Corrosion risk assessment of geothermal reinjection wellbore in Xining Basin, China

•The corrosion risk of reinjection wellbore with air mixed in the geothermal water was investigated.•Oxgen corrosion rates of different types of steels in geothermal water were evaluated experimentally.•A novel oxygen corrosion rate prediction model for reinjection wellbore was established.•The corrosion law in wellbore was analyzed when different amounts of air enter into reinjection well. A potential corrosion risk exists during the re-injection of geothermal water in Xining Basin of China due to air entering the well caused by it being open. In this paper, a series of corrosion experiments using metal coupons and geothermal waters was conducted to evaluate the corrosion risk for steels in different typical geothermal waters by dissolved air. A novel and comprehensive prediction model for O2 corrosion rate in reinjection wellbores was established, and a sensitivity analysis of factors influencing air corrosion risk in reinjection wellbores was carried out. The results show that the main corrosive components in the geothermal water from the Xining Basin are Cl−, SO42−, H+ and the O2 added, which can cause a uniform corrosion and a small amount of local corrosion on the surface of carbon steel. The main corrosion products are FeO(OH) and Fe3O4. A high O2 partial pressure and flow velocity, a moderate temperature and salinity, and low pH promote a high corrosion rate for carbon steel. When the geothermal water is injected with a small amount of air, only the deeper part of the wellbore with a high temperature and pressure will suffer a serious corrosion risk, while when a large amount of air is mixed with geothermal water and injected, a huge corrosion risk will be imposed on the entire wellbore. Anti-corrosion measures should be taken if air is added to a wellbore. Removing the O2 from the geothermal water is the most effective solution for carbon steel. In addition, the established corrosion rate prediction model can consider more influencing factors and specializes in the situation of a low O2 partial pressure, which can be used as an effective tool to assess the corrosion risk in reinjection wells.