Influence of Grease on High-Pressure Gas Tightness by Metal-to-Metal Seals of Premium Threaded Connections

Abstract

A premium threaded connection for oil country tubular goods (OCTG) has a metal-to-metal seal portion to ensure its gas tightness under high pressure and high temperature. The metal surfaces are coated with grease and then integrated by rotary sliding. Despite several studies, the gas tightness mechanism of the metal-to-metal seal has not yet been clarified. In this study, as the first step to clarify the mechanism, we evaluated how grease affects the gas tightness by fundamental tests considering rotary sliding and grease degradation due to high temperature. Our results showed that grease is mandatory for the gas tightness of the metal-to-metal seals and the gas tightness is affected by both types of base oils and solid additives of grease. The seals coated with the grease composed of mineral oil had better gas tightness than those coated with that composed of plant oil, even after degradation due to high temperature. In addition, grease containing soft metal particles had better gas tightness than grease containing hard inorganic particles because the extended soft metal particles adhering to the rough parts of the seal surfaces reduced the size of the leakage paths.

Running-in of metal-to-metal seals and its influence on sealing ability with application to the design of biodegradable thread compounds

“…Thread compounds are made according to API specifications [4] and are different from normal greases [42] because of their high solids content of up to 60%. They need to give repeatable friction for the torque based assembly [43], avoid galling during assembly [44][45][46][47] and help in sealing ability once the casing connection is assembled [18,[48][49][50]. In addition, the thread compound needs to perform under the conditions present in the well.…”

“…In addition, the thread compound needs to perform under the conditions present in the well. This means under elevated temperatures and exposure to well fluids or gasses [48,49,51]. Both are known to degrade the sealing performance of the thread compound because of evaporation and dissolving of the base oil.…”

“…Inose et al [48] show that thermal degradation of the thread compounds plays a significant role in reduction of the sealing performance of metal-to-metal seal. They exposed API modified and an environmentally acceptable thread compound with inorganic particles to temperatures of 180 • C and monitored sealing ability.…”

“…As discussed (Section 2.4), sealing performance of contacting elements has been shown to be determined by the surface texture and changes thereof [48,59,62], the shape distortion [55], the interaction with the lubricant [50] and degradation thereof [48]. The initial configuration and subsequent evolution because of friction and wear of the tribosystem is thus important for successful sealing.…”

“…Moreover the wear should be minimum to stay as close as possible to the intended design contact stresses of the connection manufacturer. A metal-to-metal seal can for instance be successfully made-up but fail during load cycling because of thermal degradation of the lubricant [48] at the temperatures imposed during testing (Section 2.3). However, the current thread compound testing methodologies [4] will not reveal this behaviour as discussed in Section 2.4.…”