Metallic Sealing Technology in Downhole Completion Equipment
Authors: Blizzard William A. 
Published: December 2007

Summary.
Although metal seals provide excellent service in many currently marketed products, they are not the panacea for downhole sealing requirements. Certain downhole products are not yet easily adaptable to metal configurations. Incorporating metal seals downhole requires use of basic seal-design considerations and considerable time and money. Metal-seal technology generally is more costly to develop than nonmetal technology. Progress has been made toward developing all-metal equipment. The subsurface safety valve, the first significant dynamic product to use all-metal seal technology, is field-proven and readily available. It uses all aspects of metal seal design and has laid groundwork for products that will follow.

Introduction
Higher temperature and pressure, greater well depth, and severe corrosion significantly affect downhole products. These factors combine to create severe performance and longevity problems that downhole equipment manufacturers must solve. Polymeric seals in downhole equipment are the leading cause of equipment failure. 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, in the aerospace, high-pressure and -temperature autoclaves, cryogenic, and chemical industries. This paper examines why metal-seal technology exists and how it affects downhole well-completion equipment.

Sealing Theory
General. To understand what makes a "good" seal, one must realize that there is no currently accepted definition of "zero leakage." An acceptable seal in the oil field 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 (General Electric) defines zero leakage as less than 10(-8) cm3/s of helium at atmospheric pressure. The Natl. Aeronautics and Space Administration (NASA) defines it as no more than 1.4 × 10(-3) cm3/s of nitrogen at 300 psi and ambient temperature. To place this rate in proper perspective, the NASA leakage rate proper perspective, the NASA leakage rate requires about 3 days to fill a 12-oz bottle. Clearly, these acceptance rates exceed oil-field requirements. Most manufacturers measure for leakage by submerging the products in water and observing for bubbles products in water and observing for bubbles at test pressures. The "bubble approach" is certainly acceptable, as evidenced by its incorporation into the API's testing document for downhole safety valves.
Limitations of Elastomeric Seals. Polymeric seals have several limitations. Polymeric seals have several limitations.1. Elastomers are generally limited to lower pressures (i.e., less than 6,000 psi when not contained). 2. Temperatures greater than 400 degrees F adversely affect the thermal stability of many elastomers. 3. Elastomers are adversely affected by severe corrosion (high H2S or CO2). 4. Amine-based completion treatments may be detrimental to some elastomer compounds.5. Elastomers are affected by explosive decompression in high-pressure gas environments. 6. Many seal designs cannot tolerate excessive pressure pulsations or reversals. 7. Many elastomers have low wear resistance. 8. Frictional values are difficult to predict. predict.
However, using elastomers has many advantages. They are very forgiving of manufacturing tolerances and can adapt to rougher surface finishes on mating parts. Handling, storage, and assembly precautions are minimal. Only extreme combinations of conditions require metal seals.

Metallic Seals-Microscopic.
Metal-seal theory is neither well-documented nor regulated by any group within the petroleum industry. Work was undertaken recently to quantify the effect of bending and tensile loading on API metal flange seals. Most design information comes from in-house research projects by equipment suppliers. Design guidelines are based on years of experience and are highly empirical. Some general observations, however, can be reached. 1. Metal seals differ from elastomers in that they cannot easily flow into and fill all the microscopic voids between the two mating surfaces to prevent fluid (liquid or gas) passage. Thus, leakage occurs through channels and passageways. Metal must be forced into these voids by application of large loads to the mating parts.
JPT
P. 1244