Abstract

This work describes an automated combinatorial process for selecting EOR surfactants. It is well known that low interfacial tension (IFT) required to mobilize oil corresponds to the appearance of middle phase microemulsions (Winsor III). Instead of systematic measurements of IFT, simple observation of phase behaviour in vials allows to select best formulations. As the number of parameters to be screened incorporate surfactants, co-surfactants, alkali and electrolytes, we have developed an automated method to accelerate the screening productivity. Among other requirements, critical issues we want to address concern compatibility with electrolyte, thermal stability and low cost.

Sample preparation is done in 48 well format using a robotic liquid handling platform that supports stirring and heating. In a first step, the brine concentration above which the surfactant is not soluble anymore is determined optically using a digital camera. In a second step, we take images of the phase behaviour of surfactant formulations in the presence of model oil (dodecane, vaseline). Image analysis is used to detect middle phase microemulsions. An estimate of interfacial tension can be derived from the volume of the middle phase microemulsion. Phase diagram observations can be recorded from room temperature up to 80°C. Results are in good agreements with classical methods used to measure low interfacial tension. An example study of phosphate esters will be discussed to illustrate the method.

This technique provides a powerful tool for testing formulations prior to more expensive core flood experiments.

Introduction

Surfactant formulations providing a low interfacial tension (IFT) with the oil phase are known to effectively displace oil trapped in porous media (Stegemeier G.L., 1976; Green D. W. and Willhite G.P., 1998). Basically when the surfactant formulation contacts residual oil, drops under a pressure gradient are deformed as a result of low interfacial tension and displaced through the pore throats. In a recent paper a process based on phase behavior screening has been described for evaluating potential EOR surfactants (Levitt D.B. et al., 2006). This approach is based on a well established relationship between low interfacial tension and a microemulsion phase behavior as originally described by Winsor (1954). Type I (oil in water), type II (water in oil) and type III (bicontinuous oil and water) microemulsions can be found. The type III microemulsion also referred to as middle phase exhibits the lowest interfacial tension. The larger the volume of oil and water per unit volume of surfactant in this middle phase, the lower is the interfacial tension (Hug C., 1979). From a practical point of view this means that rather than performing systematic measurements of interfacial tension, simply observing the microemulsion phase behavior in transparent vials allows for pre-screening of a large number of compositions. This paper describes a highthroughput (or combinatorial) workflow where surfactant formulations are automatically prepared and analyzed using a robotic platform. Results on phosphate ester surfactants are used to illustrate the approach.

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