Abstract:
A tool for removal of magnetic debris from a well bore, having a plurality of magnets retained in a plurality of recesses by a plurality of retainer caps which are threadable into the recesses. The retainer caps can be made of non-magnetic material, and non-magnetic spacers can be used, to isolate the magnets from the tool body and the surroundings. The retainer caps can be positioned entirely within the recesses, to streamline the tool body. The magnets can be removed from the recesses and replaced.

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 
     This invention is in the field of apparatus used to remove debris from a well bore. Specifically, this invention applies magnetic force to attract magnetic debris to the tool, after which the tool can be withdrawn from the well bore to remove the debris. 
     2. Background Art 
     In the process of drilling an oil or gas well, producing oil or gas from the well, or refurbishing an existing well, tool failures and normal operation often result in the depositing in the well bore of various kinds of debris. Downhole milling of metal items produces metallic mill cuttings which often are not completely removed from the well bore by circulation of milling fluid. Furthermore, bit cones, bearings, slips, tong pins, and hammers, or fragments thereof, can collect at the bottom of the well bore. Several devices have been developed for the removal of such debris from the well bore. 
     Tools incorporating a basket often circulate fluid up the annulus at a rapid rate, to carry the debris upwardly to an area of reduced flow rate where the debris falls back, to settle into a basket for retrieval. Other basket tools use a venturi effect to draw debris into the tool, usually at the bottom end, then it is caught in a basket within the tool. Debris which is magnetic, or susceptible to magnetic attraction, can be attracted to a tool which incorporates magnets, followed by removal of the tool and its attached debris from the well bore. Some such tools have a magnet in the lower end of the tool, for the purpose of attaching magnetic debris to the lower end of the tool. 
     Another tool has been devised which has a plurality of magnets aligned in a string in a cavity near the outer surface of the tool, covered by a thin metal housing. Yet another tool has been devised which has a plurality of magnets individually embedded into a plurality of recesses in the outer surface of the tool, around its periphery. In this type of tool, nonmagnetic cups may be pressed or brazed into the recesses, and a set pattern of magnets are essentially permanently retained in the nonmagnetic cups by means of epoxy, or some other adhesive. The magnets are typically exposed to the well bore environment surrounding this tool, subjecting them to physical damage or corrosion in the well bore. Further, the exposed magnets are subject to physical damage during the process of cleaning debris from the tool after it is withdrawn from the well bore. Still further, the exposed magnets may even be lost in the well bore when the retaining epoxy becomes soft because of high well bore temperature. 
     It would be desirable to have a magnetic debris retrieval tool in which magnets could be placed in a choice of patterns on the outer perimeter of the tool and easily removed therefrom. It would also be desirable to cover the magnets with protective covers, and to magnetically isolate the magnets from the remainder of the tool to enhance their performance. 
     BRIEF SUMMARY OF THE INVENTION 
     By way of example, the preferred embodiment of the present invention is a tool for removal of magnetically susceptible debris from a well bore, by causing the debris to adhere to the body of the tool and removing the tool from the well bore. The tool body has a plurality of recesses in its outer surface. A plurality of magnets can be inserted in selected recesses to form a desired pattern of magnets. The magnets are retained in the recesses by retainer caps which thread into the recesses. The retainer caps are designed to fit entirely within the recesses, to give the tool a streamlined aspect, or an unobstructed outer surface. The retainer caps may be small enough to fit entirely within the recesses, threaded therein by the use of some low profile drive contour, such as a hexagonal head, a recessed hexagonal shape, or a screwdriver slot. Alternatively, the retainer cap may be threaded into the recess by the use of a drive member which can subsequently be removed from the retainer cap, by the application of higher torque to shear the drive member away, or by some similar operation. The retainer caps can be constructed of a non-magnetic material, and non-magnetic spacers can be placed beneath the magnets, to isolate the magnets from the tool body, thereby enhancing the performance of the magnets. 
     The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a longitudinal section of a tool according to the present invention, showing one arrangement of some of the recesses, magnets, and retainer caps; 
     FIG. 2 is an elevation view of one embodiment of a pattern of recesses that may be used in the present invention; 
     FIG. 3 is a section view of a hexagonal head retainer cap that may be used in the present invention; 
     FIG. 4 is a partial section view of a recess, magnet, hexagonal head retainer cap, and spacer, according to the present invention; 
     FIG. 5 is an elevation view of a retainer cap with a shearable drive member, that may be used in the present invention; and 
     FIG. 6 is a partial section view of a recess, magnet, shearable drive retainer cap, and spacer, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, the retrieval tool  10  of the present invention has an elongated, generally cylindrical, tool body  12 . The tool body  12  has an upper end  14  adapted to be threadedly attached to a work string (not shown) for lowering into a well bore. The tool body  12  also has a lower end  16  adapted to be threadedly attached to a downhole tool (not shown), such as a drill bit or downhole motor. Several of the retrieval tools  10  could also be attached in tandem to a work string, and they could be attached at an intermediate location in the work string. 
     The tool body  12  has a generally cylindrical outer surface  18 , which can have several different diameters. Positioned on the tool outer surface  18  are a plurality of recesses  20  into the tool body  12 . Each of the recesses  20  can have installed therein a magnet  22 , a retainer cap  24 , and a spacer  26 . Typically the magnet  22  is a permanent magnet, although an electromagnet would be within the scope of the invention. The spacer  26  can be constructed of a non-magnetic material, such as stainless steel. The retainer cap  24  is threaded into the recess  20  over the spacer  26  and the magnet  22 , in such a way as to retain the magnet  22  and the spacer  26  in place in the recess  20 . It can be seen that, after installation, the retainer cap  24  and all the elements thereunder are configured to fit entirely within the recess  20 . Different types of retainer caps and spacers can be used to accomplish this objective, as will be discussed later. The pattern in which the recesses  20  are formed in the outer surface  18  of the tool body  12  can vary as desired for a particular application. Further, magnets  22  can be inserted into selected recesses  20  as desired, to form a preferred pattern of magnets  22  for a given application. 
