Research On Collapse Strength Of Bending Casing With Compound Wear

In recent years,the deep wells,large displacement wells,horizontal wells and the three-dimensional directional wells have increased year by year.Compared with conventional vertical wells,on the one hand,the casing in the bending section bends with the borehole,resulting in bending and flattening;on the other hand,the casing is worn by the drill pipe during the drilling process.The collapse strength of casing in bending section much weaker,based upon the interaction of these factors,which will lead to a serious threat for subsequent wellbore integrity.For the collapse strength of bending casing,the studies conducted in recent years have mainly focused on the influence caused by single factor,such as bend or wear the study of the extrusion strength of the casing under of bending or wear.However,there has been very few studies that considers multiple factors such as bending,deformation and casing wear.Meanwhile because of the whole drill string size is not invariable,the wear error caused by the change of drill pipe size also needs further study.This paper will report the study of the collapse strength of bending casing considering the various factors simultaneously.The main contents are as follows:(1)Bending casing mechanical model was established according to the theory of material mechanics,considering the influence of the bending stress to the casing collapsing strength.Based on the casing collapsing strength formula,we derived the bending casing collapsing strength formula by using Von Mises yield failure criterion,which can provide the theoretical foundation for further research.(2)The elastoplastic casing mechanical model was established according to the theory of the flattening bend casing and the ultimate curvature of the casing in each stage was analyzed.The formula of the casing collapsing strength was derived after the casing was flattened and deformed and it was expounded that the variation law of the casing collapsing strength with borehole curvature before and after being flattened.Meanwhile,we have also analyzed the influence of such factors as bending,flattening,borehole curvature and axial load on the casing collapsing strength.(3)The rigid plastic casing model has been built in view of the buckling characteristic of the bend casing with large diameter-thick ratio and the stress strain relationship between the ovalization deformation and the buckling fold deformation of the casing cross-section has been analyzed,based on which we can not only obtain the buckling critical pressure and critical curvature of the rigid plastic casing,but also analyze the diameter-thick ratio of the casing and the influence of other factors such as length on the critical pressure of the casing.(4)According to the different sizes of the drill pipe joints involved in the wear,two geometric models of compound acute wear and compound blunt wear were established on the basis of the single size crescent wear hypothesis,and the influence of compound wear on the casing extrusion strength was analyzed by analytical method and finite element method.At the same time,we obtained a casing wear prediction model and expounded the influence of composite wear on depth,width and speed of the wear and casing collapsing strength.In this paper,we obtained the wear depth and wear width,axial load and borehole curvature,and other factors on the bending effect mechanism of the casing collapsing strength by establishing the curved casing mechanical model,considering the influence of the bending,flat,buckling and compound wear on casing collapsing strength and improving the composite wear casing section geometric model,which can enrich the theory of bending casing collapsing strength,provide the bending and deformation mechanism of casing section strength analysis a reference basis in complex cases and supply the theoretical guidance for optimal design of bending casing collapsing strength.