Fundamental Evaluations of the Sealability of Premium Joints in CCS Wells

Abstract

Abstract This study investigated the sealability of premium threaded joints (PJs) in carbon dioxide capture and storage (CCS) wells using finite element analysis (FEA) and small-scale fundamental gas tightness tests. During the operation of CCS wells, the inner surface of the tubing can reach extremely low temperatures. This cooling may decrease the metal-to-metal seal contact energy and/or change the viscoelastic properties of the applied lubricants, affecting the sealability of the PJs. Firstly, the extreme temperature differential, simulating the worst-case transient state in a tubing, was evaluated using FEA, in which the PIN inner surface was simulated at -80 °C and BOX outer surface was maintained at 25 °C. Secondly, the impact of different lubricating systems on sealability was evaluated via small-scale fundamental gas tightness tests under extremely low-temperature conditions. Additionally, dynamic viscoelasticity measurements for lubricants under extremely low-temperature conditions were conducted. The FEA results indicated that the impact of the temperature change on the minimum seal contact energy while applying a combined loading cycle was limited, even though the temperature of the metal seal surface was cooled to approximately -40 °C under an extreme temperature differential. Small-scale tests demonstrated that the sealability of an environmentally friendly thread compound (the so-called yellow dope) and solid lubricating film were equivalent to or higher at -40 °C than those observed at 25 °C. By contrast, an API dope, which is the standardized thread compound for PJs specified by the American Petroleum Institute (API), has a low sealability at -40 °C. By measuring the dynamic viscoelasticity of these lubricants, we demonstrated that an API dope can exhibit a glass transition point at approximately -50 °C, while the yellow dope transition occurred at approximately -75 °C. This difference in viscoelasticity properties at extremely low temperatures corresponds to the changing sealability characteristics. Specifically, an excessive increase in viscosity (API dope) would deteriorate the sealability, while a minor increase in viscosity (yellow dope) would improve the sealability. Dynamic viscoelasticity measurements also demonstrated that the properties of these dopes once cooled to -80 °C returned to their original states after heating to 25 °C. This indicates that PJs are also expected to retain their original sealability performances after being subjected to extreme temperature hysteresis.