in the art to drill wellbores while maintaining a selected fluid pressure in an annular space between the wellbore wall and the pipe used to drill the wellbore. See, for example, U.S. Pat. No. 6,904,981 issued to van Riet and U.S. Pat. No. 6,352,129 issued to Best. Drilling techniques such as those disclosed in the foregoing patents typically require the use of a “rotating control head” at the upper end of the wellbore in order to control the pressure in the body of fluid in the annular space. A rotating control head is a device which closes the annular space while simultaneously enabling longitudinal and rotational movement of the pipe therethrough. Using a rotating control head with centralizers affixed to the exterior of the pipe may present particular difficulties in providing sufficient restoring force while maintaining the ability to sealingly move the pipe through the rotating control head. 
   SUMMARY OF THE INVENTION 
   A centralizer according to one aspect of the invention includes a first stop collar configured to be affixed to an exterior surface of a pipe. A second stop collar is spaced apart from the first stop collar and is configured to be affixed to the exterior surface of the pipe. A plurality of circumferentially spaced apart longitudinal spacers is included and each is coupled at each longitudinal end to one of the stop collars. A first plurality of circumferentially spaced apart bow springs is each coupled at one longitudinal end thereof to the first stop collar and at an opposite longitudinal end thereof to a first floating collar disposed between the first and second stop collars. A second plurality of circumferentially spaced apart bow springs is each coupled at one longitudinal end thereof to the second stop collar and at an opposite longitudinal end thereof to a second floating collar disposed between the first and second stop collars. 
   Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a typical wellbore operation in which centralizers according to the invention may be used on a pipe inserted into the wellbore. 
       FIG. 2  shows one example of a centralizer. 
   

   DETAILED DESCRIPTION 
   An example wellbore operation in which centralizers according to the invention can be used is shown schematically in  FIG. 1 . A wellbore  12  may be drilled into subsurface Earth formations  13  to a depth to which a protective pipe or casing  14  is intended to be set. The pipe or casing  14  may include one or more centralizers  10  which will be explained in more detail below with reference to  FIG. 2 . The pipe  14  may be lowered into the wellbore  12  by a hoisting system such as a drilling rig  16  or the like. The drilling rig  16  may include a drawworks  20  or similar winch that extends and retracts a drill line  21 . Movement of the drill line  21  cooperates with sheaves or “blocks”  18  to cause upward and downward motion of a top drive  22  or similar device to provide rotational motion to the pipe  14 . 
   Typically during operations, the wellbore  12  is filled with fluid  11  such a “drilling mud” or other fluid used to drill and/or complete the wellbore  12 . The fluid  11  is typically lifted from a pit or tank  26  disposed at the surface. The tank  16  may include a supply  28  of cleaned or conditioned fluid. The fluid  28  is lifted by a pump  24  which discharges the fluid to the top drive  22 . Internal rotating seal elements in the top drive  22  enable the fluid to be pumped through the interior of the pipe  14 . 
   The wellbore  12  typically includes a casing  33  (“surface casing”) set to a relatively limited depth near the surface. An upper end of the surface casing  33  is coupled to a sealing element called a rotating control head  34 . The rotating control head  34  seals against the exterior of the pipe  14  to prevent escape of fluid  11  from the wellbore  12 . The rotating control head  34  may include a fluid discharge outlet  34 A coupled through a controllable choke  32  or similar variable restriction flow control device that ultimately can return the fluid  11  to the tank  26 . In some examples, the fluid discharge outlet  34 A may include a pump  30  coupled thereto at its discharge side so that fluid pressure in the wellbore  12  outside the pipe  14  may be maintained at a selected amount. Methods for controlling such pressure and devices therefor are well described in U.S. Pat. No. 6,904,981 issued to van Riet and U.S. Pat. No. 6,352,129 issued to Best, incorporated herein by reference. 
   The example shown in  FIG. 1  includes pipe in the form of a casing being inserted into the wellbore  12 . It should be clearly understood that the invention is equally applicable to any type of pipe being inserted into a wellbore, including as non-limiting examples drill pipe, coiled tubing, production tubing and rod strings. Accordingly, the invention is not limited in scope to being used with casing. A purpose for a centralizer according to the invention, to be explained with reference to  FIG. 2 , is to enable increased restoring force without increasing starting or running force. When centralizers are used with a pipe in the example operation shown in  FIG. 1 , it is desirable to minimize starting and running force in particular because of the rotating control head  34 . By minimizing such starting and running force, wear and possible damage to the rotating control head  34  may be reduced. 
   A side view of an example centralizer according to the invention is shown in  FIG. 2 . The centralizer  10  may include a first “stop collar”  40  and a second stop collar  42  disposed at a selected longitudinal distance from the first stop collar  40 . The stop collars  40 ,  42  may be made from steel or similar high strength material. The stop collars  40 ,  42  may be generally cylindrically shaped and may have an internal diameter selected to fit about the exterior of the particular pipe (e.g.,  14  in  FIG. 1 ) to which they are to be affixed. The stop collars  40 ,  42  may be similar in configuration to stop collars used in centralizers known in the art, for example, centralizers described in U.S. Pat. No. 6,457,519 issued to Buytaert. The stop collars  40 ,  42  may be affixed to the exterior of the pipe ( 14  in  FIG. 1 ) using set screws  44  or any other device known in the art for such purpose. The stop collars  40 ,  42  may be maintained at a fixed selected distance from each other along the exterior of the pipe ( 14  in  FIG. 1 ) by longitudinally extending, circumferentially spaced apart longitudinal spacers  46 . The longitudinal spacers  46  may be positioned longitudinally between the stop collars  40 ,  42  and may be affixed to the stop collars  40 ,  42  at each longitudinal end of each longitudinal spacer  46 . A selected number of longitudinal spacers, typically two or more, may be circumferentially evenly spaced about the circumference of the stop collars  40 ,  42 . As may be inferred by reference to  FIG. 2 , the longitudinal spacers  46  may be in the form of strips and may be made from the same material used to make the stop collars  40 ,  42 . The longitudinal spacers  46  need not traverse a great width (“width” as used herein the direction transverse to the length in the direction along the circumference of the centralizer) because they are substantially not load bearing. The stop collars  40 ,  42  may be affixed to the exterior of the pipe ( 14  in  FIG. 1 ) so that any longitudinal loading is transferred to the device used to affix the stop collars to the exterior of the pipe ( 14  in  FIG. 1 ). Thus, the longitudinal spacers  46  may be made of sufficiently small material thickness and width so as to provide only the required strength to fix the relative positions of the stop collars  40 ,  42  during shipment and assembly to the pipe ( 14  in  FIG. 1 ). 
