Releasable mill

Abstract

A mill assembly having a milling head which is releasable from the mill body, such as by shifting a ball clutch mechanism. The ball clutch mechanism can be shifted by dropping a pumpable plug through the work string to block fluid flow through the releasable milling head, or by increasing fluid flow through a constriction in the releasable milling head to increase the back pressure above the milling head. A check valve in the milling head can prevent uphole flow through the work string in the event of a pressure excursion. A fishing neck can be attached to the milling head.

1 . A milling head assembly for use on a work string, comprising: 
 a milling head;    at least one cutting structure on said milling head;    a fluid path through said milling head; and    a ball clutch mechanism fixedly attached to one of said milling head or a work string, said ball clutch mechanism being adapted to attach said milling head to a work string, said ball clutch mechanism being adapted to selectively release said milling head from a work string, thereby completely separating said milling head from a work string.    
   
   
       2 . The milling head assembly recited in  claim 1 , further comprising a torque transfer device on said milling head, said torque transfer device being adapted to transfer torque from a work string to said milling head, said torque transfer device being adapted to allow said separation of said milling head from a work string.  
   
   
       3 . The milling head assembly recited in  claim 1 , further comprising a flow restricting device, said flow restricting device being adapted to impede fluid flow through said fluid path at a location on said milling head, thereby creating a predetermined hydraulic force on said milling head to release said ball clutch mechanism.  
   
   
       4 . The milling head assembly recited in  claim 1 , further comprising a check valve in said fluid path of said milling head, oriented to prevent fluid flow in the uphole direction through said fluid path.  
   
   
       5 . The milling head assembly recited in  claim 4 , wherein said check valve is a ball check valve.  
   
   
       6 . The milling head assembly recited in  claim 4 , wherein said check valve is a flapper check valve.  
   
   
       7 . A mill assembly for use on a work string, comprising: 
 a mill body adapted to be attached to a work string and lowered into a well bore;    a milling head;    at least one cutting structure on said milling head;    a fluid path through said mill body and said milling head; and    a ball clutch mechanism fixedly attached to one of said milling head or said mill body, said ball clutch mechanism being adapted to attach said milling head to said mill body, said ball clutch mechanism being adapted to selectively release said milling head from said mill body at a predetermined hydraulic force on said milling head, thereby completely separating said milling head from said mill body.    
   
   
       8 . The mill assembly recited in  claim 7 , further comprising a torque transfer device on said milling head, said torque transfer device being adapted to transfer torque from said mill body to said milling head, said torque transfer device being adapted to allow said complete separation of said milling head from said mill body.  
   
   
       9 . The mill assembly recited in  claim 7 , further comprising a flow restricting device, said flow restricting device being adapted to impede fluid flow through said fluid path at a location on said milling head, thereby creating said predetermined hydraulic force on said milling head.  
   
   
       10 . The mill assembly recited in  claim 7 , further comprising a check valve in said fluid path of said milling head, oriented to prevent fluid flow in the uphole direction through said fluid path.  
   
   
       11 . The milling head assembly recited in  claim 10 , wherein said check valve is a ball check valve.  
   
   
       12 . The milling head assembly recited in  claim 10 , wherein said check valve is a flapper check valve.  
   
   
       13 . A method for milling an object in a well bore, comprising: 
 providing a mill body attached to a work string and a milling head mounted to said mill body with a ball clutch mechanism, said milling head having a cutting structure;    lowering said mill body and milling head into a well bore on the work string;    rotating said mill body and milling head to mill an object in the well bore with said cutting structure;    preventing any fluid flow in the uphole direction through said fluid path; and    shifting said ball clutch mechanism to completely release said milling head from said mill body.    
   
   
       14 . The method recited in  claim 13 , further comprising: 
 providing a separate torque transfer device on said milling head;    transferring torque from said mill body to said milling head with said torque transfer device; and    disengaging said torque transfer device upon said complete separation of said milling head from said mill body.    
   
   
       15 . The method recited in  claim 13 , further comprising impeding fluid flow at a location on said milling head, to increase hydraulic pressure and shift said ball clutch mechanism.  
   
   
       16 . The method recited in  claim 15 , further comprising pumping a plug through said work string with said fluid flow, to land at said location on said milling head to at least partially block said fluid flow, and achieve said hydraulic pressure increase at said location on said milling head.  
   
   
       17 . The method recited in  claim 15 , further comprising: 
 providing a constriction at said location in the path of said fluid flow; and    increasing said fluid flow into said work string to achieve said hydraulic pressure increase at said constriction in said fluid flow path.    
   
   
       18 . The method recited in  claim 13 , further comprising: 
 providing a fishing neck attached to said milling head; and    retrieving said milling head from a well bore by attaching to said fishing neck.