Abstract:
An apparatus for, and method of, centering downhole well tools within the wellbore of a pipe comprises at least a pair of discs secured, respectively, to the distal end of a tool in a plane normal to a longitudinal tool axis, with an arc of each disc extended past the outer perimeter of the tool to at least an internal perimeter of an applied pipe bore and flexing to centralize the tool. In alternative embodiments, the discs are replaced by blades that are secured by a plurality of attachment points and fasteners, or by spring steel wires that are secured in radial apertures through an end boss by interference fit, soldering, swaging, or gluing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to tools and methods for earth boring, well completion and production. More particularly, the invention relates to apparatus and methods for maintaining downhole tools approximately concentric with a pipe or tubing bore axis. 
       DESCRIPTION OF RELATED ART 
       [0003]    In the process of well drilling, completion and production, there are numerous tools that require substantial centralization along the axis of a pipe or tube bore. In a frequently arising example, it becomes necessary to cut a pipe or tube at a point deep within a borehole. Such remote pipe cutting is often performed with a shaped charge of explosive. 
         [0004]    Briefly, shaped charge explosives for pipe cutting generally comprise a disc of highly compressed explosive material, such as RDX or HMX, having a V-groove channel formed about the disc perimeter. A thin cladding of metal is intimately formed against the V-groove surface. When ignited at the center of the disc, the opposite flanks of the V-groove expansively explode against each other to produce a rapidly expanding jet of metal material where the impact of this jet material, upon the surrounding pipe or tubing wall, is to sever the pipe wall by hydrodynamically splashing the material out of the way. 
         [0005]    Although reliable and effective when expertly applied, the radial cutting capacity of shaped charge cutters is usually limited to only a few inches from the perimeter of the explosive material disc. Moreover, this radial cutting capacity may be further limited by downhole fluid pressure. When detonated under a downhole fluid pressure of 18,000 psi, the cutting capacity of a shaped charge cutter may be reduced by as much as 40%. If the cutter alignment within the pipe is eccentric with the pipe axis, an incomplete cut may result. 
         [0006]    Other examples of required axial position control for downhole tools include well measurement and logging processes, where the radial proximity of the pipe wall is influential upon the measured data. 
         [0007]    As a functional method, well tool centralizers are known in the prior art. U.S. Pat. No. 7,073,448 to W. T. Bell describes a shaped charge cutter housing having a centralizer comprising four blades in a single plane attached by a single fastener at the distal end of the housing. U.S. Pat. No. 5,046,563 to W. T. Engel et al describes three flat springs formed into bows with one end of each attached to the end of a shaped charge cutter housing. U.S. Pat. No. 4,961,381 to P. D. McLaughlin describes a borehole centering device for blasthole primers comprising a plurality of thin, radially extending spikes secured to a central ring. The spikes are made of a semi-conducting plastic and the central ring is sized to fit over a primer case. A further example of centralizers is disclosed by S. T. Graham et al, in U.S. Pat. No. 3,599,567, including plastic wing members radiating from a drive point for attachment over the end of a stick of explosive. The wing members have the purpose of holding the buoyant explosive down as well as centralizing the charge within a shothole. The explosive casing cutter disclosure of U.S. Pat. No. 3,053,182, to G. B. Christopher, describes a plurality of backswept spring wires secured to the cutter housing in borings directed angularly to the tool axis. Clamping screws engage portions of the spring wires extending into the housing boring 
         [0008]    In adapting prior art centralizing devices to downhole tools, such as pipe and tubing cutters, difficulties arise in the form of excess material usage for forming multiple centering blades from a single sheet of spring steel. Centralizers with elaborate designs present fabrication/assembly difficulties. 
         [0009]    One object of the present invention, therefore, is to provide the art with an inexpensively fabricated and easily attachable well tool centralizer. 
       SUMMARY OF THE INVENTION 
       [0010]    One embodiment of the present invention comprises two or more thin, resilient metal discs attached to a tool housing end. Each disc is secured, preferably, by a single pin fastener through the disc center. The fastener is placed near the perimeter of the tool housing, whereby only an arcuate portion of a disc projects, substantially normally to the longitudinal tool axis, beyond the tool perimeter to engage a pipe or tubing inside wall surface. 
