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BACKGROUND  
       [0001]     The present invention relates generally to operations performed in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a method of forming sealed wellbore junctions.  
         [0002]     Many systems have been developed for connecting intersecting wellbores in a well. Unfortunately, these systems typically involve methods which unduly restrict access to one or both of the intersecting wellbores, restrict the flow of fluids, are very complex or require very sophisticated equipment to perform, are time-consuming in that they require a large number of trips into the well, do not provide secure attachment between casing in the parent wellbore and a liner in the branch wellbore and/or do not provide a high degree of sealing between the intersecting wellbores.  
         [0003]     For example, some wellbore junction systems rely on cement alone to provide a seal between the interior of the wellbore junction and a formation surrounding the junction. In these systems, there is no attachment between the casing in the parent wellbore and the liner in the branch wellbore, other than that provided by the cement. These systems are acceptable in some circumstances, but it would be desirable in other circumstances to be able to provide more secure attachment between the tubulars in the intersecting wellbores, and to provide more effective sealing between the tubulars.  
       SUMMARY  
       [0004]     In carrying out the principles of the present invention, in accordance with an embodiment thereof, a method of forming a wellbore junction is provided which both securely attaches tubulars in intersecting wellbores and effectively seals between the tubulars. The method is straightforward and convenient in its performance, does not unduly restrict flow or access through the junction, and does not require an inordinate number of trips into the well.  
         [0005]     In one aspect of the invention, a method is provided for forming a wellbore junction which includes a step of expanding a member within a tubular structure positioned at an intersection of two wellbores. This expansion of the member may perform several functions. For example, the expanded member may secure an end of a tubular string which extends into a branch wellbore. The expanded member may also seal to the tubular string and/or to the tubular structure.  
         [0006]     In another aspect of the invention, the tubular string may be installed in the branch wellbore through a window formed through the tubular structure. An engagement device on the tubular string engages the tubular structure to secure the tubular string to the tubular structure. For example, the engagement device may be a flange which is larger in size than the window of the tubular structure and is prevented from passing therethrough, thereby fixing the position of the tubular string relative to the tubular structure.  
         [0007]     In yet another aspect of the invention, a whipstock may be used to drill the branch wellbore through the window in the tubular structure. Thereafter, the whipstock is used to install the tubular string in the branch wellbore. After installation of the tubular string, the whipstock may be retrieved from the parent wellbore, thereby permitting full bore access through the wellbore junction in the parent wellbore. The tubular string may be installed and the whipstock retrieved in only a single trip into the well using a unique tool string.  
         [0008]     In still another aspect of the invention, the window may be formed in the tubular structure prior to cementing the tubular structure in the parent wellbore. To prevent cement flow through the window, a retrievable sleeve is used inside the tubular structure. After cementing, the sleeve is retrieved from within the tubular structure.  
         [0009]     Various types of seals may be used between various elements of the wellbore junction. For example metal to metal seals may be used, or elements of the wellbore junction may be adhesively bonded to each other, etc.  
         [0010]     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  
       [0011]      FIG. 1  is a cross-sectional view of a method of forming a wellbore junction which embodies principles of the present invention and wherein a tubular structure has been cemented within a parent wellbore;  
         [0012]      FIG. 2  is an enlarged cross-sectional view of the method wherein a branch wellbore has been drilled through the tubular structure utilizing a whipstock positioned in the tubular structure;  
         [0013]      FIG. 3  is a cross-sectional view of the method wherein a tubular string is being installed in the branch wellbore;  
         [0014]      FIG. 4  is an enlarged cross-sectional view of the method wherein a sleeve is being expanded within the tubular structure to thereby secure and seal the tubular string to the tubular structure;  
         [0015]      FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 4 , showing the sleeve expanded within the tubular structure;  
         [0016]      FIGS. 6 &amp; 7  are cross-sectional views of the sleeve in its radially compressed and expanded configurations, respectively;  
         [0017]      FIGS. 8-13  are cross-sectional views of a second method embodying principles of the present invention;  
         [0018]      FIGS. 14-17  are cross-sectional views of a third method embodying principles of the present invention;  
         [0019]      FIGS. 18-20  are cross-sectional views of a fourth method embodying principles of the present invention;  
         [0020]      FIGS. 21-25  are cross-sectional views of a fifth method embodying principles of the present invention;  
         [0021]      FIGS. 26 &amp; 27  are cross-sectional views of a sixth method embodying principles of the present invention;  
         [0022]      FIGS. 28 &amp; 29  are cross-sectional views of a seventh method embodying principles of the present invention;  
         [0023]      FIG. 30  is a cross-sectional view of an eighth method embodying principles of the present invention; and  
         [0024]      FIGS. 31-35  are cross-sectional views of a ninth method embodying principles of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0025]     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 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.  
         [0026]     As depicted in  FIG. 1 , several steps of the method  10  have already been performed. A parent wellbore  12  has been drilled and a tubular structure  14  has been positioned in the parent wellbore. The tubular structure  14  is part of a casing string  16  used to line the parent wellbore  12 .  
         [0027]     It should be understood that use of the terms “parent wellbore” and “casing string” herein are not to be taken as limiting the invention to the particular illustrated elements of the method  10 . The parent wellbore  12  could be any wellbore, such as a branch of another wellbore, and does not necessarily extend directly to the earth&#39;s surface. The casing string  16  could be any type of tubular string, such as a liner string, etc. The terms “casing string” and “liner string” are used herein to indicate tubular strings of any type, such as segmented or unsegmented tubular strings, tubular strings made of any materials, including nonmetal materials, etc. Thus, the reader will appreciate that these and other descriptive terms used herein are merely for convenience in clearly explaining the illustrated embodiments of the invention, and are not used for limiting the scope of the invention.  
         [0028]     The casing string  16  also includes two anchoring profiles  18 ,  20  for purposes that are described below. The lower profile  20  may be an orienting latch profile, for example, a profile which serves to rotationally orient a device engaged therewith relative to the window  28 . The upper profile  18  may also be an orienting latch profile. Such orienting profiles are well known to those skilled in the art.  
         [0029]     A tubular shield  22  is received within the casing string  16 , and seals  24 ,  26  carried on the shield are positioned at an upper end of the tubular structure  14  and at a lower end of the anchoring profile  20 , respectively. The shield  22  is a relatively thin sleeve as depicted in  FIG. 1 , but it could have other shapes and other configurations in keeping with the principles of the invention.  
         [0030]     The shield  22  serves to prevent flow through a window  28  formed laterally through a sidewall of the tubular structure  14 . Specifically, the shield  22  prevents the flow of cement through the window  28  when the casing string  16  is cemented in the parent wellbore  12 . The shield  22  also prevents fouling of the lower profile  20  during the cementing operation, and the shield may be releasably engaged with the profile to secure it in position during the cementing operation and to enable it to be retrieved from the casing string  16  after the cementing operation, for example; by providing an appropriate convention latch on the shield.  
