Research On Potential Failure Mechanism Of Tubing String In High-yield Gas Wells In Tarim Oilfield

In recent years,a series of major accidents have occurred in high-yield gas wells in China and the world.Many high-yield gas wells in the Tarim Oilfield have caused leakage and fracture of the string,causing major economic losses.Based on a large number of domestic and foreign literature investigations and statistical analysis of on-site tubing string failures in Tarim Oilfield,this paper uses the combination of theory and numerical simulation to carry out buckling,vibration and containment of high-yield gas well tubing columns in Tarim Oilfield.The failure mechanism of corrosion pit defects is studied.Based on this,the fuel column failure prevention measures are proposed.The main results achieved are as follows:(1)Established a mechanical mathematical model of high-yield gas wells,packer and column piston effect,bulging effect,temperature effect,spiral bending effect and fluid friction effect.Based on gas dynamics theory,natural gas fluid in tubing string was established.The mathematical model of the dynamic control equation provides a mathematical model for the analysis of the column buckling problem.(2)The mathematical model of the lateral vibration,longitudinal vibration and natural frequency of the tubing string and the mathematical model of the transient vibration of the tubing string are established.It can be used to analyze and study the displacement and vibration velocity of the tubular string under the alternating load.The variation of vibration acceleration provides a mathematical model for the induced column vibration of the gas in the high-production gas well and the coupled vibration analysis of the tubing string.At the same time,it provides a theoretical basis for the study of the potential failure mechanism of the high-yield gas well tubing string in the Tarim Oilfield.(3)The finite element model of the buckling and vibration analysis of the tubing string at the wellhead to the packer of the high-production gas well was established.Through the study of gas production of 60-1.5 million squares,it can be concluded that as the output increases,the axial pressure at the bottom of the tubing increases,the buckling of the pipe string gradually increases,and the tubing gradually changes from a straight state to a sinusoidal buckling,with a high yield of 1.5 million.Under the square working condition,the tubing string and even the spiral buckling state,the bending stress and axial stress of the pipe string increase,the contact pressure between the oil pipe and the well wall gradually increases,and the contact point gradually increases,resulting in the friction between the oil pipe and the well wall gradually.Increase.(4)When the bottom axial force is 223kN(1.5 million square),the critical buckling load of the tubing string is 172.94kN,the contact pressure maximum reaches 53kN,and the contact pressure is unevenly distributed in the contact section.Sinusoidal buckling occurs at the top of the oil jacket contact position,and a spiral buckling self-locking phenomenon occurs at the bottom.(5)Using the theory of elastic mechanics and finite element method,the finite element model of the tubing string vibration of the whole well is established.The modal analysis method is used to consider the influence of the packer.The mode and vibration of the lateral vibration of the tubing string are studied.Using the transient dynamic response analysis method,the longitudinal vibration displacement,velocity and acceleration of different positions of the tubing string were analyzed.It is concluded that the longitudinal vibration displacement,velocity and acceleration of the tubing string change periodically,and the alternating load is applied near the neutral point of the tubing string,which may lead to fatigue failure of the tubing string.(6)According to the investigation of the shape of the corrosion pit in the corrosion failure well on site,a tubing string model with single corrosion pit and adjacent corrosion pit defects was established.The corrosion pit depth,the distance between the corrosion pits and the wall thickness of the tubing were analyzed.The impact of tubing damage.It is concluded that the deeper the depth of the corrosion pit,the closer the distance between the corrosion pits,the smaller the wall thickness of the tubing,the greater the stress around the corrosion pit,and even exceed the yield stress of the material,resulting in plastic deformation or even cracking of the material,causing the column to break.(7)Based on the above research on the failure mechanism of tubing,the failure prevention measures of tubing string are proposed,and the effects of different materials and column structures on the failure of tubing string are compared.Increase the wall thickness of the tubing that is prone to damage;add telescopic short joints at the bottom of the tubing to reduce the axial load at the bottom of the tubing string;use corrosion-resistant titanium alloy tubing to prevent the failure of the tubing string.It provides technical support and scientific basis for the safety of high-yield gas well tubing string in Tarim Oilfield,which is of great significance for the rational design of gas well tubing string and the improvement of the service life of tubing string.