One trip milling system

Abstract

The side tracking system includes a window mill having a full diameter cutting surface and a reduced diameter tapered cutting surface and a whipstock having a ramp engaging the reduced diameter cutting surface. The cutting tool has a body with diamond cutters initiating cutting of a steel casing, milling a window through the steel casing and drilling a secondary borehole. The materials of the whipstock have a first cutability and the materials of the casing have a second cutability. The reduced diameter cutting surface contacts the whipstock ramp at a first contact area and the full diameter cutting surface contacts the wall of the casing at a second contact area. As weight is applied to the mill, there is a first contact stress at the first contact area and a second contact stress at the second contact area. A cutability ratio is the first cutability divided by the second cutability and a contact stress ratio is the first contact stress divided by the second contact stress. The mill cuts the casing rather than the whipstock by maintaining the product of the cutability ratio and the contact stress ratio less than one.

1 . A cutting tool for milling a window through a steel casing and drilling a secondary borehole, comprising: 
 a body having an outer surface and a plurality of diamond cutters extending from said outer surface;    said diamond cutters initiating cutting of the steel casing, milling said window through the steel casing and drilling the secondary borehole.    
   
   
       2 . The cutting tool of  claim 1  further comprising: 
 a whipstock having a first cutability;    wherein said steel casing has a second cutability greater than said first cutability.    
   
   
       3 . The cutting tool of  claim 2  wherein said whipstock has a first contact stress and said steel casing has a second contact stress, said first cutability divided by said second cutability is a first ratio, said first contact stress divided by said second contact stress is a second ratio, and said first ratio times said second ratio is less than one.  
   
   
       4 . The cutting tool of  claim 1  wherein said diamond cutters are polycrystalline diamond cutters.  
   
   
       5 . The cutting tool of  claim 1  wherein said body has a longitudinal axis and said body rotates on-center about said longitudinal axis.  
   
   
       6 . The cutting tool of  claim 1  wherein said body has a longitudinal axis and said outer surface has a taper with respect to said longitudinal axis.  
   
   
       7 . The cutting tool of  claim 1  wherein said body has a longitudinal axis and said outer surface comprises multiple surfaces having different angled tapers with respect to said longitudinal axis.  
   
   
       8 . The cutting tool of  claim 1  wherein said diamond cutters mill a full gauge window through the steel casing.  
   
   
       9 . The cutting tool of  claim 8  wherein said diamond cutters drill a full gauge secondary borehole.  
   
   
       10 . The cutting tool of  claim 1  wherein said body has a longitudinal axis and said outer surface is substantially parallel to said longitudinal axis.  
   
   
       11 . The cutting tool of  claim 1  wherein said outer surface has a back tapered surface.  
   
   
       12 . The cutting tool of  claim 1  wherein said body further includes a generally circular bottom cutting surface.  
   
   
       13 . The cutting tool of  claim 1  wherein said outer surface further comprises a tungsten carbide material.  
   
   
       14 . A casing mill for milling a window through a steel casing and drilling a secondary borehole, comprising: 
 a body having an outer surface and a multiplicity of cutting elements extending from said outer surface, said cutting elements including tungsten carbide material and diamond cutters;    said diamonds initiating cutting of the steel casing, milling said window through the steel casing and drilling the secondary borehole.    
   
   
       15 . The casing mill of  claim 14  further comprising: 
 a whipstock having a first cutability;    wherein said steel casing has a second cutability greater than said first cutability.    
   
   
       16 . The casing mill of  claim 15  wherein said whipstock has a first contact stress and said steel casing has a second contact stress, said first cutability divided by said second cutability is a first ratio, said first contact stress divided by said second contact stress is a second ratio, and said first ratio times said second ratio is less than one.  
   
   
       17 . The casing mill of  claim 14  wherein said diamond cutters include natural or polycrystalline diamond cutters.  
   
   
       18 . The casing mill of  claim 14  wherein said body has an axis and said cutting elements collectively form an external cutter profile comprising: 
 an upper cylindrical gage portion; and    a lower conical portion extending downwardly from said gage portion to an angle to the axis of said body.    
   
   
       19 . A cutting tool for milling a window through steel casing in a well bore and being adapted to cooperate with a whipstock having a whipstock axis and ramp surface disposed at a ramp angle to the whipstock axis, the cutting tool comprising: 
 a tool body having a body axis and an outer surface; and    a plurality of cutting faces extending from said outer surface and having diamond material to initiate and mill the window through the steel casing and drill borehole;    said cutting faces collectively forming an external profile having a gage portion with a diameter corresponding to the window to be milled through the casing, and a conical portion having a length and extending from said gage portion at an angle.    
   
   
       20 . The cutting tool of  claim 19  wherein said whipstock has a first cutability and said steel casing has a second cutability greater than said first cutability, and wherein said whipstock has a first contact stress and said steel casing has a second contact stress, said first cutability divided by said second cutability is a first ratio, said first contact stress divided by said second contact stress is a second ratio, and said first ratio times said second ratio is less than one.