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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The device described herein relates generally to wellhead assemblies, and in particular to provide a pressure seal for use with a wellhead assembly. 
         [0003]    2. Description of Related Art 
         [0004]    Wellheads used in the production of hydrocarbons extracted from subterranean formations typically comprise a wellhead assembly. Wellhead assemblies are attached at the opening of wellbores that intersect hydrocarbon producing formations. Wellhead assemblies also provide support for casing inserted into the wellbore. The casing lines the wellbore, thereby isolating the wellbore from the surrounding formation. Tubing typically lies concentric within the casing and provides a conduit for producing the hydrocarbons entrained within the formation. Wellhead assemblies also typically include production trees that connect to the upper end of the tubing and distribute the produced fluids. The tubing may be supported by a tubing hanger in the wellhead housing or in the production tree. 
         [0005]    Hardware within the wellheads for suspending the tubing and casing is arranged in a concentric arrangement. If the hanger is in the wellhead housing an isolation sub extends between the tubing hanger and a production bore in the production tree. Various seals are employed between the sub and its mating parts. 
       SUMMARY OF INVENTION 
       [0006]    The present disclosure includes a wellhead assembly comprising, a housing, a tubular within the housing, a support shoulder, and a seal assembly disposed between the tubular and the housing where the seal assembly is configured to engage the shoulder. The seal assembly comprises an annular seal having a lower portion, an upper portion an inner side wall and an outer side wall, wherein a portion of the outer surface of the seal is inverted. One of the lower portion or upper portion may be inverted. The wellhead assembly may further comprise a lower support ring formed for mating engagement with the lower portion and an upper support ring for mating engagement with the upper portion. The support rings may include raised portions for engagement with the inverted contours of the portions. Optionally, a vent may be formed through a wall of the annular seal and solid particles may be included in the annular seal. The annular seal is energized into sealing engagement between the tubular and housing in response to pressure applied to its outer surface. The annular seal may be a metal face seal. 
         [0007]    Also disclosed herein is a pressure energized seal assembly for sealing between a tubular and a corresponding member. In this embodiment the seal comprises an annular element configured to circumscribe the tubular member, the annular element having an upper portion, a lower inverted portion, and side walls, wherein the lower inverted portion is formed for pressure communication with a pressure source, and wherein pressure applied to the lower inverted portion urges the side walls into sealing engagement with the tubular and the corresponding member, and a supporting shoulder formed for compressive engagement with the upper portion. 
         [0008]    A method of sealing between a tubular and a housing in a wellhead assembly is further included herein, the method comprising, forming a shoulder within the wellhead assembly, wherein the shoulder circumscribes the tubular, disposing a seal assembly adjacent the shoulder, wherein the seal assembly comprises an annular sealing element circumscribing the tubular having an axis, an upper portion, an inner side wall, an outer side wall, a lower portion wherein the lower portion is inverted towards the axis, and putting a pressure source in pressure communication with the lower portion thereby imparting a compressive force onto the annular sealing element that outwardly urges the inner side wall into sealing engagement with the tubular and the outer side wall into sealing engagement with the housing. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  is a side cross sectional view of an embodiment of a wellhead assembly having a pressure energized seal. 
           [0011]      FIG. 2  is a side cross sectional view of an embodiment of a pressure energized seal. 
           [0012]      FIG. 3  is a side cross sectional view of another embodiment of a pressure energized seal. 
       
    
    
       [0013]    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 
       [0014]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. This invention may, however, be embodied 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 the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
         [0015]    Referring now to  FIG. 1 , one embodiment of a wellhead assembly having a pressure activated sealing element is provided.  FIG. 1  shows a cross sectional view of a wellhead assembly  5  comprising a production tree  7  mounted atop a wellhead housing  9 . A production bore  11  is formed within the production tree  7  that provides fluid communication with a production flow outlet  15  extending from the production tree  7 . In the embodiment shown, a portion of the production bore  11  extends laterally within the production tree  7  to the production flowline  15 . Control valves  13  are provided in the primary portion of the production bore  11  and also on the production outlet  15 . Selectively opening and closing the control valves  13  selectively allows wellbore fluid flow through the production outlet  15 . 
