Study On Safety Performance Evaluation Of Vulnerable Parts Of Downhole Casing String

With the continuous development of oil and gas development technology,the number of high-temperature and high-pressure wells and ultra-deep wells continues to increase.The load on the downhole casing string during its service period is more complicated,and the casing string and its accessories are more prone to damage.Therefore,this paper selects the two most representative vulnerable positions of downhole casing strings in non-API casing joints and perforated section casings as the research objects.The theoretical analysis method and finite element method are used to make the downhole string vulnerable.The safety performance of the part is evaluated and studied.The research on non-API casing joints is mainly carried out from two aspects: connection performance and sealing performance.First,the different loads on the downhole casing joints are analyzed,and the calculation formulas for the critical load of the casing joints under different failure modes and the axial force distribution formulas of the threads are deduced.The calculation results show that the thread threads of the non-API casing joints are most likely to occur Shear failure failure and the axial load distribution law is "high at both ends and low at the middle";through the finite element analysis of the equivalent stress of the joint under different loads,it is found that the joint sealing structure and the end of the thread are the most likely to fail.Secondly,according to the characteristics of the main seal structure of the nonAPI casing,the calculation formula for the contact pressure of the cone-to-cone surface seal and the calculation formula for the gas leakage rate of the metal rough surface are derived,and the law of the influence of different parameters on the gas leakage rate is obtained by calculation;The sealing contact pressure and contact length of non-API casing joints under different loads are obtained.Combined with the sealing contact strength,the sealing criterion is given.The sealing performance of the joint can be ensured when the average contact stress reaches 4.5-5times the gas pressure in the pipe..When analyzing the mechanical properties of the perforation section casing with different perforation shapes and hole edge cracks,firstly,the stress value at each point of the hole side is solved according to the calculation model of the perforation section casing hole edge stress with different perforation shapes.Through comparison,it is found that the mechanical properties of the perforated section casing under the circular perforation are least affected;the comparison between the finite element calculation results and the theoretical value shows the rationality of the model.Then,according to the calculation model of the stress intensity factor of the hole edge crack under different number of cracks,the type I stress intensity factor at the crack tip of the elliptical hole with different number of cracks is solved respectively.The results show that the greater the difference between the long and short axis of the elliptical hole,The greater the type I stress intensity factor at the crack tip,the more significant the influence of the crack length along the axial direction of the casing on the type I stress intensity factor.Finally,analysis and calculation of the loading conditions and safety performance of the casing string of the whole well under different working conditions are carried out,and the safety performance analysis software of the vulnerable parts of the downhole casing string is compiled based on the mechanics theory and the programming language Visual Studio.