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BACKGROUND OF INVENTION 
   Background Art 
   A well bore may be drilled in the earth for various purposes, such as hydrocarbon extraction, geothermal energy, or water. After a well bore is drilled, the well bore is typically lined with casing. The casing preserves the shape of the well bore as well as provides a sealed conduit for fluid to be transported to the surface. 
   A common problem in well bores is the accumulation of metallic debris. The metallic debris can be in the form of small metal shavings. Metal shavings can enter the hydrocarbon producing formation and reduce production. Metallic debris may be generated by tools on a work string scraping against the inside of the casing. Also, metallic debris is created while milling metal objects downhole, such as a bridge plug or packer. Some of the metallic debris may be brought back to the surface by well fluids that are circulated in the well bore, but a significant amount may still remain in the well bore. 
   Corrosion and other damage degrades the interior of the metal casing over time, which leaves a rough surface. This condition is typically cured by running tools in and out of the well bore with wire brushes and scrapers to abrade the inside of the casing. A scraper typically includes steel blades disposed on the outside of a cylindrical tool. The blades are biased radially outward by springs so that the scraper abrades the inside of the casing. The scraper helps to dislodge rough particles that are magnetically attracted to the casing or embedded in the casing wall. Wire brushes serve a similar purpose, but typically remove smaller particles. Some of the removed material is in the form of small metallic shavings and flakes of metal. Fluid is circulated during this operation to lift the removed material to the surface, but some metallic debris is left in the well bore. 
   Many tools exist that use magnets to attract and hold metallic debris, allowing the metallic debris to be removed from the well bore. Typically, permanent magnets in the form of buttons or bars are spaced apart to cover the outside of the magnetic tool. Metallic debris is attracted to each magnet allowing the removal of debris. Increased removal of metallic debris is accomplished by using more and larger magnets. 
   An example of a magnetic tool used to remove metallic debris is provided in U.S. Pat. No. 6,591,117 B2, entitled “Apparatus for Retrieving Metal Debris from a Well Bore.” In the &#39;117 patent, large bar magnets are spaced apart around and along a tool body to attract metal debris. The bar magnets are fitted into recesses in the tool body and arranged to have an area between each magnet for metallic debris to settle. 
   SUMMARY OF INVENTION 
   In one aspect, the present invention relates to a downhole tool for removing metallic debris from a well bore. The downhole tool includes a body that is able to connect to a work string. Two or more hoop magnets are disposed coaxially along the length of the body, and arranged in a bucking arrangement. 
   In one aspect, the present invention relates to a downhole tool for removing metallic debris from a well bore. The downhole tool includes a body with a mandrel and a central opening. The body is able to connect to a work string. A magnet assembly is disposed on the mandrel. The magnet assembly includes an inner sleeve designed to fit around the mandrel. A plurality of hoop magnets are disposed on the inner sleeve and spaced apart along the length of the inner sleeve. The plurality of hoop magnets are arranged in a bucking arrangement. 
   In one aspect, the present invention relates to a downhole tool for removing metallic debris from a well bore. The downhole tool includes a body that is able to connect to a work string. A plurality of magnets are distributed azimuthally around the circumference of the body. The plurality of magnets are arranged in a bucking arrangement. 
   In one aspect, the present invention relates to a downhole tool for removing metallic debris from a well bore. The downhole tool includes a body with a mandrel and a central opening. The body is able to be connect to a work string. A magnet assembly is disposed on the mandrel. The magnet assembly includes an inner sleeve designed to fit around the mandrel. A plurality of magnets are distributed azimuthally around the circumference of the inner sleeve. The plurality of magnets are arranged in a bucking arrangement. 
   Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a quarter-section of a magnet carrier in accordance with one embodiment of the invention. 
       FIG. 2A  shows a cross-section of a magnet in accordance with an embodiment of the present invention. 
       FIG. 2B  shows a cross-section of a magnet in accordance with an embodiment of the present invention. 
       FIG. 2C  shows a cross-section of a magnet in accordance with an embodiment of the present invention. 
       FIG. 3  shows an arrangement of magnets in accordance with one embodiment of the invention. 
       FIG. 4A  shows a tool body in accordance with one embodiment of the invention. 
       FIG. 4B  shows a part adapted to attach to the tool body of  FIG. 3A  in accordance with an embodiment of the present invention. 
       FIG. 4C  shows a downhole tool having a magnet carrier in accordance with one embodiment of the present invention. 
       FIG. 5  shows a downhole tool having a magnet carrier with attached metallic debris in accordance with one embodiment of the present invention. 
       FIG. 6A  shows an arrangement of magnets in accordance with one embodiment of the invention. 
