The invention is a system for retrieving loose metallic debris, such as stray radioactive marker bullets, from a borehole in a subterranean formation. The system can be used to retrieve stray radioactive marker bullets during placement of the radioactive marker bullets in the subterranean formation. The system utilizes a downhole fishing tool having at least one recessed magnetic surface for attracting and holding stray radioactive marker bullets. The recessed magnetic surface allows the fishing tool to be removed from the borehole without scraping off retrieved marker bullets that are held against the magnetic surface.

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to systems which monitor the subsidence and compaction of subterranean formations by measuring the shifting of spaced-apart radioactive marker bullets implanted in the formation. In another aspect, the invention concerns a method and apparatus for retrieving stray radioactive marker bullets loosely positioned in a subterranean borehole. 
     2. Description of the Prior Art 
     The production of water and hydrocarbon fluids from subterranean formations depletes reservoir pressure and removes the fluids from the interstatial pores of the subterranean formation. As fluids are removed, the reduction of pressure in the subterranean formation may cause collapsing (i.e., subsidence) and compaction of the subterranean formation. This subsidence and corresponding compaction is heightened when the subterranean formation comprises chalk or other rock having relatively high porosity and a low compressive strength. An excessive amount of subsidence may result in well casing failure or rig collapse. Further, excessive compaction can permanently damage the permeability and hydrocarbon producing capability of a subterranean formation if the interstatial pores are irreparably closed. It is therefore desirable to monitor the subterranean formation to detect the onset of subsidence and compaction. 
     One method of monitoring formation subsidence and compaction involves implanting radioactive bullets within the subterranean formation as depth markers. The positions of the implanted bullets can then be monitored over the years using a downhole logging tool which detects the gamma rays emitted by the radioactive bullets. 
     One common method of implanting radioactive marker bullets into a subterranean formation involves lowering a radioactive marker bullet gun into the borehole and firing radioactive marker bullets radially out from the radioactive marker bullet gun and into the formation. When the subterranean formation is formed of a high density rock, bullets fired from the radioactive marker bullet gun may bounce off the subterranean formation and can be lost in the borehole. Some of these “stray” radioactive marker bullets in the borehole may be partially lodged in the wall of the borehole or may be resting on ledges of the borehole wall. Other of the stray radioactive marker bullets may simply fall to the bottom of the borehole. These stray radioactive marker bullets are typically left in the borehole and cemented. When the next section of the borehole is drilled, some of the stray radioactive marker bullets may be crushed, thereby exposing the drilling mud to radioactive particles, and risking human exposure to radiation. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a downhole fishing tool for retrieving stray radioactive marker bullets from a borehole during placement of radioactive marker bullets in a subterranean formation. 
     A further object of the invention is to provide a downhole tool assembly for implanting radioactive marker bullets into a subterranean formation and retrieving any stray radioactive marker bullets from the borehole. 
     A still further object of the invention is to provide a wellbore comprising a borehole, a stray radioactive marker bullet located in the borehole, and a downhole fishing tool for retrieving the stray radioactive marker bullet from the borehole. 
     Another object of the invention is to provide a method of retrieving stray radioactive marker bullets that does not require a fishing tool to be separately run down the borehole after the radioactive marker bullets are implanted into the subterranean formation. 
     It should be noted that the above-listed objects are exemplary, and not all of the above objects need to be accomplished by the invention described and claimed herein. 
     Accordingly, in one embodiment of the present invention, there is provided a downhole fishing tool comprising a main body and a side magnet. The main body extends along a longitudinal axis and presents a pair of axially spaced wide outer surfaces. The side magnet is coupled to the body and axially positioned between the wide outer surfaces. The side magnet presents a generally radially facing side surface. The wide outer surfaces are radially spaced from the longitudinal axis by a first maximum radial distance, while the side surface is radially spaced from the longitudinal axis by a second maximum radial distance. The first maximum radial distance is at least 10 percent greater than the second maximum radial distance. 
