Casing Integrity: Modeling Strength Degradation

Abstract

Abstract Casing integrity management is crucial, especially in wells experiencing severe casing wall degradation. It is highly desirable to predict the threshold pressure for degraded casing burst and collapse strength and to design the casing strings considering these wear effects. Knowledge of stress distribution in worn casing helps predict where casing failure occurs first. This study focuses on building a model for stress distribution along the casing circumference based on "crescent-shape" casing wear. A common method is to estimate the reduction of the casing burst strength in worn casing using the API burst strength equation with a linear reduction by the remaining wall thickness or wear percentage equivalent to a "uniformly-worn" casing model. This paper presents a rigorous engineering method to calculate the hoop strength directly, including the local bending in the thinner portion of the "crescent-worn" casing. A model and correlation to calculate varying hoop strength in the crescent-worn shape and non-worn casing have been developed with force and moment balance equations. The stress predicted in this paper is similar to the results obtained from the finite element method (FEM), which validates equations and results obtained from this paper. The developed model is generic and can be used for casings, risers, and tubings.