Study On Corrosion Behavior Of X70 Steel In CO₂-Containing Formation Water Under High Temperature And Pressure

With the development of the oil and gas industry,research on CO₂corrosion will receive more and more attention,especially in the downhole environment,the problem of CO₂corrosion is more serious.Due to factors such as temperature,partial pressure of CO₂,and flow rate,etc.,the corrosion problem will be more complicated.Therefore,the corrosion behaviour of X70pipeline steel in CO₂?containing formation water at different temperatures?partial pressure of CO₂,and flow rate were studied in a high?temperature,high?pressure reaction kettle via weight loss measurements,scanning electron microscope,X?ray diffraction,energy dispersive spectroscopy,and electrochemical methods.(1)In formation water containing CO₂,when the partial pressure of CO₂is 1.5MPa and the temperature is 30?150?,the corrosion rate of X70 steel decreases first and then increases,and the corrosion rate is the smallest at90?.When the temperature is 30?,the produced corrosion product Fe CO₃cannot adhere to the surface of the sample,and it cannot play a protective role.When the temperature is 60??90?,the corrosion products increase and can completely cover the surface of the sample,which can effectively protect the substrate,and the corrosion form is a comprehensive corrosion form.When the temperature is 120??150?,the corrosion product film is broken,the protection performance of the substrate is reduced,and the corrosion rate is increased.The Fe CO₃corrosion product film forms galvanic corrosion with the exposed steel substrate,and the"big cathode"and"small anode"accelerate the local corrosion rate.The corrosion form is localized.(2)when the temperature is 90?and the partial pressure of CO₂is 0?2MPa,the corrosion rate of X70 steel increases first and then decreases,and the corrosion rate is the largest at 1.5MPa.When the partial pressure of CO₂is0MPa,the solubility of CO₂is small,the crystallinity of the corrosion product Fe CO₃is larger,and the corrosion rate is lower.When the partial pressure of CO₂is 0.5MPa?1.5MPa,the solubility of CO₂increases,the crystallinity of Fe CO₃crystals decreases,and the corrosion rate increases.When the CO₂partial pressure is 2MPa,the high CO₂partial pressure makes the corrosion product film denser,which plays an effective protective role and reduces the corrosion rate.(3)when the temperature is 60?,the partial pressure of CO₂is 1.5MPa,and the flow rate is 0?2m/s,the corrosion rate of X70 steel increases first and then decreases,and the corrosion rate is maximum at 1m/s.when the flow velocity is 0 m/s,the charge transfer between the corrosive ions in the solution and the substrate is slow,and the corrosion rate is low.When the velocity is 0.5m/s?1m/s,as the stirring effect increases,the boundary shear stress increases and the corrosion rate increases.When the flow velocity is2m/s,the high flow rate promotes the passivation of the corrosion product film.At the same time,the corrosion rate of X70 steel at high flow rate is mainly affected by the diffusion flow rate.The diffusion flow rate decreases and the corrosion rate decreases.