     In another embodiment of the present invention, there is provided a downhole fishing tool for retrieving radioactive marker bullets from a borehole extending into a subterranean formation. The downhole fishing tool comprises a main body, a first group of magnets, a second group of magnets, and a third group of magnets. The main body extends along a longitudinal axis and includes axially spaced proximal and distal ends. The body presents a first wide outer surface proximate the distal end, a third wide outer surface proximate the proximal end, and a second wide outer surface axially spaced from and positioned between the first and second wide outer surfaces. The first group of magnets is coupled to the body, circumferentially spaced from one another, and axially positioned between the first and second wide outer surfaces. The second group of magnets is coupled to the body, circumferentially spaced from one another, and axially positioned between the second and third wide outer surfaces. The magnets in the third group of magnets are coupled to the distal end oft he body and are spaced-apart from one another. 
     In still another embodiment of the present invention, there is provided an elongated downhole tool for inserting radioactive marker bullets into a subterranean formation and retrieving stray radioactive marker bullets not properly penetrating into the formation. The downhole tool comprises a radioactive marker bullet gun having a normally upper end and a normally lower end and a magnetic fishing tool having a proximal end coupled to the normally lower end of the radioactive marker bullet gun. The fishing tool includes a side magnet axially positioned between the proximal and distal ends of the body, and an end magnet positioned proximate the distal end of the tool. The side magnet presents an exposed side surface facing in a first direction and the end magnet presents an exposed bottom surface facing in a second direction. The first direction is within 30 degrees of perpendicular to the direction of extension of the longitudinal axis of the downhole tool. The second direction is within 30 degrees of parallel to the direction of extension of the longitudinal axis. 
     In a further embodiment of the present invention, there is provided a wellbore system in a subterranean formation comprising a borehole, a stray radioactive marker bullet, and a fishing tool. The borehole extends downwardly into the subterranean formation. The stray radioactive marker bullet is loosely positioned in the borehole. The fishing tool is positioned in the borehole and includes a plurality of magnets operable to attract and hold the stray radioactive marker bullet so that the stray radioactive marker bullet can be retrieved from the borehole along with the fishing tool. 
     In still a further embodiment of the present invention, there is provided a process comprising the steps of: (a) lowering a fishing tool into a borehole in a subterranean formation; (b) traversing the borehole with the fishing tool, thereby causing the fishing tool to be positioned proximate a stray radioactive marker bullet located in the borehole; (c) coupling the stray radioactive marker bullet to the fishing tool by magnetic attraction; and (d) retrieving the fishing tool and the radioactive marker bullet coupled thereto from the borehole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGS. 
     A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein: 
     FIG. 1 is an elevation view of a wellbore system in a subterranean formation, particularly illustrating a downhole tool for implanting radioactive marker bullets and retrieving stray radioactive marker bullets lost in the borehole; 
     FIG. 2 is a side view of a downhole magnetic fishing tool constructed in accordance of the present invention, particularly illustrating the shape of the main body of the fishing tool as well as the position of the magnets on the body; 
     FIG. 3 is an end view of the downhole magnetic fishing tool illustrated in FIG. 2, particularly illustrating the end magnets attached to the distal end of the tool; and 
     FIG. 4 is an isometric view of the downhole magnetic fishing tool illustrated in FIGS.  2  and  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, a wellbore system  10  is illustrated as generally comprising a borehole  12  extending downward into a subterranean formation  14 , implanted radioactive marker bullets (RMBs)  16 , stray side RMBs  18 , stray bottom RMBs  20 , stray floating RMB  21 , and a downhole tool assembly  22 . Downhole tool assembly  22  is received in borehole  12  and is supported by a cable  24  which extends into borehole  12  and is coupled to surface equipment  26 . Cable  24  and surface equipment  26  cooperate to raise and lower tool assembly  22  in borehole  12 . Preferably, cable  24  is a wireline which not only physically supports tool assembly  22 , but also includes electrical conductors for carrying power and/or electrical control signals from surface equipment  26  to tool assembly  22 . 