       FIG. 6B  shows a magnet in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In one aspect, the present invention relates to an arrangement of magnets for removing metallic debris from a well bore. More specifically, embodiments of the present invention have a plurality of magnets spaced apart so that the magnetic field of each magnet interacts with the magnetic field of its neighbor to increase the effectiveness of the magnet arrangement to remove metallic debris from a well bore. 
     FIG. 1  shows a magnet carrier  110  in accordance with one embodiment of the present invention. A plurality of ring shaped magnets  101  are disposed on an inner sleeve  104 . Each magnet  101  may comprise an assembly of individual magnet rings  102  and spacer rings  103 . The magnet rings  102  may be permanent magnets made of any suitable magnetic material, such as neodymium iron boron, ceramic ferrite, samarium cobalt, or aluminum nickel cobalt. In one embodiment, spacer rings  103  may be made of a carbon steel with magnetic properties, or any other material that exhibits magnetic properties. The magnet rings  102  are aligned within each magnet  101  by their magnetic poles to attract each other. The spacer rings  103  are magnetized and attracted to the magnet rings  102 . The spacer rings  103  may be used to indicate the magnetic pole of each magnet. For example, two spacer rings  103  separated by a magnet ring  102  may represent the magnetic north of the magnet  101 , while one spacer ring  103  may represent the magnetic south of the magnet  101 . Alternatively, the spacer rings  13  may have markings to indicate the poles of the magnet  101 . Coloring magnet rings  102  or otherwise marking the magnets may accomplish the same purpose. This feature is used to clearly indicate which ends of the magnet  101  will attract other magnets for assembly and safety purposes. 
   In the embodiment shown in  FIG. 1 , the inner sleeve  104  is designed to accommodate four magnets  101 . The inner sleeve  104  may be sized to fit around a mandrel (not shown). To prevent dissipation of the magnet strength, the inner sleeve  104  may be formed from an austenitic stainless steel, or other material that exhibits little or no magnetic susceptibilities. A center ridge  111  may be formed on the inner sleeve  104  for assembly purposes. 
   To assemble this embodiment, a first magnet  101 B may be placed against the center ridge  111 . The first magnet  101 B may be fixed in place by a retaining device, such as a retaining ring  106  or a snap ring. A second magnet  101 A may be installed on the same side of the center ridge  111 . The second magnet  101 A is oriented so that the same magnetic pole faces the first magnet  101 B, such as north to north. Both magnets  101 A and  101 B are in close proximity to each other so that their magnetic fields repulse each other, resulting in a substantial repulsive force. The second magnet  101 A may also be secured in place by a retaining ring  106 . The same procedure may be repeated for magnets  101 C and  101 D. 
   After all four magnets  101 A– 101 D are secured, an outer sleeve  105  may be placed around the magnets  101 A– 101 D. The outer sleeve  105  is preferably formed of a material exhibiting little or no magnetic susceptibility, such as an austenitic stainless steel, to prevent interference with the magnetic fields of the magnets  101 . The outer sleeve  105  provides protection for the magnets  101  and a gathering surface for magnetic debris. In one embodiment, grooves  107  are formed in the outside surface of the outer sleeve  105 . The grooves  107  help to retain metallic debris. 
   After the outer sleeve  105  is installed, end caps  108  may be placed on the magnet carrier  110 . The end caps  108  may be secured by an interference fit between the outer sleeve  105  and inner sleeve  104 . Alternatively, the end caps  108  may be threaded or secured by any other means known in the art. The end caps  108  are preferably formed of a material exhibiting little or no magnetic susceptibility to prevent interference with the magnetic fields of the magnets  101 . The individual features in this particular embodiment are intended to illustrate how a magnet carrier may be assembled in accordance with one embodiment of the present invention. However, they are not intended to limit the scope of the invention. For example, the magnets may be held in place by other means, such as an adhesive. In one embodiment, the magnets are assembled from one end of the inner sleeve without a center ridge. One of ordinary skill in the art will appreciate that magnets may be assembled into a magnet carrier in different ways without departing from the scope of the invention. Furthermore, some embodiments may not include the magnet carrier. Instead, the magnet arrangement may be disposed directly onto a tool body, for example. 
   While the above embodiment combines separate magnet rings to form a magnet, other magnet forms may be selected to use in a similar manner. For example, the magnet  101  may be a single piece instead of a combination of magnet rings  102 . Furthermore, the magnet  101  or magnet rings  102  need not be in a contiguous ring shape. Instead, they may comprise sections that substantially form a ring. 
