Patent Publication Number: US-6032739-A

Title: Method of locating wellbore casing collars using dual-purpose magnet

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to a method of locating casing collars of a wellbore, and more specifically to a method of using the same magnet for both sensing collars and positioning a tool. 
     2. Description of Related Art 
     Tools for perforating or dismantling wellbore casings often include a magnet for sensing the presence of a casing collar. Some of the more compact tools also include a second magnet for properly orientating (both radially and circumferentially) the tool within the casing. This second magnet, however, adds bulk to the tool. This is a great disadvantage, as these tools often need to slip through narrow constructions deep within a wellbore. In addition, two magnets are obviously more expensive than one. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations of current methods of sensing casing collars, perforating casings, and splitting casing collars; it is an object of the invention to minimize the number of components used in sensing the location of casing collars. 
     Another object of the invention is to employ a single magnet for both sensing the location of a casing collar and for orientating a tool circumferentially and radially (standoff). 
     Another object is to minimize the size of a tool needed in servicing a wellbore. 
     These and other objects of the invention are provided by a novel method of employing a single magnet for both sensing the location of a casing collar and for orientating a tool circumferentially and radially. The method includes offsetting the position of the magnet relative to the tool&#39;s centerline and placing an inductive pickup coil within the magnet&#39;s magnetic field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the invention taken along line 1--1 of FIG. 2. 
     FIG. 2 is a cross-sectional view of the invention taken along line 2--2 of FIG. 1. 
     FIG. 3 is a cross-sectional view of a second embodiment of the invention taken along line 3--3 of FIG. 4. 
     FIG. 4 is a cross-sectional view of the second embodiment of the invention taken along line 4--4 of FIG. 3. 
     FIG. 5 is a cross-sectional view of the second embodiment of the invention taken along line 5--5 of FIG. 3. 
     FIG. 6 is a cross-sectional view showing the lowering of a tool into a wellbore. 
     FIG. 7 is a cross-sectional view showing the repositioning of a tool within a wellbore. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A wellbore 10 is shown to include a string of casing pipes 12 interconnected by casing collars 14. An inner string of tubing 16 interconnected by pipe couplings 18 runs through the interior of casing pipes 12. 
     A perforation tool 20 is situated between casing pipes 12 and tubing 16. Perforation tool 20 includes a housing 22 having a longitudinal centerline 24, a dual-purpose magnet 26 having a center of gravity 28 and a magnetic field 30 that exerts a magnetic force 32, an electrical coil 34 exposed to magnetic field 30, and several explosive charges 36 each being denoted by an electrically ignitable cap 38. Tool 20 is suspended by a coaxial cable 40 having an inner conductor 42 and an outer conductor 44 (grounded outer armor). Inner conductor 42, coil 34, housing 22, and outer conductor 44 are electrically connected in series. Cap 38 and coil 34 are connected in parallel with a reflective dual-diode that has a significantly high threshold voltage to prohibit the induced voltage in coil 34 from reaching cap 38. 
     The center of gravity 28 of magnet 26 is radially offset from centerline 24 in a forward direction 46. The primary discharge of explosive charges 36 also generally faces forward direction 46. In this arrangement, magnet 26 circumferentially orientates the discharge faces of explosives 36 and draws them up against the inner wall 50 of casing 12. Providing a proper circumferential position 48 and establishing a proper radial distance 52 (standoff) of explosives 36 is the first purpose of dual-purpose magnet 26. 
     The second purpose of magnet 26 is to identify the location of a casing collar 14. As magnet 26 is lowered (depicted by arrow 54) past casing collar 14, magnetic field 30 is disturbed. This disturbance induces an electrical current in coil 34, thereby generating an electrical signal 56. Coaxial cable 40 conveys signal 56 to an operator that monitors signal 56 and compares it to the depth that housing 22 has been lowered. This allows the operator to identify the location of casing collar 14. 
     Once a casing collar 14 is located, the operator can position charges 36 to perforate casing 12 at a location somewhere other than exactly at casing collar 14. Each of charges 36 are generally round for producing several round holes 57 through which a fluid (e.g., oil, water) can be drawn into casing 12. 
     FIGS. 3, 4 and 5 illustrate another embodiment of the invention that is similar to the one just discussed, except round charges 36 are replaced by a linear charge 58 that is detonated by an electrically ignitable cap 38&#39;. Unlike round charges 36, linear charge 58 is aimed directly at casing collar 14. Charge 58 splits collar 14 lengthwise to facilitate the dismantling of casing pipes 12. 
     FIG. 6 shows signal 56 being registered on an instrument 60 (e.g., combination DC power supply and microampmeter) as magnet 26 is being lowered past collar 14. Once casing collar 14 is located, housing 22 is raised slightly (see FIG. 7) to align linear charge 58 with collar 14. Switch 62 trips a conventional circuit (well known to those skilled in the art) to deliver a current of sufficient amplitude through inner conductor 42 for igniting cap 38&#39; which in turn detonates charge 58 which splits collar 14. 
     Other items worth mentioning include a set screw 64 that in assembly allows charge 36 and 58 to face in the same forward direction 46 as magnet 26. Offsetting magnet 26 from centerline 24 tends to direct its magnetic field 30 away from pipe coupling 18. This minimizes the likelihood of mistakenly identifying an inner pipe coupling 18 as an outer casing collar 14. Details of wellbore tools are found in U.S. Pat. No. 5,720,344 which is specifically incorporated by reference herein. 
     Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those skilled in the art. Therefore, the scope of the invention is to be determined by reference to the claims which follow.