Research On The Lateral Vibration Characteristics Of The Fluid-solid Coupling String System In The Wellbore

The pipe string in the wellbore is a common type of column structure in petroleum industry.The movements of these super-thin structures are limited in a narrow wellbore filled with fluid,and that are prone to instability of transverse vibration,and collision with the pit shaft under the action of various loads.That can create hidden danger in the course of normal operation of column.On the basis of previous studies,the ideas and methods concerning researching on lateral vibration of the conveying-fluid pipes are applied to that of the pipe string within the wellbore,in this paper.The stepped pipe involving fluid-structure interaction within wellbore is abstracted from the oil drilling and production engineering,and the transverse vibration control equation of that is established,considering itself gravity,the bottom force,and the fluid pressure and interaction forces both inside and outside pipe string.Taking two types of especial pipe string-the string with uniform section and the single stepped string for instance,the fourth-order partial differential equation of lateral vibration of that is discretized into ordinary differential equations by using the Galerkin method and finite element method,respectively.By means of the Bolotin’s method,the effects of the velocity and density of fluid within pipe string,bottom axial force,pipe length and wellbore diameter on the stability of system are studied,when the bottom axial force is constant and varies harmonically with time.Research shows that:when the axial force is constant,the influences of the velocity and density of fluid,axial force,and pipe length on the pipe string with uniform section are same as the single stepped pipe string.That is,increasing the velocity and density of fluid,axial force,and pipe length,the stability of system decreases,and increasing the wellbore diameter,the stability increases.When the axial force varies harmonically with time,increasing the average value and fluctuation amplitude of axial force and wellbore diameter,the stability of the string with uniform section and the single stepped pipe string decreases.Increasing the velocity and density of fluid and pipe length,the stability of pipe string with uniform section increases,however that of the single stepped pipe string decreases.Based on the researches on lateral vibration of the stepped pipe string in the wellbore,the similar theories and methods are applied to study lateral vibration and stability of the flow string and regulating separate stratum production string that the oil in different reservoirs respectively flows up through both oil tube and casing tubing annulus.Research shows that:for flow string,increasing the velocity and bottom pressure of fluid within oil tube,the stability of system decreases,and increasing the height and density of cushion within tubing casing annulus,the stability increases.For the regulating separate stratum production string,increasing lower reservoir pressure and annular fluid velocity,the stability of system reduces,and increasing internal fluid velocity and upper reservoir pressure,the stability is enhanced.