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BACKGROUND 
     The present invention relates generally to operations performed and equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides diameter based tracking for a window milling system. 
     In a typical re-entry window milling system, a milling assembly including a lead mill and a follow (or watermelon) mill are deflected laterally relative to a casing or liner string by a milling whipstock. This lateral deflection causes the mills to cut through the casing string to thereby form a window in the casing sidewall. The mills may also be used to drill through cement and/or an earth formation surrounding the casing string, thereby starting a branch wellbore extending outward from the window. 
     Generally, the lead mill is used to initiate penetration of the casing sidewall, while the follow mill is used to enlarge the window and form it to the desired final shape and dimensions. For reduced resistance to penetration of the casing sidewall, the lead mill may have a smaller diameter than the follow mill, although the mills could have the same diameter. The whipstock deflects both of the mills using the same inclined surface, so that the mills displace along substantially the same path relative to the casing string. 
     Unfortunately, certain problems arise from use of such prior window milling systems. For example, large bending stresses are experienced when mills having different diameters are guided using the same deflection surface. As another example, substantial wear is experienced when both mills traverse the same surface during the milling operation. Furthermore, prior systems do not take advantage of the unique qualities of the different mills which could be made possible by guiding the mills along respective different paths. 
     Accordingly, it may be seen that improvements are needed in the art of window milling systems. 
     SUMMARY 
     In carrying out the principles of the present invention, a window cutting system and associated methods are provided which solve at least one problem in the art. One example is described below in which the system includes a whipstock or diverter which independently guides the different mills used to cut a window. Another example is described below in which multiple independent guide paths and wear surfaces are formed on the diverter, which is then installed in a well. 
     In one aspect of the invention, a window cutting system is provided which includes a cutting assembly with multiple cutting faces for cutting the window. A diverter is configured for guiding displacement of the cutting assembly relative to the window. The diverter includes multiple separate guide paths for the respective multiple cutting faces. 
     In another aspect of the invention, a method of cutting a window through a tubular string in a subterranean well includes the steps of: positioning a diverter in the tubular string, the diverter including multiple separate guide paths for respective multiple cutting faces of a cutting assembly; and then contacting the guide paths with the cutting faces to thereby form the window. 
     In yet another aspect of the invention, a method of cutting a window through a tubular string in a subterranean well includes the step of: positioning a diverter in the tubular string, the diverter including multiple separate guide paths for respective multiple cutting faces of a cutting assembly, and with each of the guide paths providing a separate wear surface for the respective cutting face. 
     These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings, in which similar elements are indicated in the various figures using the same reference numbers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (Prior Art) is a partially cross-sectional view of a known window milling system; 
         FIG. 2  is an enlarged scale cross-sectional view of a diverter for use in a window cutting system embodying principles of the present invention; 
         FIG. 3  is an enlarged scale schematic cross-sectional view of the diverter, taken along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is an elevational view of the diverter; 
         FIG. 5  is a schematic cross-sectional view of a first alternate configuration of the diverter; 
         FIG. 6  is a schematic cross-sectional view of a second alternate configuration of the diverter; 
         FIG. 7  is a schematic cross-sectional view of a third alternate configuration of the diverter; 
         FIG. 8  is an elevational view of a cutting assembly for use in the window cutting system; 
         FIG. 9  is an elevational view of a first alternate configuration of the cutting assembly; and 
         FIG. 10  is an elevational view of a second alternate configuration of the cutting assembly. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. The embodiments are described merely as examples of useful applications of the principles of the invention, which is not limited to any specific details of these embodiments. 
     In the following description of the representative embodiments of the invention, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. In general, “above”, “upper”, “upward” and similar terms refer to a direction toward the earth&#39;s surface along a wellbore, and “below”, “lower”, “downward” and similar terms refer to a direction away from the earth&#39;s surface along the wellbore. 
     Representatively illustrated in  FIG. 1  is a prior art window milling system  10 . In the system  10 , a whipstock  12  is used to laterally deflect a milling assembly  14 , in order to mill a window  16  through a sidewall of a casing string  18 . The milling assembly  14  includes a lead mill  20 , a follow mill  22  and a tubular extension  24  which spaces apart the two mills. 