         [0031]     The shield  22  prevents cement from flowing out to the window  28  when cement is pumped through the casing string  16 . Other means may be used external to the tubular structure  14  to prevent cement from flowing in to the window  28 , for example, an outer membrane, a fiberglass wrap about the tubular structure, a substance filling the window and any space between the window and the shield  22 , etc.  
         [0032]     At this point it should be noted that the use of the terms “cement” and “cementing operation” herein are used to indicate any substance and any method of deploying that substance to fill the annular space between a tubular string and a wellbore, to seal between the tubular string and the wellbore and to secure the tubular string within the wellbore. Such substances may include, for example, various cementitious compositions, polymer compositions such as epoxies, foamed compositions, other types of materials, etc.  
         [0033]     At the time the casing string  16  is positioned in the wellbore  12 , but prior to the cementing operation, the tubular structure  14  is rotationally oriented so that the window  28  faces in a direction of a desired branch wellbore to extend outwardly from the window. Thus, the tubular structure  14  is positioned at the future intersection between the parent wellbore  12  and the branch wellbore-to-be-drilled, with the window  28  facing in the direction of the future branch wellbore. The rotational orientation may be accomplished in any of a variety of ways, for example, by engaging a gyroscopic device with the upper profile  18 , by engaging a low side indicator with the shield  22 , etc. Such rotational orienting devices (gyroscope, low side indicator, etc.) are well known to those skilled in the art.  
         [0034]     After the tubular structure  14  is positioned in the wellbore  12  with the window  28  facing in the proper direction, the casing string  16  is cemented in place in the wellbore. When the cementing operation is concluded, the shield  22  is retrieved from the casing string  16 .  
         [0035]     Referring additionally now to  FIG. 2 , an enlarged view of the method  10  is representatively illustrated wherein the shield  22  has been retrieved. A whipstock  30  or other type of deflection device has been installed in the tubular structure  14  by engaging keys, lugs or dogs  32  with the profile  20 , thereby releasably securing the whipstock in position and rotationally aligning an upper deflection surface  34  with the window  28 .  
         [0036]     The whipstock  30  also includes an inner passage  36  and a profile  38  formed internally on the passage for retrieving the whipstock. Of course, other means for retrieving the whipstock  30  could be used, for example, a washover tool, a spear, an overshot, etc.  
         [0037]     As depicted in  FIG. 2 , one or more cutting devices, such as drill bits, etc., have been deflected off of the deflection surface  34  and through the window  28  to drill a branch wellbore  40  extending outwardly from the window. As discussed above, the term “branch wellbore” should not be taken as limiting the invention, since the wellbore  40  could be a parent of another wellbore, or could be another type of wellbore, etc.  
         [0038]     Referring additionally now to  FIG. 3 , the method  10  is representatively illustrated wherein a, tubular string  42  has been installed in the branch wellbore  40 . The tubular string  42  may be made up substantially of liner or any other type of tubular material.  
         [0039]     As depicted in  FIG. 3 , the tubular string  42  includes an engagement device  44  for engaging the tubular structure  14  and securing an upper end of the tubular string thereto. The tubular string  42  also includes a flex or swivel joint  46  for enabling, or at least enhancing, deflection of the tubular string from the parent wellbore  12  into the branch wellbore  40 . Alternatively, or in addition, the swivel joint  46  permits rotation of an upper portion of the tubular string  42  relative to a lower portion of the tubular string in the rotational alignment step of the method  10  described below. The tubular string  42  is deflected off of the deflection surface  34  as it is conveyed downwardly attached to a tool string  48 .  
         [0040]     The tool string  48  includes an anchor  50  for releasable engagement with the upper profile  18 , a running tool  52  for releasable attachment to the tubular string  42 , and a retrieval tool  54  for retrieving the whipstock  30 . The running tool  52  may include keys, lugs or dogs for engaging an internal profile (not shown) of the tubular string  42 . The retrieval tool  54  may include keys, lugs or dogs for engagement with the profile  38  of the whipstock  30 .  
         [0041]     When the anchor  50  is engaged with the profile  18 , the tubular string  42  is rotationally aligned so that the engagement device  44  will properly engage the tubular structure  14  as further described below. In addition, the anchor  50  is preferably spaced apart from the engagement device  44  so that when the anchor is engaged with the profile  18  and a shoulder  56  formed on a tubing string  58  of the tool string  48  contacts the anchor, the engagement device is properly positioned in engagement with the tubular structure  14 .  
         [0042]     Specifically, the tubing string  58  is slidably received within the anchor  50 . When the shoulder  56  contacts the anchor  50 , the engagement device  44  is a predetermined distance from the anchor. This distance between the anchor  50  and the engagement device  44  corresponds with another predetermined distance between the profile  18  and the tubular structure  14 . Thus, when the tubular string  42  is being conveyed into the branch wellbore  40 , the engagement device  44  will properly engage the tubular structure  14  as the shoulder  56  contacts the anchor  50 .  
         [0043]     The running tool  52  may then be released from the tubular string  42 , the tool string  48  may be raised into the parent wellbore  12 , and then the retrieval tool  54  may be engaged with the profile  38  in the whipstock  30  to retrieve the whipstock from the parent wellbore. Note that the installation of the tubular string  42  and the retrieval of the whipstock  30  may thus be accomplished in a single trip into the well.  
         [0044]     The engagement device  44  is depicted in  FIG. 3  as a flange which extends outwardly from the upper end of the tubular string  42 . The engagement device  44  includes a backing plate or landing plate  60  which is received in an opening  62  formed through a sidewall of a guide structure  64  of the tubular structure  14 . Preferably, the opening  62  is complementarily shaped relative to the plate  60 , and this complementary engagement maintains the alignment between the tubular string  42  and the tubular structure  14 . For example, engagement between the plate  60  and the opening  62  supports the upper end of the tubular string  42 , so that an annular space exists about the upper end of the tubular string for later placement of cement therein.  
         [0045]     The guide structure  64  is more clearly visible in the enlarged view of  FIG. 2 . In this view it may also be seen that the opening  62  includes an elongated slot  66  at a lower end thereof. Preferably, the plate  6 o includes a downwardly extending tab  68  (see  FIG. 3 ) which engages the slot  66  and thereby prevents rotation of the engagement device  44  relative to the window  28 .  
         [0046]     The engagement device  44  is larger in size than the window  28 , and so the engagement device prevents the tubular string  42  from being conveyed too far into the branch wellbore  40 . The engagement device  44  thus secures the upper end of the tubular string  42  relative to the tubular structure  14 . Of course, other types of engagement devices may be used in place of the illustrated flange and backing plate, for example, an orienting profile could be formed on the tubular structure and keys, dogs or lugs could be carried on the tubular string  42  for engagement therewith to orient and secure the tubular string relative to the tubular structure.  
