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BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The invention is directed to devices for milling a window in casing disposed in an oil or gas wellbore and, in particular, to four-mill bottom hole assemblies for cutting a window in the wellbore casing such as for allowing a lateral, offshoot, horizontal, or branch wellbore to be drilled. 
         [0003]    2. Description of Art 
         [0004]    Bottom hole assemblies, or casing window milling assemblies, for use with whipstocks disposed within wellbore casing are known in the art. In general, these assemblies operate by lowering the assembly into a wellbore casing until a cutting end, or mill head or window mill, contacts the whipstock. As the assembly is further lowered, the window mill is forced into the wellbore casing by the whipstock. As a result, the window mill begins cutting the wellbore casing to form a window. 
         [0005]    Contemporaneously, two additional, or secondary, mills such as a reaming mill and a honing mill, begin cutting the wellbore casing above the window formed by the window mill. As the window mill moves further downhole, and is further forced into the wellbore casing by the whipstock, the opening in the casing, or window, is enlarged, usually by the two secondary mills cutting additional openings in the casing above the whipstock and gradually moving toward the window formed by the window mill until the openings and the window connect. To assist with the bending moment caused by the window mill being forced by the whipstock into the wellbore casing, a flex-joint or flexible section within the upper mills is usually disposed above the window mill. 
         [0006]    Although prior assemblies are effective at ultimately forming the desired opening in the wellbore casing, they have several shortcomings. For example, the size of the window ultimately cut in the casing should, theoretically, be as long as the ramp of the whipstock. The length of the ramp of the whipstock is defined as the distance along the angled portion of the whipstock from the point where the window mill is first moved toward the casing wall to the bottom of the angled portion. However, the window formed by the typical three-mill bottom hole assemblies have difficulty cutting a window that is as long as the ramp length of the whipstock because of the loss of appreciable restraining force on the window mill during its traverse on the bottom quarter section of the whipstock ramp. As a result, the length of the window is shortened such that longer and larger diameter assemblies and other equipment which, in most cases, are more desirable, cannot pass through the opening. 
         [0007]    Current casing window milling assemblies also experience problems with the cutting structure on the mills wearing out prematurely while cutting a window in large size casings with large size whipstocks. In many instances, three mills in three-mill assemblies do not ensure enough cutting structure to create a full gauge window while sustaining the long ramp lengths of large size whipstocks. The vibration impact can also cause the cutters to breakdown and the mills loose their cutting ability prematurely. This can lead to the considerable expense of a second milling operation with a fresh set of mills. 
         [0008]    Also, in many situations, disposition of a full gauge secondary reaming/honing mill at a location too close to a full gauge window mill produces large bending stresses in the bottom hole assembly, especially between the window mill and the secondary mill. 
       SUMMARY OF INVENTION 
       [0009]    Broadly, the bottom hole assemblies or casing window milling assemblies disclosed herein comprise four separate mills disposed at particular locations along the length of the bottom hole assembly. The locations of each of the mills allow for a window to be cut in the casing that is substantially equal to or greater than the length of the ramp of the whipstock. “Substantially equal to” is used herein as meaning at least 95% of the length of the ramp of the whipstock. 
         [0010]    The bottom hole assemblies comprise a window mill at a lower end of the bottom hole assembly. In some embodiments, the window mill is releasably connected to a whipstock so that the whipstock and the bottom hole assembly are run into the wellbore together. A first upper mill is disposed above the window mill, a second upper mill is disposed above the first upper mill, and a third upper mill is disposed above the second upper mill. The first upper mill is an under-gauged mill disposed at a distance measuring approximately 20-37% of the distance measured from the window mill to the third upper mill. In one particular embodiment, the first upper mill is at a distance that is 25% of the distance measured from the window mill to the third upper mill. 
         [0011]    The second upper mill is disposed above the first upper mill and, thus, the window mill, at a distance measuring approximately 55% to 75% percent, and in one embodiment 65% percent, of the distance measured from the window mill to the third upper mill. The third upper mill is disposed above the second upper mill and, thus, the first upper mill and the window mill, at a distance measuring approximately 120% to 130%, and in one embodiment, 125% of the length of the ramp of the whipstock. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a cross-sectional view of one specific embodiment of a casing window milling assembly disclosed herein and a whipstock shown disposed in a cased wellbore during run-in. 
           [0013]      FIGS. 2-5  are cross-sectional views of the assembly shown in  FIG. 1  showing the progression of the assembly shown in  FIG. 1  as a window is cut in the casing of the wellbore. 
       
    
    
       [0014]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF INVENTION 
       [0015]    Referring now to  FIGS. 1-5 , in one specific embodiment, casing window milling assembly of bottom hole assembly  20  includes window mill  22  secured, such as through threads (not shown), to lower joint  26 . Window mill  22  may be a conventional carbide mill or PDC mill known in the art. Lower joint  26  may be a rigid joint or have flexibility to assist in reducing stresses in bottom hole assembly  20 . Window mill  22  includes lower end  23  and mill head housing or body  27 . Lower joint includes under-gauged portion  28  to which first upper mill  30  is secured, or which forms first upper mill  30 . As is readily understood by persons of ordinary skill in the art, first upper mill  30 , as well as any other mills discussed herein, may be separate components secured to the joints of bottom hole assembly  20  or they may be formed integral with the joints of bottom hole assembly  20 . 