     Downhole tool assembly  22  comprises a radioactive marker bullet gun  28  and a magnetic fishing tool  30 . Gun  28  is operable to propel radioactive marker bullets (RMBs) outward and into subterranean formation  14 . Properly implanted RMBs  16  are snugly received in subterranean formation  14  so that any local shifting of subterranean formation  14  causes corresponding shifting of implanted RMBs  16  in that location. Although gun  28  is designed to fire radioactive marker bullets into subterranean formation  14 , several factors, such as improper operation of gun  28  and/or extremely high density of subterranean formation  14 , may cause inadequate lodging of the radioactive marker bullets in subterranean formation  14 . Such stray RMBs  18 ,  20 ,  21  may partially lodge in formation  14  (e.g., stray side RMB  18   b ), may fall (e.g., stray floating RMB  21 ) in borehole  12  and come to rest on a ledge of the sidewall of borehole  12  (e.g., stray side RMB  18   a ), or may simply fall to the bottom of borehole  12  (e.g., stray bottom RMBs  20 ). 
     Magnetic fishing tool  30  is operable to attract and hold stray RMBs  18 ,  20  which are relatively loosely positioned in borehole  12 . Magnetic fishing tool  30  comprises a first group of side magnets  32 , a second group of side magnets  34  axially spaced from the first group of side magnets  32 , and a group of end magnets  36 . Side magnets  32 ,  34  face generally outward toward the side walls of borehole  12  so that stray RMBs  18 ,  21  can be pulled into contact with and held against the outwardly facing surface of side magnets  32 ,  34  by magnetic force. End magnets  36  face generally downward toward the bottom of borehole  12  so that stray bottom RMBs  20  can be pulled into contact with and held against the downwardly facing surface of end magnets  36  by magnetic force. 
     RMBs  16 ,  18 ,  20 ,  21  can be any conventional bullet capable of being coupled to magnets  32 ,  24 ,  36  of fishing tool  30  by magnetic force. Preferably, RMBs  16 ,  18 ,  20 ,  21  include a quantity of a radioactive substance, for example, a pellet of Cesium 137 of about 100-150 micro-curies. The radioactive substance is preferably encased in a durable, metallic casing (shaped as a conventional bullet) which can be attracted to and held in contact with magnets  32 ,  24 ,  36  of fishing tool  30 . Radioactive marker bullet gun  28  can be any conventional radioactive marker bullet gun capable of propelling radioactive marker bullets into subterranean formation  14 . An example of a suitable radioactive marker bullet gun is the “E-Gun (Bullet) Perforating System” available from Baker Atlas, Houston, Tex. Another radioactive marker bullet gun which can be modified for use with fishing tool  30  is described in U.S. Pat. No. 4,916,312 (assigned to Schlumberger Technology), the entire disclosure of which is incorporated herein by reference. Other suitable radioactive marker bullet gun configurations are well known in the art. 
     Referring now to FIGS. 2-4, magnetic fishing tool  30  generally comprises an elongated main body  38  extending along a longitudinal axis  39  and three groups of magnets  32 ,  34 ,  36  coupled to body  38  at specific axial locations. Body  38  includes a proximal end  40  adapted to be coupled to a radioactive marker bullet gun and a distal end  42  which normally faces downward when fishing tool  30  is inserted into a borehole. Proximal end  40  preferably includes a male threaded member  44  for coupling fishing tool  30  to a normally lower end of a radioactive marker bullet gun. 
     The outer surface of main body  38  includes first, second, and third axially spaced wide outer surfaces  46 ,  48 ,  50 , first and second axially spaced narrow outer surfaces  52 ,  54 , a plurality of tapered axially spaced transition surfaces  56 , a terminal end surface  58 , and a terminal tapered surface  60 . First wide outer surface  46  is located proximate distal end  42 . Third wide outer surface  50  is located proximate proximal end  40 . Second wide outer surface  48  is axially spaced from and positioned between first and third wide outer surfaces  46 ,  50 . First narrow outer surface  52  is axially positioned between first and second wide outer surfaces  46 ,  48 . Second narrow outer surface  54  is axially positioned between second and third wide outer surfaces  48 ,  50 . Each transition surface  56  provides a tapered transition between one of narrow outer surfaces  52 ,  54  and an adjacent one of wide outer surfaces  46 ,  48 ,  50 . Terminal tapered surface  60  provides a tapered transition between first wide outer surface  46  and terminal end surface  58 . Preferably, wide outer surfaces  46 ,  48 ,  50  and narrow outer surfaces  52 ,  54  are substantially cylindrical in shape, transition surfaces  56  and terminal tapered surface  60  are substantially frustoconical in shape, and terminal end surface  58  is substantially flat. 
