Abstract: In high-pressure, high-temperature (HPHT) completions, elastomeric seals often reach their functional limits. This entry analyzes the mechanical principles of metal-to-metal (MTM) sealing technology.

Technical Analysis:

MTM seals rely on the controlled elastic and plastic deformation of contact surfaces. For a gas-tight seal at pressures exceeding $15,000$ PSI, the contact stress between the seal and the pocket must significantly exceed the fluid pressure.

• Material Hardness Ratios: Successful sealing requires a specific hardness differential between the seal ring and the seating surface. Typically, the seal ring is manufactured from a slightly softer alloy (e.g., Inconel 625 inlay) to ensure it conforms to the more rigid wellhead body without causing galling.
• Surface Topography: Surface finish is measured in micro-inches ($R_a$). In MTM interfaces, an $R_a$ of 16 to 32 is often required. Any microscopic ridges can become leak paths for low-molecular-weight gases like methane.
• Thermal Cycling: Engineers must account for the coefficient of thermal expansion (CTE). As wellbore fluids heat the assembly, the expansion rates of dissimilar metals can either tighten the seal or, if improperly engineered, induce fatigue that leads to “weeping” at the interface.