A metal to metal seal for downhole tools. A sealing device includes a metal seal. A nonmetal seal may be used to bias the metal seal in a radial direction in response to a pressure differential applied to the sealing device. A well tool includes a housing assembly, a closure member and the sealing device. Both of the metal and nonmetal seals contact a selected one of the housing assembly and closure member when the closure member blocks flow through the housing assembly. A method of sealing between the housing assembly and closure member includes the step of displacing the closure member to relieve the pressure differential, the metal seal continuing to seal against the pressure differential until the nonmetal seal no longer seals between the housing assembly and the closure member.

BACKGROUND 
       [0001]    The present invention relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a metal seal for downhole tools. 
         [0002]    Metal seals are sometimes used to seal between structures in well tools, and in equipment used in other environments. However, several problems are frequently encountered when metal seals are used. For example, metal seals require very smooth and clean surfaces to seal against, and most metals can only be elastically deformed to a limited extent (which thereby limits the biasing force available from elastically deforming a metal seal), etc. 
         [0003]    Elastomeric and other types of nonmetal seals may provide the ability to seal against irregular and unclean surfaces, and may provide sufficient resilient biasing force for urging the seals against the surfaces. However, nonmetal seals tend to degrade rapidly when used in dynamic configurations, i.e., where the seal must contact a moving surface while sealing against a pressure differential, or where the seal loses contact with the surface while the pressure differential still exists across the seal. 
         [0004]    Therefore, it may be seen that improvements are needed in the art of sealing devices. 
       SUMMARY 
       [0005]    In carrying out the principles of the present invention, a sealing device is provided which solves at least one problem in the art. One example is described below in which the sealing device includes both a metal seal and an elastomer seal. Another example is described below in which elastomer seals are used to energize metal seals in response to pressure differentials in different directions. 
         [0006]    In one aspect of the invention, a sealing device is provided. The sealing device includes at least one metal seal. A nonmetal seal may be used to bias the metal seal in a radial direction in response to a pressure differential applied to the sealing device. 
         [0007]    In another aspect of the invention, a well tool is provided which includes a housing assembly and a closure member. A sealing device is used for sealing between the housing assembly and closure member. The sealing device includes at least one metal seal and at least one nonmetal seal. Both of the metal and nonmetal seals contact one of the housing assembly and closure member when the closure member blocks flow through the housing assembly. 
         [0008]    A method of sealing between a housing assembly and a closure member is also provided by the invention. The method includes the steps of: providing a sealing device including at least one metal seal and at least one nonmetal seal; applying a pressure differential across the sealing device while the sealing device seals between the housing assembly and the closure member; and displacing the closure member to relieve the pressure differential. The metal seal continues to seal against the pressure differential until the nonmetal seal no longer seals between the housing assembly and the closure member. 
         [0009]    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, in which similar elements are indicated in the various figures using the same reference numbers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic partially cross-sectional view of a well system embodying principles of the present invention; 
           [0011]      FIG. 2  is an enlarged scale cross-sectional view of a closure mechanism of a flow control device in the well system; 
           [0012]      FIG. 3  is a further enlarged scale cross-sectional view of a sealing device for use in the closure mechanism; 
           [0013]      FIG. 4  is an enlarged scale cross-sectional view of an alternate configuration of the closure mechanism; and 
           [0014]      FIG. 5  is a further enlarged scale cross-sectional view of an alternate configuration of the sealing device for use in the closure mechanism of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    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. The embodiments are described merely as examples of useful applications of the principles of the invention, which is not limited to any specific details of these embodiments. 
         [0016]    In the following description of the representative embodiments of the invention, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. In general, “above”, “upper”, “upward” and similar terms refer to a direction toward the earth's surface along a wellbore, and “below”, “lower”, “downward” and similar terms refer to a direction away from the earth's surface along the wellbore. 
         [0017]    Representatively illustrated in  FIG. 1  is a well system  10  which embodies principles of the present invention. In the well system  10 , a tubular string  12  (such as a production tubing string) is positioned in a wellbore  14  lined with casing  16 . The tubular string  12  includes well tools  18 ,  20 . 
         [0018]    The well tool  18  is a packer, and the well tool  20  is a flow control device (such as a valve or choke). The packer provides an annular seal between the tubular string  12  and the casing  16 , and the flow control device regulates fluid communication between the interior of the tubular string and an annulus  22  formed between the tubular string and the casing. The flow control device includes a closure mechanism  24  which is operated to regulate flow. 
         [0019]    At this point, it should be reiterated that the invention is not limited to any of the details of the well system  10  described herein. For example, it is not necessary for the invention to be used in a wellbore, in a well tool, in a cased wellbore, in a flow control device, in a tubular string, etc. The closure mechanism  24  could, as another example, be used in a hydraulic setting device of the packer  18 , or could be used in another type of well tool. Thus, it should be clearly understood that the well system  10  is only a single example of a wide variety of uses for the principles of the invention. 
