Pipe corrosion in the cavity building process of nitrogen dissolution inhibitor used in salt-cavern gas storage

Due to the influence of nitrogen preparing process, there is a certain amount of oxygen in the injected nitrogen in the process of nitrogen dissolution inhibitor, which results in the corrosion of downhole and wellhead metal materials, so as to impact the safe and smooth operation of underground gas storage (UGS) in the late stage. The gas-liquid two-phase corrosion rates of N80 casing in the environment of high-salinity brine caused by nitrogen concentration, downhole temperature and downhole pressure under the working condition of the nitrogen dissolution inhibitor were determined by means of indoors high-temperature, high-pressure corrosion weight loss experiment, combined with surface analysis technologies, e.g. scanning electron microscope (SEM) and X-ray diffraction (XRD). What’s more, the influence degree of different factors on string corrosion was quantified. It is experimentally indicated that under the field working condition, the liquid corrosion rate is decreased from 1.362 9 mm/a to 0.041 9 mm/a and the gas corrosion rate is decreased from 0.017 6 mm/a to 0 when the nitrogen concentration is increased from from 80 % to 99.99 %. When the temperature is increased from 10 ℃ to 40 ℃, the liquid corrosion rate is increased from 0.341 4 mm/a to 0.482 9 mm/a and the gas corrosion rate is increased from 0.005 1 mm/a to 0.008 9 mm/a. When the pressure is increased from 10 MPa to 14 MPa, the liquid corrosion rate is increased from 0.544 2 mm/a to 0.605 9 mm/a and the gas corrosion rate is increased from 0.006 6 mm/a to 0.008 9 mm/a. These experimental results show that under the actual working condition of nitrogen dissolution inhibitor, the gas-liquid two-phase corrosion rates of N80 solution mining string can satisfy the requirements of 3-5 years’ solution mining when the nitrogen concentration is higher than 95%. It is concluded that the nitrogen concentration of 95 % is the lower limit of nitrogen blanket, which can provide the basis for the field operation of nitrogen dissolution inhibitor and the selection of nitrogen preparing methods.