Justin Galle and John Stachowiak
Production tubing failure is a common issue in reciprocating rod lift wells due to high-friction, metal to metal contact with sucker rod couplings during the pumping stroke. This excessive tubing wear typically leads to a hole in the tubing wall resulting in high workover costs. A sucker rod coupling with a unique low-friction and high-hardness diamond-like coating (DLC) was tested in a side-loaded reciprocating environment. Because it is well known that both hardness and coefficient of friction are directly related to frictional wear it can be concluded that longer production tubing life can be achieved by utilizing a coating with these properties. The study was performed by replicating side load forces against tubing typically seen in deviated rod lift downhole conditions. Multiple industry available couplings were tested and then compared against the results from the DLC coated coupling. Additional laboratory testing also included coupling surface wear and coefficient of friction for each tubing/coupling combination. The result of the study showed varying material loss on both the tubing and the coupling surface with high frictional wear characteristics on the current industry offerings. In comparison, the DLC coated coupling test results showed insignificant material loss on both the coupling and the tubing. Lower sustained coefficient of friction over time was also observed thus eliminating the adhesive wear between the tubing wall and the coupling surface. These results prove that using an enhanced DLC coated sucker rod coupling in a reciprocating, rod lifted application will result in reduced tubing wear, reduced coupling surface wear, and longer production tubing life. The novelty of the DLC coated coupling relates to the reduced frictional and high hardness properties which will both extend tubing life and reduce coupling wear.