Milling system and method of milling

Abstract

A mill assembly includes a shaft; a lead mill secured to a first end of the shaft, the lead mill including a first body having a plurality of first blades and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; and a second mill secured to the shaft a selected distance from the lead mill, the second mill including a second body having a plurality of second blades and a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades.

1 . A mill assembly, comprising:
 a shaft;   a lead mill secured to a first end of the shaft, the lead mill comprising:
 a first body having a plurality of first blades; and 
 a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; and 
   a second mill secured to the shaft a selected distance from the lead mill, the second mill comprising:
 a second body having a plurality of second blades; and 
 a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades. 
   
   
   
       2 . The mill assembly of  claim 1 , wherein a gage diameter of the second mill is substantially the same as a gage diameter of the lead mill. 
   
   
       3 . The mill assembly of  claim 1 , wherein the plurality of second cutters are complimentary to the plurality of first cutters. 
   
   
       4 . The mill assembly of  claim 1 , wherein at least a portion of at least one of the plurality of first and/or second cutters have a protective coating thereon. 
   
   
       5 . The mill assembly of  claim 1 , wherein at least a portion of at least one of the plurality of first and/or second blades have a protective coating thereon. 
   
   
       6 . The mill assembly of  claim 1 , wherein at least a portion of at least one of the first and/or second body have a protective coating thereon. 
   
   
       7 . The mill assembly of  claim 1 , wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body have a protective coating thereon, and wherein the protective coating comprises at least one of AlTiN, AlCr, SiTiN, and SiN. 
   
   
       8 . (canceled) 
   
   
       9 . The mill assembly of  claim 1 , wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body have a protective coating thereon, and wherein the coating has a thickness ranging from about 6 to 8 microns. 
   
   
       10 . The mill assembly of  claim 1 , wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body have a protective coating thereon, and wherein the coating has a hardness of at least 1000 HV. 
   
   
       11 . The mill assembly of  claim 1 , wherein at least one of the plurality of first cutters and second cutters has a workface angle ranging from about −5 to −40 degrees. 
   
   
       12 . (canceled) 
   
   
       13 . The mill assembly of  claim 11 , wherein at least one of the plurality of first cutters and second cutters has a workface angle of about −15 to −18 degrees. 
   
   
       14 . The mill assembly of  claim 1 , wherein the distance between a gage of the lead mill and a gage of the second mill is selected based upon the relationship:
   F=( 3 E*I* δ max )/ L 3        wherein F is load; E is modulus of elasticity of the shaft; I is the moment of inertia; δ max  is the maximum deflection of the shaft; and L is the length of the shaft from a gage of the lead mill to the second mill.     
   
   
       15 . A method of milling a window in a tubular in a wellbore, comprising:
 engaging a lead mill of a mill assembly against an interior surface of the tubular, the lead mill secured to an end of a shaft and comprising:   a first body having a plurality of first blades; and   a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades;   rotating the mill assembly;   moving the mill assembly along a surface of a whipstock assembly as the lead mill cuts the window in the tubular, thereby deflecting the lead mill and shaft outwardly through the window in the tubular; and   engaging a second mill of the mill assembly against the window in the tubular, the second mill secured to the shaft a selected distance from the lead mill and comprising:   a second body having a plurality of second blades; and   a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades.   
   
   
       16 . The method of  claim 15 , wherein the lead mill engages the tubular such that at least one of the plurality of first cutters form a workface angle with the tubular ranging from about −5 to −40 degrees. 
   
   
       17 . The method of  claim 15 , wherein the second mill engages the tubular such that at least one of the plurality of second cutters form a workface angle with the liner ranging from about −5 to −40 degrees. 
   
   
       18 . The method of  claim 15 , wherein a gage diameter of the second mill is substantially the same as a gage diameter of the lead mill. 
   
   
       19 . The method of  claim 15 , wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body comprise a protective coating. 
   
   
       20 . A method of designing a mill assembly, comprising:
 determining characteristics of a lead mill, the lead mill comprising:
 a first body having a plurality of first blades; and 
 a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; 
   determining characteristics of a shaft having a first and second end, wherein the first end is adapted to receive the lead mill and the second end is adapted to be threadably connected to a drill assembly;   determining characteristics of an engagement point; and   selecting a location on the shaft for the engagement point to be placed.   
   
   
       21 . The method of  claim 20 , wherein the location is selected based upon the relationship:
   F=( 3 E*I* δ max )/ L 3      wherein F is load; E is modulus of elasticity of the shaft; I is the moment of inertia; δ max  is the maximum deflection of the shaft; and L is the length of the shaft from a gage of the lead mill to the engagement point.   
   
   
       22 . The method of  claim 20 , wherein the engagement point is selected from the group of a second mill, a stabilizer, a motor, and drill string. 
   
   
       23 . A mill assembly, comprising:
 a shaft;   a lead mill secured to a first end of the shaft, the lead mill comprising:
 a body having a plurality of blades; and 
 a plurality of cutters having substantially cylindrical bodies secured to the plurality of blades; and 
   a protective coating disposed on at least a portion of at least one of the body, the plurality of blades, and the plurality of cutters.   
   
   
       24 . The mill assembly of  claim 23 , further comprising:
 at least one second mill secured to the shaft a selected distance from the lead mill.   
   
   
       25 . The mill assembly of  claim 23 , wherein at least a portion of the second mill comprises a protective coating thereon. 
   
   
       26 . The mill assembly of  claim 23 , further comprising:
 at least one of a motor and a stabilizer secured to the shaft a selected distance from the lead mill.   
   
   
       27 . The mill assembly of  claim 23 , wherein the protective coating comprises at least one of AlTiN, AlCr, SiTiN, and SiN.