Abstract
The depth of the wells at the Minami-Nagaoka Gas Field located in Niigata Prefecture in Japan is over 14, 000ft with up to 355°F of bottom-hole temperatures. The produced gas contains approximately 6% CO2, 2-5ppm H2S, and up to 200ppm chloride ion. During the workover operation on some of the wells, 13Cr tubings were retrieved and investigated from the corrosion stand point. The following conclusions are drawn from the investigation:1. With temperatures above 248°F and the existence of free water, pitting corrosion was observed on the 13Cr tubings of the well producing 6, 700MMSCF with 866days. The depthrange of the corrosion was 300-5, 200ft.2. Severer pitting corrosion was caused by leakage of the annular brine than that without the leakage. The depth-range of the corrosin was expanded to 330-8, 200ft.3. Crevice corrosion was observed at the faces of thread seals. The cause would be the production fluid that penetrates through opening spaces between tubular thread seals near bottom-hole.4. Scratching surface protective films on the tubings by, for example, wirelines caused localized corrosion.
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