Failure analysis of P110 tubing string in the ultra-deep oil well

A number of P110 carbon steel tubing, one of the very important parts of an oil well pipe system, suffered corrosion and scaling after being used for a relatively short times, which would result in the leakage of tubing string and the decrease of productivity. Optical metallographic microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (XRD), combined with the weight loss method and electrochemical characteristic technology were used to determine the most probable causes of the failure. The results showed that the composition and structure of tubing and coupling were in accordance with the related parameter requirements of API Spec 5CT standards, and the circumferential annular corrosion groove at the roots of male buckle side resulted from the crevice corrosion, while the axial corrosion wide slot at the outer surface of coupling and corrosion perforation of tubing were due to the reasons that the corresponding parts were mechanical damaged firstly and then suffered the severe local corrosion, in addition, the composition of corrosion products were mainly FeCO3 and Fe3O4, and scaling layer were composed of the heavy components of the crude oil, CaCO3 and a little of corrosion products. Finally, the proper annulus protection fluid, inhibitors and paraffin removers were advised to be used for preventing this type of failure from happening again in the further.