Theoretical and numerical study on residual strength of worn casing considering wellbore curvature

Abstract

Casing wear induced by frictional contact of rotating drill string with casing, will lead to casing structure integrity failure, casing strength degradation, well life shortening and huge economic loss. Logging practices and theoretical calculations have found that casing wear is mainly concentrated in the curved section with large wellbore curvature. The additional bending load in curved section will result in further reduction of casing strength along with wear defects. Therefore, it is of great significance to quantitatively evaluate the residual strength of worn casing, especially in curved section. To solve this problem, mechanical analysis is carried out for the cross section of both single size wear and compound wear. The expressions for residual burst strength, collapse strength and tensile strength are derived. Considering the axial stress generated by casing bending in curved section, the modified expressions of residual strength are obtained. Finite element verification indicates that the theoretical calculation model provides satisfactory prediction accuracy under different wellbore curvature. Using the established models, the effects of wear depth and wellbore curvature on stress distribution and bearing capacity are revealed systematically. Remaining burst strength and collapse strength steadily decline as casing wear depth increases, and the change law is in good agreement with a linear connection. When inner pressure and outer pressure are both constant, von Mises stress increases with increasing of wellbore curvature, indicating that the bearing capacity of casing will be greatly reduced under the combined effect of wear and wellbore curvature. Negligence of wellbore curvature will contribute to over-estimation of casing strength. Field application of a direction well is also carried out, which indicates that the existence of bending stress will lead to further reduction of casing strength. But the effect of wellbore curvature is obvious only if wellbore curvature is large enough. This research may serve as both a theoretical framework and a practical foundation for the assessment of worn casing residual strength in directional wells.