Full Gauge Milling Bottom Hole Assembly with Optimal Contact Force and Build Rate Capability

Abstract

A milling bottom hole assembly (BHA) for use in cutting a full gauge window in a wellbore casing wall, the resultant length of the window being greater than or equal to the whipstock ramp length. A milling BHA is described which includes two shaft portions, a window mill and two bearing mills. The design, which involves strategically placed bearing mills, allows the milling BHA to stay on the whipstock ramp for the entire casing window milling operation and, thereafter, to optimally rapidly build angle and move laterally away from the whipstock and casing, creating a significantly long window which allows for easy passage of directional drilling BHAs through the milled window.

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 shaft providing a bottom hole assembly length;   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;   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; and   the first distance is from about 75% to about 90% of the bottom hole assembly length.   
     
     
         2 . The bottom hole assembly of  claim 1  wherein the first distance is from about 80% to about 85% of the bottom hole assembly length. 
     
     
         3 . The bottom hole assembly of  claim 1  wherein the second distance is about one-third of the first distance. 
     
     
         4 . 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. 
     
     
         5 . The bottom hole assembly of  claim 1  wherein the first distance is greater than the whipstock ramp length. 
     
     
         6 . The bottom hole assembly of  claim 1  wherein the first bearing mill has an arrowhead-shaped configuration. 
     
     
         7 . The bottom hole assembly of  claim 6  wherein:
 the angled ramp presents a whipstock scoop angle; 
 the first bearing mill includes a lower portion which presents a mill blade taper angle; and 
 the mill blade taper angle is from 1.5 times to 3 times the whipstock scoop angle. 
 
     
     
         8 . The bottom hole assembly of  claim 1  wherein the second bearing mill presents a cross-section having a substantially flat bearing surface. 
     
     
         9 . 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. 
     
     
         10 . The bottom hole assembly of  claim 1  wherein the window mill contacts the whipstock to experience a contact force which gradually increases until the window mill reaches halfway across the whipstock ramp and then gradually declines as the first bearing mill passes a top of the ramp. 
     
     
         11 . The bottom hole assembly of  claim 1  wherein the shaft is formed of first and second shaft sections that are threaded together and wherein:
 the first bearing mill is carried by the first shaft section; and 
 the second bearing mill is carried by the second shaft section. 
 
     
     
         12 . The bottom hole assembly of  claim 1  wherein:
 the second distance is less than half the length of the ramp; and 
 the first distance is greater than half the length of the ramp. 
 
     
     
         13 . 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 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; 
 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; and 
 the first distance is from about 75% to about 90% of the bottom hole assembly length. 
   
     
     
         14 . The window cutting arrangement of  claim 13  wherein the first distance is from about 80% to about 85% of the bottom hole assembly length. 
     
     
         15 . The window cutting arrangement of  claim 13  wherein the second distance is about one-third of the first distance. 
     
     
         16 . The window cutting arrangement of  claim 13  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. 
     
     
         17 . The window cutting arrangement of  claim 13  wherein the first distance is equal to or greater than the whipstock ramp length. 
     
     
         18 . The window cutting arrangement of  claim 13  wherein the first bearing mill has an arrowhead-shaped configuration. 
     
     
         19 . The window cutting arrangement of  claim 13  wherein the second bearing mill presents a cross-section having a substantially flat bearing surface. 
     
     
         20 . The window cutting arrangement of  claim 13  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. 
     
     
         21 . The window cutting arrangement of  claim 13  wherein the first distance is from about 1.0 to about 1.25 times the whipstock ramp length. 
     
     
         22 . The window cutting arrangement of  claim 13  wherein the first distance is from about 1.15 to about 1.20 times the whipstock ramp length. 
     
     
         23 . The window cutting arrangement of  claim 13  wherein:
 the angled ramp presents a whipstock scoop angle; 
 the first bearing mill includes a lower portion which presents a mill blade taper angle; and 
 the mill blade taper angle is from 1.5 times to 3 times the whipstock scoop angle.