         [0011]    In another invention embodiment, ends of thin, spring steel wires can be inserted into corresponding apertures in a base of the tool housing and secured by an interference fit or other securing methods. The interference fit may be obtained by swaging or by thermal shrinkage. In an alternative embodiment, the spring steel wires can be inserted into corresponding apertures of a base ring having a different diameter and, then, secured by such methods as interference fit. Alternatively, other securing methods may be used, including, but not limited to, soldering or gluing the spring steel wires directly to the base of the tool housing. Then, the secured spring steel wires can engage the inside of the wellbore during insertion/withdrawal of the tool. 
         [0012]    In another invention embodiment, a plurality of thin, spring steel blades are attached via a plurality of fasteners to the end of the tool housing, the plurality of fasteners acting to prevent rotation of the centralizers during insertion/withdrawal of the tool, and the length of the blades cut to ensure contact with (and thus centralization relative to) the wellbore. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention is hereafter described in detail and with reference to the drawings wherein like reference characters designate like or similar elements throughout the several figures and views that collectively comprise the drawings. Respective to each drawing figure: 
           [0014]      FIG. 1  is a longitudinal section of pipe enclosing a shaped charge pipe cutting tool fitted with one embodiment of the present invention. 
           [0015]      FIG. 2  is a cross section of the  FIG. 1  illustration showing a plan view of an embodiment of the invention. 
           [0016]      FIG. 3  is a sheet metal die cutting pattern for centralizing discs, illustrating the material utilization efficiency of this invention. 
           [0017]      FIG. 4  is a plan view of an alternative configuration of the invention. 
           [0018]      FIG. 5A  is an operative detail of an embodiment of the invention in a tool withdrawal mode. 
           [0019]      FIG. 5B  is an operative detail of an alternative embodiment of the invention in withdrawal mode. 
           [0020]      FIG. 6  is a partially sectioned elevation showing an alternative embodiment of the invention. 
           [0021]      FIG. 7  is a plan view of the  FIG. 6  invention embodiment. 
           [0022]      FIG. 8A  is an enlarged cross-section of one method of fitting the wires of the embodiment of  FIG. 6 . 
           [0023]      FIG. 8B  is an enlarged cross-section detail of another method of fitting the wires of the embodiment of  FIG. 6 . 
           [0024]      FIG. 9  depicts an alternative embodiment of the present invention comprising a plurality of planar, finger-like structures usable for centralizing a tubing cutter. 
           [0025]      FIG. 10  depicts an embodiment of a single blade, from the plurality of blades, for use in centralizing a tubing cutter. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Before describing selected embodiments of the present disclosure in detail, it is to be understood that the present invention is not limited to the particular embodiments described herein. The disclosure and description herein is illustrative and explanatory of one or more presently preferred embodiments and variations thereof, and it will be appreciated by those skilled in the art that various changes in the design, organization, order of operation, means of operation, equipment structures and location, methodology, and use of mechanical equivalents may be made without departing from the spirit of the invention. 
         [0027]    As well, it should be understood the drawings are intended to illustrate and plainly disclose presently preferred embodiments to one of skill in the art, but are not intended to be manufacturing level drawings or renditions of final products and may include simplified conceptual views as desired for easier and quicker understanding or explanation. As well, the relative size and arrangement of the components may differ from that shown and still operate within the spirit of the invention. 
         [0028]    As used herein, the terms “up” and “down”, “upper” and “lower”, “upwardly” and downwardly”, “upstream” and “downstream”; “above” and “below”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or other relationship as appropriate. Moreover, in the specification and appended claims, the terms “pipe”, “tube”, “tubular”, “casing”, “liner” and/or “other tubular goods” are to be interpreted and defined generically to mean any and all of such elements without limitation of industry usage. 
         [0029]    With respect to  FIGS. 1 and 2 , a special case of the invention is shown as to include a tubing cutter  10  having explosives (not shown) within a housing  12 . The cutter  10  is shown as located within a downhole tube  14 . The cutter  10  is centrally confined within the tube  14  by a pair of centralizing discs  16  having a substantially circular planform. 
         [0030]    As best shown by  FIG. 2 , the centralizing discs  16  are secured to the cutter housing  12  by anchor pin fasteners  18 , shown in this embodiment as screws. The disc plane is substantially normally oriented to the housing axis  13 . Since the discs  16  are not expected to rotate about the anchor pins  18 , swage rivets may also serve for securing the discs to the housing  12 . 
