Strength Calculation Of Extreme-line Casing Joint

In recent years,side tracking point had to be moved upward to the hydratable and collapsing mudstone formations beyond the producing zone in order to meet the requirements of avoiding water layers for some sidetracked wells in TaHe oilfield.To prevent complicated downhole wellbore instability and collapse of the shale formation,the shale interval need to be packed.The conventional casing would lead to horizontal well completion size is too small to meet the production needs.At present,solid expandable tubing was used to seal the unstable mudstone formation.But solid expandable tubing is facing low collapse resitance and hight cost.It is feasible techinical to use extreme-line casing instead of the solid expandable tubing to seal the unstable mudstone formation.In order to ensure the rational use of extreme-line casing,so the joint's strength of extreme-line casing is needed to study detail.This paper is based on the curve of buckling torque and elastic wall theory,the calculation model of buckling torque of extreme-line casing is established and analyzed the influence of interference on buckling torque.Based on the buckling torque model and considering the influence of the screw angle,deduced the axial load distribution equation of the threaded joint of the screw thread,and analyzed the Interference of shoulder and thread to the contact pressure of bearing surface of thread.Deduced the failure model of the casing joint based from the form of failure style of casing joint,calculated the tnesile strength of joints.Based on the theory of thick-walled cylinder and considering the influence of thread interference,established the collapse model of extreme-line casing and calculated the collapse strength of pipe and joint respectively.Based on the theory of elastic-plastic and considering the influence of geometrical nonlinearity and non-linearity of extreme-line casing,established the finite element model of force and deformation of extreme-line casing.This paper have simulated the contact pressure of extreme-line casing in situation of make-up,axial tensile and collapse pressure.The tensile strength and collapse pressure have been forecasted.Then compared analytical value of tensile strength and collapse pressure to the value of finite element method.The result of mechanical behavior analysis and finite element simulation shows that buckling toruqe increases with the increase of the interference.When thread is tightened,most of the axial load generated by the interference of shoulder is borne by the last few screw,and the contact pressure of the thread bearing surface increases with the incease of the interference of thread.With the increase of the interference of the thread,the collapsing pressue is decreased.The collapsing pressure of joint of CST-ZT ?139.7mm×7.72mm is greater than pipe body.The relative error of the tensile strength analysis value,the finite element simulation value and the experimental value of CST-ZT 0139.7mm×7.72mm is less than 7%.When casing is pulled,the failing of CST-ZT ?139.7mm×7.72mmm is the joint rupture,the tensile strength of the join is 93%of the tensile strength of the pipe body.Optimizing the thread pitch can improve the uneven force of the thread,and increasing the radius of the root fillet can reduce the stress concentration at the root of the tooth and improve the tensile strength of the joint.The results of this paper have important theoretical research and practical value for the optimization of the parameters of the straight type casing and the rational use of the straight type casing.