Abstract:
Milling in casing that is over 4½ inches is done with coiled tubing that is anchored against torque reaction. An improved debris catcher is part of the bottom hole assembly to capture cuttings from the milling. A thruster can be used to maintain weight on the mill during the milling. The coiled tubing supports a mud motor to drive the mill. Return fluid is separated from the cuttings and returned to the surface.

Description:
PRIORITY INFORMATION 
   This application claims the benefit of U.S. Provisional Application No. 60/589,053, filed on Jul. 19, 2004. 

   FIELD OF THE INVENTION 
   The field of the invention related to milling downhole with a bottom hole assembly delivered on coiled tubing with provisions to absorb torque reaction from milling and to collect generated debris near the milling location. 
   BACKGROUND OF THE INVENTION 
   Workovers in existing wells can require removal of packers or plugs by milling them out. Other occasions can also occur where there is a need to mill out a tool or even a casing section. If the well is not too deviated, rigid tubing has been used to support a mill and the rotation force provided from surface equipment. Alternatively, where the deviated nature of the wellbore precludes rotation from the surface, the bottom hole assembly includes a mud motor to turn the mill. The bottom hole assembly is still delivered on rigid tubing but such tubing above the mud motor remains stationary, with the output of the mud motor driving the mill below. In either alternative a workover rig must be erected over the well to handle the rigid tubing string for trips into and out of the well. There is a fair amount of expense associated with erecting the rig on site and handling the tubing to assembly and disassemble the string for trips into the well. It would be advantageous if a coiled tubing unit could be used at the surface instead of a workover rig. Being able to use coiled tubing would save time and money for the operator over using rigid tubing. However, the use of coiled tubing creates other issues that are not of concern when using rigid tubing. The main problem is that coiled tubing is considerably weaker than rigid tubing. During milling a reaction torque is created that is passed to the supporting tubing. In the past, milling on coiled tubing has been attempted in small casings that are less than 4-½ inches with equally small mud motors driving the mill. These attempts worked, after a fashion, because the torque output from the motor and the resultant torque reaction from milling was sufficiently small so as to not twist the coiled tubing. If the torque reaction turns the coiled tubing it can raise the mill off the packer being milled or bounce it, resulting in erratic milling. Worse still, the coiled tubing can fail from being over-torqued. For this reason milling with coiled tubing was limited in the past to very small applications, generally with casing sizes fewer than four inches. 
   The milling process generates debris in the wellbore. Even if a milling job in a larger casing were attempted with small coiled tubing and an equally low powered mud motor, the return flow in the larger casing sizes would reduce the velocity of the returning fluid so as to allow the debris to drop out rather than be carried to the surface for separation with surface equipment. While debris catchers of various designs are known they have operational shortcomings. Some require a separate trip. They generally let the debris-laden fluid passes through an open port on the trip downhole. When the tool is brought uphole, the bypass port is closed and the fluid passes through a screen leaving the debris inside the screen. Some examples of such tools are the H-3015 and the 10084-1 offered by Baker Oil Tools. Some debris catchers can be run in the same trip as the milling equipment but due to the way such tools operate they can&#39;t have a mud motor below them. These tools use a venturi effect to direct the cuttings into the tool and generally must be coupled with specially designed mills that create the type of cuttings that will enter this type of debris catcher. One such tool offered by Baker Oil Tool s is the VACS tool. U.S. Pat. No. 6,176,311 illustrated the concepts of central circulation, annulus diversion of debris into the tool, an interior capture area and screen. This design has been improved in the present invention to minimize issues of plugging and damage to the annulus diverter device when running in or removing the tool. Other debris removal tools are described in U.S. Pat. Nos. 5,176,208; 5,402,850; and 6,276,452. 
   Anchors for tubing downhole are known, as illustrated in U.S. Pat. No. 6,276,452. 
   The present invention permits small coiled tubing to support large mud motors for big milling jobs. The coiled tubing is anchored in position and the mud motor operates the mill in conjunction with a thruster to keep the mill on the tool being milled. Other variations are envisioned that secure the coiled tubing against torque reaction while allowing the mill to progress and mill out downhole. An improved debris catcher is incorporated into the assembly with greater debris retention capacity and other operational enhancements to improve its operation. These and other aspects of the invention will be more readily apparent to those skilled in the art from the description of the preferred embodiment and the claims that appear below. 
