For oil production systems, an interior two-part seal ring of resilient material is disposed within a coupling sleeve which receives threaded tubing ends in its opposite ends. The seal is asymmetric, with tapered ends of different lengths and taper angles. A length for mill end engagement is shorter than the length for field end engagement which may be made up and disconnected repeatedly. The seal configuration includes a principal body with a inwardly directed ridge between its tapered ends and a reinforcing ring of different, less permeable material, abutting the ridge and closing off one or more radial ports in the wall of the body. The central body also includes a circumferential gas groove about is periphery, which together with the radial ports, facilitates collection and release of gases permeating the seal body under high down-hole pressures.

BACKGROUND OF THE INVENTION 
     Intercoupled lengths of tubular elements are used throughout the international oil production industry, with a high proportion of such installations being based on standards set by the American Petroleum Institute (API). For example, although there are a number of premium tubular products used for specialized or particularly difficult applications, the majority of the production tubing and casing tubular goods, including couplings, in use are made up of standardized types, such as those with API 8 round or buttress threads. Stocks of Internally Plastic Coated and Fiberglass Lined tubing and casing are now widely employed, especially where production wells are located in more environmentally demanding locations. The standard tubing sections are interconnected by internally threaded standardized cylindrical sleeves or couplers, also, if required, internally plastic coated. In the petroleum industry, product interchangeability is essential for economy where feasible because strings are made up and disassembled as many times as conditions will permit. Thus standardization, under API criteria as to sizes, materials, thread types, and tolerances, enables widespread use of tubing and casings which, can be replaced and interchanged multiple times economically during their useful lives. 
     Usually after manufacture, a reserve of tubing or casing is held in inventory in a pipe yard or on a drilling or production site. One end of a pipe length, called the mill end, typically is pre-attached to a coupling sleeve, for storage and shipment to at a production site when needed. At the production site, the string is assembled by stabbing the pin (“field”) end of a different pipe length into the available mill end of a coupler, which is disposed vertically as the new length is engaged, rotated, and made up to proper API torque specifications. The string is built up and successively fed downhole until the production depth is reached. As production depths are historically consistently increasing, in order to gain access to new oil and natural gas sources, greater stresses and physical demands, are concomitantly being imposed on the tubular goods and especially on the threaded connections. Thus, it is now common to employ seal elements within couplers to engage each of the opposite tubing pin ends in order to seal against interior fluid and pressures, and to combat leakage. 
     Tubing strings must periodically be withdrawn for service, inspection or replacement of problem components, so that tubing strings commonly require repeated make-ups. Seals used in the string must therefore also be reusable as many times as feasible. In the present state of the art, the pipe strings are extremely long and must withstand not only high pressures and temperatures but also harsh and corrosive environments. Accordingly, minor irregularities and non-uniformities in the seals, the plastic coating or the tubing can be the source of major and costly problems. 
     While modern plastic coating methods are efficient, they do not always assure uniformity in critical thread regions. For example coating layer discontinuities can often appear in pin end regions, because uniform deposition of coating is more difficult in such transition regions. It is, in other words, sometimes dubious, as to whether available manufacturing procedures have provided a uniform corrosion barrier and effective sealing against high internal pressures, even where an internal seal has been employed. 
     Moreover, there are certain factors inherent in synthetic materials used in tubing connections which render seals formed from such materials susceptible to failure when exposed to high pressure gases, particularly under elevated temperatures. “Teflon” is a material widely used for petroleum seals, because of its chemical stability and resilient properties. The chemistry of “Teflon” and other plastic formulations, however, is such that pressurized gases will, with time, permeate into and throughout the seal. When there has been sufficient exposure to such permeating gases, a seal may in time become fully permeated and begin to distort in shape. If the internal pressure is reduced or relieved, the gas-permeated seal is apt to implode into the tubing string or distort in shape, losing its sealing capability. This loss of seal integrity requires removal and repair of the entire string. Accordingly, there exists a need for a superior seal/corrosion barrier and coupling combination which can meet the aggravated and demanding conditions imposed by high pressure, high temperature, corrosive downhole environments. 
     SUMMARY OF THE INVENTION 
     A sealing/corrosion barrier system in accordance with the invention employs a purposefully asymmetric reusable sealing element as part of an interior annular two-piece geometry employing materials of different properties in a fashion which enables long term use and repeated make-up. 
