Research On Prediction Method Of Post Buckling Wear Of Slender Rod String In Vertical Section Of Wellbore

In the process of oil exploitation,the phenomenon of rod string wear is common,which is easy to cause the wear and leakage of tubes or make rod string broken,reduce oil field output,and cause underground safety accidents.In addition,the most serious wear phenomenon is the post buckling wear of rod string,which is more severe than ordinary wear.Therefore,it is very important to study the post buckling wear of rod string.In this paper,slender rod string in the vertical section of the underground is selected as the research object,and three-dimensional beam element is used to establish the finite element model of the buckling of the rod string in the wellbore,and the buckling configuration of the rod string is calculated based on the slow dynamic method,so as to obtain the sinusoidal and helical buckling configurations of the buckling beam.In order to predict the wear of the buckling rod string and wellbore,a finite element sub-model of the buckling rod string was established by a solid model,then the displacement and the calculation results of internal force extracted from the beam element are mapped to the solid element as boundary conditions of the solid sub-model.The calculation results of contact force of the buckling sub-model and the wellbore is obtained by solving the finite element equilibrium equation of the buckling solid sub-model.Then the Archard model was introduced to calculate the wear depth of the rod string,and the moving contact boundary method was used to describe the removal process of the surface material of the rod string.In order to avoid the problem of non-convergence of equilibrium equation caused by mesh distortion which is due to wear,a prediction method for post buckling wear of rod string in wellbore was established.Based on the above calculation method,the buckling configuration and wear depth of downhole compression rod string,compression torsion rod string and compression torsion belt joint rod string were predicted.The research results show that:(1)When the buckling configuration is sinusoidal,the wear parts of the rod string are less,and the compression load and annular clearance have little influence on the wear of the string;When the buckling configuration is spiral,the wear parts of the rod string are more and they were continuous,besides,the compression load and annulus clearance have a great influence on the wear of the string.The wear depth of the rod string increases with the increase of the load and the annular clearance.(2)When the rod string is subjected to compression load and torque at the same time in the mine,the increase of torque and compression load will make the rod string buckle seriously and increase the wear depth.(3)Under the action of a certain compression load and torque,compared with the rod string without joint,the spiral buckling pitch of the rod string with joint is larger,and the contact points are fewer than those without joint.But the local contact pressure is larger,so the wear depth is larger.Based on the wear depth and other results,the residual strength of the rod string with joint is calculated,and it is concluded that the fracture resistance and tensile strength of the rod string are reduced due to the wear of the rod string and the wellbore caused by buckling.And the residual strength of rod string is different with different wear depth.The results can provide theoretical basis for the safety evaluation of underground rod string.