Abstract:
A debris catching device for downhole milling features modular debris receptacles that are held in the housing in a manner that facilitates stacking and a generally undulating flow path to facilitate dropping of the debris into the receptacles as the remaining fluid travels up the tool for ultimate screening before the fluid exits the tool to flow up to the surface or in a reverse circulation pattern back to the mill below the debris catcher. The modules can also be aligned with flapper valves at the top of each module to prevent debris in the tool from falling to the mill if circulation is turned off. The mill is configured to have an off-center return path preferably as large as the passage through the mill body to aid circulation and cutting performance.

Description:
PRIORITY INFORMATION 
   This application is a divisional application claiming priority from U.S. patent application Ser. No. 12/029,228, filed on Feb. 11, 2008. 

   FIELD OF THE INVENTION 
   The field of this invention is downhole debris catching tools and more specifically those that reverse circulate into a mill to capture the cuttings as they come up through the tool. 
   BACKGROUND OF THE INVENTION 
   Milling Operations downhole generate cuttings that a captured in tools associated with a mill frequently referred to in the industry as junk catchers. There are many configurations for such tools. Some have external seals that direct cuttings coming up from a mill around the outside of the tool back into the tool so that the circulating fluid can exit while the debris is captured in the tool body. Examples of this design are U.S. Pat. Nos. 6,176,311 and 6,607,031. Another design involves establishing a reverse circulation with jets that discharge outside a tool body toward a mill below and act as eductors to draw fluids through the mill and into a screened section central passage. Once the debris laden fluid exits the central passage the velocity slows and debris drops into an annular passage and the fluid keeps going toward the top of the tool. On the way out the top the remaining debris is left on a screen and can drop into the same annular space that caught the larger debris further down the tool as the now screened fluid is drawn by the jets at the top of the tool to go right back down around the outside of the tool toward the mill so that the cycle can repeat. 
     FIG. 1  illustrates the basics of this known design. A mill  10  generates cuttings that are removed with reverse circulation that goes up passage  12  and exits at  14  into a wide spot  16  in the tool body  18 . The heavier debris falls into annular space  20  around the passage  12  while the fluid stream with some smaller debris continues up the tool body  18  until it reaches a screen  22 . The debris remaining is caught outside the screen  22  and eventually falls to annular space  20 . The clean fluid is drawn by the jets  24  fed by fluid pumped from the surface through a string (not shown). Exhaust from the jets  24  combined with fluid drawn by those jets now goes back down around the tool body  18  toward mill  10  and the rest goes up to the surface outside the tubular string that runs from the surface (not shown). 
     FIG. 2  shows a detail of the junk catcher of  FIG. 1 . What is depicted is the lower end just above the mill  10 . A threaded connection  26  holds the bottom sub  28  to the tool body  18 . Debris  30  typically falls down in annular space  20  and wedges tube  32  that defines the passage  12  and prevents the ability to relatively rotate the bottom sub  28  with respect to body  18  to get the threaded connection  26  to let loose. That threaded connection  26  has to get undone so that the debris  30  can get flushed out of the tool when it is brought to the surface. Note that the tube  32  is attached to the bottom sub  28  and in the past efforts to get the threaded connection undone have sheared the tube  32  or have otherwise caused it to crack or fail when debris  30  got compacted in annular space  20 . 
   Another issue was that tube  32  was prefabricated to a predetermined length which limited the volume of the annular space  20 . Yet another issue occurred when the surface pumps were shut off and debris on the screen  22  can fall through the hat  34  through the side openings  36  under it. 
   Turning now to  FIG. 7 , a detailed view of the mill  10  from  FIG. 1  is shown with a central passage  38  leading to circulation outlets  40  four of which can be seen in the associated bottom view. Passages  40  are far smaller than passage  38  that feeds them. This layout worked well for normal downhole milling with circulation going down passage  38  to outlets  40  when a tool or other wellbore obstruction was milled out in a traditional way. However, in conjunction with the debris catcher shown in  FIG. 1  there was a problem since the circulation patterns are reversed for the debris catcher in  FIG. 1  and cuttings are reverse circulated into the body of mill  10  which leads to plugging of the passages  40 . The mills of  FIG. 7  had blades  42  featuring inserts  44  and textured carbide faces in between to assist in the milling operation. 
