Evaluation of casing integrity defects considering wear and corrosion - Application to casing design

Casing integrity is an important category of well integrity in drilling and well operations. Casing integrity defect due to wear and corrosion can cause casing strength degradation, casing deformation and even well abandonment. In this paper, a theoretical model for casing strength degradation due to wear is established in bipolar coordinate system. Another model is established to calculate stress concentration factor of casing with corrosion pit at inner wall. The effects of relevant parameters on residual strength of defective casing are analyzed according to parametric study. According to the stress distribution of casing after wear and corrosion under tri-axial stress, strength check is carried out based on Von Mises yield criterion and Lame thick-walled solution for the pipe. Then, in order to evaluate casing integrity of a real well, casing wear and corrosion experiments are conducted to study mechanisms and obtain relevant parameters for calculation. Finally, according to the established models and experimental results, a modified casing design of this well is proposed considering the effects of wear and corrosion on casing strength degradation. The work presented in this paper can provide a theoretical foundation and technological basis for casing integrity evaluation and casing design of highly-deviated wells or extended-reach wells in sour environment.