     In accordance with the invention, a sealing/corrosion barrier system includes a principal body element of typical seal material, e.g. “Teflon” having a central body region of substantially constant inner diameter that is longitudinally slightly off center. The central body region is asymmetrically divided by an internally directed retainer shoulder. Adjacent to, but spaced from the retainer shoulder in the constant I.D. section, are one or more radial differential pressure elimination ports extending through the wall of the body element at at least one circumferential location. Around the periphery of a generally central region of the body also is a gas groove of limited depth that intercepts the differential pressure elimination ports. 
     The “Teflon” body element is tapered longitudinally from its central region in opposite directions to minimum thickness at its longitudinal ends. Again, however, these wings or tapers are asymmetric in that the length of taper on the mill end side is significantly less than the length of taper on the field end side. Within the “Teflon” body, in engagement with, and about, the constant inner diameter portion of the body, and further in abutment with the retainer shoulder, is a reinforcing ring of high tensile and corrosion resistant properties. In this example the reinforcing ring is of polyether ether ketone (“Peek”) but it can also be made of suitable metallic materials. The reinforcing ring has an outer circumference flush with the inner surface of the central seal/corrosion barrier body and an inner circumference substantially flush with the inner surfaces of the pin ends of inserted pipe. The “Teflon” body also includes circumferentially outwardly directed ridges at longitudinally spaced apart locations that are of low height relative to the outer circumference of the central body but engage in mating grooves in the opposing face of the coupling to prevent longitudinal shifting of the seal when a pin end is torqued into position. To this end, a shaped tool mating with the interior of the seal element may be inserted within the seal interior to prevent the formation of wrinkles in the seal as torque is applied to the pin and the seal is correspondingly deformed. 
     The combination of the “Teflon” seal and the interior “Peek” reinforcing ring provide, by virtue of their material properties and geometry, a combination of structural integrity under high pressure and also non-reciprocal response to internal pressurization. With the ring body in place, the tapered longitudinal end wings receive the inserted threaded pin ends and deform in compliance to the pin end threads. This engagement occurs first at the mill end wing as the pin end is being threaded into a selected depth with chosen torsional force. The “Teflon” body is held in place longitudinally by the engagement of the outer pressure ridges in the receiving grooves in the coupling and the inserted mating tool. The “Peek” retainer ring is then installed and consequently closes off the radial ports through the body. 
     At the field site, the tubular elements with attached coupling sleeves can then be assembled into a string. To this end, a new length of tubing is stabbed into an open end of the coupling sleeve and is tightened until it engages into the tapered wing at the field end side to a desired torque level. During torquing, the external pressure ridges prevent longitudinal displacement of the ring body relative to the coupling sleeve, so the desired final geometric disposition of components is achieved, thus virtually eliminating the possibility of explosive decompression of the seal material. 
     In operation, internal gas pressures that are encountered may cause, over time, permeation of the gases through the matrix of the “Teflon” until the gases build up in the small but adequate gas groove. If the tubing string is then removed from the well site, or the internal pressure in the tubing is suddenly relieved, the gases which have permeated through the “Teflon” are readily able to flow in a low impedance path from the gas groove through the radial ports and then to the interior of the tubing. Thus any differential gas pressures on the “Teflon” body and the “Peek” reinforcing ring are almost instantaneously relieved. Also during operations, minute amounts of fluid can work their way between the “Peek” ring and “Teflon” ring, passing through the differential pressure elimination ports, then filling the gas groove and making contact with the steel surface at the center of the coupling sleeve. The form and fit of the internal diameter of the “Teflon” seal and the external diameter of the “Peek” ring allow the fluid in, but eliminate any possibility of exit. Any corrosive activity that takes place between the Teflon ring and steel coupling internal diameter is also minute, resulting merely in the discoloration of the steel surface and no structural or mechanical damage to the steel coupling. Since the fluid electrolytes cannot be circulated or otherwise renewed, a dead corrosion cell is established at the conclusion of one ion exchange, and further corrosion is eliminated. 
