Wanru Shang, Pablo E. Barajas and Reza Eghtesad
Norris
In today’s ever-more complex and corrosive downhole environment, especially in well sections with high dog leg severities, high water cuts and high sand content, it is challenging to develop a “one size fits all” coupling. Such coupling would need to be engineered to increase the life of the tubing and the coupling itself from basin to basin. Couplings may slap and then rub against the tubing, which consequently causes extremely aggressive angle contact to the tubing by the rod string, leading to coupling wear, coupling-to-tubing wear, and hole-in-tubing fail. Multiple stress corrosion cracking mechanisms can occur depending on the wellbore fluid pH and coupling material condition. Coating techniques have been developed and applied since 1960, such as spray metal. Since there is a trend that more advanced coatings have been researched and developed, it will be beneficial to develop a systematic approach to design, characterize, justify and validate emerging coating materials and techniques to measure their true value vs. conventional API couplings. This paper presents a 3D performance matrix, including corrosion resistance, coating hardness and abrasion resistance, and coating coefficient of friction (COF), contributing to the development of new coupling technology and improvement of existing ones. Among three factors, COF values are used by rod design software to calculate rod loads and drag force. The balance of coupling hardness and abrasion resistance is supposed to reduce tubing damages. As for corrosion resistance, corrosion mechanism and the study of residual stresses and carbide precipitation of base materials will be introduced, along with its influence on stress corrosion cracking susceptibility. Coupling samples with different coatings were collected and tested in the aspects of hardness, abrasion resistance, COF, wear rate and corrosion resistance. Results have been plotted in 3D diagram. This newly developed performance matrix serves as a reference for coupling selection according to various well conditions and empowers the operators to select proper and suitable couplings that fit their specific wells demand to reduce frequency of failures, reduce workover cost and downtime cost, and improve production. 3D scanning technology for tubing and coupling wear was used to quantify the mass loss during the wear tests.