13%Cr steel tubing corroded at shallow depths in deep, hot Minami-Nagaoka Gascondensate wells onshore Japan. The produced gas contains 6% CO2 and 2~3ppm H2S, and the produced water contains 400 ppm acetate. The localized corrosion apparently initiated from mechanical damages produced by wire line operations. The corrosion damage was more severe in wells with longer shut in periods. These observations are contradictory to the traditional understanding that the corrosion performance of 13%Cr steel deteriorates at high temperatures in wet CO2 environments. And abovementioned observations suggest that the corrosion initiated during well's shut in period at shallow depths due to pH lowering by higher CO2 solubility at low temperatures and the existence of acetic acid. Based on this understanding, corrosion performance of Super 13Cr steel containing Ni and Mo was evaluated as a potential countermeasure in comparison with 13% Cr and 22Cr steels at a low temperature in addition to at high temperatures using flow loop and rotating cage. The effect of Clconcentration and the influence of trace amounts of H2S on the corrosion performance of Super 13Cr steel was elucidated.
Development of Minami-Nagaoka gas field started in the late 1970's. Minami-Nagaoka gas wells are as deep as 5000 m and the bottom hole temperature is about 180oC. The produced gas contains 6% CO2 and 2~3 ppm H2S, and the produced water contains approximately 400 ppm acetic acid and other organic acids. 13% Cr stainless steel (SS) was used as a tubing material in some wells. After using this steel for less than 4 years, corrosion problems were found at both shallow and bottom depths of all wells. Many reports provided by steel manufacturers indicate that the corrosion resistance of 13% Cr SS in CO2-H2S environments strongly depends on temperature1-3. The corrosion resistance is very high and stable at temperature below 150 oC. At the temperature range above 150 oC, the corrosion resistance decreases apparently with increasing the temperature. Therefore 150 oC has been defined as a critical temperature for using 13%Cr SS by steel manufacture4-6. Nevertheless, as described above, in the case of Minami-Nagaoka gas field, only mild uniform corrosion was found around bottom of the well, where the temperature was about 180 oC. The severe localized corrosion shown in Figure 1 was observed at shallow depths where temperature was lower than 120 oC. This fact seems to contradict the results which have been presented by steel manufacturers, since it is reported that 13%Cr has a good corrosion performance under 150 oC2-6. Because of localized corrosion of 13%Cr SS, 22Cr SS, which contains Ni and Mo in addition to Cr, has been used as tubing material in Minami-Nagaoka gas wells. The 22Cr SS shows an excellent corrosion performance under the conditions of Minami-Nagaoka gas wells. However, it is very difficult to be widely accepted by oil and gas industry for using the 22Cr SS as a tubing or a pipe material because of its high price. Based on the reasons mentioned above.