Patent Publication Number: US-7219724-B2

Title: Downhole magnetic retrieval tool

Description:
FIELD OF THE INVENTION 
   The present invention relates to wellbore tools and methods for retrieving metal debris, such as cuttings and other metal objects, that may be formed by milling downhole objects, by unintentionally dropping an object in a wellbore, or by intentionally releasing a portion of a tool to fall in the well and subsequently be removed. More specifically, the tool of the present invention utilizes magnetic forces to attract ferrous metal debris, so that the tool effectively captures the debris and may then be retrieved to the surface from the wellbore. 
   BACKGROUND OF THE INVENTION 
   When drilling an oil or gas well, or when refurbishing an existing well, normal operations may result in various types of metal debris in a well. Downhole milling produces cuttings which often are not completely removed by circulation. Other metallic objects may drop into and collect near the bottom of the well, or on intermediate plugs placed within the well. 
   Methods have been used to circulate fluids up the annulus at a rapid rate and thereby carry debris upward, with expectations that the debris will then settle into the basket for retrieval when circulation is reduced. Some basket tools utilize a venturi action to drawn debris into the tool. 
   Other tools utilize magnets mounted within a housing for being lowered into the well. Some tools practically may be limited to retrieving cutting since magnetization is only at the bottom of the tool. Other tools utilize a plurality of magnets aligned in cavities near the outer surface of the tool. Each magnet may be recessed in the tool body. Exposed magnets are subject to physical damage during the process of cleaning debris from the well. Conventional metal debris retrieving tools are relatively expensive, and it is difficult or impossible to effectively clean and change out the magnets of most tools in the field. 
   U.S. Pat. No. 6,655,462 discloses magnets within an interior of a rotatable protective sleeve. U.S. Pat. Nos. 6,216,787, 6,308,781, and 6,491,117 disclose magnetic tools and magnet protectors for deflecting a striking force on the magnets. U.S. Pat. No. 6,439,303 discloses the retainer caps for mounting each of a plurality of magnets within a recess in the tool body. Publication 2001/4013413 discloses in one embodiment a magnet which covers substantially the entire surface of the tool body. Other patents of interest include U.S. Pat. Nos. 2,089,724, 2,709,104, 2,729,494 and 2,918,323, 3,905,631, 4,226,285, 5,178,757, 5,453,188, 5,944,100, 6,269,877, and 6,629,562. 
   The disadvantages of the prior art are overcome by the present invention, and an improved magnetic retrieval tool and method are hereinafter disclosed for retrieving debris from a well. 
   SUMMARY OF THE INVENTION 
   In one embodiment, a tool for suspending in a well to retrieve ferrous metal debris from the well includes an elongate tool body having a plurality of circumferentially arranged slots. Each slot has a radially outward portion with a circumferential width less than a circumferential width of a radially inward portion. A plurality of magnets may be positioned in each slot, and one or more stop rings, such as centralizers, used for axially retaining the plurality of magnets in the slots. Each of the stop rings is removable from the tool body, thereby allowing removal of the plurality of magnets from the plurality of the slots in a field operation. 
   In a preferred embodiment, the tool body includes a central bore for pumping fluid through the tool body, and the upper end of the tool body is configured for attaching to a tubular extending into the surface. The outer surface of each centralizer has an effective diameter greater than the outer surface of the plurality of magnets. 
   In one embodiment, each of the plurality of slots has substantially a dovetail configuration, while in another embodiment, each of the plurality of slots has a substantially T-shaped configuration. Each of the plurality of magnets in substantially enclosed within a magnet carrier, with the non-ferrous carrier having an outer configuration for slidably fitting within a respective one of the plurality of slots. In one embodiment, each magnet has a substantially cylindrical configuration, and the carrier is formed from one of stainless steel, aluminum and a high wear plastic material. 
   According to the method, a tool body is provided with a plurality of slots as discussed above, and magnets are positioned within each slot and are held in place by one or more stop rings removable from the tool body. The tool with the magnets may then be positioned in the well for collecting and subsequently retrieving metal debris. 