         [0020]    Referring additionally now to  FIG. 2 , an enlarged scale cross-sectional view of a portion of the well tool  20  is representatively illustrated. In this view it may be seen that the closure mechanism  24  includes a tubular closure member  26  which is displaced relative to a housing assembly  28  to thereby regulate flow through openings  30  in the housing assembly. 
         [0021]    To completely block flow through the openings  30 , the closure member  26  engages a sealing device  34 . The sealing device  34  operates to provide a seal between the closure member  26  and the housing assembly  28  to thereby prevent flow through the openings  30 . 
         [0022]    In one important feature of the sealing device  34 , both metal seals  32   a ,  32   b  and nonmetal seals  36   a ,  36   b  are included in the device. These seals  32   a ,  32   b ,  36   a ,  36   b  contact and seal against the closure member  26  when the closure member is in the position depicted in  FIG. 2 . However, it will be appreciated that the sealing device  34  could be carried on, and displace with, the closure member  26 , so that the seals  32   a ,  32   b ,  36   a ,  36   b  could contact and seal against the housing assembly  28  when the closure member is in the position depicted in  FIG. 2 , if desired. 
         [0023]    Note that a separate seal  38  is shown sealing between the sealing device  34  and the housing assembly  28 . However, it will be appreciated that this seal  38  could be incorporated into the sealing device  34 , if desired. For example, the nonmetal seals  36   a ,  36   b  could extend further radially outward into sealing contact with the housing assembly  28 , and/or a seal could be formed by metal to metal contact between the housing assembly and an outer ring  40  of the device  34 . 
         [0024]    Referring additionally now to  FIG. 3 , a further enlarged cross-sectional view of the sealing device  34  is representatively illustrated. In this view it may be more clearly seen that the metal seals  32   a ,  32   b  each include an inclined beam or arm  42   a ,  42   b  extending between a seal surface  44   a ,  44   b  and the ring  40 . It may also be seen that each of the nonmetal seals  36   a ,  36   b  includes a generally wedge-shaped portion  46   a ,  46   b  positioned between the ring  40  and a respective one of the arms  42   a ,  42   b.    
         [0025]    The metal seals  32   a ,  32   b  are preferably made of strong, durable and resilient metals, such as Inconel 718, 13-chrome steel, etc. The nonmetal seals  36   a ,  36   b  are preferably made of high temperature and well fluid resistant, strong and elastomeric materials, such as NBR, HNBR, fluoroelastomers, etc. Non-elastomeric materials, such as PEEK, etc., may additionally or alternatively be used in the nonmetal seals  36   a ,  36   b . It should be clearly understood that any metal materials may be used for the metal seals  32   a ,  32   b , and any nonmetal materials may be used for the nonmetal seals  36   a ,  36   b , in keeping with the principles of the invention. 
         [0026]    Note that the nonmetal seals  36   a ,  36   b  are not necessary for the sealing device  34  to seal between the housing assembly  28  and the closure member  26 . The sealing device  34  could be provided without the nonmetal seals  36   a ,  36   b , in which case the metal seals  32   a ,  32   b  would still provide sealing engagement with the closure member  26 . Use of the nonmetal seals  36   a ,  36   b  is preferred when a bubble-tight sealing engagement is required. 
         [0027]    When the closure member  26  engages the sealing device  34  as depicted in  FIG. 2 , the seal surfaces  44   a ,  44   b  contact the outer surface of the closure member and the arms  42   a ,  42   b  are deflected radially outward somewhat. This deflection causes elastic deformation of the arms  42   a ,  42   b , resulting in a biasing force being applied by the arms to the seal surfaces  44   a ,  44   b . Note that the seal surfaces  44   a ,  44   b  have small ridges formed thereon to concentrate this radial biasing force on a relatively small area, thereby increasing the contact pressure between the seal surfaces and the outer surface of the closure member  26 . It should be understood, however, that use of the small ridges is not required on the seal surfaces  44   a ,  44   b.    
         [0028]    The nonmetal seals  36   a ,  36   b  are also radially compressed between the ring  40  and the outer surface of the closure member  26 . In this manner, a seal surface  48   a ,  48   b  on each nonmetal seal  36   a ,  36   b  is biased into sealing contact with the outer surface of the closure member  26 . 
         [0029]    Deflection of the arms  42   a ,  42   b  as described above will compress the wedge portion  46   a ,  46   b  of each nonmetal seal between the ring  40  and the respective arm. If the nonmetal seals  36   a ,  36   b  are made of a resilient material, this compression will result in a radial biasing force being applied to each arm, thereby further biasing the seal surfaces  44   a ,  44   b  into contact with the outer surface of the closure member  26 . 
