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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the priority of U.S. Provisional Patent Application Ser. No. 60/991,432 filed Nov. 30, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to the arrangement of mills on bottom hole assemblies that are used to cut windows in casing strings for the creation of lateral wellbores. 
         [0004]    2. Description of the Related Art 
         [0005]    In modern hydrocarbon production, it is common to create one or more lateral production wellbores which extend outwardly from a central, generally vertical wellbore. In order to form a lateral production wellbore, a window must be cut into the side of casing in the central wellbore. Thereafter, drilling tools are used to form an extended lateral wellbore. Traditionally, whipstocks and milling tools are used to create the window in the central wellbore casing wall. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides an improved milling bottom hole assembly for use in cutting a window in a wellbore casing wall. An exemplary bottom hole assembly (BHA) is described which includes a central shaft. The distal end of the central shaft 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 BHA designs. The above is superior for ingress and egress of long and stiff directional drilling BHAs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein: 
           [0008]      FIG. 1  is a side, cross-sectional cutaway drawing of an exemplary milling tool bottom hole assembly constructed in accordance with the present invention depicted alongside an associated exemplary whipstock. 
           [0009]      FIG. 2  is a side, cross-sectional view of an exemplary wellbore containing a whipstock, and milling tool bottom hole assembly shown in  FIG. 1 , during an initial window cutting stage. 
           [0010]      FIG. 3  is a side, cross-sectional view of the arrangement depicted in  FIG. 2 , now with the window cutting operation further advanced. 
           [0011]      FIG. 4  is a side, cross-sectional view of the arrangement depicted in  FIGS. 2 and 3 , now with the window cutting operation further advanced. 
           [0012]      FIG. 5  is a graph depicting the correlation of side forces on the window mill with distance of the window mill from the whipstock kick-off point. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]      FIG. 1  illustrates an exemplary whipstock  10  and a milling tool bottom hole assembly  12 , which is constructed in accordance with the present invention. The bottom hole assembly  12  includes a threaded upper end  14  which is used for securing the bottom hole assembly  12  to a drill string  16 . The bottom hole assembly  12  includes a central shaft  17  formed of upper and lower string sections  18 ,  20 , which are secured together at threaded joint  22 . A window mill  24 , of a type known in the art, is secured to the distal end of the bottom hole assembly  12 . 
         [0014]    A first bearing mill  26  is located on the lower string section  20  above the window mill  24 . The first bearing mill  26  preferably presents a rounded, arcuate cross-section. A second bearing mill  28  is located on the upper string section  18 . The second bearing mill  28  preferably presents a cross-section that is curved and oblong, thereby presenting a substantially flat center segment  30  and arcuately curved end sections  32 . The second bearing mill  28  may be of the type generally known in the industry as a “Watermelon mill.” In an alternate embodiment, the second bearing mill  28  presents a cross-section that is arcuately rounded, in the same manner as the first bearing mill  26 . 
         [0015]    The overall length “L” of the bottom hole assembly  12  (the bottom hole assembly length) exceeds the longitudinal length “l” of the ramp  34  of the whipstock  10  (the whipstock ramp length). The second bearing mill  28  is preferably located at a distance “x” from the window mill  24  that is from about 1.0 to about 1.25 times the length “l” of the ramp  34 . Most preferably, the distance “x” is about 1.15 to about 1.20 times the length “l” of the ramp  34 . The first bearing mill  26  is preferably located at a distance “d” from the window mill  24  that is from about one-fifth to about one-half of the length “x”. Most preferably, the distance “d” is about one-third of the length “x”. It is further noted that the spacing (“d 1 ”) between the first and second bearing mills  26 ,  28  preferably exceeds the distance “d”. 
         [0016]      FIGS. 2 ,  3  and  4  illustrate the bottom hole assembly  12  in operation to create a window  40  in the casing  42  surrounding a primary wellbore  44 .  FIGS. 2-4  also depict the bottom hole assembly  12  exiting the primary wellbore  44  along a departure path  46  through the surrounding earth  48 . 
         [0017]    In operation, the drill string  16  and bottom hole assembly  12  are rotated within the casing  42 , and the bottom hole assembly  12  is lowered within the wellbore  44  until the bottom hole assembly  12  encounters the whipstock  10  proximate the kick-off point  43 . As  FIG. 2  illustrates, the window mill  24  is urged against the casing  42  and begins to cut the window  40 . As the milling operation continues, the window mill  24  cuts downwardly from the upper window end  50  to increase the length of the window  40  (as shown in  FIGS. 3 and 4 ). At the same time, the incline of ramp  34  urges the window mill  24  laterally outside of the wellbore  44 . The lower string section  20  remains substantially rigid between the window mill  24  and the first bearing mill  26 . However, due to the substantial distance between the first and second bearing mills  26 ,  28 , the portion of the lower string section  20  above the first bearing mill  26  and the portion of the upper string section  18  below the second bearing mill  28  will bend and flex. The first bearing mill  26  will cut away the upper end  50  of the window  40  during the milling operation, thereby increasing the length of the window  40 . It is noted that, as the milling operation progresses, the first bearing mill  26  will reach the upper end of the whipstock  10  before or at the same time as the mid-point ( 52  in  FIGS. 1 and 3 ) of the bottom hole assembly  12  due to the spacing of the first bearing mill  26  proximate to the window mill. 
