Casing Strength Degradation due to Corrosion - Applications to Casing Pressure Assessment

Abstract

Abstract Well casings are subjected to severe corrosion because of their exposure to corrosion environments - seawater, saline formation water, and the corrosive fluids that are transported. Corrosion develops pits and cavities at both the inner and outer walls of the casing. The burst and collapse loads are the most common loads for casing under operation conditions. The loads act on the pitted casing will cause stress concentration and degraded casing strength. The strength deterioration can significantly shorten the casing life; even cause failure of the well. Thus, it is highly desirable to know the stress concentration factor (SCF) for both casing designing and evaluation processes. The degree of stress concentration depends on the cavity shape. This paper derives simple formulae for SCF around cavities of various geometries including shallow-spherical (depth is less than the open radius), exact hemispherical (depth is equal to the open radius), and deep-spherical (depth is greater than the open radius) cavities. SCF graphs are generated and results of sensitivity analyses are presented. A comparison analysis of the shallow-spherical, exact hemispherical and deep-spherical cavities SCF formulae is also presented. The analysis shows that the shallow-spherical and deep-spherical formulae give almost the same SCF results. Thus, any shallow-spherical or deep-spherical formulae can be used in designing and evaluating casing with spherical cavities. Finally, application procedure for predicting the degraded casing strength is illustrated in this paper. Introduction Literatures indicate that pitting corrosion is the most insidious and common corrosion on casing and the corrosion pits act as stress risers to decrease the pressure integrity of the casing which caused casing failure1–6. Comparatively little research has assessed the effect and integrity of the casing strength based on corrosion pits geometry shapes and dimensions. It is highly desirable to predict the extent of stress concentration caused by corrosion-induced pits and cavities for both casing designing and evaluating processes. Studies on pitting corrosion indicate that pitting corrosion is a localized form of corrosion by which holes are produced in the structure wall.7–9 It is most likely to occur in the presence of chloride ions, combined with such depolarizers as oxygen or oxidizing salts. Small scratches, defects, and impurities in the steel pipe wall can initiate the pitting process. Mechanism analysis has shown that, because pits can be either hemispherical or cup-shaped, apart from the localized loss of thickness, corrosion pits on the tubing wall can cause severe local stress concentration if the casing is subjected to loads. The most damaging load for casing is the burst and collapse load.10 Burst load to the well casing is originated from the column of fluids inside of the casing. The collapse load is generated by the hydrostatic head of the fluid column outside the casing string, which is usually formation fluid. The degree of stress concentration is affected by cavity shape. Corrosion-induced pitting falls into four categories based on geometry shapes:spherical cavity as shown in Fig. 1;cylindrical cavity as shown in Fig. 2;ellipsoidal cavity as depicted in Fig. 3, andirregular shapes cavities as sketched in Fig. 4. This paper focuses on analysis of stress concentration around spherical cavities only. The corrosion cavity shown in Fig. 1 is an example of deep-spherical cavities.