         [0030]    When a pressure differential  50  is applied across the sealing device  34  in an upward direction as depicted in  FIG. 3 , the wedge portion  46   b  of the lower nonmetal seal  36   b  will be further compressed between the ring  40  and the arm  42   b  of the lower metal seal  32   b . This compression of the lower wedge portion  46   b  will result in a further radial biasing force being applied to the arm, thereby further biasing the lower seal surface  44   b  into contact with the outer surface of the closure member  26 . 
         [0031]    When a pressure differential  52  is applied across the sealing device  34  in an downward direction as depicted in  FIG. 3 , the wedge portion  46   a  of the upper nonmetal seal  36   a  will be further compressed between the ring  40  and the arm  42   a  of the upper metal seal  32   a . This compression of the upper wedge portion  46   a  will result in a further radial biasing force being applied to the arm, thereby further biasing the upper seal surface  44   a  into contact with the outer surface of the closure member  26 . 
         [0032]    Thus, it will be appreciated that each of the sealing surfaces  44   a ,  44   b  is radially biased into metal to metal sealing contact with the outer surface of the closure member  26  due to: 1) elastic deformation of the respective arm  42   a ,  42   b,  2) compression of the respective wedge portion  46   a ,  46   b  between the ring  40  and the respective arm due to deformation of the arm, and 3) compression of the respective wedge portion  46   a ,  46   b  due to the pressure differential  50  or  52 . This results in reliable metal to metal sealing between the metal seals  32   a ,  32   b  and the outer surface of the closure member  26 . 
         [0033]    If, however, the seal surfaces  44   a ,  44   b  or the outer surface of the closure member  26  should become damaged, so that metal to metal sealing therebetween cannot be achieved, sealing contact between the nonmetal seals  36   a ,  36   b  and the closure member may still be possible. 
         [0034]    In another important feature of the sealing device  34 , note that, as the closure member  26  displaces upward from its closed position depicted in  FIG. 2 , sealing contact with the closure member is progressively removed from the lower nonmetal seal  36   b , then the lower metal seal  32   b , then the upper metal seal  32   a , and then the upper nonmetal seal  36   a . This means that, if the differential pressure  50  or  52  is applied against the sealing device  34  when the closure member  26  displaces upward, the pressure differential across the lower nonmetal seal  36   b  will be relieved while the other seals  32   a ,  32   b ,  36   a  maintain sealing contact with the closure member. This prevents damage to the seal  36   b  from excessive flow when the pressure differential  50  or  52  is relieved. 
         [0035]    When the closure member  26  eventually displaces upward sufficiently far that it no longer is in sealing contact with the upper nonmetal seal  36   a , and the pressure differential across this seal is thus relieved, the closure member will still be contained within a closely fitted sleeve  66  in which the openings  30  are formed, thereby preventing damage to the seal from excessive flow. 
         [0036]    As the closure member  26  displaces downward from its open position in which flow is permitted through the openings  30 , the pressure differential  50  or  52  may be applied when the closure member sealingly engages the sealing device  34 . The pressure differential  50  or  52  will first be applied to the upper nonmetal seal  36   a  while the closure member  26  remains within the closely fitted sleeve  66 , thereby preventing damage to the seal from excessive flow. Next, in succession, the closure member  26  sealingly contacts the upper metal seal  32   a , the lower metal seal  32   b , and the lower nonmetal seal  36   b.    
         [0037]    It may now be fully appreciated that the sealing device  34  provides significant benefits in performing the sealing function in the closure mechanism  24  of the well tool  20 . For example, the metal seals  32   a ,  32   b  provide for metal to metal sealing between the closure member  26  and the housing assembly  28 , the metal seals are resiliently biased into sealing contact in multiple ways (including an increased biasing force as the differential pressure across the sealing device  34  increases), and the nonmetal seals  36   a ,  36   b  provide for additional sealing capability in the event that metal to metal sealing cannot be achieved. Pressure differentials from either direction across the sealing device  34  can be sealed against, without damage to the seals  32   a ,  32   b ,  36   a ,  36   b , whether the closure member  26  displaces to close or open while the pressure differential exists. 
         [0038]    Referring additionally now to  FIG. 4 , an alternate configuration of the closure mechanism  24  is representatively illustrated. This alternate configuration of the closure mechanism  24  includes an alternate configuration of the sealing device  34 , which is depicted in a further enlarged cross-sectional view in  FIG. 5 . 
         [0039]    The sealing device  34  as illustrated in  FIG. 5  is similar in some respects to the sealing device of  FIG. 3 , in that it includes multiple metal seals  54   a ,  54   b  with respective seal surfaces  56   a ,  56   b  and inclined beams or arms  58   a ,  58   b  extending between the seal surfaces and a ring  60 . 
