The Influence Of Type And Amount Of Alcohol On Surfactant-Oil-Brine Phase Behavior And Properties
Authors: Salter Stephen J. 
Published: December 2007

Many formulations used in surfactant flooding contain alcohol. Since microscopic oil displacement efficiency has been shown to be dependent on the phase behavior and properties of the microemulsion systems which form when these formulations contact oil, it is logical to study the effects that alcohol type and amount have on these properties. Results reported here show these effects to be both consistent and straight-forward. Using the 'salt optimization' technique, systems containing alcohols and mixtures of alcohols from C3 to C14 in size were studied. Results are presented for two different oils and three surfactants.

The optimal salinity for a given oil and the solubility of the surfactant-alcohol mixture in brine are affected by both the type and amount of alcohol. The relative solubility, partitioning of an alcohol between the oil and brine, is the partitioning of an alcohol between the oil and brine, is the key to its characterization. More water soluble alcohols result in higher optimal salinities for a given surfactant as well as increase the maximum brine salinity in which such a surfactant-alcohol mixture is soluble. It becomes convenient, therefore, to look at these systems relative to their optimal salinities. Over the range of salinity studied, the amount of alcohol determines the interfacial tension at optimal salinity, the size of the multiphase region, and the fluid phase viscosities. Increasing the ratio of alcohol to active surfactant raises the optimal interfacial tension, increases the size of the multiphase region, and decreases the microemulsion phase viscosity.

Introduction

Independent of whether surfactant flooding is viewed as a miscible or immiscible process, its success or failure in a given porous medium depends upon the phase behavior of the surfactant-oil-brine-rock phase behavior of the surfactant-oil-brine-rock system and the interfacial tension, viscosity, and adsorption of/and between the phases which result. It is thus logical to relate the compositional variables (rock, oil, brine, polymer, and surfactant fluid compositions) to overall performance.

Frequently, one of the materials added along with the surfactant is alcohol, often referred to either as a cosolvent or a cosurfactant. Alcohols as a class of materials vary widely in their properties. Methanol, for example, is a small, relatively polar, very water soluble material while hexanol is larger, less polar, and almost insoluble in water. It is not surprising, therefore, that the effects of various alcohols in surfactant-flooding systems differ greatly. Opinions regarding the reason for alcohol addition vary. Trushenski et al state that in many cases alcohol increases the water solubility of the surfactant and decreases surfactant adsorption. Healy and Reed, on the other hand, cite deviscification of the resulting microemulsion phase. Jones and Dreher concur with this fact while adding that various alcohols influence phase behavior differently. The work reported here attempts phase behavior differently. The work reported here attempts to clarify some of the effects that alcohol type and amount have on the chemical and physical properties of mircoemulsion systems.

The approach taken here is quite similar to the salt optimization' approach employed by Healy, Reed, and Stenmark. As they state, the use of pseudoternary phase diagrams greatly simplifies both understanding and phase diagrams greatly simplifies both understanding and presentation of the results. The surfactants used in this presentation of the results. The surfactants used in this study all contained varying amounts of active surfactant, unreacted oil, water, and inorganic salt, where the active surfactant and unreacted oil were both composed of a multitude of different molecular species. In most of the results reported here, crude oils composed of countless molecular species were employed. Alcohol and brines, composed of deionized water and sodium chloride, were also added. In order to consider such a system as ternary, these components must be broken down into three pseudo-components. It is in this step that the approach used here differs from Healy, Reed, and Stenmark. The compositions presented here will all be written in weight percentages.

The first component selected is active surfactant (AS). The ability of this component to solubilize oil and water is of primary concern.