     With repeated withdrawals and disassembly of the string, the relatively longer field end wing on the central body enables repeated makeovers. Each time the seal is reused the threaded pin end of the tubing can penetrate the field end wing to slightly greater depths, maintaining the sealing/corrosion bather performance and mechanical integrity as the desired operative API specified torque level is reached in each instance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which like numerals refer to like parts, and wherein: 
         FIG. 1  is a perspective view, partially broken away, of adjacent pin ends on a tubing string within a configuration of coupling sleeve and internal seals in accordance with the invention; 
         FIG. 2  is a side sectional fragmentary view of relevant portions of the seal elements, coupling sleeve, and engaged pin ends in the assembly of  FIG. 1 ; 
         FIG. 3  is a detailed but fragmentary enlarged side sectional view of a segment of the assembly of  FIGS. 1 and 2  showing the engagement of a coupling and pin ends to seal elements in accordance with the invention, and 
         FIG. 4  is a fragmentary, side sectional view of the seal elements in unstressed relation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An example of a single section  10  of a string of tubular elements with sealed connections in accordance with the invention, for a modern petroleum production application is shown in  FIG. 1  and comprises one of a number of interconnecting coupling sleeves  14  for joining the pin ends of tubing  12 ,  16  entering a coupling sleeve  14  from opposite ends. Details can more clearly be seen in  FIGS. 2-4 , to which reference is also made. The tubular items may use round, buttress or other threads, with the example here chosen being of a tubing string with 8″ round threads, although the sealing system described herein is equally applicable to tubing and casing having other thread configuration. A tubing  12  or pipe on the mill end side has pin end male threads  13  which mate with female threads in the associated mill end of a coupling sleeve  14 , often interchangeably called a coupler or collar. The connection, as fully made up in the field site, includes the field end tubing  16 , with pin end male threads  17  engaged into the sleeve  14  on the field end side. The tubing in each end of a coupling  14  is torqued to a predetermined level, or to a number of turns, in known fashion. Although make-up to an equal number of turns is shown by way of example in the Figures, it should be noted that the mill end side connection, once completed, is substantially invariant because makeup and breakout is effected on the opposite side. On the field end side, the depth of the penetration of the pin end varies because the greater the number of times the connection is made up, the greater depth of penetration of the field pin end usually needed for proper sealing as the seal deforms. 
     The combination further includes a shaped “Teflon” seal ring body  20  and an interior reinforcing ring  22  of a less permeable, but higher modulus material, here preferably a polyether ether ketone (“Peek”) material. “Peek” material in its extruded form has the following typical properties (modulus values tend to be substantially higher when the material is reinforced with glass or carbon fiber): 
                                             Tensile Strength (psi)   16,000           Tensile Modulus (psi   500,000           Tensile Elongation at Break (%)   20           Flexural Strength (psi)   25,000           Flexural Modulus (psi)   600,000           Compressive Strength (psi)   20,000           Compressive Modulus (psi)   500,000           Hardness Rockwell   M100 (R126)           IZOD Impact Notched (ft-lb/in)   1.0                    
The interior reinforcing ring  22  is generally rectangular in cross-section and mates within an interior circumferential, substantially uniform diameter, seating surface  23  on the central region of the seal ring body  20 , as seen in  FIG. 4 . This central inner diameter section  23  is longitudinally offset toward pin  12  on the mill end side and merges into a mill end tapered wing  26  converging to the transverse mill end surface over a predetermined length. At the other end of the Teflon seal ring body  20 , a tapered wing  28  on the field end side converges over a relatively longer length, as compared to the length of the mill end tapered wing  26 , to its end surface. This configuration enables repeated make-ups of the coupling, by making and breaking engagement of the pin end to the seal ring body  20  at the field end while the connection at the mill end side remains unchanged.
 
     Offset slightly from the longitudinal center of the coupling body  10  is a radially inwardly directed shoulder  30  at one longitudinal end of the slightly offset central inner diameter section  23 , which shoulder  30  defines the positional limit of an inserted “Peek” interior reinforcing ring  22 . An internal circumference of the internal reinforcing ring  22  is substantially flush with an inner surface of the inserted tubing pin ends  12 ,  16 , while the inwardly directed shoulder  30  on the seal ring body  20  terminates radially outside the inner circumferences defined by the interior reinforcing ring  22  and tubing  12 ,  16 . Consequently, with the interior reinforcing ring  22  in position on the central internal diameter section  23  of the seal ring body  20 , the internal surface presented by the coupling  10  includes a length defined by the inner surface of the reinforcing ring  22  that is flush with the interior circumferences of the tubing lengths  12 ,  16 . Short gaps exist between the inserted tubing  12 ,  16  and the ends of the reinforcing ring  22 , but these short discontinuities are bounded by surfaces of the inserted seal ring body  20  and remain sealed. 
