If chemical stability of EOR polymers has been extensively studied over the past decades, surfactants have attracted less attention except for a few reference papers. This article presents existing and new data on the chemical stability of a variety of surfactants classically used for EOR applications in aqueous solutions. The aim is to clarify under which reservoir/operational conditions each surfactant types and combinations can be safely used regarding chemical stability.

Anionic surfactants, including alkyl benzene sulfonates, internal olefin sulfonates, alkyl ether sulfates and alkyl glyceryl ether sulfonates are investigated. Their thermal stability is monitored in a wide range of conditions: temperature from 25 to 120°C, pH, oxygen level… Their performance in terms of solubility and interfacial tension reduction is monitored through classical bulk aqueous tests over several months. In addition, liquid chromatography and two phase titration are used to identify any loss of integrity of the studied products.

Results indicate that both hydrolysis and oxidation mechanisms should be considered to guarantee long term stability of EOR surfactants. Hydrolysis sensitive surfactants, e.g. alkyl ether sulfates, undergo hydrolysis with associated loss of performances at high temperatures. This is shown to be true even in alkaline conditions as pH essentially impacts degradation kinetics and not inner stability. This study also shows that properly selected EOR surfactants and blends can withstand very harsh conditions (up to 120°C) for several months allowing high confidence in EOR process performance along with time in a reservoir. Oxygen management is however critical to achieve such performance.

This paper initiates a series of articles giving a clear framework for safe use of surfactants in oil reservoirs regarding their chemical integrity. It is intended to provide reliable guidelines for products selection, lab evaluation and field application. These aspects, often underestimated or simply by-passed in some EOR studies, are critical to ensure field trial success in challenging conditions.

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