Prediction of corrosion life of nitrogen blanket casing in salt cavern gas storage

In this paper, to analyze the oxygen corrosion of the casing string under the nitrogen blanket condition of the salt cavern underground gas storage and to predict the casing's service life, an indoor corrosion weight-loss experiment was used to study the effects of oxygen concentration, temperature, and pressure on the gas and liquid phase of N80 steel pipe corrosion laws. The corrosion products were examined using scanning electron microscopy, X-ray fluorescence spectroscopy, and X-ray diffraction analyses. Finally, the corrosion life of the casing was predicted based on the uniform corrosion rate and casing strength check. The results indicated the following: (1) under the experimental conditions, the gas-liquid two-phase corrosion rate gradually decreased with the increase in nitrogen concentration, while when the concentration was 78%-95%, the liquid-phase corrosion was serious, and in the liquid nitrogen environment (nitrogen concentration is 99.9%), the liquid-phase corrosion rate was lower than the oilfield corrosion control index (0.076 mm/a); (2) the temperature and pressure had no significant effect on the casing corrosion; (3) with the increase in nitrogen concentration, the service life of the inner and outer pipes under different working conditions gradually increased, with the minimum service life equaling 28 years when the concentration was 99.99%; (4) nitrogen concentrations of 99.99%, 95%, 90%, and 78% can meet the column strength of the 3-year cavity-building cycle, while under the 5-year cavity-building cycle, it is recommended that the lower limit of the purity is 90%. Overall, the research results will provide a strong information basis for the construction of salt cavern gas storage cavities.