         [0047]     As depicted in  FIG. 3 , the engagement device  44  carries a seal  70  thereon which circumscribes the opening  62  and sealingly engages the guide structure  64 . The guide structure  64  carries seals  72 ,  74  thereon which sealingly engage above and below the window  28 . Thus, the tubular string  42  is sealed to the tubular structure  14  so that leakage therebetween is prevented.. The seals  70 ,  72 ,  74 , or any of them, may be elastomer seals, non-elastomer seals, metal to metal seals, expanding seals, and/or seals created by adhesive bonding, such as by using epoxy or another adhesive.  
         [0048]     Referring additionally now to  FIG. 4 , an enlarged view is representatively illustrated of the method  10  after the tubular string  42  is installed in the branch wellbore  40  and the whipstock  30  is retrieved from the well. Note that an alternatively constructed engagement device  44  is illustrated in  FIG. 4  which does not include the plate  60 . Instead, the flange portion of the engagement device  44  is received in the opening  62  and the engagement device is sealed to the tubular structure  14  about the window  28  using one or more seals  76 ,  78 ,  80  circumscribing the window. The seal  76  is an adhesive, the seal  78  is an o-ring and the seal  80  is a metal to metal seal.  
         [0049]     To further secure the tubular string  42  to the tubular structure  14 , a member  82  is expanded within the tubular structure using an expansion device  84 . As depicted in  FIG. 4 , the member  82  is a tubular sleeve having an opening  86  formed through a sidewall thereof. Of course, other expandable member shapes and configurations could be used in keeping with the principles of the invention.  
         [0050]     The opening  86  is rotationally aligned with an internal flow passage  88  of the tubular string  42 , for example, by engaging the expansion device  84  with the upper profile  18 . Then, the expansion device  84  is actuated to displace a wedge or cone go upwardly through the member  82 , thereby expanding the member outwardly. Such outward expansion also outwardly displaces seals  92 ,  94 ,  96 ,  98 ,  100  carried on the member.  
         [0051]     The seals  94 ,  96  sealingly engage the guide structure  64  above and below the opening  62 . The seals  92 ,  98  are metal to metal seals and sealingly engage the tubular structure  14  above and below the guide structure  64 . The seal  100  is an adhesive seal which circumscribes the passage  88  and sealingly engages the flange portion of the engagement device  44 . Of course, the seals  92 ,  94 ,  96 ,  98 ,  100 , or any of them, may be any type of seal, for example, elastomer, non-elastomer, metal to metal, adhesive, etc.  
         [0052]     After the member  82  is expanded, the expansion device  84  is retrieved from the well and the tubular string  42  is cemented within the branch wellbore  40 . For example, a foamed composition may be injected into the annulus radially between the tubular string  42  and the branch wellbore  40 . The foamed composition could expand in the annulus to fill any voids therein, and could expand to fill any voids about the structure  14  in the wellbore  12 .  
         [0053]     Note that the engagement device  44  is retained between the member  82  and the tubular structure  14 , thereby preventing upward and downward displacement of the tubular string  42 . In addition, where metal to metal seals are used, the expansion of the member  82  maintains a biasing force on these seals to maintain sealing engagement.  
         [0054]     Referring additionally now to  FIG. 5 , a partial cross-sectional view, taken along line  5 - 5  of  FIG. 4  is representatively illustrated. In this view, only the tubular string  42 , tubular structure  14 , guide structure  64  and expandable member  82  cross-sections are shown for clarity of illustration. From  FIG. 5 , it may be more clearly appreciated how the engagement device  44  is received in the guide structure  64 , and how expansion of the member  82  secures the engagement device in the tubular structure  14 .  
         [0055]     In addition, note that no separate seals are visible in  FIG. 5  for sealing between the engagement device  44  and the tubular structure  14  or expansion member  82 . This is due to the fact that  FIG. 5  illustrates an alternate sealing method wherein sealing between the engagement device  44  and each of the tubular structure  14  and expansion member  82  is accomplished by metal to metal contact between these elements.  
         [0056]     Specifically, expansion of the member  82  causes it to press against an interior surface the engagement device  44  circumscribing the passage  88 , which in turn causes an exterior surface of the engagement device to press against an interior surface of the tubular structure  14  circumscribing the window  28 . This pressing of one element surface against another when the member  82  is expanded results in metal to metal seals being formed between the surfaces. However, as mentioned above, any type of seal may be used in keeping with the principles of the invention.  
         [0057]     Referring additionally now to  FIGS. 6 and 7 , the expansion member  82  is representatively illustrated in its radially compressed and radially expanded configurations, respectively. In  FIG. 6 , it may be seen that the expansion member  82  in its radially compressed configuration has a circumferentially corrugated shape, that is, the member has a convoluted shape about its circumference. In  FIG. 7 , the member  82  is radially expanded so that it attains a substantially cylindrical tubular shape, that is, it has a substantially circular cross-sectional shape.  
         [0058]     Referring additionally now to  FIGS. 8-13 , another method embodying principles of the invention is representatively illustrated. In the method lo, a tubular structure  112  is interconnected in a casing string  114  and conveyed into a parent wellbore  116 . The tubular structure  112  preferably includes a tubular outer shield  118  outwardly overlying a window  120  formed through a sidewall of the tubular structure. The shield  118  is preferably made of a relatively easily drilled or milled material, such as aluminum.  
         [0059]     The shield  118  prevents cement from flowing outwardly through the window  120  when the casing string  114  is cemented in the wellbore  116 . The shield  118  also transmits torque through the tubular structure  112  from above to below the window  120 , due to the fact that the shield is rotationally secured to the tubular structure above and below the window, for example, by castellated engagement between upper and lower ends of the shield and the tubular structure above and below the window, respectively.  
         [0060]     The tubular structure  112  is rotationally aligned with a branch wellbore-to-be-drilled  122 , so that the window  120  faces in the radial direction of the desired branch wellbore. This rotational alignment may be accomplished, for example, by use of a conventional wireline-conveyed direction sensing tool (not shown) engaged with a key or keyway  124  having a known orientation relative to the window  120 . Other rotational alignment means may be used in keeping with the principles of the invention.  
         [0061]     In  FIG. 9  it may be seen that a work string  126  is used to convey a mill, drill or other cutting tool  128 , a whipstock or other deflection device  130  and an orienting latch or anchor  132  into the casing string  114 . The drill  128  is releasably attached to the whipstock  130 , for example, by a shear bolt  134 , thereby enabling the drill and whipstock to be conveyed into the casing string  114  in a single trip into the well.  
         [0062]     The anchor  132  is engaged with an anchoring and orienting profile  136  in the casing string  114  below the tubular structure  112 . Such engagement secures the whipstock  130  relative to the tubular structure  112  and rotationally orients the whipstock relative to the tubular structure, so that an upper inclined deflection surface  138  of the whipstock faces toward the window  120  and the desired branch wellbore  122 .  
         [0063]     Thereafter, the shear bolt  134  is sheared (for example, by slacking off on the work string  126 , thereby applying a downwardly directed force to the bolt), permitting the drill  128  to be laterally deflected off of the surface  138  and through the window  120 . The drill  128  is used to drill or mill outwardly through the shield  118 , and to drill the branch wellbore  122 . Of course, multiple cutting tools and different types of cutting tools may be used for the drill  128  during this drilling process.  
