The Analysis Of Fluid-solid Coupling Vibration Characteristics Of Tubing String In Fracturing Condition

In the fracturing process,the pipe string is used as the main bearing component and the power transmission component,and it needs to bear the mutual effect of its weight,axial force,internal and external fluid pressure.These effects cause the pipe string to vibrate.When the fracturing fluid flows through the tubing string at high speed,the fluid-structure interaction vibration will occur in the tubing string.Severe vibration will lead to wear and fatigue of the tubing string,and even downhole accidents.The problem of the failure of the tubing string during the fracturing process is increasingly prominent.Therefore,the following researches have been conducted on the fluid-structure interaction vibration of the fracturing tubing string.Firstly,the failure mechanism of fracturing tubing string is discussed,and the flow field at the fracturing tubing collar is analyzed.Then the fluid-structure interaction vibration equation of fracturing tubing string is deduced by the equations of motion and continuity of fluids and the vibration and constitutive equation of solids.The established equations are decoupled by the characteristic line method and discrete by spatial interpolation.And the dynamic analysis of tubing string and fracturing fluid under start-stop pump operating conditions is carried out.Finally,the finite element software is used to simulate the downhole conditions.The effect of axial force,internal pressure and the presence of fluid-structure interaction on the natural frequency and mode shape of the tubing string are analyzed.The results show: The fracturing fluid flow rate,pressure fluctuation,and axial additional stress increases as the displacement increases;the pressure fluctuation value of the fracturing fluid and the axial vibration velocity of the tubing string decrease with the depth of the well;the shorter the time to stop the pump,the greater the increase in the pressure fluctuation value of the fracturing fluid and the tubing axial vibration speed;the natural frequency of fractured tubing string is reduced by 7% compared with no fluid-structure interaction when considering fluid-structure interaction;the natural frequency of the fracturing string increases with the axial force and decreases with the increase of the internal pressure.When the internal pressure is 80 MPa,the first-order natural frequency of the fracturing tubing string drops to 0.The tubing string is prone to resonance because it is in a state of instability.