Window Mill for Sidetracking

Abstract

A mill for cutting a window in wellbore casing has cylindrical carbide or PDC cutters on its blades and non-cylindrical carbide cutting inserts located on the body of the mill where it first contacts the casing or has a relatively low cutter density. The mill may also include one or more elements made of softer material than the non-cylindrical carbide inserts, which are positioned on the blades, such as behind the non-cylindrical cutters on the same blade, where the window mill first contacts the casing during milling a window.

1 . A window mill configured to mill a window in a casing made of steel that cases a wellbore for sidetracking by rotating the window mill while urging it against an inside surface of well casing using a whipstock, the window mill comprising:
 a body having a center axis around which the window mill rotates when milling and elongated blades that extend generally along a length of the body, each blade having a leading edge; and   cutting elements mounted in pockets along leading edges of the blades, the cutting elements each having a cutting profile that collectively defines a cutting profile for the window mill;   wherein the cutting elements comprise non-cylindrical carbide inserts with non-circular cross-sections and cutters with cutting faces of ultra-hard material.   
     
     
         2 . The window mill of  claim 1 , wherein one or more of the non-cylindrical carbide inserts are mounted on a section of the window mill where the window mill will first contact the casing when it is lowered into the casing and the window mill is urged against casing by a whipstock. 
     
     
         3 . The window mill of  claim 2 , wherein a gauge portion of the window mill is the where the window mill first makes contact. 
     
     
         4 . The window mill of  claim 2  wherein one or more cylindrical carbide cutters are also mounted on the section of the window mill where the window mill first contacts the casing, and wherein the one or more non-cylindrical carbide inserts have a wider cutting profile than the one or more cutters. 
     
     
         5 . The window mill of  claim 2 , wherein two or more of the non-cylindrical carbide inserts are mounted adjacent to each other in a pocket on one of the elongated blades along a nose section of the window mill with no webbing between them. 
     
     
         6 . The window mill of  claim 2 , wherein at least one of the non-cylindrical carbide inserts has a back-raked cutting face and a cutting edge defined between the cutting face and a top surface of the non-cylindrical carbide insert. 
     
     
         7 . The window mill of  claim 2 , wherein one or more of the non-cylindrical carbide inserts has a contoured cutting edge adapted to expose initially only a portion of the cutting edge. 
     
     
         8 . The window mill of  claim 2 , wherein the window mill further comprises one or more protectors mounted on the body behind one or more of the cutting elements on the cutting profile, each of the one or more protectors being mounted with a portion extending above the cutting profile of the window mill and adapted to be worn down progressively to expose the one or more cutting elements as the window mill is rotated. 
     
     
         9 . The window mill of  claim 8 , wherein one or more of the non-cylindrical carbide inserts are mounted on a section of the window mill where the window mill first contacts casing when the window mill is urged against casing by a whipstock. 
     
     
         10 . The window mill of  claim 9 , wherein a gauge portion of the window mill is the where the window mill first makes contact. 
     
     
         11 . The window mill of  claim 8 , wherein at least one of the one or more protectors is mounted on one of the elongated blades behind one of the non-cylindrical carbide inserts on said blade. 
     
     
         12 . The window mill of  claim 8 , wherein at least one non-cylindrical insert from the one or more non-cylindrical inserts and at least one protector from of the one or more protectors are brazed into a shared pocket on one of the blades. 
     
     
         13 . The window mill of  claim 12 , wherein the at least one protector from of the one or more protectors has a flat front surface that fits against a flat back surface of the at least one non-cylindrical cutter. 
     
     
         14 . The window mill of  claim 10 , wherein at least one of the one more protectors is mounted on the gauge portion of the window mill and is configured to protrude beyond a predetermined, nominal gauge diameter of the window mill. 
     
     
         15 . The window mill of  claim 14 , wherein at least one of the cutting elements is mounted in the gauge portion and wherein each of the at least one cutting element mounted in the gauge portion is set at or below the nominal gauge diameter of the window mill. 
     
     
         16 . The window mill of  claim 8 , wherein at least one of the one or more protectors comprises an insert that is brazed to a pocket formed in the body of the window mill. 
     
     
         17 . The window mill of  claim 8 , wherein at least one of the one or more protectors comprises a replaceable wear pad mechanically joined to the body of the window mill. 
     