         [0064]     As depicted in  FIG. 9 , the casing string  114  has been cemented within the wellbore  116  prior to the drilling process. However, it is to be clearly understood that it is not necessary for the tubular structure  112  to be cemented in the wellbore  116  at this time. It may be desirable to delay cementing of the casing string  114 , or to forego cementing of the tubular structure  112 , as set forth in further detail below.  
         [0065]     In  FIG. 10  it may be seen that the branch wellbore  122  has been drilled extending outwardly from the window  120  of the tubular structure  112  by laterally deflecting one or more cutting tools from the parent wellbore  116  off of the deflection surface  138  of the whipstock  130 .  
         [0066]     In  FIG. 11  it may be seen that a liner string  140  is conveyed through the casing string  114 , and a lower end of the liner string is laterally deflected off of the surface  138 , through the window  120 , and into the branch wellbore  122 . An engagement device  142  attached at an upper end of the liner string  140  engages a tubular guide structure  144  of the tubular structure  112 , thereby securing the upper end of the liner string to the tubular structure. This engagement between the device  142  and the structure  112  forms a load-bearing connection between the casing string  114  and the liner string  140 , so that further displacement of the liner string into the branch wellbore  122  is prevented.  
         [0067]     Engagement between the device  142  and the structure  144  may also rotationally secure the device relative to the tubular structure  112 . For example, the slot  66  and tab  68  described above may be used on the device  142  and structure  144 , respectively, to prevent rotation of the device in the tubular structure  112 . Other types of complementary engagement, and other means of rotationally securing the device  142  relative to the tubular structure  112  may be used in keeping with the principles of the invention.  
         [0068]     Note that the device  142  is depicted in  FIG. 11  as a radially outwardly extending flange-shaped member which inwardly overlaps the perimeter of the window  120 . The device  142  inwardly circumscribes the window  120  and overlaps its perimeter, so if one or both mating surfaces of the device and tubular structure  112  are provided with a suitable layer of sealing material (such as an elastomer, adhesive, relatively soft metal, etc.), a seal  146  may be formed between the device and the tubular&#39;structure due to the contact therebetween. The device  142  may be otherwise shaped, and may be otherwise sealed to the tubular structure  112  in keeping with the principles of the invention.  
         [0069]     In  FIG. 12  it may be seen that the whipstock  136  and anchor  132  are retrieved from the well and a generally tubular expandable member  148  is conveyed into the tubular structure  112  and expanded therein. For example, the expandable member  148  may be expanded radially outward using the expansion device  84 , from a radially compressed configuration (such as that depicted in  FIG. 6 ) to a radially extended configuration (such as that depicted in  FIG. 7 ).  
         [0070]     The member  148  preferably has an opening  150  formed through a sidewall thereof when it is conveyed into the structure  112 . In that case, the opening  150  is preferably rotationally aligned with the window  120  (and thus rotationally aligned with an internal flow passage  152  of the liner string  140 ) prior to the member  148  being radially expanded. Alternatively, the member  148  could be conveyed into the structure  112  without the opening  150  having previously been formed, then expanded, and then a whipstock or other deflection device could be used to direct a cutting tool to form the opening through the sidewall of the member.  
         [0071]     Note that the method  110  is illustrated in  FIG. 12  as though the casing string  114  is cemented in the wellbore  116  at the time the member  148  is expanded in the structure  112 . However, the structure  112  could be cemented in the wellbore  116  after the member  148  is expanded therein.  
         [0072]     After being expanded radially outward, the member  148  preferably has an internal diameter D 1  which is substantially equal to, or at least as great as, an internal diameter D 2  of the casing string  114  above the structure  112 . Thus, the member  148  does not obstruct flow or access through the structure  112 .  
         [0073]     Note that a separate seal is not depicted in  FIG. 12  between the member  148  and the device  142  or the structure  112 . Instead, seals  154 ,  156  between the member  148  and the structure  112  above and below the guide structure  144  are formed by contact between the member  148  and the structure  112  when the member is expanded radially outward. For example, one or both mating surfaces of the member  148  and tubular structure  112  may be provided with a suitable layer of sealing material (such as an elastomer, adhesive, relatively soft metal, etc.), so that the seals  154 ,  156  are formed between the member and the tubular structure due to the contact therebetween. The member  148  may be otherwise sealed to the tubular structure  112  in keeping with the principles of the invention.  
         [0074]     To enhance sealing contact between the member  148  and the structure  112  and/or to ensure sufficient forming of the internal diameter D 1 , the structure may be expanded radially outward somewhat at the time the member is expanded radially outward, for example, by the expansion device  84 . This technique may produce some outward elastic deformation in the structure  112 , so that after the expansion process the structure will be biased radially inward to increase the surface contact pressure between the structure and the member  148 . Such an expansion technique may be particularly useful where it is desired for the seals  154 ,  156  to be metal to metal seals. If this expansion technique is used, it may be desirable to delay cementing the structure  112  in the wellbore  116  until after the expansion process is completed.  
         [0075]     Similarly, a seal  158  between the member  148  and the device  142  outwardly circumscribing the opening  150  is formed by contact between the member  148  and the device when the member is expanded radially outward. For example, one or both mating surfaces of the member  148  and device  142  may be provided with a suitable layer of sealing material (such as an elastomer, adhesive, relatively soft metal, etc.), so that the seal  158  is formed between the member and the device due to the contact therebetween. The member  148  may be otherwise sealed to the device  142  in keeping with the principles of the invention. Radially outward deformation of the structure  112  at the time the member  148  is expanded radially outward (as described above) may also enhance sealing contact between the member and the device  142 , particularly where the seal  158  is a metal to metal seal.  
         [0076]     The expandable member  148  secures the device  142  in its engagement with the guide structure  144 . It will be readily appreciated that inward displacement of the device  142  is not permitted after the member  148  has been expanded. Furthermore, in the event that the device  142  has not yet fully engaged the guide structure  144  at the time the member  148  is expanded (for example, the device could be somewhat inwardly disposed relative to the guide structure), expansion of the member will ensure that the device is fully engaged with the guide structure (for example, by outwardly displacing the device somewhat).  
         [0077]     Referring additionally now to  FIG. 13 , an alternate procedure for use in the method  110  is representatively illustrated. This alternate procedure may be compared to the illustration provided in  FIG. 8 . Instead of the outer shield  118 , the procedure illustrated in  FIG. 13  uses an inner generally tubular shield  160  having an inclined upper surface or muleshoe  162 . Although no separate seals are shown in  FIG. 13 , the inner shield  160  is preferably sealed to the tubular structure  112  above and below the guide structure  144 , so that cement or debris in the casing string  114  is not permitted to flow into the window  120  from the interior of the structure  112 . Preferably, the inner shield  160  is made of metal and is retrievable from within the structure  112  after the cementing process.  
