Cause of Increasing Corrosion Rate on Tubing in a Gas Well of Changbei Gas Field

The overall corrosion rate of the tubing in changbei gas field was slow, but it was found in the multi arm caliper detection of a gas well that the local corrosion rate of the tubing increased, which was far greater than the corrosion rate of the whole gas field and the corrosion rate of this well in the early stage. The causes and mechanism of sudden acceleration of corrosion rate of tubing in this well were analyzed by carrying out the analysis of water quality and gas composition, multi arm caliper test and corrosion coupon test, analysing the macro morphology and corrosion products of corrosion coupon respectively, and combining with the corrosion degree and condition of tubing. The results show that the main cause of tubing corrosion in this well was electrochemical corrosion caused by CO₂, and the overall corrosion rate was not high. The cause for the acceleration of local corrosion rate of tubing was that the formation water production and salinity increased in the later stage, which promoted the “deep excavation” effect after the formation of local pitting corrosion, and made the corrosion rate increase nonlinearly. The higher concentration of Cl^- in water further increased the corrosion rate.