         [0016]    The wellhead housing  9  is attached to the production tree  7  by an external connector  17 . Included within the housing  9  are production tubing  22 , a tubing hanger  21 , and a casing hanger  23 . In the embodiment shown, the casing hanger  23 , which is a generally annular member, is coaxially secured within a portion of the housing  9  and supports a string of casing cemented in the well. Packoffs  19  (also referred to as casing hanger seals) are disposed between the outer circumference of the casing hanger  23  in a portion of the inner circumference of the housing  9 . An inner groove is shown formed within the casing hanger  23  formed to receive the annular tubing hanger  21 . Production tubing  22  extends downward from the tubing hanger  21  into the wellbore  3  from within the housing  9 . 
         [0017]    An annular isolation sleeve  27  coaxially resides within a portion of the production tree  7  on its upper end and extends downward terminating within the upper portion of the tubing hanger  21 . Wellbore flow from the production tubing  22  reaches the production bore  11  through the isolation sleeve  27 . Examples of seal assemblies  30  are shown circumscribing the isolation sleeve  27  on the sleeve  27  upper end and sleeve  27  lower end. For the purposes of reference and clarity, the term “upper” generally refers to a position closer to the top of the production tree  7 , and the term “lower” generally refers to a position closer to the bottom of the wellbore  5 . 
         [0018]    The seal assembly  30  on the sleeve  27  upper end resides in an upper pocket  26  formed in the sleeve  27 . The seal assembly  30  on the sleeve  27  lower end is in a lower pocket  28  formed in the sleeve  27 . In the embodiment of  FIG. 1 , seal assembly  30  provides a sealing function between the outer circumference of the isolation sleeve  27  and surrounding concentric hardware. Optionally the seal assembly  30  can be positioned in other concentric members of the wellhead assembly  5  or multiple seal assemblies  30  may be included within the wellhead assembly  5 . 
         [0019]      FIG. 2  illustrates one cross-sectional view of an embodiment of a seal assembly  30  for use within a wellhead assembly  5 . In this embodiment, the seal element  34  is in a pocket  26  formed by an upward facing shoulder  31  formed on the isolation sleeve  27  and a downward facing shoulder  36  on a threaded retainer ring  29 . Optionally and as noted above, the pocket  26  may be formed in any one of a number of the concentric members making up the wellhead assembly  5 . The seal assembly  30  of  FIG. 2  comprises a seal element  34  in the annular space between a pair of concentric wellhead assembly elements. In the embodiment shown, the seal element  34  is an annular member comprising a metal, elastically deformable outer wall  39  circumscribing an inner hollow space  43 . The hollow space  43  does not have to be sealed. The wall  39  forms a pressure barrier around the hollow space  43  whereby applying a force at a first location on the outer surface of the wall  39  causes an outward bulge on the wall  39  at a second location. The wall  39  may be formed from a pliable and elastic metal allowing it to deform under applied force and in some situations return to its original un-deformed shape. Optionally the seal element  34  may be a metal faced seal. 
         [0020]    With reference to the specific embodiment illustrated in  FIG. 2 , the seal element  34  comprises an upper portion  40 , a lower portion  42 , an outer sidewall  46 , and an inner sidewall  44 . The outer sidewall  46  is shown in contacting engagement with a wall of the tree production bore  11 . The inner wall of the pocket  26  in this embodiment is the isolation sleeve  27 , thus, the inner sidewall  44  is illustrated in contact with a cylindrical exterior surface of the isolation sleeve  27 . The upper and lower portions  40 ,  42  of the seal element  34  of  FIG. 2  are inverted wherein the mid section of these portions  40 ,  42  bows inward toward the axis of the seal element  34 . Inverting each of the upper and lower portions  40 ,  42  creates a “W” shaped cross section of these respective portions  40 ,  42 . Inverting the upper and lower portions  40 ,  42  fashions an inwardly protruding space on the outside of the wall  39  at the upper and lower portions  40 ,  42 . 
