As is well known in the industry, the search for oil and gas has required drilling wells in excess of 20,000 feet and into zones of extremely high pressure and corrosive environments. Consequently, there have been numerous improvements in well liner connection, which must occur every thirty to forty feet into the well, thus relegating the simple threaded connection of earlier times to small scale operations in shallow formations. The time, inconvenience and expense of removing the string of pipe to replace a seal which has failed at some depth greater than 10,000 feet is readily apparent.
By way of example, metal-to-metal pressure seals were first disclosed in U.S. Pat. No. 2,006,520 wherein a seal surface formed on the male end of a pin of a tube cooperates with a seal surface formed in the female or box portion of the joinder tube to effect a seal just prior to shouldering of the threaded connection. This basic metal-to-metal seal provided an effective seal; however there was no back-up seal and the seal surfaces were subject to contamination, pitting due to storage deterioration, and damage occurring during handling of the tubing.
U.S. Pat. No. 3,489,437 disclosed two adjacent metal-to-metal seals formed at a shoulder within the female member. While the dual seal is an improvement, this arrangement is susceptible to degradation of the seal performance due to contamination or deterioration of the seal surfaces.
Teflon seal elements or other thermoplastic seal elements have been used as back-up sealing elements when compressed in the connection; however these type elements typically resist radial compression and therefore tend to impart internal forces within the connection which act to urge the male and female elements apart thus reducing the effectiveness of the metal-to-metal seals and may contribute to hydrogen sulfide corrosion failures which are often associated with high stress levels in the metal connection.
Numerous seal arrangements are in existence; however, none are known to present the particular combination claimed herein.