The long-term corrosion behaviour of abandoned wells under CO2 geological storage conditions: (2) Experimental results for corrosion of casing steel

摘要

Regarding the geological storage of CO2 in aquifers or in depleted gas fields, the reactivity of the well casing left in the abandoned well is important in order to investigate the potential for leakage. The corrosion behavior of the casing steel tube was investigated in highly pressurized CO2 environments. The corrosion test of the J55 and N80 steel tube in API specification 5CT was conducted. Those steel tubes have been frequently used as casing tube and are found in abandoned natural gas wells. The following test conditions were achieved with the autoclave apparatus: temperature at 50 to 70 °C, CO2 pressure at 5 to 18 MPa with/without the impurities of 20 ppm H2S and 2000 ppm CO, in/above stagnant 0.5 M NaCl solution of simulated formation water. The test durations were 100, 300 and 1000 hours. After the corrosion test, the specimens were analyzed by Energy dispersive X-ray spectrometry and X-ray diffractometry. The specimens were found to be covered mainly with FeCO3, which is considered to be formed from dissolved Fe~(2+) and environmental CO2. The corrosion rate was evaluated via the weight loss of the specimens after the removal of the surface corrosion products, containing FeCO3. No significant differences were observed between J55 and N80 in terms of FeCO3 formation and corrosion weight loss. It was found that the weight loss by corrosion, around 100 mg/cm~2 after 100 hours, was remarkably large when the specimen was placed in the solution and was, in contrast, two orders of magnitude smaller when the specimen was above the solution. In the case with the impurities of 20 ppm H2S and 2000 ppm CO, the weight loss in the solution was less relative to the pure CO2 condition. The weight loss in the solution reached a plateau in initial 100 hours, and was only slightly increased in the 300- and the 1000-hour tests. The 'plateau' was considered to be due to the protective effect of the FeCO3 formed on the specimen. Considering the protective effect of the FeCO3, the corrosion of the steel casing over a long period of time is small under the stagnant condition, and would be expected to remain in place with little structural degradation.