Corrosion inhibitors are a proven cost-effective method for internal corrosion protection of carbon steel piping in the presence of CO2-containing brines. The appropriate selection, deployment and performance monitoring are all essential elements ensuring successful application. However, despite many years of use within the oil and gas industry, a clear understanding of the impact of the physical chemistry of surfactants and surfactant mixtures still eludes the industry, and chemical law is often replaced with oilfield lore.

In this work, the effect of brine salinity (0.1, 1 and 10 wt.% NaCl) on the partitioning/distribution behavior of a homologous series of alkyldimethylbenzyl-ammonium chloride (or BAC) is studied for chain lengths of C12, C14 and C16, between toluene and NaCl brine. High performance liquid chromatography-mass spectrometry (HPLC-MS) is used to quantify surfactant concentration in both the toluene and aqueous phase, while critical micelle concentrations (CMC) are determined using a lipophilic dye method. These two techniques are used in conjunction with partitioning experiments to generate a comprehensive understanding of the effect of partitioning/distribution characteristics of the individual chain lengths within the BAC homologous series as a function of aqueous phase salinity. Subsequently, the distribution characteristics of a multi-component surfactant mixture (70% C12, 25% C14 and 5% C16) is considered and compared to the behavior of the single surfactant experiments.

For the BAC components studied as pure molecules, the results show that alkyl-chain length and aqueous phase salinity change critical micelle concentration (CMC) and partition coefficient (Pw/o) by several orders of magnitude over the range tested. Comparison between mixture and single surfactant experimental data demonstrates that the formation of micelles can be moved from one phase to another, influencing surfactant distribution between hydrocarbon and aqueous phases under certain conditions. This has significant implications for the behavior of formulated products which contain not only a mixture of homologues but also a mixture of functionalities.

Subject
Water, Sodium chloride, Micelles, Surfactants, Hydrocarbon phase, Mixtures, Partitioning, Salinity, Fluids, Salt concentration, Corrosion inhibitors, Toluene, Brines
Keywords:
CO2 corrosion, partitioning, corrosion inhibitors, micelle, CMC, salinity
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2025
Association for Materials Protection and Performance (AMPP)
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