   The present invention provides for greater capacity variation for the tool illustrated in  FIG. 1  leading to a modular design with passages that feature dog legs to promote dropping of debris into annular catch volumes located below dog legs. An alternative uses a modular approach with aligned modules that have flapper valves that can fall shut when circulation stops to prevent debris from falling back to the mill. The mill configuration has been changed to accommodate reverse circulation without the plugging issues of prior designs illustrated in  FIG. 7 . These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiments and associated drawings that appear below while understanding that the full scope of the invention is given by the claims. 
   SUMMARY OF THE INVENTION 
   A debris catching device for downhole milling features modular debris receptacles that are held in the housing in a manner that facilitates stacking and a generally undulating flow path to facilitate dropping of the debris into the receptacles as the remaining fluid travels up the tool for ultimate screening before the fluid exits the tool to flow up to the surface or in a reverse circulation pattern back to the mill below the debris catcher. The modules can also be aligned with flapper valves at the top of each module to prevent debris in the tool from falling to the mill if circulation is turned off. The mill is configured to have an off-center return path preferably as large as the passage through the mill body to aid circulation and cutting performance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a section view of an existing design of a debris catcher that uses reverse circulation flow patterns; 
       FIG. 2  is a detailed view of the lower end of  FIG. 1  showing the way the single debris catching structure and the passage along side of it and the manner of its fixation to the housing; 
       FIG. 3  is one version of a modular design of internals for debris catching showing an undulating flow path up the tool body; 
       FIG. 4  is a detailed view of two modules shown in  FIG. 3 ; 
       FIG. 5  shows an aligned modular design featuring flapper type valves at the top of each module; 
       FIG. 6  is a further detailed view of the module of  FIG. 5  showing how it is attached to the tool body; 
       FIG. 7  is a section and end view of a mill used in conjunction with a debris catching device such as is shown in  FIGS. 1 and 5 ; 
       FIG. 8  is a section and an end view of a mill that can be used in conjunction with a debris catcher, for example, as shown in  FIG. 3  or  5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 5  shows a mill  50  with one embodiment of the debris catching tool  52  mounted above it. In this embodiment there are modules  54  and  56  shown in housing  55  although additional modules can be used. The modules  54  and  56  are shown in larger scale in  FIG. 4  and without the housing  55  so that the flow pattern can be more easily seen. Debris laden fluid from the mill  50  enters passage  58  in module  54 . Sitting beside passage  58  is passage  60  with both passages open at the upper end  62  of module  54 . Upper end  62  is beveled and lower end  64  of module  56  is also beveled in a conforming way leaving a gap  66  between ends  62  and  64 . Passage  58  continues up the tool into passage  66  of module  56 . Passage  60  in module  54  has a closed bottom  68 . When debris laden fluid exits passage  58  at the top  62  the velocity slows and the fluid stream has to negotiate a double bend to continue into passage  66 . The combination of a slowing velocity and making the double bend to a position over the passage  60  allows debris to fall into passage  60  where they are collected until the tool  52  is removed to the surface. 
   Meanwhile flow continues up the tool  52  through passage  66  until the fluid stream reaches the upper end  70  where there is another velocity reduction so that any even lighter debris still being taken along can have another chance to drop out into passage  72  that has a closed bottom  74  all of which are part of module  56 . Note that the upper end  70  is squared off rather than beveled because in this example it is the top module. The idea is that between modules there is a cross-over effect to allow the combination of reduction of velocity by entering a larger cross-section area of the tool to work in conjunction with gravity to let the debris fall down into a receptacle in position right below the flowing stream. After the flowing stream passes the upper end  70  it enters an enlarged cross-section zone  72 , shown in  FIG. 3 . It then goes through a screen  74  and is then drawn by eductors  76  whose exhaust goes two ways; uphole in an annular space represented by arrow  78  or downhole around the annular space outside the tool body  52  toward the bit  50 . String  80  feeds fluid to the eductor jets  76  as the process of milling continues and ultimately the tool  52  is removed from the well and taken apart at joints that are disposed between the modules such as  54  and  56 . 