     
         18 . The window mill of  claim 17  wherein the wear pad is mechanically joined to a rear portion of one of the blades behind cutters that are mounted on the blade. 
     
     
         19 . The window mill of  claim 8 , wherein the exposed portion of each of the one or more protectors is made of a material that wears more quickly than the one or more cutting element. 
     
     
         20 . The window mill of  claim 1 , wherein the non-cylindrical carbide inserts have a generally rectangular cross-section. 
     
     
         21 . The window mill of  claim 1 , wherein the cutters have cylindrical cross-sections. 
     
     
         22 . A bottom hole assembly for sidetracking from a cased wellbore, the bottom hole assembly comprising:
 a milling assembly releasably connected with a whipstock assembly, the whipstock assembly including a whipstock with a surface slanted with respect to an axis of the bottom hole assembly and configured for urging the milling assembly, after disconnected from the whipstock assembly, toward an inside surface of casing in a wellbore to mill a window the casing;   wherein the milling assembly comprises at least one window mill that is configured to mill a window in the casing of a wellbore for sidetracking by rotating the window mill while urging it against an inside surface of well casing using the whipstock, the window mill comprising:   a body having a center axis around which the window mill rotates when milling and elongated blades that extend generally along a length of the body, each blade having a leading edge; and   cutting elements mounted in pockets along the leading edges of the blades, the cutting elements each having a cutting profile that collectively defines a cutting profile for the window mill;   wherein cutting elements comprise non-cylindrical carbide inserts with non-cylindrical cross-sections and cutters with cutting faces of ultra-hard material.   
     
     
         23 . The bottom hole assembly of  claim 22  wherein one or more of the non-cylindrical carbide inserts are mounted on a section of the window mill that first contacts the casing when the window mill is urged against the casing by a whipstock. 
     
     
         24 . The bottom hole assembly of  claim 22 , wherein the window mill further comprises one or more protectors mounted on the body in a shadow of one or more of the cutting elements on the cutting profile, the protectors being mounted with an exposed portion of it extending above the cutting profile, and wherein the exposed portion of the protector is made of a material that is softer than the one or more cutting elements and adapted to be worn down progressively to expose the one or more cutting elements as the window mill is rotated. 
     
     
         25 . The bottom hole assembly of  claim 22 , wherein one or more of the non-cylindrical carbide inserts are mounted on a section of the window mill that first contacts the casing when the window mill is urged against casing by a whipstock. 
     
     
         26 . The bottom hole assembly of  claim 22  wherein each of the cutters has a cylindrical cross-section. 
     
     
         27 . A window mill configured to mill a window in casing in a wellbore for sidetracking by rotating the window mill while urging it against an inside surface of the casing using a whipstock, the window mill comprising:
 a body having a center axis around which the window mill rotates when milling and elongated blades that extend generally along a length of the body, each blade having a leading edge;   cutting elements mounted in pockets along the leading edges of the blades, the cutting elements each having a cutting profile that collectively defines a cutting profile for the window mill, wherein cutting elements comprise cutters with cutting faces of ultra-hard material; and   one or more protectors mounted on the body in a shadow of one or more of the cutters on the cutting profile, wherein each of the one or more protectors are mounted with an exposed portion of it extending above the cutting profile of the window mill and adapted worn down progressively to expose one or more cutters as the window mill is rotated.   
     
     
         28 . The window mill of  claim 27 , wherein one or more of cutters are mounted on a section of the window mill where the window mill is configured to first contact the casing when the window mill is urged against the casing by a whipstock. 
     
     
         29 . The window mill of  claim 28 , wherein a gauge portion of the window mill is the where the window mill first makes contact. 
     
     
         30 . The window mill of  claim 27 , wherein at least one of the one or more protectors are mounted on a gauge portion of the window mill and is configured to protrude beyond a predetermined, nominal gauge diameter of the window mill. 
     
     
         31 . The window mill of  claim 30 , wherein at least one of the cutters is mounted in the gauge portion and each of the cutters mounted in the gauge portion is set at or below the nominal gauge diameter of the window mill. 
     
     
         32 . The window mill of  claim 27 , wherein at least one of the one or more protectors comprises an insert that is brazed to a pocket formed in the body of the window mill. 
     
     
         33 . The window mill of  claim 27  wherein each of the cutting elements has cylindrical cross-section.