Characteristics and formation-growth model of corrosion scales in oil wellbore

Scaling formation in the wellbore is a serious problem and affects the corrosion behaviour of the pipeline materials by changing the morphology and the physiochemical properties of the surface layer. This paper presents the results of a comprehensive study of scale found in oil well bore used in North Sea oilfield. Scales were removed from the inside of pipes which were taken from different depth under sea level and investigated by several analytical methods including visual observation and digital photography of the interior surface of the pipe and scales, high resolution scanning electron microscopy, X-ray diffraction, Energy Dispersive X-Ray Spectroscopy etc. The characteristic images of scale were obtained by microscopic analysis and results showed scale with characteristic structural features including a dense layer close to the corroded steel surface, a porous layer of iron hydroxide and chloride-rich products, bulk layer of akagneite, a thin layer of copper sulphide and a granular layer. A conceptual model was developed in this paper to describe the formation and growth of scale based on the history of the well. According to the developed model, corrosion scales were expected to grow from inside the scale via the corrosion reaction. Scale structure and composition played important roles in the migration of chloride ions and the reactions of corrosion scales that lead to iron release.