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CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Prov. App. No. 60/535,553, filed Jan. 08, 2004, for “Single Mill Casing Window Cutting Tool”. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     This invention is in the field of methods and apparatus used to cut an exit or side window in a casing, downhole in a well bore, typically for the purpose of drilling a lateral well bore angled away from the main bore.  
         [0005]     2. Background Art  
         [0006]     Often, it is desired to cut a window in a downhole well casing in an oil or gas well, for the purpose of exiting the casing, to drill a lateral well bore off of the main bore. Typically, multiple mills are used to form such a window having the desired full gage diameter. Often, three separate mills are used, as shown in  FIG. 1 .  
         [0007]     The downhole direction will be shown to the right herein, and the uphole direction to the left, but it should be understood that the borehole could be oriented vertically, horizontally, or anywhere in between. Further, it should be understood that, as used herein, the term “full gage” signifies a window having a diameter which is at least 96% of the full inside diameter of the casing which is being exited. Since the window will have an elliptical shape, the lesser or transverse diameter of such a full gage window will be at least 96% of the full inside diameter of the casing. The greater or longitudinal diameter of the window may be even greater than the inside diameter of the casing. Finally, when the term “separate mills” is used herein, it should be understood that two groups of cutting inserts separated by a substantial axial distance can be considered to constitute two “separate mills”. For example, in some such mills, an axial distance of greater than one inch between groups of inserts could be considered substantial.  
         [0008]     Often, the three mills that are typically used to cut such a window are all mounted on a work string WS together, one above the other. The lower or lead mill LM penetrates the casing wall C; then it is followed by the upper mills, commonly referred to as string mills or backup mills B 1 , B 2 . In such an apparatus, all three mills LM, B 1 , B 2  typically have the same outer diameter, at least in their original condition. However, by the time the lead mill LM penetrates the casing C, it will usually have a slightly smaller diameter than it originally had, because of damage to the cutting inserts, resulting from severe vibration and impact against the casing C and the whipstock W. The string mills above it are then required to pass through the window in order to open it up to a substantially full gage diameter.  
         [0009]     The lead mill LM cuts a relatively small window in the casing C as it is guided therethrough by the guide surface GS of a whipstock W. Rotation of the work string WS then continues as the second mill B 1 , and eventually the trailing mill B 2 , pass through the window, as shown in  FIG. 2 . Each mill slightly enlarges the window as it passes through. Typically, at least two and often three mills must be used, because the cutting inserts on the lead mill LM and even on the second mill B 1  often deteriorate rapidly, with the result that the lead mill LM and the second mill B 1  lose diameter during the window cutting process. This loss in mill diameter results in the cutting of a window that is slightly smaller than the original diameter of the mills. The mills can not have an original diameter that is larger than the full casing gage, of course. So, the second and third mills B 1 , B 2  must be used to bring the casing window diameter up to full gage.  
         [0010]     This typical apparatus having three separate mills on the work string suffers from several disadvantages. First, it is quite costly, because three mills, or at least three spaced apart groups of cutting inserts, are required. Second, the three mills are typically mounted together in a longitudinal arrangement, with the arrays of cutting inserts on the different mills being axially separated by a distance of greater than one inch. This creates a relatively long three mill assembly with a rigid axis. As a result, the apparatus can experience a substantial amount of vibration. Third, because of the long rigid axis of the three mill assembly, it is difficult to turn the mill in a direction away from the whipstock after forming the window and exiting the casing.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention can include a method and apparatus for cutting a substantially full gage casing exit window with a single mill, rather than multiple mills. The mill of the present invention can have multiple blades, with cutting inserts mounted on the blades. The cutting inserts can be of a very hard, wear resistant, material, such as polycrystalline diamond. Although very hard, such inserts can be susceptible to damage from vibration and impact against the casing and especially against the hard guide surface of the whipstock. The original condition of the relatively hard cutting inserts can be preserved by providing a secondary set of relatively more durable inserts behind each row of the harder primary cutting inserts. These secondary inserts provided for the purpose of protecting the primary cutting inserts can be made of a relatively more durable material such as tungsten carbide.  
         [0012]     To further counteract the susceptibility of the cutting inserts to damage, each blade can have a tapered or tapering section which tapers outwardly from its leading end toward its trailing end, with the angle of this taper being less than the angle of the wedge or guide surface on the whipstock with which the mill is used. The trailing end of the blade can be at substantially full gage diameter. A substantially full gage section of the blade above the tapered section can also be provided. A narrow band of the mill body or the blades can also be provided as a buffer section, preferably in the full gage section, with this narrow buffer section being at the full diameter of the mill body, to protect the cutters on the full gage section from shock or impact.  
