Failure analysis, corrosion rate prediction, and integrity assessment of J55 downhole tubing in ultra-deep gas and condensate well

Abstract

Several carbon steel tubing suffered severe corrosion in service which resulted in the leakage of production fluids from tubing string and, consequently, a decrease of well productivity. Optical metallographic microscopy, X-ray diffraction (XRD), combined with weight loss and characterization methods were used to determine the most probable causes of the failure. The results showed that the composition and structure of the tubing joints and couplings were in accordance with the parameter requirements of API 5CT for grade J55. Upon visual inspection, the corroded pipe exhibited significant thickness reduction in multiple locations, which justifies utilizing the triaxial yield of pipe body formula, assuming the minimum measured thickness as the nominal thickness of the pipe. When compared to the upper bound internal pressure requirements in API 5CT, the equivalent stress calculation showed significantly lower failure pressure levels, which provide strong evidence that the pipe could not withstand actual service conditions. The composition of corrosion products was mainly FeCO3 and Fe3O4, and scaling layer were composed of the heavy components of the crude oil, CaCO3 and corrosion products. The scaling layer was protecting the steel surface from corrosion and reducing the corrosion rate. Once the scaling layer was broken, the corrosion rates increased. Also, pitting corrosion rates simulations were conducted at locations with the highest corrosion risk. Results show that at the first 30 days, pitting corrosion rates are the highest at these locations due to many factors including high CO2 partial pressures, acidic pH at these locations, and larger production volumes resulting in an increase of wall shear stresses and higher velocities.