     FIG. 2 shows a sample pattern of recesses  20  and retainer caps  24  that might be used on a tool body  12 . If desired, some of the recesses  20  could be left empty, or they could have retainer caps  24  installed, with no magnets  22 . FIG. 2 is a flat representation of a pattern that might be wrapped around the cylindrical outer surface  18  of the tool body  12 . As shown, adjacent rows of recesses  20  can be offset from each other. Alternatively, they could be aligned with each other, or some other pattern could be used. The angular spacing D between adjacent rows could be designed to cause the pattern to wrap entirely around the cylindrical surface  18  of the tool body  12 . For instance, in the embodiment shown, with six rows of recesses  20 , placing the rows with an angular spacing D of 60° would result in the pattern wrapping entirely around the surface  18 . Other angular spacing D could also be used, and the pattern could be placed on a selected portion of the outer surface  18 . 
     FIG. 3 is a section view of one type of retainer cap  24  that might be used in the present invention. The embodiment shown here has a thin upper bulkhead  28 , thereby minimizing the spatial separation between the magnet  22  and the surrounding environment. The retainer cap  24  can be constructed of a non-magnetic material, such as stainless steel. The upper shoulder of the retainer cap  24  has an external hexagonal drive contour  30 , facilitating the threading of the retainer cap  24  into a recess  20  with a wrench or similar tool. Other low profile drive contours could also be used instead of the external hexagonal drive contour, such as a hexagonal recess, or a screw driver slot. An external thread  32  is provided on the periphery of the retainer cap  24 , to facilitate the threading of the retainer cap  24  into a recess  20 . An internal cavity  34  is provided within the retainer cap  24 , to provide room for a magnet  22 , and if desired, a spacer  26 . 
     FIG. 4 is a section view of one recess  20 , with a magnet  22 , a spacer  26 , and a low profile retainer cap  24  installed therein. It can be seen that the retainer cap  24 , when installed, fits entirely within the recess  20 , leaving the tool body  12  with a streamlined or unobstructed outer profile. The recess  20  has an enlarged diameter unthreaded portion  33  to accommodate the drive contour  30  of the hexagonal drive retainer cap  24 . The retainer cap  24  is the sole means of retaining the magnet  22  and the spacer  26  within the recess  20 , by being threaded into internal threads  35  within the recess  20 . This facilitates the removal and relocation or replacement of the magnet  22  and the spacer  26 . In this embodiment, the magnet  22  and the spacer  26  fit entirely within the cavity  34  within the retainer cap  24 . The spacer  26  can be constructed of a non-magnetic material, such as stainless steel. When a non-magnetic retainer cap  24  and a non-magnetic spacer  26  are used, the magnet  22  is isolated from the remainder of the tool  10 , and the magnetic performance of the magnet  22  is enhanced. 
     FIG. 5 shows another type of retainer cap  24 ′ and spacer  26 ′ which can be used. This embodiment of the retainer cap  24 ′ has an external thread  32 ′, similar to the first embodiment, except that the external thread  32 ′ in this embodiment runs all the way up to the upper bulkhead  28  of the retainer cap  24 ′. The drive contour, furthermore, is different in this embodiment. That is, this embodiment of the retainer cap  24 ′ has a removable drive member  36 . This particular removable drive member  36  is a circular disc, which is attached to the upper bulkhead  28  of the retainer cap  24 ′ by means of a drive shaft  38 . The drive shaft  38  has a small cross-section, making the drive member  36  shearable from the retainer cap  24 ′. Other types of removable drive members could also be used, such as a shearable drive bar or a shearable hexagonal head. The spacer  26 ′ shown with this embodiment of the retainer cap  24 ′ is a disc which fits under the lower end of the retainer cap  24 ′, rather than within the inner cavity. This leaves the entire inner cavity available for installation of a magnet  22 . 
     FIG. 6 shows the second embodiment of the retainer cap  24 ′ installed within a recess  20 ′. This embodiment of the recess  20 ′ has threads  35 ′ up to the outer surface  18  of the tool body  12 , rather than having an enlarged diameter unthreaded portion like the first embodiment of the recess  20 . The spacer  26 ′ can be inserted first into the recess  20 ′, and the magnet  22  can be placed into the retainer cap  24 ′. Then, the retainer cap  24 ′ is threaded into the recess  20 ′, by means of the drive member  36 . Excess torque can then be applied to the drive member  36  to shear the drive shaft  38  from the upper bulkhead  28  of the retainer cap  24 ′, thereby removing the drive member  36  from the retainer cap  24 ′. Preferably, the drive shaft  38  is designed to shear near the upper bulkhead  28  of the retainer cap  24 ′, to leave the tool body  12  with a streamlined or unobstructed outer profile. One or more recessed drive countours (not shown) can be provided in the upper bulkhead  28  of the retainer cap  24 ′, to facilitate the removal of the retainer cap  24 ′ from the recess, if desired. Here again, the retainer cap  24 ′ and the spacer  26 ′ can be constructed of a non-magnetic material, such as stainless steel. When a non-magnetic retainer cap  24 ′ and a non-magnetic spacer  26 ′ are used, the magnet  22  is isolated from the remainder of the tool  10 , and the magnetic performance of the magnet  22  is enhanced. 
     While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.