Friction Force Analysis of Simultaneous Buckling of Tubing and Rod Strings

<p>It has recently been shown that the rod string is bent by the buckled tubing during pumping cycle (Vagapov 2016a). This means that the buckled sucker rod string moves inside a helically buckled tubing during pumping cycle. The simultaneous buckling of tubing and rod strings must have an effect on sucker rod/tubing wear between the pump and the neutral point. However, the normal contact force between the buckled tubing and buckled rod string and constraining casing has not yet been investigated. In this paper the normal contact forces and the maximum normal contact forces are determined. <br></p><p>The friction between the rod and tubing strings affects the shape of the surface dynamometer card and can cause significant changes in the maximum and minimum loads imposed at the polished rod. Obviously, the friction force due the simultaneous buckling is added to the conventional friction forces. The friction force due to simultaneous buckling is in many regards very similar to fluid load and it creates nearly the same type of the load on a polished rod at the surface. Unfortunately, it is impossible to distinguish between the friction forces and fluid load by the shape of the surface card. As a result, the friction can greatly increases the difficulty of interpreting what is happening downhole. If the pressure differential across the plunger is not known, the friction forces due to the simultaneous buckling may go unnoticed. In these wells the friction force can be confused with fluid load.