科研之友
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摘要
Wellbore systems are designed to isolate fluids in the subsurface and are typically engineered for a 30-50 year service life. In the geologic sequestration of CO2, wellbores will have to perform for 100s of years. As a consequence, one of the key questions in the viability of sequestration is whether long-term wellbore integrity is feasible in the high-salinity, high-CO2 fluids likely to be present in CO2 storage reservoirs. Isolation in wellbores is usually accomplished by a combination of Portland cement and steel. In this study, we focus on predicting the corrosion rate of steel under typical CO2 sequestration conditions. We have developed a mechanistic model for predicting corrosion rates of mild steel used in most wellbore systems. The model includes an aqueous geochemistry and an electrochemistry module. The water chemistry module uses the Pitzer formulation for activity and Duan et al.'s (2006) model for CO2 solubility. The electrochemical module accounts for both mass transfer processes and electrochemical kinetics. The electrochemistry includes the primary oxidation reaction (the dissolution of iron) and the primary reduction reactions (the formation of H-2 gas from carbonic acid, hydrogen ion, and/or water). At high CO2 pressures, the dominant corrosion reaction is Fe +2H(2)CO(3) = Fe2+ + 2HCO(3)(-) + Hz(g) and is driven by CO2 solubility rather than solution acidity. This result shows that typical buffering reactions between dissolved CO2 and minerals (e.g., carbonates) will not significantly reduce corrosion rates in contrast to many mineral reaction rates that are strongly dependent on pH. For similar reasons, high salinity solutions reduce corrosion rates significantly mainly due to the "salting-out" effect of reduced CO2 solubility. For example, an increase in salinity from 5 to 20 wt% salt results in a 50% reduction in corrosion rate as confirmed in our experiments. Numerical simulations are tabulated that provide predicted rates of corrosion in wellbore environments over a wide range of temperature, partial pressure of CO2 and salt concentration.
- 出版日期2011-7
