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BACKGROUND OF THE INVENTION 
     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 methods of casing multilateral wells and apparatus for use therein. 
     It is well known to those skilled in the art that it is very difficult to provide a seal between large diameter linings (casings and liners) for lateral or branch wellbores extending outwardly from an intersection between the lateral or branch wellbore and a main or parent wellbore. This due to the fact that it is generally deemed favorable to position a wellbore “junction” at the intersection of the wellbores to provide sealing for the formations intersected by the wellbores, to stabilize the wellbore intersection, to provide facilities for sealing and connecting linings extending into the wellbores, etc. In order for the junction to be installed at the wellbore intersection, it must pass through the main or parent wellbore and must, therefore, be smaller in diameter than the main or parent wellbore. 
     Several solutions to this problem have been proposed. One solution is to collapse the junction before it is installed in the well, and then inflate or otherwise expand the junction after it is positioned at the wellbore intersection. Unfortunately, it is difficult to ensure that the expanded junction will have a particular desired geometry after it is expanded, and the expanded junction, by its very nature, is susceptible to being again collapsed due to forces experienced in the well. 
     Another solution is to provide the junction with a relatively short lateral branch tube extending from a side thereof. The branch tube must be short so that the junction will fit within the diameter of the main or parent wellbore. However, this short lateral branch tube makes it difficult to reliably secure and seal casings or liners thereto. Furthermore, the fact that the branch tube extends laterally from the junction restricts its diameter (due to the necessity of transporting the junction through the main or parent wellbore), such that more than one standard casing size must be skipped in the transition to the lateral or branch wellbore as compared to a normal well casing program for normally pressurized formations. Still further, the wall thicknesses of the branch tube and the new lining of the main or parent wellbore take up space and restrict the inner diameter available through them. 
     A third solution is to create a junction by attaching two or more divergent tubes to the bottom of a casing. This solution utilizes an orienting cam type seal assembly that stabs into a seal bore located on top of each of these tubes, and extends back to surface to direct access to a desired tube. These tubes are limited in size by their divergence, their wall thickness, the upset created by the seal bore, and by the necessity that the inside of the casing must completely circumscribe the attached seal bores in order for an orienting cam type seal assembly to stab into a seal bore. Several standard casing sizes must be skipped in the transition from the casing to the lateral or branch wellbore as compared to a normal well casing program for normally pressurized formations. 
     Accordingly, it may be clearly seen that a method of casing multilateral wells is needed which resolves the problems in the art at present. Specifically, it would be highly advantageous to provide a method of casing multilateral wells in which it is not necessary to initially collapse the wellbore junction and then to expand the wellbore junction downhole, the wellbore junction has a known geometry which is, for example, well suited for connecting and sealing casings and/or liners thereto, the wellbore junction does not require the skipping of more than one standard casing size, and the wellbore junction conveniently utilizes standard wellbore and casing sizes to achieve these objectives. 
     SUMMARY OF THE INVENTION 
     In carrying out the principles of the present invention, in accordance with an embodiment thereof, a method of sealing the junction of subsea multilateral wells which have a large 20″ surface casing is provided in which a uniquely configured casing splitter forms a wellbore junction. The casing splitter is dimensioned so that only one standard casing size is skipped in providing the wellbore junction, and yet the casing splitter conveniently fits through standard main or parent wellbores, does not require collapse or expansion thereof, and provides ample facilities for connecting and sealing casing strings thereto. 
     In one aspect of the present invention, the casing splitter is dimensioned so that it fits within and through an existing 18.565″ wellhead bore and 20″ surface casing. This size of wellhead and surface casing is typical of those installed on the ocean floor for subsea production. In this manner, the casing splitter may be attached at a lower end of an otherwise standard 13⅜″ intermediate casing string. The casing splitter has two 8½″ diameter bores formed in a lower end thereof, of a length sufficient to anchor and seal subsequent liners therein, and which share and are separated by a single wall thickness of preferably no more than 0.435″. The splitter is attached to the bottom of the 13⅜″ casing string before it is installed. 
     In another aspect of the present invention, wellbores may be drilled through one or more lower connections of the casing splitter. Since only one standard casing size is skipped when utilizing the casing splitter, multiple standard liner sizes are available for use in wellbores drilled below the casing splitter. Because the lower bores of the casing splitter are parallel, they may be of any convenient length to anchor the liners therein. Multiple liner strings in the wellbores drilled through the casing splitter would typically be telescoped in progressively stepped down wellbore diameters. 
