Corrosion Failure Analysis of Tubing Couplings and the Mechanism of Flow-Accelerated Corrosion

Tubing couplings are recognized as a vulnerable link in pipe string integrity, accounting for over 60% of all failure incidents. In this study, a perforation-failed N80 double-male short sub retrieved from a coalbed methane jet pump well was systematically investigated. A multi-faceted analytical approach was adopted, integrating macroscopic morphological examination, scanning electron microscopy (SEM), and X-ray diffraction (XRD) for material characterization, in conjunction with computational fluid dynamics (CFD) numerical simulations based on the Realizable k-ε turbulence model. The objective was to elucidate the causes of corrosion failure and the underlying mechanism of flow-field-induced corrosion acceleration. The results show that the corrosion products on the inner wall of the short sub are mainly composed of FeCO3 and iron oxides, resulting from the synergistic electrochemical corrosion of CO2 and dissolved oxygen. The “J”-shaped groove formed at the junction between the double-male short sub and the adjacent upper and lower couplings introduces a geometric discontinuity that significantly perturbs the local flow field, leading to a binary partitioned flow structure. This structure comprises a high-shear-stress zone at the groove edges, with a peak wall shear stress reaching up to 58.5 Pa, and a vortex-stagnation dead zone at the groove center, where the flow velocity drops to as low as 0.13 m/s. This flow-field structure is the primary cause of local corrosion. High shear stress continuously scours and removes the protective corrosion film, accelerating the corrosion process, while the vortex-stagnation zone facilitates localized enrichment of corrosive species. The synergistic interplay between these two mechanisms ultimately results in localized perforation within the threaded region of the short sub. The results provide theoretical support for optimizing oil pipe coupling structures and suppressing erosion corrosion in oil and gas well pipe strings.