     First and second groups of side magnets  32 ,  34  are coupled to main body  38  proximate first and second narrow outer surfaces  52 ,  54 , respectively. End magnets  36  are coupled to main body  38  proximate terminal end surface  58 . Preferably magnets  32 ,  34 ,  36  are embedded in main body  38  by inserting each individual magnet into a respective bore in main body  38 . Preferably, magnets  32 ,  34 ,  36  are generally cylindrical in shape. Magnets  32 ,  34 ,  36  can be fixed to main body  38  by any means known in the art such as, for example, glueing, soldering, or welding. Magnets  32 ,  34 ,  36  can be any magnet (permanent or electromagnet) of suitable strength for attracting and holding radioactive marker bullets thereto. Magnets  32 ,  34 ,  36  are preferably permanent magnets, more preferably rare earth permanent magnets, and most preferably samarium cobalt magnets. 
     First and second groups of side magnets  32 ,  34  each comprise a plurality of individual magnets which are circumferentially spaced around a respective narrow outer surface  52 ,  54  of main body  38 . Preferably, each of first and second groups of side magnets  32 ,  34  comprises from 3 to 12 individual magnets, more preferably from 4 to 8 individual magnets. The individual magnets in each of first and second groups of side magnets  32 ,  34  are preferably substantially symmetrically spaced around longitudinal axis  39 . Each magnet in first and second groups of side magnets  32 ,  34  presents a substantially flat outwardly-facing side surface  62 . Side surfaces  62  preferably face generally radially away from longitudinal axis  39  of main body  38 . Preferably, each side surface  62  of each individual magnet in the groups of side magnets  32 ,  34  faces in a direction which is within 30 degrees of perpendicular to the direction of extension of longitudinal axis  39 , more preferably within 15 degrees of perpendicular to the direction of extension of longitudinal axis  39 , and most preferably substantially perpendicular to the direction of extension of longitudinal axis  39 . 
     Wide outer surfaces  46 ,  48 ,  50  are preferably substantially cylindrical and have substantially the same width. Wide outer surfaces  46 ,  48 ,  50  are radially spaced from longitudinal axis  39  by a maximum radial distance which is greater than the maximum distance between longitudinal axis  39  and side surfaces  62  of side magnets  32 ,  34 . It is important for wide outer surfaces  46 ,  48 ,  50  to extend a greater radial distance from longitudinal axis  39  than side surfaces  62  of side magnets  32 ,  34  in order to protect a captured side RMB  64  from being disengaged from (i.e., “scraped off”) side surface  62  by the sidewall of the borehole when fishing tool  30  is shifted up and down in the borehole. Preferably, the maximum radial distance between longitudinal axis  39  and wide outer surfaces  46 ,  48 ,  50  is at least 10 percent greater than the maximum radial distance between longitudinal axis  39  and side surfaces  62  of side magnets  32 ,  34 , more preferably at least 20 percent greater, and most preferably at least 40 percent greater. As used herein, the term “maximum radial distance” means the maximum distance from a central axis (e.g., longitudinal axis  39 ) to any point on a particular surface or group of surfaces, measured perpendicular to the direction of extension of the central axis. The maximum radial distance between longitudinal axis  39  and wide outer surfaces  46 ,  48 ,  50  is preferably in the range of from about 1 inch to about 6 inches, more preferably in the range of from about 1.5 inches to about 4 inches, and most preferably in the range of from 2 inches to 3 inches. 