         [0031]    In the  FIGS. 1 and 2  embodiment, the discs are mounted along a diameter line  20  across the cutter housing  12 , with the most distant points on the disc perimeters separated by a dimension that is preferably at least corresponding to the inside diameter of the tubing  14 . In many cases, however, it will be desirable to have a disc perimeter separation slightly greater than the internal diameter of the tubing  14 . This configuration is illustrated by the upward sweep in the discs in contact with the tubing  14  inside wall. 
         [0032]    Attention is particularly directed to the geometric consequence of two, relatively small diameter discs  16  secured on the diametric centerline of a larger diameter circle with opposite extreme locus points of the disc  16  perimeter coinciding with diagonally opposite locus points on the larger circle perimeter. Any force on the tool housing  12  substantially normal to the diameter  20  can be opposed by a wedging reaction against the inside wall curvature of the tube  14 . This wedging reaction can be applied to the disc  16  perimeters and, ultimately, to the housing  12  by the mounting pins  18  to maintain the axial center of the housing  12  in directions transverse to the diameter  20 . 
         [0033]    In another embodiment of the invention as shown by  FIG. 4 , three discs  16  are secured by pin fasteners  18  to the housing at approximately 120° arcuate spacing about the housing axis  13  (shown in  FIG. 2 ). In this embodiment, the most distant elements of the disc  16  perimeters from the housing axis  13  at least coincide with the inside perimeter locus of the tubing  14 . 
         [0034]    The  FIG. 4  embodiment is representative of applications for a multiplicity of centering discs on a tool housing  12 . Depending on the relative sizes of the tool  10  and pipe  14 , there may be three or more such discs distributed at substantially uniform arcs about the tool circumference. 
         [0035]    Regarding the disc  16  properties, the terms “thin”, “resilient” and “metallic” are used herein to generally describe gage thickness of high carbon and heat treated “spring” steels. Although other metal alloys are functionally suitable, the parameter of economics is a strong driver of the invention, and exotic alloys are relatively expensive. 
         [0036]    Within this triad of material properties for a specific disc  16  application, gage thickness and bending modulus are paramount for the reason best illustrated by  FIG. 5A . In the event a well tool  10  must be withdrawn from a downhole location, the projecting arc of the disc  16  can be compressively deformed to reverse the drag sweep against the tubing wall. If the tool  10  is suspended in the tube  14  by the use of a wireline or slick line, not shown, potential exists for exceeding the tensile strength of the support line. A well tool supported by a tubing or pipe string is not as limited. Nevertheless, the disc  16  design limitations of “thin” and “resilient” have particular meaning for specific applications of the invention. 
         [0037]    Furthermore, as illustrated in  FIG. 5B , such designs have advantages in that they can be provided in a “stack” configuration, illustrated here as a pair of discs,  16   a  and  16   b , each having a thickness less than the thickness of the disc  16  illustrated in  FIG. 5A . Such configurations, it has been discovered, provide centralizing force nearly equivalent to a single disc thickness while reducing the force required to insert or withdraw the tool  10  from the tube  14 , due to the reduction in compressive stress along the diameter of the discs  16   a ,  16   b.    
         [0038]    While the centralizing force created by the arcuate projection of discs  16  beyond the tool housing  12  perimeter is an operative element of the invention, the economics of fabrication is an equally driving feature. Configurations other than a full circle may also provide an arcuate projection from the tool  12  perimeter. However, many alternate configurations are either more expensive to form or waste more fabrication material. Shown by  FIG. 3  is a disc  16  stamping pattern as imposed against a stock sheet of thin, resilient metal material  22 . When compared to single plane cross or star pattern centralizers, the percentage of material waste for a disc pattern is minimal. 
         [0039]    Referring now to  FIG. 6 , another economically driven embodiment of the invention is illustrated which includes spring steel centralizing wires  30  of small gage diameter. A plurality of these wires are arranged radially from an end boss  32 , seated within and extending from apertures  34  (shown in  FIGS. 8A-8B ). Such wires may preferably be formed of high-carbon steel, stainless steel, or any metallic or metallic composite material with sufficient flexibility and tensile strength. 
         [0040]    The end boss  32  is machined as an integrated part of the tool housing  12 , and the diameter of the end boss  32  will always be smaller than the diameter of the tool housing  12 . Note that the scale and angle of end boss  32  is depicted for clarity; in alternative embodiments, end boss  32  may be any configuration of the distal end of tool housing  12 . 