   SUMMARY OF THE INVENTION 
   Milling in casing that is over 4-½ inches is done with coiled tubing that is anchored against torque reaction. An improved debris catcher is part of the bottom hole assembly to capture cuttings from the milling. A thruster can be used to maintain weight on the mill during the milling. The coiled tubing supports a mud motor to drive the mill. Return fluid is separated from the cuttings and returned to the surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a - e  are a sectional elevation of the bottom hole assembly for coiled tubing milling. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1   a , coiled tubing  10  is run into casing  12 . At the lower end  14  is a threaded connection  16  to which an anchor  18  is attached. The anchor  18  is preferably of a known design as described in U.S. Pat. No. 6,276,452. It features extending gripping members  20  and  22  that are hydraulically actuated by fluid circulation down the coiled tubing  10 . A connection  24  is at the lower end of the anchor  18  to attach the debris catcher  26 . The debris catcher  26  runs from upper end  28  to lower end  30  in  FIG. 1   c . Continuing with the preferred assembly, a jet sub  34  is connected to lower end  30 . A mud motor  36  (shown schematically) is connected to jet sub  34 . A thruster  38  (shown schematically) is connected to mud motor  36 . A mill  40  is connected to the thruster  38 . Mill  40  comes in contact with the object  42  (shown schematically) to be milled in the wellbore. That object  42  could be a packer, a bridge plug, another downhole tool, or a section of casing or tubular. Depending on the specific attributes of the components selected they can be attached in different orders. The thruster  38  can be optionally omitted and instead the anchor  18  can be repositioned periodically during the milling by cutting circulation to release the anchor  18  and letting the assembly move down to a new position. At that time the circulation can begin again and the anchor  18  will take another grip of the casing  12 . Of course, the anchor  18  is above the mud motor  36  to isolate the coiled tubing  10  from reaction torque from the mill  40  milling the object  42 . The coiled tubing  10  can be sized as small as practicable to not only support the load of the bottom hole assembly but also to keep the pressure drop in flow passage  32  at a reasonable level. Initiating flow through the coiled tubing  10  will set the anchor  18  first before any significant milling by mill  40  can take place. At that point the coiled tubing is protected from reaction torque transmitted through the mud motor  36 . The mud motor  36  can be of a type known in the art as well as the thruster  38  whose purpose is to keep weight on the mill  40  to hold it against the object  42  for efficient milling. As long as the anchor  18  is properly sized for the casing  12 , the other components simply need to be small enough to easily pass through the casing  12 . As a result, the illustrated assembly can be rapidly deployed at the surface without a workover rig and the trip time to reach the object  42  to be milled can be greatly reduced as compared to running the bottom hole assembly on rigid tubing. Objects  42  in casing sizes larger than 4-½″ can be easily milled out with coiled tubing smaller than 3-½ inches in diameter. It is conceivable that coil tubing as small as 1-¼″ could be used to support milling equipment in casing as large as 9-⅝″ or larger. 
   The details of the debris catcher will now be described. Flow enters near the top  28  through passage  32 . A diverter sub  44  has downhole-oriented passages  46  spaced apart from uphole return passages  48 . Arrow  50  shows the flow beyond passages  46  and around the outside of sleeve  52  that is secured at thread  54  to the diverter sub  44 . Flow continues through annular space  56  between the sleeve  52  and the outer screen housing  57  and emerges in  FIG. 1   c  as arrow  50 . The flow  50  emerges in an annular space  58  around a diverter tube  60 . Seals  62  seal around diverter tube  60 . Accordingly the pressure is directed downwardly through the inside of sleeve  64  as shown by arrow  66  and outside sleeve  64  as shown by arrow  68 . Flow  68  encounters a piston  70  that has a movable bearing  72  below it and a pack off sleeve  74  below the bearing  72 . A return spring  76  biases the pack off sleeve  74  uphole to a retracted position. Pressure on piston  70  represented by arrow  68  pushes the piston  70  and bearing  72  downhole against the pack off sleeve  74 . A stationary ramp  78  catches the lower end  80  of the pack off sleeve  74  to force it out into sealing contact with the casing  12 . In this manner, the pack off sleeve  74  is protected from damage during run in or removal because the return spring  76  keeps it retracted and away from casing  12  until circulation through passage  32  in coiled tubing  10  is established. Another bearing  82  is supported by reverse flow sub  84 . Together bearings  72  and  82  allow the pack off sleeve  74  to rotate relative to the sleeve  64  to promote sealing and to minimize wear on the pack off sleeve  74 . 
   The flow  66  through sleeve  64  emerges near lower end  30  of the debris catcher  26  in a chamber  86  between restrictor  88  and venturi jet  90 . The venturi jet  90  discharges into return path  92  in diverter tube  60  to reduce pressure in return port  94  so as to draw debris laden fluid in (as will be explained below). Restrictor  88  creates enough backpressure to supply adequate pressure to the venturi jet  90 . This restrictor is optional and can be used when the mill nozzles (not shown) are fairly large so that insufficient backpressure is available for proper operation of the venturi jet  90 . After going through the restrictor  88  the flow  66  goes to the nozzles in the mill  40  and comes back uphole laden with cuttings in annulus  96  as shown by flow arrow  98 . 
   Flow  98  with cuttings is forced into return port  94  and aided by the action of the venturi jet  90 . It passes up the diverter tube  60  and comes out of outlets  100  The top  102  of the diverter tube  60  is capped off above outlets  100 . A screen  104  has a lower end  106  capped but the annular space  108  outside the screen is left open for the debris-laden flow  98 . The debris free flow  110  goes to the surface outside of the coiled tubing  10 . The debris  112  falls down to catch plate  114  which can be many feet below the lower end  106  of screen  104 . 
   Those skilled in the art can appreciate some of the improvements in the debris catcher  26  as compared to the design shown in U.S. Pat. No. 6,176,311. The pack off sleeve  74  is retractable for run in and removal to protect it from damage. A venturi jet  90  accelerates the debris-laden flow  98 . The debris-laden flow  98  passes a screen  104  with a relatively large open area reducing the risk of plugging using the random slots of the prior design. The debris storage area below the screen  104  can be quite long to minimize the chance of plugging. 
   Those skilled in the art will now appreciate that coiled tubing milling is possible with small coiled tubing sizes in casing bigger than 4-½ inches. The coiled tubing is isolated from reaction torque by an anchor. The milling is done with a mud motor with the additional optional use of a thruster to keep weight on the mill. A debris catcher incorporates improvements to enhance performance, capacity and reliability. A hydraulically operated cutter my be used rather than a mill to sever casing. 
   While the preferred embodiment has been set forth above, those skilled in art will appreciate that the scope of the invention is significantly broader and as outlined in the claims which appear below.