Metallic seal systems are relatively new to downhole completion equipment. Their incorporation into downhole products represents a trend toward higher technology in extreme service applications. The foundations for metal seal design have long been established in other industries such as aerospace, high pressure and temperature autoclaves, cryogenic and chemical industries. This paper will examine why metal Seal technology exists and to what extent downhole well completion equipment is affected.
As new frontiers emerge in the oil and gas industry, new problems arise. Higher temperature and pressure, greater well depth and severe corrosion significantly affect downhole products. These factors combine to create severe performance and longevity problems which manufacturers of downhole equipment must work to solve. Polymeric seals in downhole equipment are the leading cause of equipment failure.1 Since reliability of the well completion is of utmost concern to today's operator, the push toward better seal systems has generated the current influx of metal seals.
Metal seals provide excellent service in many currently marketed products2. However, metal seals are not the panacea of downhole sealing requirements. As much as the industry would like to convert to metal seals, certain downhole products have yet to prove easily adaptable to metal configurations. Incorporating metal seals downhole requires basic seal design consideration. Extensive time and financial support is also required. In general, metal seal technology costlier to develop than non-metal.
Inroads have been made toward developing all-metal equipment. The subsurface safety valve is the first significant dynamic product to utilize all-metal seal technology. It is currently field proven and a readily available item. The safety valve utilizes all aspects of metal seal design and has laid the groundwork for products that follow.
TO understand what makes a "good" seal, one must realize that there is no current accepted definition of the term "zero leakage". An acceptable seal in the oilfield may be totally unacceptable by aerospace standards. In general, however, zero leakage dictates the use of polymeric seals. When metallic seals are desirable, zero leakage must be quantified. For example, Advanced Technology Labs (G.E.) defines zero leakage to be less than 10−8 cm3 per second of helium at atmospheric pressure.