         [0040]    The sealing device  34  of  FIG. 5  also includes multiple nonmetal seals  62   a ,  62   b  positioned between the metal seals  54   a ,  54   b . A wedge portion  64   a ,  64   b  of each respective nonmetal seal  62   a ,  62   b  is positioned between a respective one of the arms  58   a ,  58   b  and the ring  60 . 
         [0041]    A difference between the nonmetal seals  62   a ,  62   b  and the nonmetal seals  36   a ,  36   b  described above is that the seals  62   a ,  62   b  are formed as a single, integral element, rather than as separate elements. Indeed the nonmetal seals  62   a ,  62   b  could be formed as a single seal, if desired. Furthermore, as discussed above for the nonmetal seals  36   a ,  36   b , use of the nonmetal seals  62   a ,  62   b  is not required in the sealing device  34  of  FIGS. 4 & 5 . 
         [0042]    As with the configuration of  FIGS. 2 & 3 , the seal surfaces  56   a ,  56   b  of the metal seals  54   a ,  54   b  are radially biased into sealing contact with the outer surface of the closure member  26  due to elastic deformation of the arms  58   a ,  58   b  and resulting compression of the wedge portions  64   a ,  64   b  of the nonmetal seals  62   a ,  62   b  between the arms and the ring  60 . However, further biasing forces applied to the arms  58   a ,  58   b  due to differential pressure across the sealing device  34  occurs somewhat differently in the alternate configuration of  FIGS. 4 & 5 . 
         [0043]    When the closure member  26  is in its closed position as depicted in  FIG. 4 , the pressure differential  50  will cause the wedge portion  64   a  of the nonmetal seal  62   a  to further compress between the arm  58   a  and the ring  60 , thereby applying a biasing force to the arm and further biasing the seal surface  56   a  against the outer surface of the closure member. When the pressure differential  52  is applied across the sealing device  34 , the wedge portion  64   b  of the nonmetal seal  62   b  will be further compressed between the arm  58   b  and the ring  60 , thereby applying a biasing force to the arm and further biasing the seal surface  56   b  against the outer surface of the closure member. 
         [0044]    As the closure member  26  displaces upward from its closed position depicted in  FIG. 4 , sealing contact with the closure member is progressively removed from the lower metal seal  54   b , then the lower nonmetal seal  62   b , then the upper nonmetal seal  62   a , and then the upper metal seal  54   a . This means that, if the differential pressure  50  or  52  is applied against the sealing device  34  when the closure member  26  displaces upward, the pressure differential across the nonmetal seals  62   a ,  62   b  will be relieved (after the pressure differential is relieved across the lower metal seal  54   b ) while the upper metal seal  54   a  maintains sealing contact with the closure member. This prevents damage to the seals  62   a ,  62   b  from excessive flow when the pressure differential  50  or  52  is relieved. 
         [0045]    When the closure member  26  eventually displaces upward sufficiently far that it no longer is in sealing contact with the upper metal seal  54   a , and the pressure differential across this seal is thus relieved, the closure member will still be contained within the closely fitted sleeve  66 , thereby preventing damage to the seal from excessive flow. 
         [0046]    As the closure member  26  displaces downward from its open position in which flow is permitted through the openings  30 , the pressure differential  50  or  52  may be applied when the closure member sealingly engages the sealing device  34 . The pressure differential  50  or  52  will first be applied to the upper metal seal  54   a  while the closure member  26  remains within the closely fitted sleeve  66 , thereby preventing damage to the seal from excessive flow. Next, in succession, the closure member  26  sealingly contacts the upper nonmetal seal  62   a , the lower nonmetal seal  62   b , and the lower metal seal  54   b.    
         [0047]    It will be appreciated that the sealing device  34  in the configuration of  FIGS. 4 & 5  provides similar benefits to those of the configuration of  FIGS. 2 & 3 . For example, the metal seals  54   a ,  54   b  provide for metal to metal sealing between the closure member  26  and the housing assembly  28 , the metal seals are resiliently biased into sealing contact in multiple ways (including an increased biasing force as the differential pressure across the sealing device  34  increases), and the nonmetal seals  62   a ,  62   b  provide for additional sealing capability in the event that metal to metal sealing cannot be achieved. Pressure differentials from either direction across the sealing device  34  can be sealed against, without damage to the seals  54   a ,  54   b ,  62   a ,  62   b , whether the closure member  26  displaces to closed or open positions while the pressure differential exists. 
         [0048]    Sealing devices constructed in accordance with the principles of the invention should be capable of sealing against 15,000 psi differential pressure at 325-400° F. in a static condition (no movement of the closure member relative to the housing assembly), and should be capable of reliably sealing against 1500-5000 psi during opening and closing of the closure member. 
         [0049]    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 the specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.