Analysis Of Casing Wear During Workover Drilling And Milling Based On Drill String Dynamics

Timely workover operations are required to restore production in the event of downhole incidents in the oil and gas wells.In view of the characteristics of workover operations,the dynamic characteristics of the drill string during workover operations are more complex,and the induced casing wear can pose a threat to well integrity.Therefore,based on the characteristics of workover operations,this paper studied the dynamic characteristics of drill string and casing wear during workover drilling and milling operation,in order to provide reference for the research on dynamic characteristics of drill string,operation parameter optimization and wellbore integrity evaluation during drilling and milling operation.This paper mainly completes the research work in the following aspects:(1)Analysis on dynamic characteristics of drill string in drilling and milling operationBased on the characteristics of drilling and milling operation and the force analysis of drilling string system,the finite element modeling method of drilling string in drilling and milling operation was studied considering the characteristics of geometric nonlinearity,boundary condition nonlinearity and contact nonlinearity.Based on Hamilton’s principle and Lagrange equation,the dynamic finite element model of drill string in drilling and milling operation was established.Combined with the rub-impact judgment conditions,the dynamic characteristics of drill string and the rub-impact characteristics of different parts of drill string on casing during drilling and milling operation were obtained.The influence of key workover operation parameters on the dynamic characteristics of drill string was investigated.(2)Analysis of casing wear induced by rub-impact of drill string during drilling and milling operationOn the basis of the finite element simulation analysis of the dynamic characteristics of drill string during workover,taking casing as the research object,the wear pattern of casing induced by the rub-impact of drill string during drilling and milling operation was analyzed.The casing wear volume calculation model is used to quantitatively the degree of casing wear induced by drill string rub-impact was studied,and the influence of key workover parameters on casing wear induced by rub-impact was investigated.(3)Analysis of the influence of rub-impact wear defects on the load-bearing performance of casingFor the discontinuous and irregular casing wear defects induced by drill string rub-impact during workover drilling and milling operations,a wear casing model with such wear defects was established by means of "regularization".The evaluation method of stress concentration ratio was introduced to investigate the influence of rub-impact wear defects on the load-bearing performance of casing when subjected to external pressure,internal pressure and tensile load.