Metal-to-metal seals in premium threaded joints (PJs) for Oil Country Tubular Goods are used in severe environments such as high pressure and high temperature. Under the extreme conditions, the thermal degradation of applied grease on the seal surfaces can affect the sealability of metal-to-metal seal. In this study, changes in the sealability of greased metal-to-metal seal due to elevated temperature were investigated by fundamental gas tightness tests. In addition, changes in grease property at high temperature were also investigated. It was found that one of the causes of the loss of the sealability of metal-to-metal seal at high temperature was the decrease in grease viscous resistance against high pressure due to thermal degradation of grease. The decrease in grease viscous resistance was caused by the decrease in the kinematic viscosity of the base oil under 125°C at which the network structure of the thickener was almost maintained. Beyond 150°C, the decrease of complex viscosity of the grease due to the collapse of the network structure of the thickener decreased the grease viscous resistance. The latter significantly affected the loss of sealability of the metal-to-metal seal.
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