         [0078]     To prevent cement or debris from flowing into the structure  112  through the window  120 , a generally tubular outer shield  164  outwardly overlies the window. Preferably, the outer shield  164  is made of a relatively easily drillable material, such as a composite material (e.g., fiberglass, etc.). A fluid  166  having a relatively high viscosity is contained between the inner and outer shields  162 ,  164  to provide support for the outer shield against external pressure, and to aid in preventing leakage of external fluids into the area between the shields. A suitable fluid for use as the fluid  166  is known by the trade name Glcogel.  
         [0079]     The muleshoe  162  provides a convenient surface for engagement by a conventional wireline-conveyed orienting tool (not shown). Such a tool may be engaged with the muleshoe  162  and used to rotationally orient the structure  112  relative to the branch wellbore-to-be-drilled  122 , since the muleshoe has a known radial orientation relative to the window  120 .  
         [0080]     After the structure  112  has been appropriately rotationally oriented, the casing string  114  may be cemented in the wellbore  116 , and the inner shield  16 o may then be retrieved from the well. After retrieval of the inner shield  160 , the method  110  may proceed as described above, i.e., the whipstock  130  and anchor  132  may be installed, etc. Alternatively, the inner shield  16 o may be retrieved prior to cementing the structure  112  in the wellbore  116 .  
         [0081]     Referring additionally now to  FIGS. 14-17 , another method  170  embodying principles of the invention is representatively illustrated. The method  170  differs from the other methods described above in substantial part in that a specially constructed tubular structure is not necessarily used in a casing string  172  to provide a window through a sidewall of the string. Instead, a window  176  is formed through a sidewall of the casing string  172  using conventional means, such as by use of a conventional whipstock (not shown) anchored and oriented in the casing string according to conventional practice.  
         [0082]     One of the many benefits of the method  170  is that it may be used in existing wells wherein casing has already been installed. Furthermore, the method  170  may even be performed in wells in which the window  176  has already been formed in the casing string  172 . However, it is to be clearly understood that it is not necessary for the method  170  to be performed in a well wherein existing casing has already been cemented in place. The method  170  may be performed in newly drilled or previously uncased wells, and in wells in which the casing has not yet been cemented in place.  
         [0083]     In  FIG. 15  it may be seen that a liner string  178  is conveyed into a branch wellbore  180  which has been drilled extending outwardly from the window  176 . At its upper end, the liner string  178  includes an engagement device  182  which engages the interior of the casing string  172  and prevents further displacement of the liner string  178  into the branch wellbore  180 . Engagement of the device  182  with the casing string  172  may also rotationally align the device with respect to the casing string.  
         [0084]     As depicted in  FIG. 15 , the device  182  is a flange extending outwardly from the remainder of the liner string  178 . The device  182  inwardly overlies the perimeter of the window  176  and circumscribes the window. Contact between an outer surface of the device  182  and an inner surface of the casing string  172  may be used to provide a seal  184  therebetween, for example, if one or both of the inner and outer surfaces is provided with a layer of a suitable sealing material, such as an elastomer, adhesive or a relatively soft metal, etc. Thus, the seal  184  may be a metal to metal seal. Other types of seals may be used in keeping with the principles of the invention.  
         [0085]     In an optional procedure of the method  170 , the liner string  178  (or at least the device  182 ) may be in a radially compressed configuration (such as that depicted in  FIG. 6 ) when it is initially installed in the branch wellbore  180 , and then extended to a radially expanded configuration (such as that depicted in  FIG. 7 ) thereafter. This expansion of the liner string  178 , or at least expansion of the device  182 , may be used to bring the device into sealing contact with the casing string  172 .  
         [0086]     In  FIG. 16  it may be seen that a generally tubular expandable member  186  is conveyed into the casing string  172  and aligned longitudinally with the device  182 . The member  186  has an opening  188  formed through a sidewall thereof. The opening  188  is rotationally aligned with the window  176  (and thus aligned with a flow passage  190  of the liner string  178 ).  
         [0087]     However, it is not necessary for the opening  188  to be formed in the member  186  prior to conveying the member into the well, or for the opening to be aligned with the window  176  at the time it is positioned opposite the device  182 . For example, the opening  188  could be formed after the member  186  is installed in the casing string  172 , such as by using a whipstock or other deflection device to direct a cutting tool to cut the opening laterally through the sidewall of the member.  
         [0088]     As depicted in  FIG. 16 , the member  186  has an outer layer of a suitable sealing material  192  thereon. The sealing material  192  may be any type of material which may be used to form a seal between surfaces brought into contact with each other. For example, the sealing material  192  may be an elastomer, adhesive or relatively soft metal, etc. Other types of seals may be used in keeping with the principles of the invention.  
         [0089]     In  FIG. 17  it may be seen that the member  186  is expanded radially outward, so that it now contacts the interior of the casing string  172  and the device  182 . Preferably, such contact results in sealing engagement between the member  186  and the interior surface of the casing string  172 , and between the member and the device  182 .  
         [0090]     Specifically, the sealing material  192  seals between the member  186  and the casing string  172  above, below and circumscribing the device  182 . The sealing material  192  also seals between the member  186  and the device  182  around the outer periphery of the opening  188 , that is, sealing engagement between the device  182  and the member  186  circumscribes the opening  188 . Thus, the interiors of the casing and liner strings  172 ,  178  are completely isolated from the wellbores  174 ,  180  external to the strings. This substantial benefit of the method  170  is also provided by the other methods described herein.  
         [0091]     As depicted in  FIG. 17 , the casing string  172  is outwardly deformed when the member  186  is radially outwardly expanded therein. At least some elastic deformation, and possibly some plastic deformation, of the casing string  172  outwardly overlying the member  186  is experienced, thereby recessing the member into the interior wall of the casing string.  
         [0092]     As a result, the inner diameter D 3  of the member  186  is substantially equal to, or at least as great as, the inner diameter D 4  of the casing string  172  above the window  176 . Preferably, during the expansion process, the inner diameter D 3  of the member  186  is enlarged until it is greater than the inner diameter D 4  of the casing string  172 , so that after the expansion force is removed, the diameter D 3  will relax to a dimension no less than the diameter D 4 .  
         [0093]     Thus, the method  170  does not result in substantial restriction of flow or access through the casing string  172 . This substantial benefit of the method  170  is also provided by other methods described herein.  
         [0094]     Outward elastic deformation of the casing string  172  in the portions thereof overlying the member  186  is desirable in that it inwardly biases the casing string, increasing the contact pressure between the mating surfaces of the member and the casing string, thereby enhancing the seal therebetween, after the member has been expanded. However, it is to be clearly understood that it is not necessary, in keeping with the principles of the invention, for the casing string  172  to be outwardly deformed, since the member  186  may be expanded radially outward into sealing contact with the interior surface of the casing string without deforming the casing string at all.  
         [0095]     When the member  186  is expanded, it also outwardly displaces the device  182 . This outward displacement of the device  182  further outwardly deforms the casing string  172  where it overlies the device. Elastic deformation of the casing string  172  overlying the device  182  is desirable in that it results in inward biasing of the casing string when the expansion force is removed. This enhances the seal  184  between the device  182  and the casing string  172 , and further increases the contact pressure on the sealing material between the device  182  and the member  186 .  