   The first stop collar  40  may include affixed thereto a plurality of longitudinally extending, circumferentially spaced apart blades or bow springs  52 . The bow springs  52  may be made from spring steel and may be substantially the same configuration as used in conventional centralizers, e.g., those disclosed in the Buytaert &#39;519 patent set forth above. The bow springs  52  may be affixed to the first stop collar  40  at one longitudinal end as shown in  FIG. 2 . The opposite longitudinal end of each of the bow springs  52  may be affixed to a first floating collar  50 . The first floating collar  50  may be arranged to move longitudinally along the outside of the pipe ( 14  in  FIG. 1 ) corresponding to the bow springs  52  being laterally compressed and relaxed. Thus, the first floating collar  50  can move longitudinally along the exterior of the pipe while the two stop collars  40 ,  42  remain longitudinally fixed with respect to each other and in a fixed position along the exterior of the pipe. The bow springs  52  coupled to the first stop collar  40  extend longitudinally in the direction of the second stop collar  42  as shown in  FIG. 2 , and typically terminate at a position between the two stop collars  40 ,  42 . In some examples, the floating collars  48 ,  50  are disposed radially inside those of the bow springs coupled to the opposed stop collar  40 ,  42 . 
   Correspondingly, bow springs  52  may be affixed at one longitudinal end thereof to the second stop collar  42 . The other longitudinal end of such bow springs  52  may be affixed to a second floating collar  48  disposed between the two stop collars  40 ,  42 . The second floating collar  48  moves longitudinally along the exterior of the pipe as the coupled bow springs  52  are compressed and relaxed in a manner corresponding to movement of the first floating collar  50 . The bow springs  52  extending between the respective first  40 ,  50  and second  52 ,  48  collars each have length such that there is longitudinal overlap between the bow springs  52  coupled to the first stop collar  40  and those coupled to the second stop collar  42 , and such overlapping bow springs  52  may be circumferentially arranged to avoid interference with each other. In one example, four bow springs  52  are coupled to the first stop collar  40  and four bow springs  52  are coupled to the second stop collar  42  providing a total of eight bow springs  52 . 
   The bow springs  52  in the present example may have a tapered width (width herein being the direction transverse to the length of the bow spring and in the circumferential direction as explained with reference to the longitudinal spacers  46 ). Such taper may be observed in  FIG. 2  as having a wider central portion  56  and as narrower longitudinal ends  54  where the bow springs  52  couple to the respective stop collar  40 ,  42 . Such configuration of bow spring may reduce interference between the bow springs  52  and the longitudinal spacers  46  when the bow springs  52  are laterally compressed. In some examples, the central portion may be substantially ovoid. Such configuration may provide increased restoring force without substantially increasing the starting force and running force of the centralizer. The configuration of the bow springs  52  described above may provide the advantage of having three places along each bow spring where spring force originates. In prior art centralizers, most of the spring force is generated by flexure of the bow spring where it couples to a collar. In the present example, the central portion  56  of the bow springs  52  also provides spring force when the bow spring is compressed. 
   In some examples, the centralizer  10  may include eight bow springs, for connected to each of the first stop collar  40  and the second stop collar  42 . Configured as shown in  FIG. 2 , the centralizer will have a substantially fixed length. In some examples, the bow springs  52  may be shaped as shown in  FIG. 2  such that when the centralizer is radially fully compressed, the bow springs  52 , the collars  40 ,  50 ,  48 ,  42  and the longitudinal spacers cooperatively engage with each other to form a metal to metal seal. A centralizer configured to provide such sealing capability may be moved through a wellbore sealing device such as a rotating control head while substantially preventing movement of wellbore fluid pressure longitudinally along the centralizer as is passes through the rotating control head. Thus, a centralizer made according to some aspects of the invention may be used with managed pressure wellbore operations such as managed pressure drilling and managed pressure casing cementing. See, for example, U.S. Pat. No. 6,904,981 issued to van Riet and U.S. Pat. No. 6,352,129 issued to Best for descriptions of such apparatus and methods. 
   A centralizer made as explained herein may provide increased restoring force, while minimizing increases in starting and running force as contrasted with alternative arrangements of a centralizer. Centralizers according to the invention may have higher reliability and durability than centralizers known in the art prior to the invention. A centralizer made according to the invention may have substantially constant axial length under all conditions of radial compression. The floating collars may be positioned in a manner to avoid exposure of moving parts on the centralizer to external objects such as the wellbore wall or wellbore devices during movement of a pipe. A centralizer according to the invention may be able to provide a fluid tight seal when compressed, enabling its use with wellbore pressure control devices such as rotating control heads. The bow springs of a centralizer made according to the invention may have three points at which spring force is generated as contrasted to only one or two for centralizers known in the art prior to the present invention. 
   While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.