Bottom Hole Assembly for Casing Window Milling

Abstract

A bottom hole assembly for use in cutting a window in a wellbore casing wall. An exemplary bottom hole assembly is described which includes a shaft that carries a window mill. A pair of bearing mills is carried by the shaft above the window mill. Placement of the bearing mills permits the bottom hole assembly to cut a window having a greater length and quality (gage width and reduced burrs/slivers) than windows cut by other bottom hole assembly designs.

1 . A bottom hole assembly for use in milling a window in a wellbore casing in association with a whipstock with an angled ramp having a whipstock ramp length, the bottom hole assembly having a bottom hole assembly length and comprising:
 a central shaft;   a window mill located proximate a lower end of the shaft;   a first bearing mill upon the shaft;   a second bearing mill upon the shaft;   the window mill and the second bearing mill being spaced from each other by a first distance; and   the first bearing mill being spaced from the window mill at a second distance that is from about one-fifth to one-half of the first distance.   
   
   
       2 . The bottom hole assembly of  claim 1  wherein the second distance is about one-third of the first distance. 
   
   
       3 . The bottom hole assembly of  claim 1  wherein the first and second bearing mills are separated from each other by a third distance that is at least as great as the second distance. 
   
   
       4 . The bottom hole assembly of  claim 1  wherein the first distance is greater than the whipstock ramp length. 
   
   
       5 . The bottom hole assembly of  claim 1  wherein the first bearing mill presents a rounded, arcuate cross-section. 
   
   
       6 . The bottom hole assembly of  claim 1  wherein the second bearing mill presents a cross-section having a substantially flat bearing surface. 
   
   
       7 . The bottom hole assembly of  claim 1  wherein the second bearing mill presents a rounded, arcuate cross-section. 
   
   
       8 . The bottom hole assembly of  claim 1  wherein the bottom hole assembly length has a midpoint and wherein the first bearing mill reaches an upper end of a whipstock during a milling operation before the midpoint of the bottom hole assembly length reaches the upper end of the whipstock. 
   
   
       9 . A window cutting arrangement for forming a window within a wellbore casing, the window cutting arrangement comprising:
 a whipstock to be disposed within the wellbore casing, the whipstock presenting an angled ramp and having a whipstock ramp length;   a bottom hole assembly for contacting the angled ramp and cutting a window within the wellbore casing, the bottom hole assembly having a bottom hole assembly length and comprising:
 a central shaft; 
 a window mill proximate a lower end of the shaft and operable for cutting a window in the wellbore casing; 
 a first bearing mill upon the shaft; 
 a second bearing mill upon the shaft; 
 the window mill and the second bearing mill being spaced from each other by a first distance; and 
 the first bearing mill being spaced from the window mill at a second distance that is from about one-fifth to one-half of the first distance. 
   
   
   
       10 . The window cutting arrangement of  claim 9  wherein the second distance is about one-third of the first distance. 
   
   
       11 . The window cutting arrangement of  claim 9  wherein the first and second bearing mills are separated from each other by a third distance that is at least as great as the second distance. 
   
   
       12 . The window cutting arrangement of  claim 9  wherein the first distance is equal to or greater than the whipstock ramp length. 
   
   
       13 . The window cutting arrangement of  claim 9  wherein the first bearing mill presents a rounded, arcuate cross-section. 
   
   
       14 . The window cutting arrangement of  claim 9  wherein the second bearing mill presents a cross-section having a substantially flat bearing surface. 
   
   
       15 . The window cutting arrangement of  claim 9  wherein the second bearing mill presents a rounded, arcuate cross-section. 
   
   
       16 . The window cutting arrangement of  claim 1  wherein the bottom hole assembly length has a midpoint and wherein the first bearing mill reaches an upper end of the whipstock during a milling operation before the midpoint of the bottom hole assembly length reaches the upper end of the whipstock. 
   
   
       17 . The window cutting arrangement of  claim 12  wherein the first distance is from about 1.0 to about 1.25 times the whipstock ramp length. 
   
   
       18 . The window cutting arrangement of  claim 17  wherein the first distance is from about 1.15 to about 1.20 times the whipstock ramp length. 
   
   
       19 . A method of forming a window in wellbore casing, the method comprising the steps of:
 disposing a bottom hole assembly within a wellbore, the bottom hole assembly comprising:
 a shaft; 
 a window mill mounted upon the shaft and located proximate a lower end of the shaft; 
 a first bearing mill carried by the shaft; 
 a second bearing mill carried by the shaft; 
   contacting a whipstock ramp within the wellbore casing with the window mill so that the window mill cuts a window in the wellbore casing;   milling away an upper portion of the window with the first bearing mill; and   contacting the wellbore casing with the second bearing mill so that the first bearing mill and the window mill are retained against the whipstock ramp.   
   
   
       20 . The method of  claim 19  wherein the bottom hole assembly is disposed within the wellbore casing on a drill string.