         [0096]     The method  170  is depicted in  FIG. 17  as though the casing string  172  is not yet cemented in the parent wellbore  174  at the time the member  186  is expanded therein. This alternate order of steps in the method  170  may be desirable in that it may facilitate outward deformation of the casing string  172  above and below the window  176 . The casing and/or liner strings  172 ,  178  may be cemented in the respective wellbores  174 ,  180  after the member  186  is expanded.  
         [0097]     Referring additionally now to  FIGS. 18-20 , another method  200  embodying principles of the invention is representatively illustrated. In  FIG. 18  it may be seen that a tubular structure  202  is cemented in a parent wellbore  204  at an intersection with a branch wellbore  206 . However, it is not necessary for the tubular structure  202  to be cemented in the wellbore  204  until later in the method  200 , if at all.  
         [0098]     The structure  202  is interconnected in a casing string  208 . The casing string  208  is rotationally oriented in the wellbore  204  so that a window  210  formed through a sidewall of the structure  202  is aligned with the branch wellbore  206 . Note that the window may be formed through the sidewall of the structure  202 , and that the branch wellbore  206  may be drilled, either before or after the structure is conveyed into the wellbore  204 .  
         [0099]     A liner string  212  is conveyed into the branch wellbore  206  in a radially compressed configuration. Even though it is radially compressed, a flange-shaped engagement device  214  at an upper end of the liner string  212  is larger than the window  210 , and so the device prevents further displacement of the liner string into the wellbore  206 . Preferably, this engagement between the device  214  and the structure  202  is sufficiently load-bearing so that it may support the liner string  212  in the wellbore  206 .  
         [0100]     An annular space  216  is provided radially between the device  214  and an opening  218  formed through the sidewall of a guide structure  220 . When the liner string  212  is expanded, the device  214  deforms radially outwardly into the annular space  216 . The liner string  212  is shown in its expanded configuration in  FIG. 19 .  
         [0101]     As depicted in  FIG. 20 , a generally tubular expandable member  222  is radially outwardly expanded within the structure  202 . An opening  224  formed through a sidewall of the member  222  is rotationally aligned with a flow passage of the liner string  212 . The opening  224  may be formed before or after the member  222  is expanded.  
         [0102]     Preferably, this expansion of the member  222  seals between the outer surface of the member and the inner surface of the structure  202  above and below the guide structure  220 , and seals between the member and the device  214 . Thus, the interiors of the casing and liner strings  208 ,  212  are isolated from the wellbores  204 ,  206  external to the strings. Alternatively, or in addition, a seal may be formed between the device  214  and the structure  202  circumscribing the window  210  where the structure outwardly overlies the device.  
         [0103]     Preferably the seals obtained by expansion of the member  222  are due to surface contact between elements, at least one of which is displaced in the expansion process. For example, one of both of the member  222  and structure  202  may have a layer of sealing material (e.g., a layer of elastomer, adhesive, or soft metal, etc.) thereon which is brought into contact with the other element when the member is expanded. Metal to metal seals are preferred, although other types of seals may be used in keeping with the principles of the invention.  
         [0104]     As depicted in  FIG. 20 , the tubular structure  202 , and the casing string  208  somewhat above and below the structure, are radially outwardly expanded when the member  222  is expanded. This optional step in the method  200  may be desirable to enhance access and/or flow through the structure  202 , enhance sealing contact between any of the member  222 , device  214 , structure  202 , etc. If the casing string  208  is outwardly deformed in the method  200 , it may be desirable to cement the casing string in the wellbore  204  after the expansion process is completed.  
         [0105]     Referring additionally now to  FIGS. 21-25  another method  230  embodying principles of the invention is representatively illustrated. As depicted in  FIG. 21 , an expandable liner string  232  is conveyed through a casing string  234  positioned in a parent wellbore  236 . A lower end of the liner string  232  is deflected laterally through a window  237  formed through a sidewall of a tubular structure  238  interconnected in the casing string  234 , and into a branch wellbore  240  extending outwardly from the window.  
         [0106]     An expandable liner hanger  242  is connected at an upper end of the liner string  232 . The liner hanger  242  is positioned within the casing string  234  above the window  237 .  
         [0107]     The liner string  232  is then expanded radially outward as depicted in  FIG. 22 . As a result of this expansion process, the liner hanger  242  sealingly engages between the liner string  232  and the casing string  234 , and anchors the liner string relative to the casing string. Another result of the expansion process is that a seal is formed between the liner string and the window  237  of the structure  238 . Thus, the interiors of the casing and liner strings  232 ,  234  are isolated from the wellbores  236 ,  240  external to the strings. The seal formed between the liner string  232  and the window  237  is preferably a metal to metal seal, although other types of seals may be used in keeping with the principles of the invention.  
         [0108]     A portion  244  of the liner string  232  extends laterally across the interior of the casing string  234  above a deflection device  246  positioned below the window  237 . As depicted in  FIG. 23 , a milling or drilling guide  248  is used to guide a drill, mill or other cutting tool  250  to cut through the sidewall of the liner string  232  at the portion  244  above the deflection device  246 . In this manner, access and flow between the casing string  234  above and below the liner portion.  244  through an internal flow passage  252  of the deflection device  246  is provided.  
         [0109]     Alternatively, the liner portion  244  may have an opening  254  formed therethrough. The opening  254  may be formed, for example, by water jet cutting through the sidewall of the liner string  232 . The opening  254  may be formed before or after the liner string  232  is conveyed into the well.  
         [0110]     Preferably, the opening  254  is formed with a configuration such that it has multiple flaps or inward projections  256  which may be folded to increase the inner dimension of the opening, e.g., to enlarge the opening for enhanced access and flow therethrough. As depicted in  FIG. 25 , the projections  256  are folded over by use of a drift or punch  258 , thereby enlarging the opening  254  through the liner portion  244 .  
         [0111]     The projections  256  are thus displaced into the passage  252  of the deflection device  246  below the liner string  232 . A seal may be formed between the liner portion  244  and the deflection device  246  circumscribing the opening  254  in this process of deforming the projections  256  downward into the passage  252 . Preferably, the seal is due to metal to metal contact between the liner portion  244  and the deflection device  246 , but other types of seals may be used in keeping with the principles of the invention.  
         [0112]     Referring additionally now to  FIGS. 26 &amp; 27 , another method  260  of sealing and securing a liner string  262  in a branch wellbore to a tubular structure  264  interconnected in a casing string in a parent wellbore is representatively illustrated. Only the structure  264  and liner string  262  are shown in  FIG. 26  for illustrative clarity.  
         [0113]     In  FIG. 26  it may be seen that the liner string  262  is positioned so that it extends outwardly through a window  266  formed through a sidewall of the structure  264 . The liner string  262  would, for example, extend into a branch wellbore intersecting the parent wellbore in which the structure  264  is positioned.  
