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Corrosion leakage became serious due to cyclic sewage injection and continuous increase in water injection pressure. Corrosion behavior for N80 casing was studied during long-term service, using corrosion reaction kettle and weight loss method. The morphology and composition were characterized with laser confocal microscopy (LCM), scan electron microscopy (SEM) and energy disperse spectroscopy (EDS). These results showed that corrosion rate remained constantly at initial stage, and increased gradually, then sharply reached to 0.327mm/a at later stage, accompanying with inflection point under oxygen condition. Meanwhile, corrosion rate decreased significantly from 0.028mm/a to 0.020mm/a under oxygen-free condition. The corrosion of N80 steel was dominated by dissolved oxygen, and accelerated by chloridion. The micro-mechanism was suggested to be the transition of defect structure from crack with micron range to etch pit with millimeter range.

In view of the external thread fracture during casing running in a well in Xinjiang Oilfield, the failure reasons of thread fracture were analyzed by combining the theory with experiment, and the casing safe service window with harsh working conditions was given. The operation of the failed casing and determined the actual working conditions of the casing threaded joints during the running process were investigated in this paper. According to the theoretical method, the boundary conditions and load conditions of the fracture casing joint in service were determined. With the aid of full-scale physical simulation test device, as well as the above boundary conditions and load conditions, the same batch casing tensile bearing characteristics were determined. Through nondestructive testing, the metallographic observation, scanning electron microscopy, spectral analysis and up-and-down test, the main controlling factors of casing thread fracture were determined. By the finite element analysis, the casing threads service state under axial tension and bending loads was established, the safety performance of threads under ideal working conditions was studied, and the fatigue mechanism of threads was revealed. Combined with the analysis results, the corresponding relationship between the casing tensile bearing characteristics and safety factor was given, which provides technical support for the safe service selection of casing body and thread under harsh conditions, saving cost and shortening the well construction period.