     As depicted in  FIG. 1 , the lead mill  20  has partially traversed an inclined deflection surface  26  of the whipstock  12 , and has been thereby deflected laterally into contact with the casing string  18 . As a result, the lead mill  20  has penetrated the sidewall of the casing string  18  to initiate formation of the window  16 . 
     The follow mill  22  has also contacted the inclined surface  26  of the whipstock  12 , but has not yet contacted the casing string  18 . However, due to the larger diameter of the follow mill  22 , this contact between the follow mill and the surface  26  tends to raise the lead mill  20  off of the surface. The contact between the lead mill  20  and the casing string  18  tends to bias the lead mill toward the surface  26 . 
     Thus, it will be appreciated that an undesirable situation results from the use of a single surface  26  to guide the mills  20 ,  22  toward the casing string  18 . Very large bending stresses are induced in the extension  24  due to the deflections of the lead mill  20  relative to the surface  26  when the follow mill  22  contacts the surface. The deflections of the lead mill  20  also cause an irregular, unstable milling of the window  16 . Furthermore, in traversing the surface  26 , both of the mills  20 ,  22  cause wear of the same surface, thus requiring that the whipstock  12  be constructed of more expensive wear resistant materials, or that the whipstock be replaced during the milling operation (which is both time-consuming and expensive to accomplish) when the surface is excessively worn. 
     Referring additionally now to  FIG. 2 , a cross-sectional view of a diverter  28  which embodies principles of the present invention is representatively illustrated. The diverter  28  may be used in place of the whipstock  12  in the system  10  described above. 
     However, it should be clearly understood that the diverter  28  could be used in other systems in keeping with the principles of the invention. For example, the diverter  28  could be used in window cutting systems in which windows are formed through any type of tubular string, such as liner strings, casing strings, tubing strings, etc. The diverter  28  could be used with window cutting assemblies which include any number, combination and configurations of mills, drills, cutting faces, etc. Thus, the uses of the diverter  28 , and the principles of the invention, are not limited in any manner to the details of the system  10  described herein. 
     One unique feature of the diverter  28  is that it provides multiple guide paths  30 ,  32  for the respective multiple cutting faces of a cutting assembly. In the example depicted in  FIG. 2 , the two guide paths  30 ,  32  correspond to the respective lead mill  20  and follow mill  22  of the milling assembly  14 , although it will be appreciated that many other configurations are possible. 
     Another unique feature of the diverter  28  is that the guide paths  30 ,  32  enable the mills  20 ,  22  to be independently guided as they are displaced along the diverter. In turn, this provides the opportunity to utilize the mills  20 ,  22  in ways not previously possible, for example, to form the window  16  in unique shapes, increase the stability of the mills during the cutting operation, increase the efficiency of the cutting operation, etc. 
     Yet another unique feature of the diverter  28  is that the guide paths  30 ,  32  provide separate wear surfaces for the respective mills  20 ,  22 . As described more fully below, the guide paths  30 ,  32  may include intersecting portions at which the mills  20 ,  22  traverse the same surface, but in the example depicted in  FIG. 2 , the guide paths are at least in part independent of each other. 
     The paths  30 ,  32  separately guide the mills  20 ,  22  due to the difference in the diameters of the mills. In  FIG. 3 , an enlarged scale lateral cross-sectional view of the diverter  28  is representatively illustrated, in which the difference in curvature of the guide paths  30 ,  32  may be clearly seen. 
     The path  30  includes a surface  34  having concave curvature at radius r corresponding to the diameter of the lead mill  20 , whereas the path  32  includes a surface  36  having a concave curvature at a larger radius R corresponding to the larger diameter of the follow mill  22 . A result of this difference in curvature is that the lead mill  20  will contact and be guided by the surface  34 , while the follow mill  22  will contact and be guided by the surface  36 . 
     It is not necessary for the radius r to be exactly half of the diameter of the lead mill  20 , or for the radius R to be exactly half of the diameter of the follow mill  22 . For example, the radius r could be somewhat greater than the radius of the lead mill  20 , but less than the radius of the follow mill  22 , while the radius R could be somewhat greater than the radius of the follow mill. 
     An elevational view of the diverter  28  is representatively illustrated in  FIG. 4 . In this view the various surfaces which make up the guide paths  30 ,  32  may be clearly seen. 