         [0013]     As the mill progresses downwardly along the whipstock, because of the difference in the blade taper angle and the whipstock wedge angle, only the nose of the mill rides along the whipstock, with the cutting inserts on the tapered and full gage sections of the blade being angled away from the whipstock. Keeping the tapered and full gage sections of the blade spaced away from the whipstock preserves the condition of the cutting inserts along the tapered and full gage sections. This causes the mill to substantially maintain its full gage diameter all the way through the casing window, thereby alleviating the need for using one or more backup mills to establish a full gage window opening.  
         [0014]     The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic view of a prior art three mill assembly;  
         [0016]      FIG. 2  is a schematic view of the assembly shown in  FIG. 1  as it penetrates the casing wall;  
         [0017]      FIG. 3  is a side elevation view of the mill of the present invention; and  
         [0018]      FIG. 4  is a schematic view of the mill shown in  FIG. 3 , as its nose moves along the whipstock and the tapered and full gage portions of the mill penetrate the casing wall. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     As shown in  FIG. 3 , the mill  10  of the present invention includes a mill body  12 , and a plurality of blades  14  spaced around the periphery of the mill body  12 . The mill  10  also has a substantially rounded nose section  20 , which can be formed from the leading ends of the blades  14 , or which could be formed on the mill body  12  itself, without departing from the present invention. A plurality of primary cutting inserts  22  are mounted on the outer surface of each blade  14 , oriented to cut through a casing as the mill  10  is rotated by the work string WS. It can be seen that the outermost edges of the primary cutting inserts  22  together establish a contour which is radially outside the outer contour of the blades  14  themselves. The primary cutting inserts  22  can be formed of a very hard material such as polycrystalline diamond, for maximum wear resistance in cutting through metal. A plurality of similar primary nose inserts  26  can be mounted on the surface of the nose section  20 .  
         [0020]     While such hard materials are optimal for wear resistance, they are often less than optimal for durability, sometimes breaking prematurely under impact against hard surfaces. To protect the primary cutting inserts  22  from impact damage, a plurality of secondary inserts  24  can also be mounted on the outer surface of each blade  14 , positioned behind the plurality of primary cutting inserts  22 , relative to the rotational direction of the mill  10 . The rotational direction of the mill illustrated in  FIG. 3  is clockwise when viewed from the uphole end. The secondary inserts  24  can be formed of a material such as tungsten carbide, which is relatively more durable than the material of the primary inserts  22 . Since the secondary inserts  24  are principally for the purpose of protecting the primary cutting inserts  22  from being damaged by impact against the casing or against the whipstock, they are placed behind the primary inserts  22  in a rotational sense, leaving the primary inserts  22  exposed to rotational contact with the casing for cutting purposes. However, the secondary inserts  24  are mounted with their outermost edges lying on substantially the same contour established by the outermost edges of the primary inserts  22 , thereby absorbing impacts to protect the primary inserts  22 . A plurality of similar secondary nose inserts  28  can be mounted on the surface of the nose section  20 , behind the primary nose inserts  26  in a rotational sense.  
         [0021]     The blades  14  are shown as substantially elongated blades having a slightly helical shape curving around the mill body  12 , but they could have other shapes without departing from the present invention. Importantly, each blade  14  has a tapered section  16  which has an outer edge that tapers outwardly from a point near its leading end  17  toward a point near its trailing end  19 , as seen more easily in  FIG. 4 . The diameter established by all of the trailing ends  19  of these tapering sections  16  of the blades  14  can be a substantially full gage diameter, relative to the inner diameter of the casing through which the mill  10  is lowered downhole.  
         [0022]     The outer edge of the tapering section  16  of each blade  14  lies along a contour which is at an acute angle relative to the longitudinal axis of the mill body  12 . It can be seen that, since the blades shown are helical, their outer edges lie along a substantially conical contour, which can have a slight axial curve or bulge, or which can be a straight conical contour. In either case, the important feature is that the outer edge of the tapered section  16  of the blade  14  has an increased radial distance from the longitudinal axis of the mill body  12  as the blade section  16  tapers outwardly from its leading end  17  toward its trailing end  19 , and that this angle of taper bears a particular relationship to the angle on the whipstock, as discussed below.  