         [0016]    Under-gauged portion  28  is used herein to describe a portion of the lower joint  26  that has an outer diameter that is smaller than the outer diameter of the remainder of lower joint  26 . In alternative embodiments, the outer diameter of lower joint  26  is uniform, i.e., there is no under-gauged portion  28 , or the portion of lower joint  26  that includes mill  30  has an enlarged outer diameter to provide additional strength to lower joint  26 . In these embodiments, first upper mill  30  disposed along lower joint  26  is a mill that has an outer diameter that is smaller than the maximum outer diameter of window mill  22  and the maximum outer diameters of the mills disposed above first upper mill  30 , which are discussed in greater detail below. Regardless of whether lower joint  26  includes an under-gauged portion  28  or if the lower joint includes an under-gauged mill, first upper mill  30  is referred herein as the “under-gauge mill” because the combined outer diameter, i.e. the outer diameter of lower joint  26  and the overall thickness of first upper mill  30 , is less than the maximum outer diameters of window mill  22  and the two mills disposed above first upper mill  30 . First upper mill  30  is disposed along lower joint  26  above window mill  22  at a distance measuring approximately 20% to 37%, and in one embodiment 25%, of the distance  24  measured from window mill  22  to third upper mill  46  (discussed in greater detail below). 
         [0017]    Lower joint  26  is secured, such as through threads (not shown), to upper joint  36 . Upper joint is then secured to a tool string (not shown) such as through threads (not shown). Upper joint  36  includes second upper mill  40  and third upper mill  46 . In one embodiment, both second upper mill  40  and third upper mill  46  are “full-gauge mills” because their diameters are not increased or decreased by the outer diameter of upper joint  36 . Nor are the outer diameters of second upper mill  40  or third upper mill  46  increased or decreased to be any larger or smaller than the maximum diameter of window mill  22 . 
         [0018]    Second upper mill  40  is disposed toward a lower end of upper joint  36  and third upper mill  46  is disposed toward an upper end of upper joint  36 . Second upper mill  40  is disposed above first upper mill  30  and, thus, window mill  22 , at a distance measuring approximately 55%-75% percent, and in one embodiment 65%, of the distance  24  measured from window mill  22  to third upper mill  46 . Third upper mill  46  is disposed above second upper mill  40  and, thus, above first upper mill  30  and window mill  22 , at a distance measuring approximately 120%-130%, and in one embodiment, 125%, of the length of the ramp  82  of whipstock  80 . Referring to  FIG. 1 , the length of ramp  82  is measured from the top  84  of whipstock  80  where ramp  82  begins to the bottom  86  of ramp  82  of whipstock  80 . In certain embodiments, whipstock  80  has an over-all length greater than 20 feet and a ramp length greater than 18.5 feet. 
         [0019]    The locations of first upper mill  30 , second upper mill  40 , and third upper mill  46  with respect to window mill  22  facilitates creation of a restraining force on window mill  22  to decrease the chance of early jump-off of window mill  22  from casing  15  near the mid-section of whipstock ramp  82 . Also, under-gauge portion  28  disposed at a distance discussed above, facilitates reduction of unacceptable bending stresses in bottom hole assembly  20 . 
         [0020]    Although first, second, and third upper mills  30 ,  40 , and  46  may be any mills known in the art, in one particular embodiment, first and second upper mills  30 ,  40  are ball mills having a rounded, arcuate cross-section, and third upper mill  46  is a watermelon mill, having a substantially flat surface cross-section with bearing structure ingrained. 
         [0021]    Window mill  22 , and first, second, and third upper mills  30 ,  40 ,  46 , all may include an outer layer of, or formed completely out of, a material selected from the group consisting of carbide, aluminum bronze, tungsten carbide, or hardfacing. Alternatively, or in addition, one or more of window mill  22 , or first, second, or third upper mills  30 ,  40 ,  46  may include blades or other cutting devices known in the art. 
         [0022]    Bore  50  is longitudinally disposed through window head  22 , lower joint  26  and upper joint  36  to facilitate circulation of fluid down wellbore  10 . 
         [0023]    In operation, bottom hole assembly  20  is assembled as shown in  FIG. 1 , secured to a tool string (not shown), and lowered into wellbore  10  having casing  15 . It is to be understood, however, that although whipstock  80  is shown as part of bottom hole assembly  20  in the embodiments of  FIGS. 1-5  so that whipstock  80  can be set during a single run of bottom hole assembly  20  into cased wellbore  10 , whipstock  80  is not required to be part of bottom hole assembly  20 . To the contrary, whipstock  80  may be previously disposed within cased wellbore  10  so that bottom hole assembly  20  can be lowered into cased wellbore  10  until mill head  22  contacts whipstock  80 . 