     In a further aspect of the present invention, the casing splitter may have a 13⅜″ threaded casing connection formed on an upper end thereof, and a 9⅝″ flush joint threaded casing connection formed on a lower end thereof. Recesses may be machined outwardly from one or both of the 8½″ bores orthogonally to the other 8½″ bore to aid in locating and securing the subsequent liners. 
     In a still further aspect of the present invention, access to individual laterals may be achieved using a bent sub, or a diverter may be disposed in the casing splitter. The diverter may be positioned to permit access between the 13⅜″ casing connection and one of the 8½″ bores while preventing access between the 13⅜″ casing connection and the other bore. A latch or mule-shoe may be placed on the outside of the diverter, and a matching profile machined on the inside of the casing splitter, to assist in orientation. 
     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 a representative embodiment of the invention hereinbelow and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a method of casing a multilateral well, the method embodying principles of the present invention; 
     FIG. 2 is a cross-sectional view of a casing splitter utilized in the method of FIG. 1; and 
     FIG. 3 is a cross-sectional view of a diverter which may be installed in the casing splitter of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     Representatively 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 maybe utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. 
     In the method  10 , advantageous use is made of standard wellbore and casing sizes to provide a wellbore junction having characteristics unrealized in the past. Specifically, a casing splitter  12  is positioned in a nominal 22″ wellbore  14 . This wellbore is underreamed below a 20″ casing string in a typical casing program, for example, the 20″ casing string  16  depicted in FIG.  1 . The components of this casing string on a subsea well generally allow passage of equipment up to 18.565″ in diameter. Specifically, a wellhead  17  in this type of drilling program typically has a 18.565″ restriction therein. 
     The present inventor has discovered that, by configuring the casing splitter  12  so that it fits within the 18.565″ limitation, it will connect to a 13⅜″ casing string  18  on top and may have two linings  20 ,  22  connected on bottom extending outwardly from two 8½″ bores formed in the casing splitter, which share a common wall thickness, such that only one standard casing size is skipped in providing the wellbore junction. This is a significant advantage over prior wellbore junction methods. 
     The ability to fit the casing splitter  12  within the 18.565″ diameter-limited wellbore  14  is enhanced by providing for the lower linings  20 ,  22  to extend parallel to each other initially as they extend downwardly away from the casing splitter. In this manner, it is not necessary to have a branch tube exiting laterally from the casing splitter  12 , or for the linings  20 ,  22  to initially diverge from each other at all, which would require a larger, nonstandard wellbore size or a reduction in casing size to accommodate. In addition, the use of a flush joint casing connection in the lower end of one of the casing splitter  12  lower bores permits a casing string, such as a 9⅝″ casing string, to be installed in the well below the casing splitter, if desired. Sealing hangers (not shown) are used to connect one or both of the linings  20 ,  22  to the casing splitter. 
     Thus, in the method  10 , the casing splitter  12  is connected to a lower end of the 13⅜″ intermediate casing string  18  and positioned therewith in the underreamed wellbore  14  drilled below the 20″ surface casing string  16 . At this point, one casing string  20  may be already connected to the lower end of the casing splitter  12  (e.g., using flush joint threaded connections), or the liner strings may be connected later using conventional hangers, etc. 
     If the casing splitter  12  is run without the lower linings  20 ,  22 , then cementing equipment, such as a float shoe, etc., maybe installed in the lower end of the casing splitter. The casing splitter  12  is then cemented in the wellbore  14  with the remainder of the 13⅜″ casing string  18 . The liner strings  20 ,  22  are then installed in wellbores  24 ,  26  drilled through the lower end of the casing splitter  12 . If the lower casing string  20  is connected to the casing splitter  12  when it is initially installed in the wellbore  14 , the casing string  20  may also be cemented into the wellbore  14  along with the casing splitter. 
     As depicted in FIG. 1, the liner strings  20 ,  22  have been installed in the wellbores  24 ,  26  drilled through the lower end of the casing splitter  12 . One or both of the wellbores  24 ,  26  may deviate from vertical, for example, the wellbore  26  as shown in FIG.  1 . Additional smaller diameter wellbores  28 ,  30  may be drilled below the liner strings  20 ,  22 , and liners  32 ,  34  or other liner strings may be installed in the wellbores. Thus, progressively smaller wellbores may be drilled below the liner strings  20 ,  22 , with additional liner strings being telescopingly received in these wellbores. 
