Analysis Of Casing Wear And Residual Strength During Workover Milling

In case of well falling objects,it is necessary to put the drill string into the wellbore for workover drilling and grinding.When the drill string rotates in the wellbore,it will whirl and collide with the inner wall of the casing,resulting in casing wear and reducing the safety of the wellbore.It is of great significance to study casing wear caused by drill string whirl during workover milling to understand casing working condition and improve wellbore integrity.Considering the nonlinear contact of drill pipe joint casing and grinding shoe casing,the dynamic model of drill string is established by ANSYS,and the enhanced Lagrange method is used to solve the contact problem.The whirling trajectory and collision characteristics of drill string are obtained by simulating the whirling of drill string in vertical wellbore.The whirling characteristics of drill string and the impact and rolling wear of drill string whirling on casing are discussed based on the analysis of wear volume during collision,the influence of drill string speed on casing wear is revealed.The results show that: the contact of drill pipe joint,grinding shoe and casing caused by whirl has two forms: point to line and line to line,and the main contact is point to line;the wear of casing caused by grinding shoe and joint is mainly discontinuous wear along the circumference of casing,and the wear caused by grinding shoe is much higher than that of drill pipe joint;except the results near the critical speed,the higher the rotation speed of drill string system,the better the drilling performance.The higher the bit pressure of the drill string,the greater the wear of the drill string caused by whirl.After getting the casing wear during milling.The analytical model of casing wear volume caused by drill string whirling during workover milling is established.The residual tensile strength,internal pressure resistance and external extrusion resistance of the worn casing are analyzed.It is found that the wear caused by high speed makes the casing strength decrease seriously.The tensile strength decreases by 15.6%,the internal pressure resistance decreases by 16.3% and the external extrusion resistance decreases by 15.5% On the premise of milling footage,reduce the rotation speed of drill string as much as possible.