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BACKGROUND  
         [0001]    The present invention relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a reduced debris milled multilateral window.  
           [0002]    In multilateral wells it is common practice to drill a branch or lateral wellbore extending laterally from an intersection with a main or parent wellbore. A casing string is typically installed in the parent wellbore, a whipstock is positioned in the casing string at the desired intersection, and then one or more mills are deflected laterally off of the whipstock to form a window through the casing sidewall.  
           [0003]    Unfortunately, this milling process usually produces a large amount of debris, such as small pieces of the metal casing, which accumulate in the parent wellbore. This debris may make the whipstock difficult to retrieve after the milling process is completed. Even after the whipstock is retrieved, the debris may cause other problems, such as plugging flow control devices, damaging seals, obstructing seal bores, interfering with passage of equipment past the intersection, etc.  
           [0004]    One proposed solution is to pre-mill the window in the casing, that is, form the window through the casing sidewall prior to installing the casing in the parent wellbore. However, if the casing is to be cemented in the main wellbore, the window should be closed during the cementing operation, such as by using an internal or external sleeve. Typically, the sleeve is made of an easily milled material, such as aluminum or a composite material, or is made so that it can be retrieved after the cementing operation.  
           [0005]    Although such sleeves have achieved some success, they also have their problems. For example, the sleeve material may be incompatible with fluids used in the well. The use of an external sleeve increases the casing outer diameter, requiring either a smaller casing size to be used, or a larger wellbore to be drilled. The use of an internal sleeve reduces the casing inner diameter, restricting the passage of fluids and equipment through the casing. The use of a shiftable or retrievable inner sleeve requires another operation in the well and increases the complexity of the equipment and the procedure.  
           [0006]    From the foregoing, it can be seen that it would be quite desirable to provide improved apparatus, systems and methods for forming windows in casing.  
         SUMMARY  
         [0007]    In carrying out the principles of the present invention, in accordance with an embodiment thereof, a window joint is provided for interconnection in a casing string. The use of the window joint reduces the debris created when a window is milled through the window joint.  
           [0008]    In one aspect of the invention, a window joint is provided which includes a generally tubular body having a sidewall. A window portion of the sidewall is configured for forming a window therethrough. A thickness of the sidewall is reduced in the window portion using a variety of techniques.  
           [0009]    In another aspect of the invention, a window joint system is provided which includes a window joint interconnected in a casing string and positioned in a parent wellbore. The window joint includes a sidewall having a window portion through which a window is formed to drill a branch wellbore. The window portion has a reduced thickness of the sidewall prior to forming the window through the window portion.  
           [0010]    In yet another aspect of the invention, a method of drilling a branch wellbore extending laterally from an intersection with a parent wellbore is provided. The method includes the steps of: interconnecting a window joint in a casing string, the window joint including a sidewall having a window portion with a reduced thickness of the sidewall; positioning the casing string in the parent wellbore; aligning the window joint with the window portion facing toward the desired branch wellbore; cutting through the window portion of the window joint, thereby forming a window through the sidewall; and drilling the branch wellbore through the window.  
           [0011]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic partially cross-sectional view of a method embodying principles of the present invention;  
         [0013]    [0013]FIG. 2 is a schematic cross-sectional view of a first window joint embodying principles of the invention;  
         [0014]    [0014]FIG. 3 is a cross-sectional view of the first window joint, taken along line  3 - 3  of FIG. 2;  
         [0015]    [0015]FIG. 4 is a schematic cross-sectional view of a second window joint embodying principles of the invention;  
         [0016]    [0016]FIG. 5 is a cross-sectional view of the second window joint, taken along line  5 - 5  of FIG. 4;  
         [0017]    [0017]FIG. 6 is a schematic cross-sectional view of a third window joint embodying principles of the invention;  
         [0018]    [0018]FIGS. 7A &amp; B are alternate cross-sectional views of the third window joint, taken along line  7 - 7  of FIG. 6; and  
         [0019]    [0019]FIG. 8 is an elevational side view of a fourth window joint embodying principles of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]    Representatively and schematically illustrated in FIG. 1 is a method  10  which embodies principles of the present invention. In the following description of the method  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, 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.  
         [0021]    In the method  10 , a main or parent wellbore  12  is drilled and a casing string  14  is installed and cemented in the wellbore. The terms “parent” and “main” wellbore are used herein to designate a wellbore from which another wellbore is drilled. A parent or main wellbore does not necessarily extend directly to the earth&#39;s surface, but could instead be a branch of yet another wellbore.  