   A significant feature of the invention is that the magnets are reliably held within the tool body, yet may be easily removed from the tool body in a field operation for repair or cleaning. 
   Yet another feature of the invention is that the tool protects each magnet within a magnet carrier which slides within a respective slot. 
   A significant advantage of the tool and method is that the tool may be used and easily cleaned and repaired in a field operation. 
   These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates one embodiment of a magnetic tool according to the present invention. 
       FIG. 2  is a cross-sectional view of the magnetic tool shown in  FIG. 1 . 
       FIG. 3  illustrates a tool as shown in  FIG. 2  with the centralizer removed and one of the magnet strips removed. 
       FIG. 4  is a cross-sectional view of the tool as shown in  FIG. 1 . 
       FIG. 5  is a top view of the centralizer generally shown in  FIG. 1 . 
       FIG. 6  is a side view of the centralizer shown in  FIG. 5 . 
       FIG. 7  is a front view of a plurality of magnets in a magnet carrier. 
       FIG. 8  is a side view of the magnets and carrier shown in  FIG. 7 . 
       FIG. 9  is a partial cross-section view of another slot in the tool body. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  depicts a magnetic tool according to one embodiment of the invention. The tool  10  includes an elongate tool body  12  which has circumferentially arranged slots  14  (see  FIG. 4 ) each for receiving a plurality of magnets  16 . With the embodiment pictured in  FIG. 1 , a centralizer  18  is provided near the central portion of the tool  10 , and is spaced below a plurality of upper slots and above a plurality of lower slots. 
   As shown more clearly in  FIG. 2 , a tool body  12  includes an upper box end  20  for threading the tool to a conventional running string, and a lower pin end  22  for attaching a continuation of the running string or another tool to the lower end of the tool body  12 . A central bore  24  is provided through the tool body, and passes fluid from the string through the tool body for washing the well, which substantially contributes to an upward flow of debris and the collection of debris on the magnets. 
   The magnets may be provided in elongate carrier strips, and  FIG. 3  illustrates the centralizer  18  removed from the tool body  12  so that each carrier strip may be lowered or raised through a slot and easily removed from the tool body. One of the carrier strips  26  is thus shown removed from a respective slot in  FIG. 3 . As shown in  FIG. 4 , each magnet  16  may be substantially enclosed within a non-ferrous carrier strip  26 , which may consist of a non-ferrous radially interior spaced plate  30  and a side and cover plate  32  with a substantially U-shaped cross-sectional configuration. The entirety of the magnet  16  is thus protected from contact with both the tool body  12  and from debris and other objects exterior of the tool body.  FIG. 4  illustrates that the slots  14  may each have a substantially dovetail configuration, with the radially inner portion having a circumferential width wider than a radially outer portion, thereby capturing the carrier and the magnets within the carrier within the tool body. Each carrier strip  26  may thus be slid within a respective slot, and may be retained within the slot by the centralizer. 
   Returning again to  FIG. 3 , each dovetail slot  14  thus is open to a recess portion  34  within the tool body configured to receive the centralizer  18 . If desired, the end of each slot adjacent to the recessed portion  34  may be removed, or may have a uniform circumferential width, so that a carrier  26  axially longer than the axial length of the recessed portion  34  may still be slid within a respective slot. 
     FIG. 5  illustrates the centralizer  18 , which as shown in  FIG. 6  has a plurality of ribs  36  with an effective diameter greater than the diameter of the magnets or the carrier, so that ribs  36  prevent the carrier from engaging a sidewall of the well and retain collected debris on the outer surface of the tool. The space between the ribs  36  provides fluid flow in the annulus surrounding the tool body. Conventional bolts or other securing members may be used to connect the two halves of the centralizers together when on the tool body. The centralizer  18  may be configured for being rotatably fixed to the tool body, or may rotate relative to the tool body. The centralizer  18  may be easily removed from the tool body by removing bolts  33 . 
