3D Mechanical Characteristics And Sealing Performance Analysis Of Casing Premium Threaded Connections With Eccentric Wear

The exploitation of oil and gas resources moves further to super depth and high pressure environments,which make casing strings face with more complicated loading conditions,including huge tension,possible compression,large bending,high internal and external pressure.This puts forward high requirements for the performance of the casing string.In the actual borehole,the inner surface of the casing string is subjected to circumferential and axial friction due to the contact between the drill string or other operation string and the casing string,which causes the casing string to wear easily.Worn casing string is easy to fail under complex downhole loads and there is a great risk of huge economic losses.Field statistical results show that most of casing failure accidents occur at threaded connections.Therefore,the threaded connection is the weakest part of the casing string,so it is of great significance to analyze the threedimensional mechanical characteristics and sealing performance of casing premium threaded connection with eccentric wear.This is also an issue that is rarely concerned in the research of well integrity under complex load conditions.It is difficult to obtain an analytical solution of casing premium threaded connections with eccentric wear due to its complicated mechanical characteristics.There are some problems such as high cost,difficulty in measurement and difficulty in applying complex loads in the full-scale experiment analysis.The numerical simulation method has been proved to be a powerful tool for analyzing the stress characteristics of casing premium threaded connections.However,because the stress analysis of the casing threaded connection with eccentric wear involves geometric nonlinearity,material nonlinearity and contact nonlinearity,the three-dimensional stress analysis is very difficult.The current studies mainly focuses on the two-dimensional finite element analysis of unworn casing premium threaded connections,and the results of threedimensional research related to this aspect are almost blank.In this thesis,the three-dimensional elastic-plastic finite element method was used to study the stress distribution of dynamic wear casing premium threaded connection under complex loads with full consideration of geometric nonlinearity,material nonlinearity and contact nonlinearity.The influence of wear amount on its mechanical characteristics and sealing performance was emphatically studied.The influence of different wear patterns was also discussed.On this basis,considering the roughness of the sealing contact surface,a simplified three-dimensional finite element microscopic contact mechanics model for the main sealing structure of threaded connections was proposed.A criterion for evaluating the sealing performance of casing premium threaded connections was obtained and the sealing performance of cone to cone and ball to cone casing premium threaded connections were evaluated.The influence of the wear amount on the sealing performance of the sealing surface of the worn connections was discussed.Finally,the obtained sealing criterion was applied to evaluate the sealing performance of the worn connection in the actual wellbore,and the reliability of the method is verified.The main work and research results of this thesis are as follows:1)On the basis of optimizing the element type and mesh,the three-dimensional mechanical characteristics analysis of the casing premium threaded connection was realized,and the stress distribution characteristics on the critical path of the thread teeth,sealing surface and shoulder surface were obtained.Compared with the results of twodimensional analysis,it is found that the contact stress obtained by two-dimensional analysis method is too large because the plastic deformation of the material is not considered.2)Considering the material nonlinearity,geometric nonlinearity and contact nonlinearity,the three-dimensional finite element modeling of dynamic worn casing premium threaded connection was realized by using the "cutting method",and the related three-dimensional mechanical characteristics were analyzed.The results show that under complex loads,von Mises stress on the whole and wear area of the casing premium threaded connection with eccentric wear increases greatly,and local strength failure is easy to occur.The contact stress of the thread teeth is small in the middle and large at both ends.The contact stress of the thread teeth near the shoulder and the big end of pin is relatively large,which is easy to cause thread gluing.The average contact stress and the contact width of the sealing belt on the axial path of the least resistance to leakage of the sealing surface decrease with the increase of the wear amount,and the sealing failure is easy to occur.With the increase of wear depth,the uneven distribution of circumferential contact force on the shoulder surface increases,and the risk of leakage increases.3)The influences of different wear patterns on the mechanical characteristics and sealing performance of casing premium threaded connections under complex loads were studied.The results show that due to the wide wear range,the overall von Mises stress of eccentric cylinder wear connection is the highest,followed by the crescenttype wear connection,and then the unworn connection.That is to say,eccentric cylinder wear connection is most prone to local strength failure.On the axial path of the least leakage resistance of the sealing surface,the average contact stress and contact width of the crescent-type wear connection are less than the unworn connection.The eccentric cylinder wear connection has zero contact force and zero contact stress area,and has the lowest sealing ability.The contact force on the shoulder surface of eccentric cylinder wear connection is the most non-uniform,and the contact force at the wear position is the lowest,leading to the worst auxiliary sealing ability.The unworn connection has the most uniform contact force on the shoulder surface,followed by the crescent-type wear connection.4)A simplified three-dimensional finite element microscopic contact mechanics model for the sealing surface was established to obtain a criterion for evaluating the sealing performance of casing premium threaded connections,and the variation of the sealing performance of the connection with the wear amount was studied.The results show that the convex peak of the contact surface is high stress zone and the concave valley is relatively low stress zone under external load.The plastic deformation and displacement of convex peak are larger than that of concave valley.When the material is in the elastic deformation stage,the displacement of the connection in the microcontact mechanics model of the sealing surface changes very little.Once the material has plastic deformation,the displacement of the connection increases rapidly.Under the loading of " make-up torque + 110.0 MPa internal pressure",the sealing surface of1.94 ?m cone to cone and ball to cone casing premium threaded connections are leaking.It is necessary to add thread grease as an auxiliary seal in actual use.The casing premium threaded connection with surface roughness of 0.1 ?m has good sealing performance in the same loading condition.With the increase of wear amount,the critical leakage pressure difference of the sealing surface of the casing premium threaded connection decreases gradually,and the sealing ability decreases.With 5.0mm crescent wear,the critical leakage pressure difference of the sealing surface decreases by 40.85% compared with the unworn connection.5)A three-dimensional elastic-plastic finite element model was established for the casing premium threaded connection with eccentric wear in the actual wellbore,and the three-dimensional stress characteristics were analyzed.The sealing performance of the connection was evaluated by the proposed criterion.The results show that von Mises stress and plastic strain increase with the wear of casing premium threaded connection,which increases the risk of strength failure.The contact stress on the critical path of the thread teeth increases,and the average contact stress on the path near the top of the tooth and near the root of the tooth increases by 17.8% and 11.5% respectively,which increases the load of the thread teeth.When the material deforms plastically,it is easy to cause thread gluing.The uniformity of the contact force on the sealing surface decreases when the casing connection is worn.The sealing performance of casing premium threaded connections before and after wear were evaluated by the sealing criterion proposed in this paper.It is found that the connection before wear has good sealing performance,after wear of 4.0 mm,the calculated leakage volume is greater than the leakage threshold specified by ISO,and the connection begins to leak,which is consistent with the actual logging data.