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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 12/325,184 filed Nov. 29, 2008 which claims priority to provisional patent application Ser. No. 60/991,432 filed Nov. 30, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to the arrangement and design 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 (BHA) for use in cutting a window in a wellbore casing wall. An exemplary milling BHA is described which includes a shaft that is made up of two shaft sections. The distal end of the shaft carries a window mill. A pair of bearing mills is carried by the shaft sections above the window mill. Preferably, each of the bearing mills is carried by a different shaft section. Placement of the bearing mills permits the milling BHA to cut a window having a greater length and quality as it allows the milling BHA to stay on the whipstock ramp for the entire milling operation and then exit the ramp and casing rapidly, such that the lateral build rate of the milling BHA away from the whipstock and its anchor is optimum and both risks of casing reentry of the milling BHA and excessive damage to the milling BHA are mitigated. The resultant milled casing exit window is superior for subsequent ingress and egress of long and stiff directional drilling BHAs. A full gauge arrowhead-shaped mill is preferably used for the lower bearing mill. A full gauge watermelon-shaped mill is preferably used for the upper bearing mill. All three mills, the window mill, the arrowhead-shaped mill and the watermelon-shaped mill, present the same full gauge diameter. 
     
    
     
       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 BHA constructed in accordance with the present invention depicted alongside an associated exemplary whipstock. 
           [0009]      FIG. 1A  is a side view of an exemplary arrowhead-shaped mill used with the milling BHA shown in  FIG. 1 . 
           [0010]      FIG. 1B  illustrates an exemplary relationship between the angle of the lower portion of the first bearing mill blades and the associated whipstock scoop angle. 
           [0011]      FIG. 2  is a side, cross-sectional view of an exemplary wellbore containing the whipstock, and the milling BHA shown in  FIG. 1 , during an initial window cutting stage. 
           [0012]      FIG. 3  is a side, cross-sectional view of the arrangement depicted in  FIG. 2 , now with the window cutting operation further advanced. 
           [0013]      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. 
           [0014]      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. 
           [0015]      FIG. 6  is a graph depicting an exemplary contact force on a window mill as the milling BHA is moved along a whipstock ramp. 
           [0016]      FIG. 7  is a graph depicting exemplary contact forces versus distance along a whipstock ramp. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]      FIG. 1  illustrates an exemplary whipstock  10  and a milling BHA  12 , which is constructed in accordance with the present invention. The milling BHA  12  includes a threaded upper end  14  which is used for securing the milling BHA  12  to a drill string  16 . The milling BHA  12  includes a shaft  17  formed of upper and lower shaft sections  18 ,  20 , which are secured together at threaded joint  22 , and a window mill  24 . The window mill  24 , of a type known in the art, is secured to the distal end of the milling BHA  12 . 
         [0018]    A first bearing mill  26  is located on the lower shaft section  20  above the window mill  24 . The first bearing mill  26  is preferably of full gauge and is preferably of an arrowhead-shaped configuration, as illustrated in  FIG. 1A . The blades of the first bearing mill  26  present an enlarged, full gauge cutting diameter  25  that is located within the upper half of the length of the mill  26 . As a result, the portion  27   a  of the first bearing mill  26  that is located above the full gauge diameter  25  quickly increases from the mill&#39;s shaft  17  diameter radially outwardly to the full gauge diameter  25 . The portion  27   b  of the mill  26  that is located below the full gauge diameter  25  decreases gradually from the full gauge diameter to the diameter of the shaft  17 . The tapered lower portion  27   b  facilitates easy movement and entry of the mill  26  onto a whipstock ramp and reduces chances of getting stuck. Also, the positioning of the cutting structures on the gauge section of the mill  26  allows effective cutting. The tapered lower portion  27   b  is designed to improve the longevity of the cutting portion of the arrowhead-shaped first bearing mill  26 . In an embodiment, the milling BHA  12  with all milling sections at full gauge diameter is designed such that, as the first bearing mill  26  transitions from the primary wellbore  44  into the window  40 , the contact forces between the first bearing mill  26  and the surrounding casing  42  are increased. 
         [0019]    Also of note is that the angle of the taper on the lower portion  27   b  of the arrowhead-shaped first bearing mill  26  is derived from the predicted angular position between the centerlines of the first bearing mill  26  and the whipstock  10  when the first bearing mill  26  transitions from the primary wellbore  44  into the window  40 . Because maximum forces are encountered at this transition point, the angle of the taper is such that the surface area on the cutting surface is optimized, damage to the mill  26 &#39;s cutting structure is minimized, and cutting structure life expectancy is maximized.  FIG. 1B , depicts an exemplary whipstock scoop angle “X,” which is the angle between the vertical axis of the whipstock  10  and the inclination of ramp  34  (i.e., the whipstock scoop angle).  FIG. 1B  also illustrates an angle “Y” which is the angle at which the blades of the lower portion  27   b  of the first bearing mill  26  are disposed from the vertical axis of the milling BHA  12  (i.e., the mill blade taper angle). In a currently preferred embodiment, the angle “X” is derived from angle “Y” such that Y=(1.5 to 3)X. 