     The surfaces  34 ,  38 ,  40 ,  42  have a relatively small curvature (corresponding to the smaller diameter of the lead mill  20 ) and are only contacted by the lead mill. The surfaces  36 ,  44 ,  46 ,  48  have a relatively large curvature (corresponding to the larger diameter of the follow mill  22 ) and are only contacted by the follow mill. The surfaces  50 ,  52 ,  54  are designed to be contacted by both of the lead and follow mills  20 ,  22 . 
     Thus, the guide path  30  for the lead mill  20  includes the surfaces  34 ,  38 ,  50 ,  40 ,  42 ,  52 ,  54 , and the lead mill will traverse these surfaces in that order. The guide path  32  for the follow mill  22  includes the surfaces  44 ,  46 ,  36 ,  50 ,  48 ,  52 ,  54 , and the follow mill will traverse these surfaces in that order. Note that the guide paths  30 ,  32  intersect at the surfaces  50 ,  52 ,  54 . 
     In one unique feature of the guide paths  30 ,  32  as configured in  FIG. 4 , the lead mill  20  contacts and is deflected laterally outward by the surface  38  at the same time that the follow mill  22  contacts and is deflected laterally outward by the surface  46 . In this manner, both of the mills  20 ,  22  are laterally supported by the diverter  28  when they contact and cut through the casing string  18 . This lateral support is also provided by the diverter  28  for both of the mills  20 ,  22  when they are traversing the respective surfaces  40 ,  36  which are not laterally inclined and do not deflect the mills outward. 
     It may now be more fully appreciated how the use of different guide paths enables enhanced shaping of windows. For example, the lead mill  20  may be deflected laterally at a different point along the diverter  28  as compared to the lateral deflection of the follow mill  22 . As another example, one of the mills  20 ,  22  may traverse a surface which is not laterally inclined at a point on the diverter  28  where the other mill traverses a surface which is laterally inclined (e.g., the surface  38  traversed by the lead mill is laterally inclined at a point longitudinally along the diverter where the surface  36  traversed by the follow mill is not laterally inclined). 
     This ability to independently deflect and guide the cutting faces of the mills  20 ,  22  opens up a wide variety of possibilities for creating uniquely shaped windows. Furthermore, the multiple guide paths  30 ,  32  provide increased support to the mills  20 ,  22  during the cutting operation (for example, the lead mill  20  can be supported by the guide path  32  while the follow mill  22  simultaneously contacts and is guided by the path  30 ), which reduces bending stresses in the extension  24 , and the multiple guide paths can increase the stability of the mills during the cutting operation. These features are of substantial benefit especially when both of the mills  20 ,  22  are cutting through the casing string  18 , and also when only the lead mill is cutting through the casing string. 
     Referring additionally now to  FIG. 5 , a schematic cross-sectional view of a longitudinal portion of an alternate configuration of the diverter  28  is representatively illustrated. In this view it may be seen that the guide path  32  includes an undulating surface  56 . For example, the surface  56  may have a sinusoidal shape with a period of length L. 
     Preferably, the period L of the surface  56  is less than the length of an outer cutting face  58  of the follow mill  22 . In this manner, the cuts made by the cutting face  58  will overlap along the length of the window  16 , producing a substantially constant width of the window. However, it is not necessary for the surface  56  to be shaped so that the cuts made by the cutting face  58  overlap, since in some circumstances it may be desired to produce windows with other than constant widths. 
     One benefit of using the undulating surface  56  in the guide path  32  is that it produces a lateral oscillating (in-and-out) displacement of the follow mill  22  relative to the casing string  18  as the follow mill traverses the surface. It is known that when the centerline of the follow mill  22  is inline with the sidewall of the casing string  18 , an unstable situation results. The oscillating motion of the follow mill  22  produced by the surface  56  permits the follow mill to displace back and forth through the sidewall of the casing string  18 , cutting a width of the window  16  at least as great as the diameter of the follow mill along a substantial length of the window, without the follow mill centerline having to remain inline with the sidewall of the casing string for any substantial amount of time. 
     In addition, note that the guide path  30  as depicted in  FIG. 5  does not include a surface which also produces a similar oscillating motion of the lead mill  20 . This is due to the fact that the guide paths  30 ,  32  are independent of each other. However, it should be understood that the guide path  30  could include a surface which produces an oscillating motion of the lead mill  20  in keeping with the principles of the invention. 