         [0018]    During the milling operation, as illustrated by  FIG. 4 , the flat portion  30  of the second bearing mill  28  will contact the surrounding casing  42  and be urged to remain radially inside of the casing  42 . This urging results in additional lateral forces to be imparted to the lower portion of the bottom hole assembly  12 , causing the bottom hole assembly  12  to hold against the whipstock  10  for a longer time, thus leading to a longer window  40 . 
         [0019]    The design of the bottom hole assembly  12  provides high constraining forces at the window mill  24  while traversing the midsection of the ramp  34  of the whipstock  10 . The use of a bottom hole assembly  12  constructed in accordance with the present invention produces a milled window  40  having an extended length, as measured from the upper end  50  to the lower end  52 . The proximity of the first bearing mill  26  to the window mill  24  creates restraining forces on the window mill  24  to urge it properly along the departure path  46  from the primary wellbore  44 . Additionally, the proximity of the first bearing mill  26  to the window mill  24  helps in harnessing the efficiency of the cutters of the first bearing mill  26  for is additional cutting of the upper end  50  of the window  40 . This results in a longer window  40  than with many conventional techniques.  FIG. 3  depicts the upper end  50  of the window  40  being milled away by the first bearing mill  26 . At the same time, the first bearing mill  26  is spaced at an optimum distance from the window mill  24  to avoid an early jump-off of the window mill  24  from the casing  42  near the mid-point of the whipstock ramp  34 . 
         [0020]    As noted, the first bearing mill  26  preferably has an arcuate cross-section, thereby providing for point-type contact between the bearing mill  26  and the surrounding casing  42  or the whipstock  10 . Point-type contact results from the fact that the surface of the curved bearing mill  26  cross-section will contact the surrounding casing  42  or whipstock  10  at a single point.  FIG. 3  illustrates the mill  26  contacting the casing  42  at point  54 . In addition, the bottom hole assembly  12  can pivot with respect to the surrounding casing  42  about the point  54 . Binding of the bottom hole assembly  12  as it turns while moving onto the upper end of the whipstock ramp  34  is dramatically reduced as a result of this point-type contact between the first bearing mill  26  and the casing  42 . The combination of these advantages results in a longer service life for the bottom hole assembly  12 . 
         [0021]      FIG. 5  depicts the side forces imparted to the window mill  24  as it is moved along the whipstock ramp  34  from the kick-off point  43 . It can be seen by reference to  FIG. 5  that the side forces imparted to the window mill  24  by the bottom hole assembly are kept within a reasonable range throughout the milling operation.  FIG. 5  is a chart wherein the amount of side force (in kip-force, or klbf) imparted to the window mill (bit)  24  is represented by curve  60 . As can be seen, the side forces are within an acceptable limit and are higher at locations along the whipstock ramp  34  where the window mill  24  has maximum chances of early jump-offs. In  FIG. 5 , areas where the curve  60  presents a positive side force (1, 2, 3, 4, etc.) indicate that the window mill  24  is being urged against the ramp  34  of the whipstock  10 . Conversely, areas where the curve  60  depicts negative side force (−1, −2, −3, etc.) indicate that the window mill  24  is being diverted away from the ramp  34  of the whipstock  10 .  FIG. 5  indicates that the BHA  12  causes the window mill  24  to be continually urged against the ramp  34  until point  62 , which generally coincides with the point at which the window mill  24  has moved entirely outside of the casing  42 . As a result of this continuous positive side force, the possibility of the window mill  24  tending to undesirably “jump off” of the ramp  34  during initial phases of window cutting is minimized. More specifically, when the gage O.D. of the window mill  24  clears the casing  42 , because of which the casing  42  no longer provides a restraining force urging the window mill  24  against the ramp  34 , side forces are maximized. A thorough finite element analysis of the proposed design predicts the trajectory of the lateral bore hole created in the surrounding earth formation  48  after the window mill  24  has moved past the ramp  34  (i.e., beyond point  62  of curve  60 ). This analysis shows that the window mill  24  and hence the bottom hole assembly  12  will tend to desirably hold or build an angle that is more normal to the casing  42  than with other bottom hole assembly designs, which tend to drop angle. This improved trajectory is desirable for the creation of a lateral wellbore using drilling assembly. 
         [0022]    It can be seen that the bottom hole assembly  12  and the whipstock  10  collectively provide a window cutting arrangement that is operable to form a window in surrounding wellbore casing. It should also be understood that the invention provides an improved method for forming a window within wellbore casing. 
         [0023]    The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention.