Prediction To Residual Strength Of Defective Casing And Its Safety Evaluation

During the drilling process,the friction and collision between the drilling string and the casing are inevitable,which causing casing wear.In the process of production,the casing strings are exposed to the acidic environment for a long time,which causes the casing corrosion defects.When the defective casings are subjected to external loads,the stress concentration may be generated at the defect,resulting in a decrease in the strength and safety performance of the casing.Therefore,it has a very important significance to ensure the safety of production that accurately predicting the residual strength of the defective casing and evaluating its safety and reliability.In this study,the remaining strength and reliability of casing with crescent-shaped wear and corrosion cavity were studied.Firstly,for the single-point defect,different models were selected to calculate the residual burst strength and collapse strength of the defective casing under different defect depth,which were compared with the experimental datum.The results show that the residual strength of the casings is reduced with the increase of the depth of the defect,and the trend is linear.In these computational models,the results of the eccentric cylinder model are in good agreement with the selected test values,which was used in reliability analysis later.Secondly,the problem with multiple defects cannot be solved by theoretical analysis.Therefore finite element method was used to study the bearing capacity of defective casing.The results show that the influence of the radius,depth and quantity of the defects on the mechanical properties of the casing is different,and the load capacity of the casing is not sensitive to the change of the number of defects.But the change of the depth and radius of the defects has a great influence on the load carrying capacity of the casing.Finally,the probability distribution model of the residual strength and external load of the defective casing are obtained by the method of probability theory and mathematical statistics.The safety and reliability evaluation method of the defective casing is established,and it was used to the analysis of an existing well,which provides a new method for strength design and safety evaluation of casing.