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CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of co-pending U.S. Provisional Application Ser. No. 61/769,541 filed Feb. 26, 2013 the full disclosure of which is hereby incorporated by reference herein for all purposes. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The present disclosure relates in general to a wellhead assembly with an isolation sleeve through which a fracturing string with an associated hanger can be retrieved. 
     2. Description of Prior Art 
     Hydrocarbon producing wellbores are sometimes stimulated to increase the production of hydrocarbons. Hydraulic fracturing, or fracing, is one example of stimulation, which involves pressurizing all or a portion of the wellbore to improve communication between the surrounding formation and the wellbore. Generally, a fracturing fluid is pressurized at surface by a pump, which passes through a fracturing tree then enters a fracturing string. The fracturing string extends into the well and is supported by a string hanger in the wellhead. When the fracturing process is completed, a bridge plug is installed in the wellhead and the fracturing tree is replaced with a blowout preventer. A bored out tubing spool is utilized to allow full bore opening. The fracturing string and string hanger are retrieved through the blowout preventer. The blowout preventer and bored out tubing spool can then be removed and replaced with a standard tubing spool and a subsequent wellhead member, such as a tubing head. The bridge plug can be retrieved. 
     SUMMARY OF THE INVENTION 
     Embodiments of the system and method of this disclosure eliminate the steps of adding a bridge plug and blowout preventer to the casing head before retrieving the string hanger and fracturing string, as was previously required. The step of having to remove the bridge plug and blowout preventer after retrieving the string hanger and fracturing string are also eliminated. The need for a bored out spool is also eliminated. 
     Disclosed herein is an example of a wellhead assembly having a casing head mounted on a wellbore and a tubing head on the casing head. A main bore extends axially through the casing head and tubing head. A landing area in the main bore is profiled to selectively receive a fracturing string hanger. An annular isolation sleeve is coaxially set in the main bore. The isolation sleeve has an inner radius greater than an outer radius of the fracturing string hanger, so that the fracturing string hanger selectively passes through the isolation sleeve. 
     In an alternative embodiment, a wellhead assembly has a casing head mounted on a wellbore and a tubing head on the casing head. A main bore extends axially through the casing head and tubing head. An annular packoff assembly is disposed in the main bore, the annular packoff assembly having a landing area that is profiled to selectively receive a fracturing string hanger. An annular isolation sleeve is coaxially set in the main bore, a first portion of the annular isolation sleeve being located in the tubing head and a second portion of the annular isolation sleeve being located in the casing head. The annular isolation sleeve has an inner radius greater than an outer radius of the fracturing string hanger, so that the fracturing string hanger selectively passes through the isolation sleeve. An inner radius of the main bore of the tubing head is greater than an outer radius of the fracturing string hanger, so that the fracturing string hanger selectively passes through the tubing head. 
     In yet another alternative embodiment, a method for retrieving a fracturing string hanger from a wellhead assembly includes installing an annular isolation sleeve and tubing head on a casing head of a wellbore, the annular isolation sleeve having an inner radius greater than an outer radius of the fracturing string hanger. The fracturing string hanger is passed through the isolation sleeve. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side partial sectional view of an example embodiment of a wellhead assembly set over a wellbore and in accordance with the present invention. 
         FIG. 2  is a side partial sectional view of a portion of the wellhead assembly of  FIG. 1  configured for drilling the wellbore in accordance with the present invention. 
         FIG. 3  is a side partial sectional view of the wellhead assembly of  FIG. 1  with a fracturing tree and fracturing string in accordance with the present invention. 
         FIG. 4  is a side partial sectional view of an embodiment of the wellhead assembly of  FIG. 3 , having a tubing head in place of the fracturing tree and in accordance with the present invention. 
         FIG. 5  is a side partial sectional view of an embodiment of the wellhead assembly of  FIG. 4 , with the fracturing string being removed and in accordance with the present invention. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
     An example of a wellhead assembly  10  is shown in a side sectional view in  FIG. 1 , wherein the wellhead assembly  10  is mounted over a wellbore  12  that projects into a subterranean formation  14 . A base plate  16  (wellhead housing) makes up a lower portion of the wellhead assembly  10 , and sits on the surface  18  of the formation  14 . An annular casing head  20  mounts on top of the base plate  16 ; from which a length of conductor casing  22  extends downward into wellbore  12 . Inserted within conductor casing  22  is a string of intermediate casing  24  supported within casing head  20  on a casing hanger  26 , which is shown landed within casing head  20 . Optionally, an emergency casing hanger  28  (casing slip) is shown within casing head  20  and provides an alternative means of securing and supporting the intermediate casing  24 . Valve  30  is shown mounted on a side wall of casing head  20  and provides selective communication between the area ambient to wellhead assembly  10  and the inside of casing head  20  via port  32 . Similarly, a plug assembly  34  provides selective communication to the inside of casing head  20  via port  36 . An additional valve  38  mounts into the side wall of the casing head  20  and communicates with inside of casing head  20  via port  40 . 
