Releasable mill

Abstract

A mill assembly having a milling head which is releasable from the mill body, such as by shearing a shear pin. The release mechanism can be released 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;    a check valve in said fluid path of said milling head, oriented to prevent fluid flow in the uphole direction through said fluid path;    a fishing neck attached to said milling head; and    a releasable fastening mechanism fixedly attached to one of said milling head or a work string, said fastening mechanism being adapted to attach said milling head to a work string, said fastening 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 fastening mechanism.  
   
   
       4 . The milling head assembly recited in  claim 1 , wherein said check valve is a ball check valve.  
   
   
       5 . The milling head assembly recited in  claim 1 , wherein said check valve is a flapper check valve.  
   
   
       6 . 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;    a check valve in said fluid path of said milling head, oriented to prevent fluid flow in the uphole direction through said fluid path;    a fishing neck attached to said milling head; and    a releasable fastening mechanism fixedly attached to one of said milling head or said mill body, said fastening mechanism being adapted to attach said milling head to said mill body, said fastening 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.    
   
   
       7 . The mill assembly recited in  claim 6 , 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.  
   
   
       8 . The mill assembly recited in  claim 6 , 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.  
   
   
       9 . The milling head assembly recited in  claim 6 , wherein said check valve is a ball check valve.  
   
   
       10 . The milling head assembly recited in  claim 6 , wherein said check valve is a flapper check valve.  
   
   
       11 . 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 releasable fastening mechanism, said milling head having a cutting structure and a fishing neck;    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;    releasing said releasable fastening mechanism to completely release said milling head from said mill body; and    retrieving said milling head from the well bore by attaching to said fishing neck.    
   
   
       12 . The method recited in  claim 11 , 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.    
   
   
       13 . The method recited in  claim 11 , further comprising impeding fluid flow at a location on said milling head, to increase hydraulic pressure and release said releasable fastening mechanism.