     In this position, the reinforcing ring  22  covers the interior aperture of one or more radial ports  34  in the seal ring body  20  which each such radial port  34  has a diameter that is a fraction of the longitudinal length of the reinforcing ring  22 . In this example, there are two such radial ports,  34 , located on opposite circumferential sides of the seal ring body  20 . These radial ports  34  provide openings for elimination of differential pressure and eliminate circulation of electrolytes. The seal ring body  20  also includes a small peripheral circumferential gas groove  36  about the body, and intersecting the radial ports  34 . The gas groove  36  is accordingly slightly offset from the longitudinal center of the seal ring body  20  and has a depth of about ⅛″ and a generally hemispherical shape, although the shape is not critical. The seal ring body  20  also includes, on its outer surface  24 , a pair of longitudinally spaced apart circumferential ridges or sealing pressure points  38 ,  39  which have relatively sharply angled side walls and which are less than about ¼″ height relative to the outer diameter of the seal ring body  20 . To receive these ridges  38 ,  39 , the collar or coupling sleeve  14  includes receiving grooves  41  and  42  at correspondingly longitudinally spaced positions. Therefore, once the seal ring body  20  is inserted longitudinally to engage in the collar  14  by overcoming the mechanical resistance offered by the slightly smaller diameter of the collar  14 , the sealing pressure points or circumferential ridges  38 ,  39  fit into the receiving grooves  41 ,  42 . The torque exerted on the seal, as a first mill pin end is rotated in against the collar, can act to distort or wrinkle the seal ring body  20 . To avoid this distortion, it is convenient to insert a retainer tool shaped to mate with the seal ring body interior, and to hold it constrained as the pin threads force into and deform the seal ring body  20  where they engage the resilient material. Once the mill end pin of tubing  12  is in place, the seal ring body  20  does not shift or wrinkle (after the retainer tool is removed). The seal ring body  20  thereafter retains its desired longitudinal position as the field pin end of the tubing  16  is torqued into position against the adjacent wing  28  of the seal ring body  20 . 
     With this configuration, the connection between the intercoupling sleeve  14  and the tubing lengths  12 ,  16  can be made up and disassembled a number of times, breaking only the engagement between the field end tubing  16  and the field end wing  28  in the coupling sleeve  14 . The internal pressures that normally distort a “Teflon” seal by long-term build-up of pressure and permeation through the seal ring body  20  do not result in distortion by permeation because of the presence of the interior reinforcing ring  22 . The gas permeation through the “Teflon” which does occur under pressure is effectively accumulated in the gas groove  36  over a period of time but the inwardly directed forces generated, as the permeated gases collect, do not cause substantial distortion of the seal ring body  20 . Instead because of the presence of the less permeable high modulus “Peek” reinforcing ring  22  in the central region of the seal ring body  20 , inward displacement is effectively opposed. If the internal pressure within the tubing string suddenly drops, or the string is to be removed for service or because of other reasons, the built-up pressure discharges through the radial ports  34  in a few seconds time, retaining the integrity and shape of the seal ring body  20  and preventing displacement of the seal ring body  20  and of the interior reinforcing ring  22  from position. Consequently, the dual mating seal geometries and the materials employed sharply restrict the tendencies toward seal distortion and failure. 
     Those skilled in the art will appreciate how variations in materials and geometries can be employed to like or other effects, as needed. Structural reinforcement of a pressure distortable sealing material by an associated conforming element in accordance with the invention can limit failures and field maintenance problems. Providing flow collection and escape paths for through-permeating gases also are of benefit to on-site operation. 
     Although there have been described above and illustrated in the drawings various alternatives in accordance with the invention, the invention is not limited thereto but encompasses all forms and variations in accordance with the appended claims.