     A flanged coupling sealingly engages an upper end of casing head  20  to an annular tubing head  42 , creating an interface  51  between the upper end of casing head  20  and the annular tubing head  42 . Similarly, a production tree  44  is flange mounted on an upper end of tubing head  42  that is distal from casing head  20 . A main bore  46  axially intersects casing head  20  and tubing head  42 . 
     An annular packoff assembly  48  is shown coaxially in the portion of main bore  46  that is within casing head  20 . A transition on an inner surface of annular packoff assembly  48  is formed where its inner radius projects outward and defines a circular groove and a landing area or shoulder  49  on its upper end. A lower end of an annular isolation sleeve  50  (isolation bushing) is illustrated mated with and landed on shoulder  49 . In an example, an inner radius of isolation sleeve  50  is at least as large as an inner radius of the annular packoff assembly  48  below shoulder  49 . An inner radius of the main bore  46  of tubing head  42  above isolation sleeve  50  is sized at least as large as an inner radius of the annular packoff assembly  48  below shoulder  49 . The inner radius of the main bore  46  of tubing head  42  is enlarged proximate to the casing head  20  and is as least as large as an outer radius of the isolation sleeve  50  to accommodate the annular isolation sleeve  50 . 
     In the example of  FIG. 1 , isolation sleeve  50  intersects the interface  51  between casing head  20  and tubing head  42  so that opposing portions of isolation sleeve  50  are respectively circumscribed by tubing head  42  and casing head  20  so that a first portion  41  of isolation sleeve  50  is located within tubing head  42  and a second portion  43  of isolation sleeve  50  is located in casing head  20 . Seals on an outer radius of isolation sleeve  50  provide sealing contact between isolation sleeve  50  and annular packoff assembly  48 . Additionally, seals also create a fluid and pressure barrier between the outer radius of isolation sleeve  50  and inner radius of casing head  42 . Thus the isolation sleeve  50  blocks communication between the main bore  46  and fluid lines shown formed through the side walls of casing head  20  and tubing head  42 . Isolation sleeve  50  also seals the main bore  46  from the interface  51  of casing head  20  and tubing head  42 . 
     Still referring to  FIG. 1 , a valve  52  shown registering with port  54  through a side wall of tubing head  42  provides selective communication to main bore  46  from outside wellhead assembly  10 . Additionally, plug assembly  56  which registers with port  58  in a side wall of tubing head  42  allows for selective communication into main bore  46 . Further shown in main bore  46  is a second annular packoff  60  that seals around a string of production tubing  62  shown coaxially within main bore  46  and extending downward into wellbore  12 . In an example, production tubing  62  communicates wellbore fluids produced from within wellbore  12  to the production tree  44 . A production tubing hanger  64  is shown mounted within production tree  44  and supports production string  62 . 
     The example of the wellhead assembly  10  of  FIG. 1  is functional and produces fluids from the wellbore  12 , wherein the wellbore  12  is completed. Referring now to the example of  FIG. 2 , the wellbore  12  is shown in a stage of being formed and prior to being completed. The wellhead assembly  10  of  FIG. 2  includes an annular wear bushing  66  coaxially mounted within main bore  46  and landed on the annular packoff assembly  48 . In this example, wear bushing  66  protects annular packoff assembly  48  from a drill string (not shown), that inserts through the main bore  46  and bores through the formation  14  to form the wellbore  12 . A blowout preventer  67 , which is shown coupled on an upper end of casing head  20  with a flange connection, can be used for pressure control of the wellbore  12 . Further in the example of  FIG. 2 , an upper end of the wear bushing  66  is coaxially disposed within a portion of the main bore  46  in the blowout preventer  67 , while its lower end is circumscribed by the annular packoff assembly  48 , which is within casing head  20 . 