         [0114]     An upper end  268  of the liner string  262  remains within the tubular structure  264 . To secure the liner string  262  in this position, a packer or other anchoring device interconnected in the liner string may be set in the branch wellbore, or a lower end of the liner string may rest against a lower end of the branch wellbore, etc. Any method of securing the liner string  262  in this position may be used in keeping with the principles of the invention.  
         [0115]     As depicted in  FIG. 26 , the upper end  268  is formed so that it is parallel with a longitudinal axis of the structure  264 . The upper end  268  may be formed in this manner prior to conveying the liner string  262  into the well, or the upper end may be formed after the liner string is positioned as shown in  FIG. 26 , for example, by milling an upper portion of the liner string after it is secured in position. If the upper end  268  is formed prior to conveying the liner string  262  into the well, then the upper end may be rotationally oriented relative to the structure  264  prior to securing the liner string  262  in the position shown in  FIG. 26 .  
         [0116]     In  FIG. 27  it may be seen that the upper end  268  of the liner string  262  is deformed radially outward so that it is received in an opening  270  formed through the sidewall of a generally tubular guide structure  272  in the tubular structure  264 . The opening  270  is rotationally aligned with the window  266 .  
         [0117]     The upper end  268  is deformed outward by means of a mandrel  274  which is conveyed into the structure  264  and deflected laterally toward the upper end of the liner string  262  by a deflection device  276 . The mandrel  274  shapes the upper end  268  so that it becomes an outwardly extending flange which overlaps the interior of the structure  264  circumscribing the window  266 , that is, the flange-shaped upper end  268  inwardly overlies the perimeter of the window.  
         [0118]     Preferably, a seal is formed between the flange-shaped upper end  268  and the interior surface of the structure  264  circumscribing the window  266 . This seal may be a metal to metal seal, may be formed by a layer of sealing material on one or both of the upper end  268  and the structure  264 , etc. Any type of seal may be used in keeping with the principles of the invention.  
         [0119]     The flange-shaped upper end  268  also secures the liner string  262  to the structure  264  in that it prevents further outward displacement of the liner string through the window  266 . After the deforming process is completed, the mandrel  274  and deflection device  276  may be retrieved from within the structure  264  and a generally tubular expandable member (not shown) may be positioned in the structure and expanded therein. For example, any of the expandable members  82 ,  148 ,  186 ,  222  described above may be used.  
         [0120]     After expansion of the member in the structure  264 , the member further secures the liner string  262  relative to the structure by preventing inward displacement of the liner string through the window  266 . Various seals may also be formed between the expanded member and the structure  264 , the flange-shaped upper end  268 , and/or the guide structure  272 , etc. as described above. Any types of seals may be used in keeping with the principles of the invention.  
         [0121]     Referring additionally now to  FIGS. 28 &amp; 29 , another method  280  of sealing and securing a liner string  282  in a branch wellbore to a tubular structure  284  interconnected in a casing string in a parent wellbore is representatively illustrated. In  FIG. 28 a  generally tubular expandable member  286  used in the method  280  is shown. The member  286  has a specially configured opening  288  formed through a sidewall thereof. The opening  288  may be formed, for example, by waterjet cutting, either before or after it is conveyed into the well.  
         [0122]     The configuration of the opening  288  provides multiple inwardly extending flaps or projections  290  which may be folded to enlarge the opening. As depicted in  FIG. 29 , the opening  288  has been enlarged by folding the projections  290  outward into the interior of the upper end of the liner string  282 . The projections  290  are deformed outward, for example, by a mandrel and deflection device such as the mandrel  274  and deflection device  276  described above, but any means of deforming the projections into the liner string  282  may be used in keeping with the principles of the invention.  
         [0123]     The projections  290  are deformed outward after the member  286  is positioned within the structure  284 , the opening  288  is rotationally aligned with a window  292  formed through a sidewall of the structure, and the member is expanded radially outward. Of course, if the opening  288  is formed after the member  286  is expanded in the structure  284 , then the rotational alignment step occurs when the opening is formed.  
         [0124]     Expansion of the member  286  secures an upper flange-shaped engagement device  294  relative to the structure  284 . Seals may be formed between the member  286 , structure  284 , engagement device  294  and/or a guide structure  296 , etc. as described above. Any types of seals may be used in keeping with the principles of the invention.  
         [0125]     Furthermore, deformation of the projections  290  into the liner string  282  may also form a seal between the member  286  and the liner string about the opening  288 . For example, a metal to metal seal may be formed by contact between an exterior surface of the member  286  and an interior surface of the liner string  282  when the projections  290  are deformed into the liner string. Other types of seals may be used in keeping with the principles of the invention.  
         [0126]     Preferably, the projections  290  are deformed into an enlarged inner diameter D 5  of the liner string  282 . This prevents the projections  290  from unduly obstructing flow and access through an inner passage  298  of the liner string  282 .  
         [0127]     Referring additionally now to  FIG. 30 , another method  300  of sealing and securing a liner string  302  in a branch wellbore to a tubular structure  304  interconnected in a casing string in a parent wellbore is representatively illustrated. The method  300  is similar to the method  280  in that it uses an expandable tubular member, such as the member  286  having a specially configured opening  288  formed through its sidewall. However, in the method  300 , the member  286  is positioned and expanded radially outward within the structure  304  prior to installing the liner string  302  in the branch wellbore through a window  306  formed through a sidewall of the structure.  
         [0128]     Expansion of the member  286  within the structure  304  preferably forms a seal between the outer surface of the member and the inner surface of the structure, at least circumscribing the window  306 , and above and below the window. The seal is preferably a metal to metal seal, but other types of seals may be used in keeping with the principles of the invention.  
         [0129]     After the member  286  has been expanded within the structure  304 , the projections  290  are deformed outward through the window  306 . This outward deformation of the projections  290  may result in a seal being formed between, the inner surface of the window  306  and the outer surface of the member  286  circumscribing the opening  288 . Preferably the seal is a metal to metal seal, but any type of seal may be used in keeping with the principles of the invention.  
         [0130]     After the projections  290  are deformed outward through the window  306 , the liner string  302  is conveyed into the well and its lower end is deflected through the window  306  and the opening  288 , and into the branch wellbore. The vast majority of the liner string  302  has an outer diameter D 6  which is less than an inner diameter D 7  through the opening  288  and, therefore, passes through the opening with some clearance therebetween. However, an upper portion  308  of the liner string  302  has an outer diameter D 8  which is preferably at least as great as the inner diameter D 7  of the opening  288 . If the diameter D 8  is greater than the diameter D 7 , some additional downward force may be needed to push the upper portion  308  of the liner string  302  through the opening  288 . In this case, the liner upper portion  308  may further outwardly deform the projections  290 , thereby enlarging the opening  288 , as it is pushed through the opening.  