     FIG. 1 illustrates the liner  32  in the wellbore  28 , which intersects a formation or zone  36 , and the liner  34  in the wellbore  30 , which intersects the same or another formation or zone  38 . The use of the casing splitter  12  in the method  10  provides a stabilized, secure, collapse resistant, sealed and large diameter junction for production from, or injection into, each of the zones  36 ,  38 . This result is accomplished using standard wellbore sizes and standard casing sizes, with only one standard casing size being skipped in providing the junction between the three wellbores  14 ,  24 ,  26 . 
     Note that the method  10  is depicted in FIG. 1 as being performed in a subsea well environment, but it is to be clearly understood that surface wells may also benefit from the principles of the present invention. In addition, although the lower liner strings  20 ,  22  are preferably standard 7 or 7⅝″ liner strings as described above, other sizes of casing or liner strings may be utilized, without departing from the principles of the present invention. For example, smaller sized liner strings may be used in some circumstances if the liner strings are installed through the casing splitter  12 . 
     Referring additionally to FIG. 2, a cross-sectional view of the casing splitter  12  embodying principles of the present invention is representatively illustrated. The casing splitter  12  includes a cylindrical housing  40  having an outer diameter somewhat smaller than the inside diameter of a nominal 18.565″ wellhead. A standard 13⅜″ threaded casing connection  42  is formed in the top of the housing  40  at an end of a main bore  44  formed in the housing. 
     Two parallel 8½″ diameter bores  46 ,  48  are formed extending upwardly from the bottom of the housing  40 . The bores  46 ,  48  preferably share a common wall  52  having a thickness of no more than 0.435″. A flush joint threaded connection  50  may be provided in the case described above wherein a casing string is initially connected to the bottom of the casing splitter  12 . 
     If the liner strings are connected to the casing splitter  12  after it is installed in the well, hangers or other devices may be used to hang the liner strings in the bores  46 ,  48 , without the need for threaded connections. Note that a particular benefit of the parallel arrangement of the bores  46 ,  48  in that case is that the bores may be made as long as needed to provide a secure and adequately sealed connection between the casing splitter and the liner strings. Recesses  53  may be machined outwardly from one or both of the 8½″ bores orthogonally to the other 8½″ bore to aid in locating and securing the subsequent liners. Profiles  55 ,  57  capable of supporting the liner strings may be formed in the bores  46 ,  48 . 
     To provide a transition between the bore  44  and each of the bores  46 ,  48 , additional bores  54 ,  56  are formed in the housing  40 . The bore  54  extends between a centerline of the bore  44  and a centerline of the bore  46 . The bore  56  extends between the centerline of the bore  44  and a centerline of the bore  48 . The bores  54 ,  56  permit passage of tools, equipment, etc. conveniently through the housing  40  between the casing and liner strings connected thereto. 
     Note that the bores  54 ,  56  may not be necessary, for example, if the bores  46 ,  48  intersect the bore  44  as depicted in FIG.  2 . However, it also is not necessary for the bores  46 ,  48  to intersect the bore  44 , in keeping with the principles of the present invention. 
     Referring additionally now to FIG. 3, a diverter  58  which may be used in conjunction with the casing splitter  12  is representatively illustrated. The diverter  58  includes a cylindrical housing  60  having an outside diameter which permits it to be installed within the bore  44  of the housing  40 . 
     An inclined bore  62  is formed in the housing  60 . The bore  62  is centered in the housing  60  at the top, but is offset from the center of the housing at the bottom. In this manner, when the diverter  58  is installed in the bore  44  of the housing  40 , the bore  62  will be aligned with the bore  44  on top, and will be aligned with one of the bores  54 ,  56  on bottom. 
     Thus, when the diverter  58  is installed in the housing  40 , it is oriented so that its bore  62  is rotationally aligned with a selected one of the bores  54 ,  56 . The selected one of the bores  54 ,  56 , combined with the bore  62 , then forms a path between the bore  44  and the corresponding selected one of the bores  46 ,  48 . The diverter  58 , therefore, permits access between the casing string connected to the top of the casing splitter  12  and a selected one of the liner or casing strings connected to the bottom of the casing splitter. A conventional latch or mule-shoe may be placed on the outside of the diverter  58 , and a matching profile machined on the inside of the housing  40 , to assist in orientation. 
     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.

Summary:
A method of casing multilateral wells and associated apparatus provide increased diameter in wellbores drilled below a casing splitter. In a described embodiment, a casing splitter is positioned in a subsea wellbore with a minimum restriction of 18.565″, with the casing splitter attached at a lower end of a 13⅜″ casing string. Two 8½″ bores extend through a lower end of the casing splitter, whereby only one standard casing size is skipped due to the wellbore junction formed by the casing splitter.