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CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of copending U.S. application Ser. No. 09/637,494 filed on Aug. 11, 2000, the disclosure of such copending application being hereby incorporated in its entirety herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates generally to operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a wellbore junction isolation method and associated apparatus.  
           [0003]    Wellbore junctions are formed at intersections of wellbores in a well. For example, a main or parent wellbore may have a branch or lateral wellbore drilled extending outwardly from an intersection between the main and branch wellbores. Of course, the main wellbore may extend below the intersection with the branch wellbore, for example, to intersect a formation from which it is desired to produce hydrocarbons into the main wellbore.  
           [0004]    Unfortunately, however, some wellbore junctions are not able to withstand substantial internal pressure applied thereto. For this reason, pressure within these wellbore junctions is limited to the fracture gradients of the respective formations in which the wellbore junctions are positioned. Thus, if stimulation operations, such as fracturing, must be performed for any formations below the wellbore junctions, expensive, time-consuming and/or complicated procedures must be used to prevent exceeding the fracture gradients of the formations at the wellbore junctions. Similar problems may also arise in other, non-stimulation types of well treatment such as, for example, circulation, washing and cleaning operations.  
           [0005]    Therefore it would be quite desirable to provide a method of isolating a wellbore junction which is convenient and easily performed, and which isolates the wellbore junction from pressures applied through the junction.  
         SUMMARY  
         [0006]    In carrying out the principles of the present invention, in accordance with an embodiment thereof, a method of isolating a wellbore junction is provided in which an isolating assembly is positioned at the wellbore junction and sealingly engaged with spaced apart wellbore portions. The assembly isolates at least one flow passage extending therethrough from the wellbore junction. Pressure may then be applied to the flow passage without that pressure being communicated to the wellbore junction.  
           [0007]    The isolating assembly may be operatively positioned in different orientations to protectively isolate the junction from fluid pressure being exerted within either selected one of two intersecting wellbores such as a main wellbore and an intersecting branch wellbore.  
           [0008]    In one aspect of the invention, the assembly includes two sealing devices which are sealingly engaged between the assembly and wellbore portions intersecting at the wellbore junction. The sealing devices are sealingly engaged straddling the wellbore junction. In this manner, an annulus formed between the assembly and the wellbore portions with which the sealing devices are sealingly engaged is divided into three portions, a middle one of which is in fluid communication with the wellbore junction.  
           [0009]    In another aspect of the invention, the other two annulus portions are in fluid communication with each other via another flow passage formed through the assembly. Thus, a circulation flowpath is formed between the annulus portions above and below the wellbore junction extending through the assembly.  
           [0010]    Preferably, the sealing structures are cup packers which permit the assembly to move longitudinally relative to the wellbore portions with which the assembly is sealingly engaged. This is particularly beneficial in subsea well applications in that the permitted assembly movement relative to the wellbore structure provides automatic compensation for rig heave. A variety of other types of sealing or slip joint structures which permit this compensation could alternatively be utilized if desired.  
           [0011]    In yet another aspect of the invention, the assembly is conveniently installed in a single trip into the well and may be positioned entirely within a main wellbore portion or operatively extended from the main wellbore into an associated intersecting branch wellbore portion. A particular embodiment described herein includes inner and outer tubular structures, with the sealing devices on the outer structure, and the inner structure sealed to the outer structure above and below the sealing devices.  
           [0012]    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  
       [0013]    [0013]FIG. 1 is a schematic view of a method embodying principles of the present invention;  
         [0014]    [0014]FIG. 2 is an enlarged scale schematic view of the method of FIG. 1, wherein an apparatus embodying principles of the present invention is being utilized in the method to isolate and protect a main/branch wellbore junction from fluid pressure being created within the main wellbore; and  
         [0015]    [0015]FIG. 3 is a schematic view similar to that in FIG. 2, but with the apparatus being used to isolate and protect the junction from fluid pressure being created within the branch wellbore. 
     
    
     DETAILED DESCRIPTION  
       [0016]    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. Specifically, the term “above” is used herein to designate a direction toward the earth&#39;s surface along a wellbore, and the term “below” is used herein to designate a direction away from the earth&#39;s surface along a wellbore, even though the wellbore may not be substantially vertical. 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.  
         [0017]    Referring now to FIG. 1, the method  10  is described herein as being performed in a subterranean well, which may be a subsea well, at a wellbore junction  16  formed by an intersection between a main wellbore  12  and a branch wellbore  14 . The wellbores  12 ,  14  are depicted in FIG. 1 as being cased or lined, but it is to be clearly understood that the principles of the invention may be incorporated into other methods performed in uncased or unlined wellbores. Furthermore, the principles of the invention are not limited to wellbore junctions formed between main and branch wellbores.  
         [0018]    AS illustrated in FIG. 1, the main wellbore  12  extends below the wellbore junction  16  to intersect a formation or zone  18 . It is desired to perform a stimulation operation, such as fracturing, on the formation  18  to thereby increase production of hydrocarbons therefrom. However, it is also desired not to apply excessive internal pressure to the wellbore junction  16 . Of course, the principles of the invention may be incorporated into other methods in which it is not desired to produce hydrocarbons from a formation, or in which it is not desired to perform stimulation operations.  
         [0019]    Referring additionally now to FIG. 2, a somewhat enlarged view of the wellbores  12 ,  14  and the junction  16  therebetween is representatively illustrated. Further steps of the method  10  have been performed in which a wellbore isolation assembly  20  is installed in the main wellbore  12  in a single trip. The assembly  20  is installed by conveying it into the wellbore  12  suspended from a work string  22 .  
