Coated Continuous Sucker Rod Reduces Fatigue Failures in Progressing Cavity Pumping Applications

Continuous sucker rod is used in progressing cavity (PC) pump applications to minimize the tubing wear and connections failures that can occur with conventional jointed sucker rods. While PC pumps are often assumed to have relatively steady state loading conditions, rod string rotation combined with local curvature results in high frequency low magnitude cyclic bending stresses. Variations in produced fluids and certain modes of pump friction also give rise to lower frequency moderate to higher magnitude cyclic torsional and axial loads. These cyclic stresses, particularly when combined with corrosive downhole environments, can result in rod fatigue failures at loads well below their specified load ratings often with short runtimes. Alloyed rod materials can slow the impact of corrosion but usually the failures persist. Corrosion inhibitors can reduce downhole equipment corrosion but their effectiveness on continuous rod can be diminished due to the rotational contact impairing the build-up of a protective layer on the rod surface. Rod coatings can provide barrier protection to the underlying bare rod material from production fluid exposure eliminating corrosion and its impact on accelerating fatigue failures. Certain coatings can also protect the rod body against mechanical damage that contributes to fatigue as well as reduce rod and tubing wear and its associated contact friction. Summarized in this paper is ten plus years of experience with a first-generation coated continuous rod product that employs a thick durable polyethylene coating. Deployment of several thousand rod strings in an area with highly directional heavy oil wells reduced rod fatigue failures from multiple breaks a year to multi-year runtimes. Broader application deployment also demonstrated reductions in fatigue failures but identified well depth and produced fluid limitations that significantly limited the products application range leading to a second-generation development. This paper provides an overview of the second-generation coated rod development including design requirements, materials development and laboratory testing, product prototyping and validation, new coated rod manufacturing processes and service equipment testing. In support of this development were evaluations using custom rotary tribology equipment and multiaxial fatigue equipment select findings of which are included. Lastly it details field testing and initial commercial deployment that confirms benefits like those with the first-generation design but over an expanded application range.