   The preferred fixation technique is shown in  FIG. 6  although it is in the context of a different modular design.  FIGS. 5 and 6  go together as an alternative modular design. The lowest module  82  is shown in both FIGS. and is typical of the preferred attachment system for each module. As shown in  FIG. 6  the tool housing  84  surrounds the tube  86 . In this embodiment there is but a single passage  88  in tube  86  with the debris caught in annular space  90  after the fluid stream pushes open the flapper valve  92  located above a screen section  94 . Centralizers  96  can be mounted to tube  86  to keep the annular space  90  around the tube  86  reasonably uniform in dimension over the length of tube  86 . Tube  86  terminates at  96  and just above that location one or more set screws or fasteners  98  are threaded through the housing  84 . A plugged cleanout hole  100  is also provided. At the surface after milling, the housing  84  is broken out at its top  102  and near its bottom at thread  104 . The plugged cleanout  100  is opened to flush debris out as much as possible to end  102 . After that is done the set screws or fasteners  98  are undone and the tube  86  should come right out. Since the tube  86  from its lower end  96  to the flapper  92  is only held in housing  84  with the set screws  98  its release is far simpler than the prior design shown in  FIG. 2  where the tube was integral to a sub  28  that was threaded at  26  and the presence of compacted debris around the tube  20  either damaged the tube or the threaded connection  26  as efforts were made to undo it. 
   The modular design of  FIGS. 5 and 6  with preferably centrally mounted modules with a screen  94  and a flapper  92  is designed to let flow go backwards bypassing the closed flapper  92  and going through the screen  94 , if circulation is cut off so that debris can still settle in the annular space  90  around each module and the liquid can go through the screen  94  because the flappers  92  are all closed and run out the mill  50  as the tool  52  is pulled out of the hole. While the tubes  86  are shown in their preferably centralized orientation, they can be offset from each other as well. 
   Turning now to the design of the mill and  FIGS. 7 and 8 , as mentioned before the problem with the  FIG. 7  design was that the outlets  40  would clog with debris which could overheat or simply just stall the mill in a tangle of cuttings. Another issue with the former design was that the blades  42  come short of the center  104  leaving just an array of ground carbide particles in that region. When milling out a packer, for example, the effect was uneven milling. Mills that simply used a central bore to accept reverse circulation flow when milling suffered from having no milling going on near their centers so as to leave a core of un-milled tool as the cutting progressed. The mill of the present invention in  FIG. 8  has a main bore  106  preferably centrally located with a bend  108  so that the entrance for cuttings  110  is near the circumference  112 . A network of passages  114  directs the cuttings from the action of the carbide particle arrays  116  to the entrance  110 . The passages  114  also direct reverse circulating fluid coming down outside the tool into the entrance  110 . There are two main advantages of this design. One is that the entrance  110  is close to or even larger than the bore  106  to reduce if not eliminate the problem of balling up of cuttings in the  FIG. 7  design from small inlets  40  as compared to the main passage above them  38 . Another advantage is that the offset inlet  110  allows for particle arrays  116  otherwise on the periphery at circumference  112  to take up the slack of a missing portion of cutting structure at or near the periphery to still get effective milling at the periphery as opposed to locating the inlet in the center which would contribute to a no milling zone or a coring effect of milling the exterior of a downhole tool without the center. 
   Those skilled in the art will appreciate that the improvements to the debris catching tool using the modular designs makes them more likely to come apart at the surface for cleaning when laden with cuttings that could be compacted. A plugged cleanout  100  allows an initial attempt to flush the cuttings clear of a surrounding modular housing before undoing the set screws  98  to allow removal with a pull out force at the opposite end such as near the centralizers  96 . The modular design can incorporate a flow path with a debris receptacle in each module and a sinuous path for flow coupled with sudden enlargements of the flow area where the bends are so that the reduced velocity will act with gravity to allow the debris to drop straight down to an aligned debris receptacle in a given module below. Alternatively, using modules as shown in  FIG. 6  the flow can come straight up through the modules and due to gaps between the modules where the velocity slows debris can still fall away and be pushed to the periphery when it will fall down into the annular collection area in part made possible by centralizers  96  around the tube  86 . When there is no circulation, the flappers  92  close and drainage to the mill  50  can occur through the screens  94  in each module. In that way a wet string is not pulled and debris is not permitted to fall back into the mill  50  when circulation stops. The mill reduces clogging with debris with the inlet  110  as large as or larger than the bore  106  and the offset from center location of it allows adjacent cutting structure near the periphery to compensate for the zone of missing cutting structure where the inlet  110  is located. This reduces the coring effect as compared to prior designs with central inlets. 
   The use of a modular design allows the ability to match the expected level of cuttings with the storage capacity to hold them until the milling is done. The mounting technique facilitates removal when the tool is laden with cuttings with minimal risk of damage to the modules and rapid reassembly is facilitated. 
   The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below.