Influence Factors on Corrosion of Wellbore in Yanchang Gas Field

The effects of corrosion factors, including salinity, Cl^-, Ca²⁺, Mg²⁺, HCO₃^-, pH value, CO₂ partial pressure and O₂, on the corrosion of N80 steel wellbore were studied by means of field coupon corrosion tests, weight loss method, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). And their corrosion mechanisms were analyzed. The results showed that the average corrosion rate of N80 steel increased linearly with the increase of salinity, Cl^- concentration and Ca²⁺ + Mg²⁺ concentration in produced water in the working condition of gas wells in Yanchang gas field. The average corrosion rate of N80 steel did not change significantly with the content of HCO₃^-, but the change rule with pH was more complicated, and there was a synergistic effect between pH and other corrosion factors. The CO₂ partial pressures in gas wells were all more than 0.21 MPa, which led to high average corrosion rate of N80 steel, and its change trend with CO₂ partial pressure was not obvious. The existence of O₂ had little effect on the average corrosion rate of N80 steel after the formation of protective film on the surface of N80 steel. The influence of salinity and ion composition on wellbore corrosion was greater than the influence of gas components.