     Narrow outer surfaces  52 ,  54  are preferably substantially cylindrical and have substantially the same width. The maximum radial distance between longitudinal axis  39  and narrow outer surfaces  52 ,  54  is less than the maximum radial distance between longitudinal axis  39  and wide outer surfaces  46 ,  48 ,  50 . The maximum radial distance between longitudinal axis  39  and narrow outer surfaces  52 ,  54  can be less than the maximum radial distance between longitudinal axis  39  and side surfaces  62  of side magnets  32 ,  34  (i.e., when side surfaces  62  of side magnets  32 ,  34  are slightly raised from narrow outer surfaces  52 ,  54 ). Alternatively, the maximum radial distance between longitudinal axis  39  and narrow outer surfaces  52 ,  54  and the maximum radial distance between longitudinal axis  39  and side surfaces  62  of side magnets  32 ,  34  is substantially the same (i.e., when side surfaces  62  of side magnets  32 ,  34  are substantially flush with narrow outer surfaces  52 ,  54 ). 
     Group of end magnets  36  comprises a plurality of spaced-apart individual magnets. Preferably, group of end magnets  36  comprises from 1 to 8 individual magnets, more preferably from 2 to 6 individual magnets, and most preferably 3 individual magnets. Each end magnet  36  presents a substantially flat downwardly-facing bottom surface  66 . Bottom surfaces  66  preferably face generally axially away from main body  38  in a direction which is within 30 degrees of parallel to the direction of extension of longitudinal axis  39 , more preferably within 15 degrees of parallel to the direction of extension of longitudinal axis  39 , and most preferably substantially parallel to the direction of extension of longitudinal axis  39 . The direction in which bottom surfaces  66  of end magnets  36  face and the direction in which each side surface  62  of side magnets  32 ,  34  face are preferably within 30 degrees of perpendicular to one another, more preferably within 15 degrees to perpendicular to one another, and most preferably substantially perpendicular to one another. Side surfaces  62  of side magnets  32 ,  34  and bottom surfaces  66  of end magnets  36  are preferably exposed (i.e., uncovered) so that radioactive marker bullets located away from side and bottom surfaces  62 ,  66  can be pulled directly into contact with and held against side and bottom surfaces  62 ,  66  by magnetic force. 
     Referring again to FIG. 1, in operation, downhole tool assembly  22  can be assembled by coupling magnetic fishing tool  30  to a normally lower end of radioactive marker bullet gun  28 . A normally upper end of radioactive marker bullet gun  28  can then be coupled to cable  24 . Downhole tool assembly  22  can then be lowered into borehole  12 . Once in borehole  12 , radioactive marker bullets (RMBs) can be propelled outwardly from gun  28  into subterranean formation  14 . Stray RMBs  18 ,  20 ,  21  fired from gun  28  but not properly lodged in subterranean formation  14  may fall downward from gun  28  and towards magnetic fishing tool  30 . As these stray RMBs (e.g., stray floating RMB  21 ) fall past fishing tool  30 , they may become attracted by and coupled to side magnets  32 ,  34  of fishing tool  30 . Some of the stray RMBs (e.g., stray side RMB  18   a ) may fall past magnetic fishing tool  30  and come to rest on ledges of the sidewall of borehole  12 . Other of the stray RMBs (e.g., stray bottom RMBs  20 ) may fall to the bottom of borehole  12 . Stray RMBs  18 ,  21  can be pulled into contact with and coupled to side magnets  32 ,  34  of fishing tool  30  when fishing tool  30  is passed by and/or contacted with stray side RMBs  18 . Stray bottom RMBs  20  can be pulled into contact with and coupled to end magnets  36  when fishing tool  30  is lowered to the bottom of borehole  12 . When downhole tool assembly  22  is removed from borehole  12 , the stray RMBs coupled to magnets  32 ,  34 ,  36  of magnetic fishing tool  30  are removed therewith. Thus, downhole tool  22  allows implanted RMBs  16  to be fired into subterranean formation  14  and stray RMBs  18 ,  20 ,  21  to be retrieved from borehole  12  during a single run of downhole tool assembly  22  in borehole  12 . 
     The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventor hereby states his intent to rely on the doctrine of equivalence to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from, but outside the literal scope of the invention as set forth in the following claims.