     FIG. 7  depicts in greater detail one of the magnet carrier strips  26  with the non-ferrous base plate  30  and the side and top protector  32 . The protector  32  may include radially outer cover  31  and rib portions  32  spaced between cover  31  and plate  30 . As shown in  FIG. 8 , each of the magnets  16  may have a substantially circular configuration, or more particularly a cylindrical configuration, as shown in  FIG. 7 . Cylindrical magnets of the type shown in  FIGS. 7 and 8  have a relatively low cost, are highly reliable, and are preferred in many applications compared to elongate magnets. In other embodiments, however, a single elongate magnet or more than one elongate magnet may be provided within the substantially enclosed non-ferrous carrier. A non-ferrous carrier may be formed from one of stainless steel, aluminum or a high-wear plastic material. 
   A non-ferrous magnet carrier  26  formed from the base plate  30  and the side and top protector  32  at least substantially encloses each of the respective magnets  16 . This enclosure is highly desired to protect the magnets from foreign objects external of the tool, since the magnets otherwise may strike objects in the well and fracture or otherwise become damaged. Carrier  32  separates the tool body  12  and the magnets by providing a non-ferrous magnet carrier between the sides of each slot and each magnet. Most importantly, the carrier formed from the base plate  30  and the protector  32  may be easily slid from the respective slot in the tool body at the surface, and disassembled, cleaned, and if necessary repaired at the well site. The base plate  30  may be removed from protector  32 , then the magnets  16  each removed from its respective pocket within protector  32 . As shown in  FIG. 4 , the carrier  26  thus has an outer configuration for slidably fitting the carrier and the magnets positioned therein within the respective one of the plurality of slots. 
     FIG. 9  illustrates a partial cross sectional view of another slot which is substantially T-shaped configuration in a tool body. Other configurations for securing the magnetic strip in the slot may be used, with the slot having a radially inner portion which is circumferentially wider than a radially outer portion. 
   A tool  10  retrieves ferrous metal debris from a well, and includes an elongate tool body  12  having a plurality of circumferentially arranged slots  14  each for receiving a plurality of magnets. One or more stop rings  18  retain the plurality of the magnets within the slot. Each stop ring is removable from the tool body, thereby allowing the removal of the plurality of the magnets from the plurality of slots. According to the method, the tool is provided with the slots and the magnets, and is suspended in a well to retrieve various metal debris. 
   The magnetic tool as disclosed above includes a single stop ring  18 , which may easily be removed from the tool body. Depending on the desired axial length of the magnets throughout the length of the tool, and in part depending upon the maximum allowed travel length of the farthest magnet to its exit point on the tool, more than one stop ring may be provided. A plurality of carrier  26  are preferably provided in each slot. Each axially spaced stop ring may be removable from the tool body so that magnets may slide out of their respective slots and into the cavity formed when the stop ring is removed. Depending on the number and configuration of the stop rings provided along the tool body, it may be desirable to position stabilizers or centralizers above, below, and possibly between the magnets to provide protection to the metal object attracted to the magnets. 
   In a preferred embodiment, the stop ring may comprise a centralizer with a plurality of ribs having an outer surface spaced outward from the magnets, thereby providing protection to objects attracted to the outer surface of the tool. The centralizer may be rotatably fixed to the tool body or may be rotable on the tool body. If desired, bearings may be provided to allow the centralizer to more easily rotate on the tool body. 
   The term “stop rings” as used herein is intended to mean a substantially ring-shaped member which fits on the tool body and axially secures the carrier strips and the magnets therein on the tool body. The stop ring may be completely circular, or in other configurations may be substantially C-shaped to satisfy the purpose as disclosed herein. 
   In the embodiment depicted, the upper end of the tool body  12  is configured for receiving a tubular for suspending the tool body in the well, and for passing fluid through a central bore in the tool body. In other embodiments, the tool may be configured for connection to a wireline, or to another type of tubular for suspending the tool in the well. 
   Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alternations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.