Milling system providing cuttings re-circulation

Abstract

Milling systems, tools, and methods include using a mill with a barrier to promote re-circulation of debris and cuttings to the face of the mill. The face of the mill may include cutting elements. A barrier may obstruct the flow of cuttings and debris away from the face to re-circulate the cutting and debris to promote re-milling of the cuttings and debris by the cutting elements of the face of the mill. The barrier may include a hydraulic barrier formed by fluid expelled from nozzles in or above the mill. A secondary attrition system uphole of the mill may also be used to stage conditioning and re-sizing of cuttings and debris. After debris and cuttings pass through the barrier, the secondary attrition system may re-mill the cuttings and debris to a finer size before allowing the cuttings and debris to pass into the wellbore annulus for transport to the surface.

What is claimed is: 
     
         1 . A downhole milling system, comprising:
 a milling bit having a face, the milling bit including at least one cutting element on the face configured to mill a downhole tool; and   a barrier configured re-circulate cuttings and debris generated by the at least one cutting element to the face of the milling bit.   
     
     
         2 . The downhole milling system recited in  claim 1 , the barrier being selectively activatable. 
     
     
         3 . The downhole milling system recited in  claim 1 , the barrier including a hydraulic barrier. 
     
     
         4 . The downhole milling system recited in  claim 3 , the hydraulic barrier including at least one nozzle configured to create a re-circulation zone. 
     
     
         5 . The downhole milling system recited in  claim 4 , the at least one nozzle being angled at least 15° relative to the longitudinal axis of the milling bit. 
     
     
         6 . The downhole milling system recited in  claim 4 , the at least one nozzle having an elongated cross-sectional shape. 
     
     
         7 . The downhole milling system recited in  claim 4 , the at least one nozzle including a plurality of first nozzles configured to cool or clean the face of the milling bit and one or more second nozzles configured to create the re-circulation zone. 
     
     
         8 . The downhole milling system recited in  claim 1 , further comprising:
 a secondary attrition system configured to re-mill or re-grind cuttings uphole of the milling bit.   
     
     
         9 . A method of milling, comprising:
 generating debris using a mill having cutting elements;   creating a re-circulation zone that promotes re-circulation of the debris to the cutting elements; and   conditioning the debris by re-milling the debris following re-circulation of the debris to the cutting elements.   
     
     
         10 . The method recited in  claim 9 , wherein creating a re-circulation zone includes using a hydraulic barrier generated using at least one nozzle to re-circulate the debris. 
     
     
         11 . The method recited in  claim 9 , wherein creating a re-circulation zone includes jetting fluid through at least one nozzle to form a fluid curtain extending at least partially around the mill. 
     
     
         12 . The method recited in  claim 9 , wherein conditioning the debris further includes:
 using the mill to re-mill the debris using the cutting elements of the mill; or   using a secondary attrition system axially offset from the mill to re-mill the debris.   
     
     
         13 . The method recited in  claim 9 , wherein creating a re-circulation zone includes creating a re-circulation zone configured to:
 allow debris smaller than a predetermined size to pass through the re-circulation zone and re-circulating debris larger than the predetermined size to the cutting elements; or   increase the residence time and length of the debris below a reference line of the mill.   
     
     
         14 . A mill, comprising:
 a face;   a plurality of blades;   a plurality of cutting elements coupled to the plurality of blades;   at least one first nozzle configured to expel fluid from the face to cool the plurality of cutting elements; and   a plurality of second nozzles offset from the face and configured to expel fluid to create a hydraulic barrier restricting flow of debris away from the face.   
     
     
         15 . The mill recited in  claim 14 , each of the plurality of second nozzles being located in one of a plurality of junk channels of the mill, the plurality of second nozzles being configured to create a hydraulic barrier restricting flow of debris within the plurality of junk channels. 
     
     
         16 . The mill recited in  claim 14 , the plurality of second nozzles having a width-to-height ratio between 2:1 and 8:1. 
     
     
         17 . The mill recited in  claim 14 , the plurality of blades including re-milling features. 
     
     
         18 . The mill recited in  claim 17 , the re-milling features being located on rear surfaces of the plurality of blades. 
     
     
         19 . The mill recited in  claim 17 , the re-milling features including recesses formed in the plurality of blades, and cutting elements within the recesses. 
     
     
         20 . The mill recited in  claim 14 , the plurality of second nozzles being oriented at an angle between 32.5° and 47.5° relative to a longitudinal axis of the mill.