         [0023]     Each blade  14  also can have a substantially full gage section  18  at its trailing end, with the full gage sections  18  having outside edges which are parallel to and equidistant from the longitudinal axis of the mill body  12 . That is, the full gage sections  18  of the blades  14  establish a substantially cylindrical contour which is coaxial with the longitudinal axis of the mill body  12 . Each such outer edge of a full gage section  18  could be parallel to the longitudinal axis, or it could be slightly helical, without departing from the present invention. Further, the full gage section  18  could be formed on a substantially cylindrical surface of the mill body  12  itself, rather than on the blades  14 , without departing from the present invention. Finally, a narrow band of the mill body  12  or blade  14  can be provided as a buffer section  36  on the full gage section  18 , to act as a shock absorber, to further protect the primary cutting inserts  22  in the full gage section  18  from impact damage. The axial dimension of this narrow buffer section  36  would preferably be at least a quarter inch. The outermost diameter of this buffer section  36  would preferable be at the full diameter of the mill  10 .  
         [0024]     For ease of illustration, the mill  10  is shown in  FIG. 4  as having its longitudinal axis parallel to the longitudinal axis of the casing C and the longitudinal axis of the whipstock W, even though in actual use, the longitudinal axis of the mill  10  will likely be slightly angled relative to the longitudinal axis of the casing C. The upper end of the whipstock W has a wedge or angled guide surface GS, which is azimuthally oriented in the desired direction for exiting the casing C. The guide surface GS is angled at a first acute angle  30  relative to the longitudinal axis of the casing C. The tapered section  16  of each blade  14  of the mill  10  is angled at a second acute angle  32  relative to the longitudinal axis of the mill body  12 . The second angle  32  is less than the first angle  30  by a third angle  34 .  
         [0025]     As the mill  10  is lowered through the casing C, the nose section  20  of the mill  10  will contact the guide surface GS of the whipstock W. The fact that the second angle  32  of the blade taper is less than the first angle  30  of the whipstock causes the outer edges of the primary cutting inserts  22  on the tapered sections  16  of the blades  14  to be angled away from the hard guide surface GS of the whipstock W. It has been found that for best effect, the magnitude of the third angle  34  should be between about 3 degrees and 10 degrees, to insure that the primary inserts  22  on the tapered sections  16  of the blades  14  will be spaced away from the whipstock guide surface GS, even when the longitudinal axis of the mill  10  is slightly angled relative to the longitudinal axis of the casing C.  
         [0026]     As the mill  10  is rotated by the work string WS, the nose section  20  of the mill  10  moves downwardly along the guide surface GS, causing the primary and secondary inserts  22 ,  24  on the full gage section  18  and the primary and secondary inserts  22 ,  24  on the tapered blade sections  16  to contact and mill through the wall of the casing C. Only the nose section  20  of the mill  10  contacts the whipstock guide surface GS.  
         [0027]     While this milling is taking place, it can be seen that the presence of the secondary inserts  24  and the narrow band  36  of body or blade protects the primary inserts  22  from being unduly damaged by impact against the casing C. It can also be seen that the existence of the third angle  34  keeps the primary inserts  22  from contacting the whipstock guide surface GS, thereby protecting the primary inserts  22  from undue damage. Further, the use of a single mill according to the present invention minimizes the rigid axial length of the tool and minimizes the vibration experienced, which further reduces the incidence of impact damage to the primary inserts  22 . This combined protection of the primary inserts  22  allows the mill  10  to maintain the substantially full gage diameter of the outer edges of the primary inserts  22  near the trailing ends of the tapered sections  16  of the blades  14  and the substantially full gage diameter of the outer edges of the primary inserts  22  on the full gage section  18  of the mill  10 . Maintaining this substantially full gage diameter allows the mill  10  to completely form a substantially full gage window in the casing wall, without the need for backup mills.  
         [0028]     Eventually, the primary and secondary inserts  26 ,  28  on the nose section  20  also contact and mill through the casing C. It can be seen that the presence of the secondary nose inserts  28  protects the primary nose inserts  26  from being unduly damaged by impact against the casing C or the guide surface GS of the whipstock W.  
         [0029]     It is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.

Summary:
An apparatus for cutting a window in a casing pipe in a well, having only a single mill with full gage primary cutting inserts of a very hard, highly wear resistant, material. The primary inserts can be protected from damage by forming each blade on the mill with an outside edge profile having a shallower angle than the angle of the whipstock which is used to guide the mill through the casing, preventing the primary inserts from riding directly on the whipstock. The primary inserts on each blade can be further protected from damage by a row of secondary inserts of a relatively more durable material, arranged behind each row of primary inserts, or by a full diameter buffer section of mill body or blade.