Reason Analysis on Leakage and Corrosion of Premium Connection Mill End of Stainless Steel Tubing in High Pressure Gas Well

The inner pressure of casing abnormally increased after completion operation in one high pressure gas well. The connection mill ends were inspected and analyzed after they were broken out. It was discovered that the few connection mill ends were leaked and galling, and the part of connection mill ends were corroded at the shoulder inside wall and disappearing position of bevel diameter on pin inner surface. The leakage and galling proportions of tubing connections were highest, and corrosion was most severe in depth of 2 612.11-1 625.19 m. The leakage was not only related to premium connection structure design and make up torque, but also related to the load borne on tubing string during production according to inspection results. The stainless steel tubing was not suitable to such high pressure well with CO2 and Cl- because of tubing corrosion, and the severe corrosion at joint illuminated that there was corrosion concentration at joint position.