         [0020]    A second bearing mill  28  is located on the upper shaft 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 . Both the first and second bearing mills  26 ,  28  extend radially outwardly to full gauge. 
         [0021]    The overall length “L” of the milling BHA  12  (the milling BHA 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”. 
         [0022]    The distance “x” of the second bearing mill  28  from the window mill  24  is also preferably from about 75% to about 90% of the overall milling BHA length “L”. More preferably, the distance “x” is from about 80% to about 85% of “L”. 
         [0023]      FIGS. 2 ,  3  and  4  illustrate the milling BHA  12  in operation to create a window  40  in the casing  42  surrounding a primary wellbore  44 .  FIGS. 2-4  also depict the milling BHA  12  exiting the primary wellbore  44  along a departure path  46  through the surrounding earth  48 . 
         [0024]    In operation, the drill string  16  and milling BHA  12  are rotated within the casing  42 , and the milling BHA  12  is lowered within the wellbore  44  until the milling BHA  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 does the mid-point ( 52  in  FIGS. 1 and 3 ) of the milling BHA  12  due to the spacing of the first bearing mill  26  proximate to the window mill  24 . 
         [0025]    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 milling BHA  12 , causing the milling BHA  12  to hold against the whipstock  10  for a longer time, thus leading to a longer window  40 . 
         [0026]    The design of the milling BHA  12  provides high constraining forces at the window mill  24  while it traverses the midsection of the ramp  34  of the whipstock  10 . The use of a milling BHA  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 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 . 
         [0027]    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 milling BHA  12  can pivot with respect to the surrounding casing  42  about the point  54 . Binding of the milling BHA  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 milling BHA  12 . 
         [0028]      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 whipstock  10  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 milling 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 gauge 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 to compensate for the lost casing-induced restraining force. 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 milling BHA  12  will tend to desirably hold or build an angle that is more normal to the casing  42  than with other milling BHA designs, which tend to drop angle. This improved trajectory is desirable for the subsequent completion of a lateral wellbore using a drilling assembly. 
         [0029]    It can be seen that the milling BHA  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. 
         [0030]    In order to achieve a high build rate, the lower mill  26 , which follows the window mill  24 , will experience a contact force/restoring force that is in a direction towards the whipstock  10  at the time after the window mill  24  has exited the casing  42 . Also, generally the magnitude of the contact force on the lower mill  26  should be equal to or greater than the maximum contact force experienced by the window mill  24 .  FIG. 6  illustrates the contact force upon an exemplary window mill  24  as the milling BHA  12  advances along the ramp  34 . The contact force of the window mill  24  against the ramp  34  increases gradually (portion  64 ) as the window mill  24  enters the whipstock ramp  34 . The contact force is substantially constant during portion  66  as the window mill  24  advances to the middle of the ramp  34 . Finally, as the window mill  24  exits the ramp  34 , the contact force falls gradually (portion  68 ). 
         [0031]    Contact forces at defined intervals are experienced by the mills  26 ,  28  (which are at drift OD) when they contact the casing  42  as they pass through the deviated well profile. The contact force plots are generated for the window mill  24 , lower mill  26  and the upper mill  28 . For comparison purposes, these respective contact forces are superimposed on the same plot in  FIG. 7 .  FIG. 7  shows that, when the window mill  24  is on the ramp  34 , the contact forces (distance  1 - 18  in  FIG. 7 ) are positive, which indicates that the window mill  24  is pressing against the whipstock  10 . At the same time, the lower mill  26  contact forces are negative, indicating that it is pressing against the casing  42 . Projected distance of the positive window mill contact force curve on the x-axis is directly proportional to the length of the window that will be milled. In the case illustrated by  FIG. 7 , the positive force distance is 19 feet. Once the contact force becomes negative, this indicates that the window mill  24  has exited the ramp  34  (distance  18 - 23  in  FIG. 7 ). The negative peak on the lower mill  26  contact force (distance  8  in  FIG. 7 ) is seen when the lower mill  26  is just about to enter the whipstock  10 . The negative direction also indicates that the lower mill  26  is pressing against the casing  42 . It will be appreciated by one of skill in the art that the window mill  24  experiences a contract force that gradually increases until the window mill  24  reaches approximately halfway across the whipstock ramp  34  and then gradually declines as the first bearing mill passes the upper end of the ramp  34 . Once the window mill  24  exits the ramp  34 , the lower mill  26  experiences positive contact forces (distance  21  in  FIG. 7 ), which indicates that the lower mill  26  is now pressing against the ramp  34 . A higher magnitude of the positive contact force on the lower mill  26  compared to the negative contact force (distance  21  in  FIG. 7 ) on the window mill  24  helps establish the desired build rate for the rat hole that is subsequently drilled. 
         [0032]    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.

Summary:
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.