Improvement to Water Speciation and FeCO<sub>3</sub> Precipitation Kinetics in CO<sub>2</sub> Environments: Updates in NaCl Concentrated Solutions
Zheng Ma, Xin Gao, Bruce Brown, Srdjan Nešić, Marc Singer
Abstract
In the oil and gas industry, the dissolution rate of metal-based materials exposed to an aqueous phase is often controlled by the presence of surface layers such as corrosion products. Knowing the characteristics and rate of formation of these surface layers can help assess the level of protection against corrosion they provide and can permit the development of a successful asset integrity program. The appropriate determination of species concentration is a key aspect of accurately predicting the precipitation rate of corrosion products, more specifically of FeCO3, which is the main product of corrosion of steel in CO2-containing environments. While aqueous speciation in low-salinity brine is well-established, the effects of high NaCl contents (NaCl ≥ 3 wt %) on water chemistry and FeCO3 precipitation are less defined. In this work, two fundamental aspects of H2O/Fe/CO2 system speciation in high salinity brine are discussed: the consumption of Fe2+ due to the formation of ferrous chloride (FeCl+) complex and the dependence of Kca, the first dissociation constant of H2CO3, on ionic strength. This leads to a comprehensive revision of the models used to calculate the solubility product of iron carbonate (FeCO3), Ksp,FeCO3. In parallel, the kinetics of FeCO3 precipitation are experimentally investigated using an electrochemical quartz crystal microbalance (EQCM) in concentrated NaCl solutions at varied temperatures (50–80 °C). This led to the development of a new kinetics model for FeCO3 precipitation that now considers the nonideality of the solutions.