         [0131]     Contact between the outer surface of the liner upper portion  308  and the inner surface of the opening  288  may cause a seal to be formed therebetween circumscribing the opening. Preferably, the seal is a metal to metal seal, but other seals may be used in keeping with the principles of the invention. An upper end  310  of the liner string  302  may be cut off as shown in  FIG. 30 , so that it does not obstruct flow or access through the structure  304 . Alternatively, the upper end  310  may be formed prior to conveying the liner string  302  into the well.  
         [0132]     Referring additionally now to  FIGS. 31-35 , another method  320  embodying principles of the invention is representatively illustrated. In  FIG. 31  it may be seen that a liner string  322  is conveyed through a casing string  324  in a parent wellbore  326 , and a lower end of the liner string is deflected laterally through a window  330  formed through a sidewall of the casing string, and into a branch wellbore  328 . The casing string  324  may or may not be cemented in the parent wellbore  326  at the time the liner string  322  is installed in the method  320 .  
         [0133]     The liner string  322  includes a portion  332  which has an opening  334  formed through a sidewall thereof. In addition, an external layer of sealing material  336  is disposed on the liner portion  332 . The sealing material  336  may be, for example, an elastomer, an adhesive, a relatively soft metal, or any other type of sealing material. Preferably, the sealing material  336  outwardly circumscribes the opening  334  and extends circumferentially about the liner portion  332  above and below the opening.  
         [0134]     The liner string  322  is positioned as depicted in  FIG. 31 , with the liner portion  332  extending laterally across the interior of the casing string  324  and the opening  334  facing downward. However, it is to be clearly understood that it is not necessary for the opening  334  to exist in the liner portion  332  prior to the liner string  322  being conveyed into the well. Instead, the opening  334  could be formed downhole, for example, by using a cutting tool and guide, such as the cutting tool  250  and guide  248  described above. As another alternative, the opening  334  may be specially configured (such as the opening  254  depicted in  FIG. 24 ), and then enlarged (as depicted for the opening  254  in  FIG. 25 ).  
         [0135]     In  FIG. 32  it may be seen that the liner string  322  is expanded radially outward. Preferably, at least the liner portion  332  is expanded, but the remainder of the liner string  322  may also be expanded. Due to expansion of the liner portion  332 , the outer surface of the liner portion contacts and seals against the inner surface of the window  330  circumscribing the window. The seal between the liner portion  332  and the window  330  is facilitated by the sealing material  336  contacting the inner surface of the window. However, the seal could be formed by other means, such as metal to metal contact between the liner portion  332  and the window  330 , without use of the sealing material  336 , in keeping with the principles of the invention.  
         [0136]     In  FIG. 33  it may be seen that the opening  334  is expanded to provide enhanced flow and access between the interior of the casing string  324  below the window  330  and the interior of the liner string  322  above the window. Expansion of the opening  334  also results in a seal being formed between the exterior surface of the liner portion  332  circumscribing the opening  334  and the interior of the casing string  324 . At this point, it will be readily appreciated that the interiors of the casing and liner strings  324 ,  322  are isolated from the wellbores  326 ,  328  external to the strings.  
         [0137]     Additional steps in the method  320  may be used to further seal and secure the connection between the liner and casing strings  322 ,  324 . In  FIG. 34 ,it may be seen that the liner string  322  within the casing string  324  is further outwardly expanded so that it contacts and radially outwardly deforms the casing string. The opening  334  is also further expanded, and a portion  338  of the liner string  322  may be deformed downwardly into the casing string  324  as the opening is expanded.  
         [0138]     This further expansion of the liner string  322 , including the opening  334 , in the casing string  324  produces several desirable benefits. The liner string  322  is recessed into the inside wall of the casing string  324 , thereby providing an inner diameter D 9  in the liner string which is preferably substantially equal to, or at least as great as, an inner diameter D 10  of the casing string  324  above the window  330 . The seal between the outer surface of the liner string  322  circumscribing the opening  334  and the inner surface of the casing string  324  is enhanced by increased contact pressure therebetween. In addition, another seal may be formed between the outer surface of the liner string  322  and the inner surface of the casing string  324  above the window  330 . Furthermore, the downward deformation of the portion  338  into the casing string  324  below the window  330  enhances the securement of the liner string  322  to the casing string. As described above, outward elastic deformation of the casing string  324  may be desirable to induce an inwardly biasing force on the casing string when the expansion force is removed, thereby maintaining a relatively high level of contact pressure between the casing and liner strings  324 ,  322 .  
         [0139]     In  FIG. 35  it may be seen that a generally tubular expandable member  340  having an opening  342  formed through a sidewall thereof is positioned within the casing string  324  with the opening  342  rotationally aligned with the window  330  and, thus, with a flow passage  344  of the liner string  322 . The member  340  extends above and below the liner string  322  in the casing string  324  and extends through the opening  334 . The member  340  is then expanded radially outward within the casing string  324 .  
         [0140]     Expansion of the member  340  further secures the connection between the liner and casing strings  322 ,  324 . Seals may be formed between the outer surface of the member  340  and the interior surface of the casing string  324  above and below the liner string  322 , and the inner surface of the liner string in the casing string. The seals are preferably formed due to contact between the member  340  outer surface and the casing and liner strings  324 ,  322  inner surfaces. For example, the seals may be metal to metal seals. The seals may be formed due to a layer of sealing material on the member  340  outer surface and/or the casing and liner strings  324 ,  322  inner surfaces. However, any types of seals may be used in keeping with the principles of the invention.  
         [0141]     The member  340  may be further expanded to further outwardly deform the casing string  324  where it overlies the member, in a manner similar to that used to expand the member  186  in the method  170  as depicted in  FIG. 17 . In that way, the member  340  may be recessed into the inner wall of the casing string  324  and the inner diameter D 11  of the member may be enlarged so that it is substantially equal to, or at least as great as, the inner diameter D 10  of the casing string. Due to outward deformation of the casing string  324  in the method  320 , whether or not the member  340  is recessed into the inner wall of the casing string, it may be desirable to delay cementing of the casing string in the parent wellbore  326  until after the expansion process is completed.  
         [0142]     Thus have been described the methods  10 ,  110 ,  170 ,  200 ,  230 ,  260 ,  280 ,  300 ,  320  which provide improved connections between tubular strings in a well. It should be understood that openings and windows formed through sidewalls of tubular members and structures described herein may be formed before or after the tubular members and structures are conveyed into a well. Also, it should be understood that casing and/or liner strings may be cemented in parent or branch wellbores at any point in the methods described above.  
         [0143]     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. For example, although certain seals have been described above as being carried on one element for sealing engagement with another element, it will be readily appreciated that seals may be carried on either or neither element. 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 sealed multilateral junction system provides fluid isolation between intersecting wellbores in a subterranean well. In a described embodiment, a method of forming a wellbore junction includes the steps of sealing a tubular string in a branch wellbore to a tubular structure in a parent wellbore. The tubular string may be secured to the tubular structure utilizing a flange which is larger in size than a window formed in the tubular structure. The flange may be sealed to the tubular structure about the window by a metal to metal seal or by adhering the flange to the tubular structure.