     FIGS. 2A ,  2 B, and  2 C show transverse cross-sections of magnets in accordance with some embodiments of the present invention.  FIG. 2A  shows a magnet having a slot  140 . Alternatively, the magnet may be a plurality of arcuate sections  150 , such as that shown in  FIGS. 2B and 2C , and disposed circumferentially about the inner sleeve resulting in a substantially 360 degree magnetic field about the magnet carrier. One of ordinary skill in the art will appreciate that other shapes or groupings of magnets may be used to provide a substantially 360 degree magnetic field about the magnet carrier without departing from the scope of the invention. For the purpose of clarity, a single magnet or a set of magnets forming a substantially 360 degree magnetic field will be referred to hereinafter as a “hoop magnet.” For example, four quarter-sections of a magnet ring disposed circumferentially about the inner sleeve at about the same longitudinal position form a hoop magnet for the purpose of this disclosure. For the present invention, a hoop magnet has two poles oriented to be substantially parallel to the axis of the hoop magnet. 
   The magnetic orientation and distance of each hoop magnet relative to a neighboring hoop magnet allows for a magnetic field with an increased radial size to be created. As is known in the art, a magnet generally has a north and a south pole. When two magnets have opposite poles facing each other (i.e., north to south), the magnets are attracted to each other. Like magnetic poles repulse each other.  FIG. 3  illustrates the effect of magnetic fields interacting in accordance with an embodiment of the present invention. The magnetic fields are represented by the lines arcing from the blocks labeled with N (north) and S (south). When magnets are oriented to repulse each other as in  FIG. 3 , the magnetic field of each magnet is deflected by the neighboring magnet. This phenomenon is commonly referred to as “bucking.” The deflection of the magnetic fields in the manner shown in  FIG. 3  results in magnetic fluxes oriented in the same direction between the neighboring magnets. The summation of the magnetic fluxes gives rise to a magnetic field that projects further outward from between the two magnets. This results in a magnetic field with greater outward reach than the magnetic field of a single magnet with the same strength. Arranging a plurality of hoop magnets in this manner allows for a larger apparent magnetic field for a magnet arrangement. The term “bucking arrangement” is utilized to clearly and concisely describe the type configuration for the two or more magnets as disclosed herein. 
   The longitudinal spacing of the hoop magnets vary depending the characteristics of the hoop magnets, such as the strength of the magnetic field. If the hoop magnets are too far apart, the bucking effect is reduced, causing the hoop magnets to act more individually. When moving the hoop magnets close together, the bucking effect increases, causing the magnetic field to expand radially. At the same time, the overall coverage of the magnetic field in the longitudinal direction is reduced for a given number of hoop magnets. Because the well bore is limited in diameter, the radial reach of the magnetic field is wasted much beyond the well bore. Therefore, it is desirable to balance the length and radial reach of the magnetic field created by the magnet arrangement. In one embodiment, six ceramic ferrite hoop magnets 1 inch in height are disposed ¾ of an inch apart longitudinally. 
   The number of hoop magnets spaced longitudinally in the magnet carrier may vary. Two or more hoop magnets may be spaced longitudinally in accordance with embodiments of the present invention. In one embodiment, six hoop magnets are used. In another embodiment, five hoop magnets are spaced apart in the magnet carrier. One of ordinary skill in the art will appreciate that the number of hoop magnets in the magnet carrier can vary without departing from the scope of the invention. 
     FIGS. 4A and 4B  show a downhole tool body in accordance with one embodiment of the present invention. The downhole tool body shown in  FIGS. 4A and 4B  is adapted to connect to a work string on both ends by a box connection  304  and a pin connection  303 . The downhole tool body includes two components illustrated apart in  FIGS. 4A and 4B . The component in  FIG. 4A  has a mandrel  301  adapted to accommodate additional components, such as a magnet carrier, scraper, brush, and centralizer. The additional components may be secured on the downhole tool body by connecting the end body in  FIG. 4A  to the tool body in  FIG. 4B  by connection  307 . While a threaded connection is shown, one of ordinary skill in the art would appreciate that other connections may be used without departing from the scope of the invention. The downhole tool body includes a central opening  306  to allow fluid to circulate through the work string. 
   Turning to  FIG. 4C , an assembled downhole tool in accordance with an embodiment of the invention is shown. Several components have been disposed on the mandrel  301  and secured by connecting the component in  FIG. 4A  to the component in  FIG. 4B . From bottom to top, the components are a lower centralizer  310 , a scraper module  312 , a magnet carrier  110 , two brush modules  311 , and an upper centralizer  314 . This is just one example of a module arrangement. An alternative arrangement may be a centralizer, two scraper modules, two magnet carriers, and a centralizer. A longer mandrel would allow for additional modules. One of ordinary skill in the art will appreciate that more or less modules with these or other known components may be used without departing from the scope of the invention. The combination of modules will vary depending on the purpose of the operation and the conditions of a particular well bore. For example, if the objective is only to remove metallic debris in the well, multiple magnet carriers may be deployed without any brush or scraper modules. In one or more embodiments, a boot basket module may be disposed on the mandrel to capture both metallic and non-metallic debris. 