         [0022]    The term “casing” is used herein to designate a tubular string used to line a wellbore. Casing may actually be of the type known to those skilled in the art as “liner”, and may be made of any material, such as steel or composite material, and may be segmented or continuous, such as coiled tubing.  
         [0023]    The casing string  14  in the method  10  includes a window joint  16  interconnected therein. An internal orienting profile  18  may be formed directly on the window joint, or it may be separately attached thereto as depicted in FIG. 1. The window joint  16  is positioned at a desired intersection  22  between the parent wellbore  12  and a branch or lateral wellbore  20  to be drilled later.  
         [0024]    The terms “branch” and “lateral” wellbore are used herein to designate a wellbore which is drilled outwardly from its intersection with another wellbore, such as a parent or main wellbore. A branch or lateral wellbore may have another branch or lateral wellbore drilled outwardly therefrom.  
         [0025]    The window joint  16  and orienting profile  18  are rotationally oriented relative to the branch wellbore  20  using techniques known to those skilled in the art, such as by using a gyroscope engaged with the orienting profile.  
         [0026]    The parent wellbore  12  below the intersection  22  may be completed before or after the branch wellbore  20  is drilled (or not at all). As depicted in FIG. 1, the lower parent wellbore  12  has been completed and has a packer  24  installed therein. The packer  24  includes an internal seal bore or PBR  26 .  
         [0027]    When it is desired to drill the branch wellbore  20 , a whipstock or deflector  28  is positioned in the casing string  14  below the intersection  22 . Keys or dogs  30  carried on the whipstock cooperatively engage the orienting profile  18 . This engagement anchors the whipstock  28  to the casing string  14  and rotationally orients an inclined deflector surface  32  so that it faces toward the desired branch wellbore  20 .  
         [0028]    One or more cutting tools, such as mills and drills, are then lowered through the casing string  14  and deflected laterally off of the deflector surface  32  to form a window  34  through the casing and to drill the branch wellbore  20 . In prior art methods, this process of forming the window  34  has resulted in a large quantity of debris accumulating in the parent wellbore  12  at and below the intersection  22 . Although the whipstock  28  might have been equipped with a debris barrier  36  in these prior art methods, the debris could still hamper retrieval of the whipstock from the well, interfere with passage of equipment through the intersection  22 , cut seals (such as packing elements on the packer  24 ), prevent sealing in seal bores (such as the seal bore  26 ), or cause other difficulties.  
         [0029]    The present invention, however, substantially reduces the debris created in milling the window  34 , which reduces or eliminates the problems described above. These advantages are achieved in the method  10  without requiring the use of an internal or external sleeve. Nevertheless, a sleeve could be used in the method  10 , if desired, without departing from the principles of the invention.  
         [0030]    To achieve these benefits, the window joint  16  used in the method  10  has a reduced thickness sidewall in a window portion of the window joint. This reduced thickness results in less debris being created when the window  34  is milled. Although reduced, the sidewall thickness in the window portion is still sufficient to prevent cement, or other fluids or gases, from flowing therethrough when the casing string  14  is installed and cemented in the parent wellbore  12 .  
         [0031]    The reduced thickness of the window portion of the window joint  16  makes the sidewall lighter in the window portion, and so the opposite side of the window joint is influenced by gravitational force to seek the lower side of the wellbore  12  when the casing string  14  is installed. The parent wellbore  12  is depicted in FIG. 1 as being substantially vertical, but those skilled in the art understand that this situation is very rare, since most wellbores are actually deviated at least somewhat from true vertical.  
         [0032]    Preferably, the branch wellbore  20  is drilled so that it extends at least partially upwardly from the parent wellbore  12 . Therefore it is a significant benefit for the side of the window joint  16  opposite the window portion to seek the lower side of the wellbore  12  when the casing string  14  is installed.  
         [0033]    Representatively and schematically illustrated in FIGS. 2-8 are various window joints which may be used for the window joint  16  in the method  10 . These various specific examples of window joints are described herein to show how the principles of the invention may be incorporated into the construction of window joints, but it is to be clearly understood that the principles of the invention are not limited to the details of these specific examples. Instead, the principles of the invention permit a wide variety of window joint constructions.  
         [0034]    In addition, it should be clearly understood that the principles of the invention may be incorporated into methods other than the method  10 , such as methods wherein a whipstock is not used. The window joint examples described below, and other window joints embodying principles of the invention, may be used in these other methods, as well.  
         [0035]    In FIGS. 2 &amp; 3 a window joint  40  having an internal orienting profile  42  formed in a tubular body  38  of the window joint is depicted. Preferably, the orienting profile  42  is formed directly on the window joint  40 , so that the separate steps of connecting the orienting profile to the window joint and rotationally aligning the profile with the window joint are avoided. However, the orienting profile  42  could be formed in a separate element, such as a collar, if desired.  