         [0020]    The assembly  20  includes an outer tubular structure  24  and an inner tubular structure  26 . For example, the outer structure  24  may include one or more lengths of liner and the inner structure  26  may include one or more lengths of tubing. A flow passage  28  is formed through the inner structure  26  and another flow passage  30  extends in the space between the inner and outer structures  24 ,  26 .  
         [0021]    The inner structure  26  is connected and sealed to the outer structure  24  at a three-way tubular connector  32 . AS will be readily appreciated by those of skill in this particular art, a variety of other structures could alternatively be utilized to form this connection if desired. The connector  32  is also the point at which the work string  22  is attached to the assembly  20 . The inner structure  26  is also sealed to the outer structure at a seal  34 . Preferably, the seal  34  is an o-ring seal or packing received in a polished bore formed in the outer structure  24 , but other types of seals may be used without departing from the principles of the invention.  
         [0022]    The outer structure  24  further includes two ported subs  36 , 38  and two sealing devices  40 , 42 . The upper ported sub  36  is positioned between the upper sealing device  40  and the connector  32 . The lower ported sub  38  is positioned between the lower sealing device  42  and the seal  34 . When the sealing devices  40 ,  42  are sealingly engaged in the main wellbore  12  as depicted in FIG. 2, the upper ported sub  36  provides fluid communication between the flow passage  30  and an annulus  44  formed between the assembly  20  and the wellbore  12  above the upper sealing device  40  via one or more ports in a sidewall of the upper ported sub, and the lower ported sub  38  provides fluid communication between the flow passage  30  and the annulus  44  below the lower sealing device  42  via one or more ports in a sidewall of the lower ported sub.  
         [0023]    Preferably, the sealing devices  40 ,  42  are of the type well known to those skilled in the art as cup packers. However, other types of sealing devices may be utilized in keeping with the principles of the invention. In the method  10 , the packers  40 ,  42  are positioned so that they straddle the wellbore junction  16  and thereby seal between the assembly  20  and the wellbore  12  above and below its intersection with the wellbore  14 . In this manner, the annulus  44  is divided into three portions, a middle one of which is in fluid communication with the wellbore junction  16  external to the assembly  20 . The upper and lower annulus  44  portions are in fluid communication with the flow passage  30  via the ported subs  36 ,  38 .  
         [0024]    To perform a fracturing operation, a slurry (indicated by arrows  46 ) including fluid and proppant is pumped down the work string  22 , through the flow passage  28  and into the formation or zone  18 . Return circulation of fluid (indicated by arrows  48 ) is directed from the annulus  44  below the lower packer  42  to the flow passage  30  through the lower ported sub  38 , and then from the flow passage  30  to the annulus  44  above the upper packer  40  through the upper ported sub  36 .  
         [0025]    Note that the method  10  permits two flow passages  28 ,  30  to be positioned across the wellbore junction  16 , the flow passages being isolated from each other and from the junction in the assembly  20 , and permits the annulus  44  above and below the assembly to be isolated from the junction  16 . This result is accomplished in only one trip into the well.  
         [0026]    AS previously mentioned, the sealing devices  40 , 42  shown in FIG. 2 are preferably cup packers. The use of the cup packers  42 , 42  permits the isolation assembly  20  to move longitudinally relative to the main wellbore  12  within which it is sealingly received, in both uphole and downhole directions, as indicated by the double-ended arrows  50  in FIG. 2. This feature of the illustrated isolation assembly  20  is particularly advantageous in subsea well applications in that it automatically compensates for rig heave. Other types of seal structures, or various types of slip joint structures such as a pressure balanced bumper sub and associated length of drill collars (not shown) above the assembly  20  could alternatively be utilized to provide this rig heave compensation if desired. Such alternate sealing or slip joint structures, as well as the illustrated cup packers  40 , 42 , may be generally characterized as means associated with the assembly  20 , for automatically compensating for rig heave without breaking he seals between the sealing devices  40 , 42  and their associated wellbore portions.  
         [0027]    Instead of being positioned entirely in the main wellbore  12  and protectively isolating the main/branch wellbore junction  16  from fluid pressure forces being exerted within the main wellbore  12  below the junction  16 , the isolation assembly  20  may also be utilized, as schematically shown in FIG. 3, to isolate the junction  16  from fluid pressure being exerted in the branch wellbore  14  downhole from the junction  16 . TO effect this junction protection task a lower longitudinal portion of the assembly  20 , when being moved downhole to operatively position it, is deflected into the branch wellbore  14  using a suitable conventional deflection device such as the schematically depicted whipstock structure  52  shown in phantom in FIG. 3.  
         [0028]    With a lower longitudinal portion of the isolation assembly extended into and operatively installed within the branch wellbore  14  as schematically depicted in FIG. 3, the upper sealing structures  40  are sealingly engaged within the main wellbore  12  above the junction  16 , and the lower sealing structures  42  are sealingly engaged within the branch wellbore  14  outwardly from the junction  16 . The installed assembly  20  is operative to isolate the junction  16  from fluid pressure being exerted in the branch wellbore  14  downhole from the sealing devices  42 —for example in conjunction with carrying out a fluid fracturing stimulation process (similar to that carried out in the main wellbore  12  as previously described herein with respect to FIG. 2) in a formation (not shown) penetrated by the branch wellbore  14 .  
         [0029]    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 this specific embodiment, 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 wellbore junction isolation method and associated apparatus provide convenient isolation of a wellbore junction while permitting certain operations to be performed in a main or branch wellbore below the junction. In described embodiments, formations intersected by a main or branch wellbore below a wellbore junction may be stimulated by fracturing after installing an assembly at the wellbore junction, in a straddling and sealing relationship therewith, to isolate it from pressures applied during the fracturing operation. The illustrated isolation assembly may be installed in a single trip into the main wellbore.