Failure Analysis on Fracture of a Tubing Used in an Oilfield

A tubing used in an oilfield for a vertical gas well fractured after 56 days regular gas production. Macroscopic examination, chemical composition analysis, mechanical property testing, metallographic examination, scanning electron microscopy and energy spectrum analysis were used to comprehensively investigate the failure reasons of the oil tube. The results show that:the fracture of the oil tube belonged to the corrosion fatigue fracture; and the decarburization on the outer surface reduced the corrosion resistance of the oil tube; during the service process, the oxygen corrosion occurred on the outer surface where the water and air contacted, and the corrosion pitting formed; the stress concentration generated in the bottom of corrosion pitting, and induced the fatigue crack source; under the action of external force, fatigue cracks propagated constantly, and finally led to the fracture failure of the tubing.