         [0036]    The window joint  40  has a sidewall  46  that is a consistent thickness at upper and lower end connections  48  of the window joint. The end connections  48  may be provided with conventional threads, seals, seal bores, or welds, etc. (not shown) for interconnection in a tubular string. However, between the end connections  48 , the window joint  40  includes a window portion  44  having a reduced sidewall  46  thickness.  
         [0037]    This reduced sidewall  46  thickness is formed by laterally offsetting an inner diameter  50  in the window portion  44  relative to inner diameters  52  at the end connections  48 . That is, a longitudinal centerline  54  of the window portion  44  is laterally offset relative to a longitudinal centerline  56  of the end connections  48 . However, note that the window joint  40  has the same outer diameter  58  at the window portion  44  and at the end connections  48 , resulting in the inner diameter  50  being also laterally offset relative to the outer diameter  58 .  
         [0038]    The offset inner diameter  50  may be formed in the window joint  40  using various methods. For example, the inner diameter  50  may be cut using a lathe, or the window joint could be cast, forged or drawn with the offset inner diameter.  
         [0039]    In FIGS. 4 &amp; 5 another window joint  60  is depicted. A sidewall  62  of the window joint  60  has a reduced thickness in a window portion  64 . The reduced thickness is due to a recess  66  formed internally on the sidewall  62 . The recess  66  may be formed by milling, casting, forging, or any other method.  
         [0040]    One advantage of using an internally formed recess is that the recess may be used for additional purposes. For example, a whipstock or deflector  68  may carry a member  70  which engages the recess  66  to position and rotationally align a deflector face  72  relative to the window portion  64 .  
         [0041]    In FIGS. 6, 7A &amp; B another window joint  80  is depicted. The window joint  80  has a sidewall  82  with a reduced thickness in a window portion  84  between end connections  86 . The window joint  80  can also include an orienting profile, such as the profile  42  described above, and can also include one or more internal recesses, such as the recess  66  described above, formed on the window portion  84 .  
         [0042]    The cross-sectional views in FIGS. 7A &amp; B depict alternate methods of forming the reduced sidewall thickness in the window portion  84 . In FIG. 7A, the reduced thickness is formed by cutting (or casting, forging, drawing, etc.) a laterally offset, but larger radius  88  on an outer radius go of the window joint  80 . The radius go has a longitudinal centerline  92 , which also corresponds to inner and outer diameters  94 ,  96  of the window joint  80 . However, a centerline  98  of the radius  88  is laterally offset relative to the centerline  92 .  
         [0043]    Thus, the window portion  84  includes multiple intersecting external radii  88 , go. One benefit of this construction is that the sidewall thickness of the window portion  84  gradually increases to either side between the radius  88  and the inner diameter  94  in the window portion, providing increased support against collapse of the window portion.  
         [0044]    Although the window joint  80  as depicted in FIG. 7A has the radius  88  greater than the radius go, it should be understood that the radii could be the same, or the radius  88  could be smaller than the radius go.  
         [0045]    In FIG. 7B the window joint  80  is depicted with the reduced sidewall thickness being due to a recess  100  formed externally on the window portion  84 . The recess  100  may be formed by milling, casting, forging, or any other method. Note that any shape of the recess  100  may be used in keeping with the principles of the invention.  
         [0046]    For example, instead of the recess  100  being curved about the circumference of the sidewall  82 , as depicted in FIG. 7B, the recess could be straight, etc. Although the recess  100  is depicted in FIG. 7B as extending only a portion of the length of the window joint  80 , the recess could extend the entire length of the window joint.  
         [0047]    In FIG. 8 another window joint  110  is depicted which is similar to the window joint  80 . However, the window joint  110  includes a window portion  116  having multiple recesses  112  formed externally thereon. Between the recesses  112 , circumferentially extending ribs  114  are disposed to support the reduced sidewall thickness resulting from the recesses.  
         [0048]    The window joint  110  may alternatively, or in addition, have one or more recesses formed internally thereon, such as the recess  66  described above, and if multiple internal recesses are used, supporting ribs may be formed between the internal recesses.  
         [0049]    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 contemplated by 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:
A reduced debris milled multilateral window. In a described embodiment, a window joint is constructed in a manner which reduces debris created when a window is milled therethrough. The window joint includes a generally tubular body having a sidewall, a window portion of the sidewall being configured for forming a window therethrough, and a thickness of the sidewall being reduced in the window portion.