         [0024]    In either of the foregoing operations, window mill  22  is freed from whipstock  80  so that whipstock  80  guides window mill  22  into the wellbore casing  15  to facilitate window mill  22  cutting window  90  in the wellbore casing  15 . As bottom hole assembly  20  is lowered downward, bottom hole assembly  20  is rotated and begins cutting window  90  in casing  15  ( FIG. 2 ). As bottom hole assembly  20  is lowered further into casing  15 , rotation of bottom hole assembly  20  continues, and cutting of window  90  continues as window mill  22  moves down ramp  82  of whipstock  80  ( FIGS. 3-5 ). In so doing, bottom hole assembly  20  is angled off of the axis  70  ( FIG. 2 ) of casing  15  so window mill  22  cuts through casing  15  and moves into the earth formation (not shown) to form an open-hole wellbore (not shown). 
         [0025]    After window mill  22  has cut into casing  15  a sufficient distance, first upper mill  30  engages casing  15  ( FIG. 3 ) above the top of whipstock, and, thereafter, starts to cut casing  15  above window  90 . First upper mill  30  continues to cut casing  15  above the top  84  of whipstock  80 , and hence enlarging the window  90 , until the enlarged portion of window  90 , i.e. the portion of casing  15  cut by first upper mill  30 , combines with the portion of window  90  cut in casing  15  by window mill  22 . Bottom hole assembly  20  then exits casing  15  through window  90  as illustrated in  FIG. 5 . 
         [0026]    During creation of window  90 , one or both of second upper mill  40  and/or third upper mill  46  contact casing  15  when window mill  22  is past half-way down the length of ramp  82  of whipstock  80 . At this point during the window cutting process, second upper mill  40  and third upper mill  46  contact casing  15  and begin to ream, i.e., clean and cut, the portion of window  90  cut by first upper mill  30 . As bottom hole assembly  20  moves downward, second upper mill  40  and third upper mill  46  continue to ream the portion of window  90  cut by window mill  22 . It is to be understood, however, that second upper mill  40  and third upper mill  46  are not required to be limited to reaming window  90  in casing  15 . In certain embodiments, second upper mill  40  and third upper mill  46  can also engage and cut casing  15  above the portion of window  90  cut by first upper mill  30 . 
         [0027]    Further down the cutting process, first upper mill  30 , second upper mill  40  and third upper mill  46 , engage the formation to continue cutting and cleaning out window  90 . Because of the location of first upper mill  30  relative to window mill  22 , the cutting ability of first upper mill  30  is best utilized to extend window  90  above the top  84  of whipstock  80  and ream/clean window  90  at later stages of window formation. As also shown in  FIG. 5 , window  90  is greater than length of ramp  82  of whipstock  80 . After this is accomplished bottom hole assembly  20  can be retrieved from the wellbore casing  15  and a drill string or another piece of equipment can be run into the wellbore casing  15  to complete the new wellbore. 
         [0028]    The four mills of bottom hole assembly  20  disposed at the locations discussed herein assist in providing a constant and appreciable restraining force on window mill  22  during its traverse on the bottom quarter section of whipstock ramp  82  leading to a longer window length, especially with large size whipstocks. The location of first upper mill  30  to window mill  22  also facilitates creation of a restraining force on window mill  22  to reduce the chance of early jump-off of window mill  22  from casing  15 . Under gauge first upper mill  30  facilitates reduction of bending stresses in bottom hole assembly  20 , especially between window mill  22  and first upper mill  30 . The appreciable distance between second upper mill  40  and third upper mill  46  facilitate reduction of bending stresses between second upper mill  40  and third upper mill  46 . 
         [0029]    It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. For example, each mill described herein can be any type of mill or milling device known to persons in the art. Each mill may comprise a separate device secured to the lower and upper joints or they may be formed integral with the lower or upper joints. Each mill may include blades or other cutting devices, or they may include abrasive surfaces. In other words, as used herein, the term “mill” is to be understood to be given its broadest meaning as being any device capable of cutting or reaming casing of a wellbore. Moreover, second and third upper mills may be designed to only ream out the window after it has been cut in the casing by the window mill and the first upper mill. Alternatively, second and/or third upper mill may also cut an upper portion of window  90  above the portion cut by first upper mill  30 . Accordingly, the invention is therefore to be limited only by the scope of the appended claims.

Summary:
Bottom hole assemblies for cutting windows in wellbore casing comprise a window mill, a first upper mill, a second upper mill, and a third upper mill. The first mill has an outer diameter that is smaller than the outer diameters of the window mill and the second and third upper mills. The first upper mill is disposed above the window mill at a distance measuring approximately twenty to thirty-seven percent of the distance measured from the window mill to the third upper mill. The second upper mill is disposed above the window mill at a distance measuring approximately fifty-five to seventy-five percent of the distance measured from the window mill to the third upper mill. The third upper mill is disposed above the window mill at a distance measuring approximately one-hundred twenty to one-hundred thirty percent of the length of a ramp of a whipstock for guiding the mills.