Corrosion Mechanism of Oil Casing in Water Injection Well in Bohai Oilfield

The downhole environment of water injection wells in Bohai oilfield is complex, and the string perforation and casing corrosion are serious. The water samples were taken from the actual water injection wells in an oilfield in Bohai, and the water quality and bacterial types were analyzed. The field water samples were used as corrosive media to carry out the simulation test. The microbial corrosion evaluation in the water injection wells was carried out. The corrosion rates of N80 steel, 3Cr steel and 13Cr steel in the corresponding environment before and after adding fungicides or corrosion inhibitors were compared and analyzed, and the long-term corrosion rate prediction model was established. The results showed that microbial corrosion was the main reason for the corrosion of water injection well string in Bohai Oilfield. The bacteria in the injected water were mainly sulfate reducing bacteria (SRB) and iron oxidizing bacteria (IOB), and the bactericide can effectively reduced the corrosion rate of tubing and casing. The uniform corrosion rates of N80 steel and 3Cr steel in microbial environment were 0.58 mm·a^-1 and 0.27 mm·a^-1, respectively. The uniform corrosion rates after adding fungicide were 0.18 mm·a^-1 and 0.13 mm·a^-1, respectively. N80 steel could not meet the production requirements in the environment without fungicide. Low concentration bactericide had obvious inhibitory effect on the microbial corrosion of oil casing in water injection wells. The selection of 3Cr material and the addition of low concentration bactericide could meet the anti-corrosion requirements of water injection wells.