       FIG. 3  illustrates in a side partial sectional view an example of fracturing the formation  14  adjacent the wellbore  12 . As shown, the blowout preventer  67  has been removed and replaced with a fracturing tree  68  coupled with the casing head  20 . An optional tubing head adapter  70  attached to a lower end of fracturing tree  68  mounts onto casing head  20  with a flange connection, thus facilitating connectivity of fracturing tree  68  with casing head  20 . Instead of production tubing  62  ( FIG. 1 ), a tieback or tubular fracturing string  72  extends into wellbore  12  from fracturing tree  68  and through casing head  20 . Supporting the fracturing string  72  is a string hanger  74  shown having an upper end coupled coaxially within tubing head adapter  70 . In this example, the wear bushing  66  ( FIG. 2 ) has been removed from within the annular packoff assembly  48  which allows a lock ring  76  of the string hanger  74  to selectively engage an inner profile  47  ( FIG. 5 ) of the annular packoff assembly  48 . An inner radius of the annular packoff assembly  48  is reduced at its lower end to form an upward facing or landing area or circular shelf  75  ( FIG. 5 ). A sloped downward facing surface  73  of string hanger  74  engages circular shelf  75 . In alternative embodiments, lock ring  76  may be omitted. An external radius of string hanger  74  is sized to fit within a portion of annular packoff assembly  48 . Fracturing string  72  delivers fracturing fluids into the wellbore  12 , which fracturing tree  68  controls a flow of the fracturing fluid to the fracturing string  72 . 
     Referring back to  FIG. 3 , after fracturing operations are complete, a back pressure valve  78  can be inserted within fracturing string  72  thereby blocking communication from within wellbore  12  into above the back pressure valve  78 . In this case, the back pressure valve  78 , string hanger  74 , and annular packoff assembly  48  are shown coaxially disposed within casing head  20 . Isolating pressure within the wellbore  12  with the back pressure valve  78  allows removal of tubing head adapter  70  and fracturing tree  68 . 
     As shown in  FIG. 4 , tubing head adapter  70  and fracturing tree  68  ( FIG. 3 ) have been removed from the wellhead assembly  10 . Instead of installing a bridge plug to casing head  20  and adding a blowout preventer to the upper end of casing head  20 , embodiments of this disclosure allow for tubing head  42  and isolation sleeve  50  to now be installed on the upper end of casing head  20 . Further in this example, the inner radius of isolation sleeve  50  exceeds the outer radius of the string hanger  74  and forms an annular gap  77  between these two members. This gap  77  allows for the required clearance for string hanger  74  and fracturing string  72  to be pulled out of casing head  20  and pass through tubing head  42 . Because the inner radius of the isolation sleeve  50  and tubing head  42  above isolation sleeve  50  is at least as large as an inner radius of the annular packoff assembly  48  below shoulder  49 , string hanger  74  and fracturing string  72  can pass through isolation sleeve  50  and tubing head  42 . Thus string hanger  74  and fracturing string  72  can be retrieved from within wellhead assembly  10  without removing tubing head  42 . 
     The example of  FIG. 5  illustrates in partial side sectional view an example of removing the string hanger  74  from within the wellhead assembly  10  using a landing joint  80  that is coupled to a lower end of a pipe string  82  with a threaded connection. The landing joint  80  is an annular member with an inner sleeve  84  Inner sleeve  84  inserts within an upper end of string hanger  74  and engages threads formed on an inner radial surface of string hanger  74 , allowing landing joint  80  and pipe string  82  to retrieve string hanger  74  and fracturing string  72  from within wellhead assembly  10 . In certain embodiments, landing joint  80  has an outer sleeve  86 . A lower end of outer sleeve  86  extends over lock ring  76  to disengage lock ring  76  from profile  47  and maintain lock ring  76  in a position that prevents lock ring  76  from moving radially outward as string hanger  74  is removed. 
     Because the inner radius of the isolation sleeve  50  and the inner radius of the main bore  46  are at least as large as the inner radius of the annular packoff assembly  48 , there is sufficient space within the isolation sleeve  50  and tubing head  42  to retrieve the string hanger  74  through tubing head  42 . Moreover, the strategically dimensioned isolation sleeve  50  eliminates the steps of adding a bridge plug and blowout preventer before retrieving the string hanger  74  and fracturing string  72  then removing the bridge plug and blowout preventer after retrieving the string hanger  74  and fracturing string  72 , as was previously required. 
     The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.

Summary:
A wellhead assembly includes a casing head, tubing head, and a production tree mounted on the tubing head. An isolation sleeve is set in a main bore of the wellhead assembly that extends across an interface between the casing and tubing heads so that a portion resides in each. The isolation sleeve is configured so that a fracturing string, and its associated hanger, can be retrieved through the isolation sleeve; which significantly reduces the time and steps required to conduct a fracturing operation in a well. Moreover, the present isolation sleeve can be used without changes to existing casing or tubing heads.