Characterization, formation and development of scales on L80 steel tube resulting from seawater injection treatment

Scale formation in oil and gas production wells is a serious problem that could cause flow reduction and increase the risk of corrosion perforation. In this paper, several characterization methods including visual observation and digital photography of the interior surface of the tubes and scales, high resolution scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), have been utilized to investigate the scales. Results show that the scale has a layered structure. Moving from the steel matrix towards the inner tube surface, the scale consists of a dense layer of iron oxy-hydroxide, a porous layer of iron hydroxide and chloride-rich products, a bulk layer of akagneite (Fe8O8(OH)8Cl1.35) and hibbingite (γ-Fe2(OH)3Cl), a thin layer of copper sulphide (Cu2S) and a granular layer of barite (BaSO4). The different layers have their unique morphological characteristics and mineral phases, implying different formation mechanisms of the layers. A mechanism is proposed for the formation and growth of the scales considering the history of the well. The mechanism suggests that corrosion scales develop from the inner tube surface via corrosive reactions. Scale growth is a result of continuous corrosion followed by a combination of deposition and oxidation of the corrosion products. Scale structure and composition play important roles in the migration of chloride ions and in the reactions of corrosion scales that lead to iron release.