Study On Vibration Mechanical Characteristics Of Tubing String In High Production Gas Well

With the exploration and development of large-scale gas fields in Tarim,Changqing,Sichuan and other places,the natural gas production in China has been greatly improved.However,the increase of production also brings new production problems.The vibration of the tubing string caused by the impact of high speed natural gas in the tubing string can't be ignored.Due to the vibration of tubing string caused by natural gas,many high production gas wells cause premature fatigue failure of tubing string,which has a serious impact on normal production.The traditional statics analysis of tubing string can not solve this problem,so the study of tubing string dynamics is very meaningful.In this paper,a coupled calculation model for the pressure and temperature of high production gas well was established,and the distributions of the temperature,pressure,velocity and density of the natural gas in the tubing string under different yield were studied.A finite element model for the flow field of natural gas flowing through the tubing collar was established.The distributions of the pressure,velocity and turbulent kinetic energy of the natural gas flow through the tubing collar in detail were analyzed.Based on the theory of elasticity and finite element method,the vibration model of the whole tubing string was established.Using the modal analysis method,considering the influence of the packer,the first ten natural frequencies of the vibration of the tubing string were calculated and the first four modes were extracted.Using the harmonic response analysis method,considering the influence of the packer,the resonances of the tubing string between the first 4 natural frequencies were analyzed.Transient dynamic response analysis method was used to analyze the vibration velocity,acceleration,displacement and equivalent stress at different positions of the tubing string under two conditions of constant output and fluctuation of output.The analyses of the temperature,pressure,flow velocity,and density of natural gas flowing in the tubing string show that different production rate will change the temperature,pressure,velocity,and density of natural gas along the flow direction.Abrupt changes in flow path will change the velocity,pressure and turbulent kinetic energy of the natural gas.The closer the tubing collar is located to the wellhead,the greater change in velocity,turbulence intensity,and turbulent kinetic energy of the natural gas,the greater force on the tubing string.This will increase the exciting force generated by natural gas flow on the tubing string,and the greater the production rate,the more pronouncing effect.The restraint of the packer will increase the overall stiffness of the tubing string,the natural frequency of longitudinal vibration of the tubing string will increase,but the increase is small.Harmonic response analysis shows that the tubing string tends to generate longitudinal resonance at low natural frequencies and the longitudinal displacement generated by the tubing string near the first-order natural frequency is relatively large.With the fluctuation of output,the tubing string will be subjected to random excitation,the excitation frequency and excitation amplitude is unstable and the tubing string always is in an unstable vibration state.As the production rate increasing,the vibration speed and dynamic stress of the tubing string increase.This studies about the vibration mechanical characteristics of the tubing string in high production gas well in this paper,provide the theoretical basis for the strength design of the tubing string,the safe production of high production gas well and the prolongation of the service life of the tubing string.