         [0041]    Referring now to  FIG. 7 , a plan view of the configuration in  FIG. 6  is shown, with the plurality of centralizing wires  30  projecting outwardly in a radial arrangement from end boss  32 . While the depicted configuration includes a total of eight centralizing wires  30 , it should be appreciated that the plurality may be made up of any number of centralizing wires  30 , or in some cases, as few as two. As can be seen in the plan view, the use of centralizing wires  30  rather than blades or other machined pieces, allows for the advantageous maximization of space in the flowbore around the centralizing system, compared to previous spider-type centralizers, by minimizing the cross-section compared to systems featuring flat blades or other planar configurations. 
         [0042]    As with the configuration in  FIGS. 1-5 , the wires  30  are normally oriented to the housing axis  13  and engaged with the sides of the tubing  14 . Wires  30  are sized such that the length of the wires  30  is slightly larger than the length between the inside terminus of apertures  34  and inside diameter of tubing  14 . Thus, wires  30  will exert compressive force to centralize tubing cutter  10 , and flex in the same fashion as the cross-section of discs  16 , shown in  FIG. 1  and  FIG. 5 a   , during insertion and withdrawal. The length of wires  30  may be sized for a specific tubing  14  inside diameter, either before or after attachment to the end boss  32 . 
         [0043]    Referring now to  FIG. 8A , the system of  FIGS. 6-7  is shown in cross-section, including the end boss  32  having the plurality of apertures  34  formed laterally and penetrating a short distance therein  32 . Apertures  34  are sized to accommodate the diameter of the wires  30  at the surface of the end boss, which are attached within the apertures  34  via glue, soldering, or other methods. 
         [0044]    Referring now to  FIG. 8B , an alternative attachment method is shown for the  FIG. 6-7  embodiment, in which the diameter of the aperture  34  is slightly smaller than the body of the wires  30 , which enables an interference fit, or press fit, between wires  30  and aperture  34 , where the proximal ends of wires  30  are inserted into the apertures, and then subjected to compressive force and deformed slightly to fit the narrower aperture  34 . 
         [0045]    Referring now to  FIG. 9 , a third embodiment of the invention is illustrated herein. This configuration comprises a plurality of planar, finger-like structures (herein “blades”) to centralize a tubing cutter  10 . The plurality  40  of blades  45   a ,  45   b  are positioned on the bottom surface of the tubing cutter  10  through a plurality of fasteners  42 , projecting outwardly therefrom. The plurality  40  of blades  45   a ,  45   b  thus flex, against the sides of the wellbore  14 , to exert a centralizing force in substantially the same fashion as the disc embodiments depicted in  FIGS. 1 and 5A-5B . Thus, it can be appreciated that the plurality  40  of blades  45   a ,  45   b  may also comprise a stacked embodiment in which the thickness is reduced to stack multiple blades  45  on the same plurality of fasteners  42 . 
         [0046]      FIG. 10  depicts an embodiment of a single blade  45  from the plurality of blades  40 . Each blade  45  comprises a plurality of attachment points  44   a ,  44   b , through which fasteners  42  secure the blade in position. As shown, each respective fastener can extend through a respective attachment point to secure the blade into position. While the embodiment in  FIG. 9  is depicted with two blades  45   a ,  45   b , and each blade  45  comprising two attachment points, for a total of four fasteners  42  and four attachment points ( 44   a ,  44   b  are pictured in  FIG. 10 ), it should be appreciated that the invention may comprise any number of fasteners and attachment points. 
         [0047]    Significantly, the multiple attachment points  44  on each blade, being spaced laterally from each other, prevent the unintentional rotation of individual blades  45 , even in the event that the fasteners  42  are slightly loose from the attachment points  44 . The fasteners  42  can be of any type of fastener usable for securing the blades into position, including screws. 
         [0048]    Each blade  45  of the plurality  40  of blades  45  can be manufactured at a low cost from a pre-selected width of coil material and simply cut for length, obviating the need in the prior art for specially designed and cut centralizer patterns. As set forth above, the plurality of blades can be spaced laterally and oriented perpendicular to each other, for centralizing a tubing cutter  10  and preventing unintentional rotation of the one or more blades  45 . 
         [0049]    Although the invention disclosed herein has been described in terms of specified and presently preferred embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.