Investigation on Insulated Tubing Corrosion in Nanbu 35-2 Multi-Thermal Fluid Project

Multi-thermal fluid simulation was conducted for offshore heavy oil reservoir such as Nanbu 35-2 in Bohai Bay Oilfield since 2008. Field well integrity testing result shows that Insulated tubing corrosion is found in many multi-thermal fluid injection wells. Insulated tubing corrosion has become one of the largest problems in the multi-thermal fluid project. It will negatively affect the development performance and also indeed a risky operation under high temperature and high pressure. In order to effectively prevent the corrosion problem of insulated tubing, the principles of insulated tubing corrosion and preventive measures are investigated. At first, temperature-pressure measurement and gas analysis were carried out in the multi-thermal fluid injection wells in Nanbu 35-2 heavy oilfield. Secondly, the corrosion laws of N80 and P110 steel in high temperature and high pressure CO2/O2 system were studied. At last, the corrosion mitigation effect of imidazoline corrosion inhibitor was evaluated through dynamic coupon test method. The results indicated that temperature, oxygen partial pressure and carbon dioxide partial pressure have effect on corrosion rate. Two peaks of corrosion rate of N80 and P110 steel appeared at 60°C and 180°C respectively, in the CO2/O2 system, even a little amount of O2 (partial pressure of 0.02MPa) could remarkably speed up corrosion. The corrosion rate drastically increased when carbon dioxide partial pressure exceeded 2MPa. The imidazoline corrosion inhibitor could lower the corrosion rate to 0.11 mm/a when the concentration was set at 750mg/L, and corrosion inhibition rate was high up 94.17%. The performance of the imidazoline corrosion inhibitor has been tested in field and results showed that the insulated tubing wasn’t corroded after using the corrosion inhibitor, which indicated that the corrosion inhibitor could meet the corrosion protection requirement of P110 steel in high temperature CO2/O2 system. The principles of insulated tubing corrosion in multi-thermal fluid injection wells is complicate because of multicomponent of multi-thermal fluid and high temperature and high pressure during its injection process. This study can provide basis for the design of insulated tubing in multi-thermal fluid project.