         [0021]    Thus in one example of use applying a distributed force, such as pressure, at the outer wall  39  where the lower portion  42  is inverted, the upward force on the inverted portion flexes the sidewalls  40 ,  46  radially inward and outward into contacting and sealing engagement with the respective walls of the pocket  26  between the wall of the tree production bore  11  and the wall of the isolation sleeve  27 . In use, normally the upper portion  40  of the seal  34  will be exposed to internal pressure in the tubing  22  via the clearance existing between the end of the isolation sleeve  27  and the tree production bore  11 . Since wellbore pressure normally exceeds ambient pressure existing below the seal assembly  30 , a pressure differential will form between the lower portion  42  and the upper portion  40 . The resulting pressure differential results in a force distribution that energizes the seal assembly  30  into sealing engagement between the isolation sleeve  27  and the tree production bore  11 . If a higher pressure occurred on the exterior of the isolastion sleeve  27 , the reverse would occur with pressure being exerted in the lower seal portion  42 . 
         [0022]    Optionally the seal assembly  30  may further comprise annular rigid conformed members  32  on top.  FIGS. 2 and 3  provide a cross sectional view of the conformed members  32 . The conformed members comprise a base  33 , wherein the base is shown roughly perpendicular to the axis of the wellhead assembly  5 . Perpendicularly extending from roughly the middle of the base  33  is a cylindrical vertical member  35  giving each member  32  a “T” shape in cross section. The members  32  include a cylindrical portion that inserts into one of the inverted portions and a cap that contacts one of the shoulders  31  or  36 . Conforming members  32  prevent the inverted portion from deflecting excessively when under pressure. 
         [0023]    In one optional embodiment, individual solid particles  38  may be included within the inner annulus of the seal  34 . These particles  38  provide a structural support to the seal element  34  without hindering the distribution of or transfer of pressure forces throughout the seal element  34 , thereby energizing the seal element  34 . The diameter of the particles  38  can vary or be substantially homogenous. In one embodiment, the particles  38  comprise a multitude of glass beads. Optionally the particles may comprise fine particles such as talc. The particles  38  would not completely fill all void space within the seal element  34 , rather room is left between the particles to allow inward and outward flexing of the seal element  34 . 
         [0024]    Another optional embodiment of the seal assembly  30   a  is provided in cross sectional view in  FIG. 3 . In this embodiment, a pressure vent  37  is shown formed through the wall  39   a.  The pressure vent allows equalization of the pressure within the seal element  34   a  and the surrounding area. This may be useful in situations when the area surrounding the seal element  34   a  may experience a pressure increase during use. If the pressure in the seal element  34   a  is substantially lower than its surrounding environment, the unequal pressure distribution may prevent it from expanding into sealing engagement as needed. 
         [0025]    The seal element  34  may be formed by combining together two metal W seals, the W seals may be seam welded along their edges at a line of symmetry. Additionally, the apex of the corresponding upper and lower portions  40 ,  42  may include added support for accommodating the presence of the ring members  32 . 
         [0026]    The member  32  may comprise other embodiments. For example, the cross section may resemble that of a triangle having rounded edges as well as a semi-circular member, where the apex of the semi-circle protrudes into the inverted portion of the seal  34 . In order to ensure proper sealing engagement of the seal member  34 , the design of the seal  34  should maintain an axial clearance between the apex of the inverted portions, even under related conditions. 
         [0027]    It is to be understood that the invention 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. For example, the seal element  34  may have a single side that is inverted and not both sides. Additionally, the inverted space may comprise a generally rectangular cross section and be positioned at any radial location on the outer surface of the wall  39 . The seal may be used in many other applications other than on an isolation sleeve of a wellhead assembly. In the drawings and specification, there have been disclosed illustrative embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.

Summary:
A seal for use in sealing on a wellhead assembly, where the seal is pressure energized and comprises an annular element having an inverted portion on its outer portion along its length. The seal may further include upper and lower ring members sandwiching the element between the ring members. The seal is formable by adjoining the open surfaces of two “W” shaped annular seals along their edges thereby forming an element responsive to pressure.