     Referring additionally now to  FIG. 6 , a schematic elevational view of a longitudinal portion of another alternate configuration of the diverter  28  is representatively illustrated. In this view it may be seen that the guide path  32  includes an undulating surface  60 . For example, the surface  60  may have a sinusoidal shape with a period of length L. 
     The surface  60  is similar to the surface  56  described above, except that it produces a lateral oscillating motion of the follow mill  22  which is orthogonal to that produced by the surface  56 . In other words, the surface  56  produces an in-and-out motion of the follow mill  22  relative to the sidewall of the casing string  18 , whereas the surface  60  produces a side-to-side motion of the follow mill relative to the casing string sidewall. 
     The surface  60  may have a sinusoidal shape with a period of length L which, similar to the surface  56  as described above, produces an overlapping of the cuts generated by the cutting face  58 . In this manner, the width of the window  16  can be greater than the diameter of the follow mill  22 , even where the centerline of the follow mill is not inline with the sidewall of the casing string  18 . It will be appreciated by those skilled in the art that this is a substantial benefit in the art of window milling, at least in part because access and flow through the window  16  is enhanced by the increased width, and a smaller diameter mill may be used for a given window width (thereby reducing the torque required to drive the mill and increasing the efficiency of the cutting operation). 
     However, it should be understood that it is not necessary for the cuts made by the follow mill  22  to overlap in the manner described above. For example, it may be desirable in some situations for the cuts to not overlap, or to only partially overlap, to thereby produce other desired shapes of the window  16 . 
     In addition, note that the guide path  30  as depicted in  FIG. 6  does not include a surface which also produces a similar side-to-side oscillating motion of the lead mill  20 . This is due to the fact that the guide paths  30 ,  32  are independent of each other. However, it should be understood that the guide path  30  could include a surface which produces an oscillating motion of the lead mill  20  in keeping with the principles of the invention. 
     It may be desirable to combine the displacements produced by the surfaces  56 ,  60  described above. For example, the guide path  32  could include a surface which is helically formed and thereby produces both an in-and-out and side-to-side motion of the follow mill  22  as it traverses the surface. The guide path  30  could include such a surface for producing a similar motion of the lead mill  20 , as well. 
     An alternate configuration of the diverter  28  as depicted in  FIG. 7  demonstrates one manner in which the guide path  30  can include a surface  62  which produces an oscillating displacement of the lead mill  20 . In this example, the surface  62  has a sinusoidal shape with a period of length L which is less than the length of an outer cutting face  64  of the lead mill  20 . This configuration enables the lead mill  20  to cut through the sidewall of the casing string  18  and produce an initial opening which has a greater width than the diameter of the lead mill. 
     It will be appreciated that the guide path  30  can also be configured with surfaces which produce a side-to-side motion of the lead mill  20 , a combination of in-and-out and side-to-side motions, or any other type or combination of displacements. In addition, the guide paths  30 ,  32  may produce any displacements of the mills  20 ,  22  separately and independently of each other. This enables any desired shape of the window  16  to be formed, and allows the unique capabilities of each of the mills  20 ,  22  to be utilized to their greatest extent, among other benefits. 
     Referring additionally now to  FIG. 8 , an elevational view of a cutting assembly  66  which embodies principles of the present invention is representatively illustrated. The cutting assembly  66  may be used in place of the milling assembly  14  in the system  10  described above. 
     However, it should be clearly understood that the cutting assembly  66  could be used in other systems in keeping with the principles of the invention. For example, the cutting assembly  66  could be used in window cutting systems in which windows are formed through any type of tubular string, such as liner strings, casing strings, tubing strings, etc. The cutting assembly  66  could be used with any whipstocks or diverters which include any number, combination and configurations of surfaces, etc. Thus, the uses of the cutting assembly  66 , and the principles of the invention, are not limited in any manner to the details of the system  10  described herein. 
     As depicted in  FIG. 8 , the cutting assembly  66  is a one-piece structure which includes a lead mill  68 , a follow mill  76  and an extension  72  which spaces apart the two mills. The lead mill  68  includes an outer cutting face  74  which has a smaller diameter than an outer cutting face  76  of the follow mill  70 . A bore (not visible in  FIG. 8 ) extends longitudinally through the cutting assembly  66 . 
     Note that the lead mill  68  includes multiple angled blades. Each blade is symmetric and includes multiple faces, one of which is parallel with the longitudinal axis of the lead mill  68 . 