   The module arrangement shown in  FIG. 4C  allows for the magnet carrier  110  to capture metallic debris (not shown) as it is dislodged from the casing (not shown) by the brush modules  311  and scraper module  312 . This reduces the amount of metallic debris that would normally settle to the bottom of the well bore and potentially reduce future production. The centralizers  310  keep the downhole tool centered in the well bore so that the inside of the casing is cleaned evenly. The centered arrangement also helps to ensure that the magnetic field of the magnet carrier is fully utilized to extract metallic debris from the well fluid. 
   Modules disposed on a mandrel as shown in the above embodiment may not be forced to rotate with the rest of the work string. The modules are confined longitudinally, but are free to rotate azimuthally. This reduces the wear on the casing and on the modules. This containment system also allows for simple replacement of modules when a module wears out or when other configurations are desired. 
     FIG. 5  shows a portion of the downhole tool of  FIG. 4C  in accordance with an embodiment of the present invention. The downhole tool has been run into a well bore to remove metallic debris, primarily metal shavings. In this embodiment, the magnet carrier has six hoop magnets. The metal shavings  501  have collected on the magnet carrier  110  at the location of each hoop magnet. The hoop magnet arrangement in accordance with an embodiment of the present invention provides a substantially continuous magnetic field around and along the length of the magnet carrier. 
   While the above embodiments have included a modular type of magnet carrier, it should be understood that the hoop magnet arrangement that has been disclosed may be used in other downhole tools for the purpose of removing metallic debris from a well bore. For example, the inner sleeve may not be required if the hoop magnets are disposed directly onto a tool body adapted to attach to a work string. Additionally, the hoop magnets may be disposed at one end of a tool body adapted to attach to a work string at the other end. Hoop magnets disposed at the end of the tool may be able to effectively remove metallic debris that has settled at the bottom of the well bore. One of ordinary skill in the art will be able to utilize the disclosed hoop magnet arrangement in other downhole tool applications to remove metallic debris from a well bore without departing from the scope of the invention. 
   While the above embodiments have used hoop magnets, one having the benefit of this disclosure could utilize the bucking phenomenon with other magnets.  FIG. 6A  shows a magnet arrangement in accordance with one embodiment of the present invention. The magnets  601  are aligned such that the poles are oriented substantially transverse to the axis  603  of the tool body  602 . The magnets  601  are distributed around the circumference of the tool body  602 . The azimuthal spacing of the magnets  601  is selected so that bucking occurs between each adjacent magnet  601 . The azimuthal spacing of the magnets  601  may vary based on several factors, such as magnetic strength, size of the tool body  602 , and quantity of magnets  601  desired. Closer azimuthal spacing of the magnets  601  does not affect the longitudinal length of the magnetic field, because that is determined by the length of each magnet  601 . A closer azimuthal spacing may result in difficulty in assembling the magnets  601  to the tool body  602 . Additionally, a closer azimuthal spacing would require additional magnets  601  to surround the tool body  602 . One of ordinary skill in the art will appreciate that the azimuthal spacing and quantity of the magnets  602  may vary without departing from the scope of the invention. 
   The magnets  601  may be secured by any means known in the art, such as a bolt, straps, or adhesive. While the magnets  601  are shown directly attached to a tool body  602 , the magnets  601  may be attached to a module similar to that shown in  FIG. 1 . To prevent depletion of the magnetic field, the tool body  602  or module may be formed from a material having little or no magnetic susceptibility, such as an austenitic stainless steel. 
   While the magnets  601  shown in  FIG. 6A  are rectangular in cross-section, other shapes of magnets may be used in a similar manner.  FIG. 6B  shows a cross-section of a magnet  605  that may be used for the magnet arrangement of  FIG. 6A  in accordance with one embodiment of the present invention. The magnet  605  shown in  FIG. 6B  has an arcuate shape that conforms to a circular tool body (not shown). One of ordinary skill in the art will appreciate that other shapes of magnets may be used in a similar manner without departing from the scope of the invention. 
   Embodiments of the present invention provide one or more of the following advantages. Metallic debris, especially small metal shavings, are suspended in the well fluid. As the magnet carrier passes by the metal shavings, the metal shavings are only attracted by the magnet carrier if they are within a strong portion of the magnetic field. To capture the metal shavings throughout the well fluid, the magnetic field must extend radially to the casing from the magnet carrier. This can be accomplished by utilizing bucking between the magnetic fields of two or more hoop magnets. As the magnet carrier passes through the well bore and well fluid flows by, metal shavings are pulled from the well fluid and attached to the magnet carrier. 
   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.

Summary:
The present invention relates to a downhole tool for removing metallic debris from a well bore. The downhole tool includes a plurality of magnets disposed on the tool body. The plurality of magnets are arranged in a bucking arrangement such that repulsing forces are generated between neighboring pairs of the plurality of magnets. The bucking arrangement results in an expanded reach of the magnetic fields of the magnets to enhance the removal of metallic debris.