The Mechanism Of High-yield Gas Well Tubing String Vibration Analysis And Fatigue Life Prediction

In the beginning period of gas recovery, the vibration of gas production and pressure, in the course of gas flow in the tubing such areas with sudden changes of flow section and flow path as flow section changing areas, tubing elbows and tubing curving will produce vibration force which will lead to the vibration of the tubing. On the effect of vibration force, the tubing will vibrate. And the tubing will have perennial dynamic stress. Because of the perennial function of dynamic load, the small faults, produced by machining, transport, thread in, running in and erodent, erosion in producing course, will extend step by step by repeatedly function of alternative stress, which will eventually lead to rupture invalidation of the tubing.In this paper, the whole tubing is the analytic project; the control equations of gas flow in the tubing are set up; combined with practicality, the causes leading to vibration of tubing in the course of gas flow are analyzed; the model to calculate the vibration force is bring forward, which supplies basic data for subsequent analyses of dynamical corresponding. Based on the mode analyses, the access model of instant dynamical corresponding of tubing is set up. Exemplified with XX well, the dynamical corresponding of tubing under the function of exterior load is analyzed; and such data as the position with maximum stress and the average stress are given, which supply basic data for subsequent fatigue life calculation. The fatigue life of tubing is predicted with rupture mechanics method; the expression to calculate the fatigue life of tubing is deduced, combined with practicality, the relationship between fatigue life and the sizes of initial flaw, production rate, tubing sizes, and rupture tenacity is analyzed. Results shows: Vibration is the direct cause which leads to the rupture of tubing. With the increasing of outer diameter of tubing, the increasing of wall thickness of tubing, the increasing of initial flaw and the increasing of production rate, the fatigue life evidently decreases. So in the beginning producing period, the high rate and unsteady production will easily lead to the rupture of tubing; in middle and evening producing period, the flaws produced by erodent and erosion will also lead to the rupture of tubing; strengthening the inspect of rupture will evidently increase the life of tubing. The measures to prolong the life of tubing are bring forward; the method for tubing design is improved. Such measures are necessary: strict quality inspections, tubing antisepsis, gas well preventing sands, reasonable work system design, absorber fixing at the position where the tubing is easy to rupture.In this paper, for the first time the fatigue life of the tubing of high gas production well is quantitatively analyzed, the relationship between product rate and fatigue life is emphatically discussed. These work supply necessary data for new well exploitation, gas well management and tubing design.