     The follow mill  76  also includes multiple angled blades, each of which has multiple faces, one of which is perpendicular to the longitudinal axis of the follow mill. A trailing face of the follow mill  76  has a concave generally frusto-conical profile and has a smaller diameter than that of the lead mill  68 . 
     The one-piece construction of the cutting assembly  66  provides a very rigid structure which is also very strong due to the absence of threads, etc. joining the various components of the assembly to each other. However, it should be understood that cutting assemblies utilizing separately formed components may be used in keeping with the principles of the invention. 
     An alternate configuration of the cutting assembly  66  is representatively illustrated in  FIG. 9 . In this configuration, the follow mill  70  includes an inclined or tapered leading cutting face  78 . In addition, the lead mill  68  includes a leading inclined cutting face  80  and the cutting face  74  is also inclined. 
     The lead mill  68  includes multiple angled blades. Each blade is symmetric and includes multiple faces, all of which are inclined relative to the longitudinal axis of the lead mill  68 . The leading cutting face  80  is inclined to match the angle of the surface  42  of the diverter  28 . 
     The follow mill  76  also includes multiple angled blades, each of which has multiple faces, all of which are inclined relative to the longitudinal axis of the follow mill. The leading cutting face  78  is inclined to match the angle of the surface  38  of the diverter  28 . Thus, when the cutting faces  80 ,  78  simultaneously traverse the respective surfaces  42 ,  38  of the diverter  28 , the cutting assembly  66  is displaced laterally outward, while the longitudinal axis of the cutting assembly remains parallel to the longitudinal axis of the diverter. 
     Therefore, it will be appreciated that any configuration of the cutting assembly  66  may be used in keeping with the principles of the invention. For example, any number, placement, type and combinations of cutting faces may be used, any of the cutting faces (e.g., the leading face) on the lead mill  68  and/or follow mill  70  may include blades, cutters, etc. 
     As yet another example, depicted in  FIG. 10  is another alternate configuration of the cutting assembly  66 , schematically illustrated in a cross-sectional view. In this configuration, the respective cutting faces  74 ,  76  of lead and follow mills  68 ,  70  are made up of an array of individual cutting teeth or elements  82 , such as carbide “buttons.” 
     In addition, an end surface  84  of the lead mill  68  and an outer surface  86  of the extension  72  are covered with a cutting material. A suitable cutting material for use on the surfaces  84 ,  86  is known as CUTRITE™, and is available from Halliburton Energy Services, Inc. of Houston, Tex., USA. 
     The use of the cutting material on the surfaces  84 ,  86  allows the sidewall of the casing string  18  to be cut by portions of the cutting assembly  66  other than the lead and follow mills  68 ,  70 . Indeed, since substantially all of the lower portion of the cutting assembly  66  is capable of cutting the sidewall of the casing string  18 , it may be considered that the lead and follow mills  68 ,  70  are not really separate components of the cutting assembly or separate “mills”—instead, the cutting assembly may be considered as being made up of multiple cutting faces. These multiple cutting faces may be independently guided by a whipstock or diverter, as described above. 
     In a method which incorporates principles of the invention, a window (such as the window  16 ) is cut through a tubular string (such as the casing string  18 ) in a subterranean well. A diverter (such as the diverter  28 ) is positioned in the tubular string. When so positioned, the diverter includes multiple separate guide paths (such as the guide paths  30 ,  32 ) for respective multiple cutting faces (such as the cutting faces  74 ,  76 ) of a cutting assembly (such as the cutting assembly  66 ). Then, the guide paths are contacted by the cutting faces to thereby form the window. The guide paths are able to independently guide the cutting faces to form the window. 
     Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are within the scope of the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.

Summary:
Diameter based tracking for a window milling system. A window cutting system includes a cutting assembly with multiple cutting faces for cutting the window; and a diverter for guiding displacement of the cutting assembly relative to the window, the diverter including multiple separate guide paths for the respective multiple cutting faces. A method of cutting a window includes the steps of: positioning a diverter in a tubular string, the diverter including multiple separate guide paths for respective multiple cutting faces of a cutting assembly; and then contacting the guide paths with the cutting faces to thereby form the window. Another method of cutting a window includes the step of: positioning a diverter in the tubular string, the diverter including multiple separate guide paths for respective multiple cutting faces of a cutting assembly, with each of the guide paths providing a separate wear surface for the respective cutting face.