Mechanical Behavior Research On Sucker Rod Of Polymer Flooding Rod-pumped Well

Polymer flooding is a mature technology to improve oil recovery. However, the fact that serious eccentric wear on sucker rod string reduces the pumping inspection period and overall economic efficiency. It is urgent to solve the problems of eccentric wear on sucker rod so that to guarantee the crude oil production and to improve the oilfield exploitation's benefit. So, it is necessary to research the mechanical behaviors of sucker rod in the pumping fluid process in order to analyze the reasons and influential factors resulting in sucker rod's eccentric wear. The current research studied by domestic and overseas scholars on mechanical behavior of sucker rod is based on the wave equation and static theory, which is far away from the sucker rod's real working situation. New methods and technologies are urgently studied to accurately describe sucker rod's mechanical movement behavior and influential factors in the limited space full of polymer fluid. Effective measures need to be put forward to prevent eccentric wear, and finally accomplish the goals of pumping system's safe and proper production, which will play significant roles in perfecting pumping system's error diagnose technologies and enriching its theories, and accordingly promoting relevant discipline's development.The key problem of sucker rod's mechanical behavior is to confirm its deformation condition under different loads in the wellbores. Therefore, studies are related to other relevant disciplines, such as production engineering, solid mechanics, fluid mechanics, computing mechanics and polymer rheology. This paper sets up sucker rod's mechanical model through mechanical analysis; and also establishes the sucker rod's Kirchhoff equation and transient dynamic equation in the limited deformation range through using momentum theorem and dynamics principle. Impenetrable equation on contact interface is deduced according to sucker rod's structure parameters and movement conditions. Meanwhile, the weak form of balanced equation is derived from variational principle and updated Lagrangian function. Using augmented Lagrange multiplier sets up the momentum equation between sucker rod and tubing interface. Sucker rod's force and deformation state under multiple coupling are calculated through combing FEM (finite element method) with other several methods, such as Newton-Raphson method, load increment method, augmented Lagrange multiplier method and Newmark method.Produced fluid in the polymer flooding pumping wells is viscoelastic non-Newton fluid. Increasing viscosity of produced fluid has great influence on the frictional resistance between sucker rod and polymer fluid, as well as that between plunger and pump wall. Polymer fluid's elasticity creates an acting force upon sucker rod along the normal direction flowing through annular, which leads to sucker rod's instability. This paper presents polymer-produced fluid's flow controlling equation through using non-Newton fluid theory. Basing on finite element theory, controlling equation is dispersed through controlling volume method, and acquiring polymer fluid's finite element form. Using SIMPLE algorithm solves the equation, polymer fluid's flowing velocity distribution through annular can be determined. Meanwhile, basing on fluid mechanics, polymer produced fluid's relaxing time can be confirmed by using Rouse-Zimn model. On the ground of past laboratory tests on polymer fluid's zero shear rate and combining with produced fluid's mutative law before and after injecting polymer fluid, the zero shear rate formula has been modified and then polymer produced fluid's zero shear rate calculating equation has been successfully acquired. Combined with the normal force formula deduced from the MAXWELL model, normal force's mutative law can be obtained along with sucker rod's movement velocity and polymer's density, which will build up a stable foundation for accurately calculating sucker rod's deformation state in the polymer wells.In order to verify the veracity and practicability of analysis approach on sucker rod, a tension and pressure sensor was fixed on the sucker rod to measure the variation curve of sucker rod's longitudinal force with time. By comparing testing results with calculating results, the longitudinal force's average error is 5.07%. Meanwhile, the producing wells whose sucker rods were broken were also calculated. The comparison results show that the calculating result is consistent with real sucker rod broken and wear location, which fully proves the validity of the diagnostic method.The main influential factors on the longitudinal vibration of sucker rod were analyzed. This longitudinal vibration makes for sucker rod's dynamic instability, which further cause sucker rod vibrating along transverse direction. As a result, fixing absorber on the sucker rod can greatly reduce sucker rod's longitudinal vibrating amplitude; lower transverse dynamic instability; and hence realize the goal of preventing sucker rod's eccentric wear.The axial force can influence the sucker rod's deformation state in the polymer wells. When polymer's concentration is more than 300mg/L, axial force can change sucker rod's deformation state and worsen its eccentric wear. On the contrary, when polymer's concentration is less than 300mg/L, axial force has little influence on sucker rod so that the result can be ignored. Aiming at adopting centralizers to prevent sucker rod's eccentric wear problem, centralizers'reasonable fixing measure need to be put forward.The analysis approach put forward in this paper has strong pertinence to sucker rod's eccentric wear problem. The analysis results and improved measures will play important roles in preventing sucker rod's eccentric wear, so that to guarantee pumping system's safe production; prolong its working cycle; enhance crude oil production and reduce operating costs. All of these works will certainly improve the oilfield exploitation level and increase overall economic benefit.