Corrosion Behavior of N80 Carbon Steel Tubing in Multi-Thermal Fluid Environment

Downhole hanging ring method was used to evaluate the corrosion behavior of wellbore tubing injected with multi-thermal fluid. And the corrosion rate and corrosion product film characteristics of hanging rings at different wellbore positions were studied by weight loss method, scanning electron microscope, energy spectrum analyzer and 3D morphology analysis of corrosion pits. The results showed that the corrosion rates of the inner and outer rings increased significantly with the increase of wellbore depth. Due to the difference of corrosive media between the inside and outside of the tubing, the corrosion rate of the inner ring was much higher than that of the outer ring. But the ratio of corrosion rate between the inner ring and the outer ring decreased gradually with the increase of wellbore depth. The corrosion products on hanging rings with a multilayer structure were composed of a lot of iron oxides and a small amount of ferrous carbonate. And the layered structure of the corrosion products on the outer ring was relatively more complex compared to the corrosion products on the inner ring. In the middle and upper part of the wellbore, corrosion was mainly dominated by the oxidation reaction of high-temperature multi-gas-phase fluid, while in the lower part of the wellbore, it was mainly dominated by the electrochemical corrosion of high-temperature CO₂-O₂ solution.