Phase Behaviour of Ph-Dependent Microemulsions At High Temperature and Salinities

Oil & Gas Science and Technology, Mar 1997

A. Hussain, P. F. Luckham, T. F. Tadros

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Phase Behaviour of Ph-Dependent Microemulsions At High Temperature and Salinities

PHASE BEHAVIOUR OF PH DEPENDENT MICROEMULSIONS AT HIGH TEMPERATURES AND HIGH SALINITIES A. HUSSAIN 0 P.F. LUCKHAM 0 T.F. TADROS 0 Imperial College of Science 0 0 (1) Department of Chemical Engineering, Technology and Medicine , Prince Consort Road, London SW7 2BY - United Kingdom - This paper describes the formulation principal for a model microemulsion system which exhibits pH dependent phase behaviour. The system investigated consists of octane, brine, alkyl ether carboxylic acid surfactants and short chain alcohols. The CMCs of these surfactants were lower in acid form of the surfactant (COOH) than for the salt form (COO-), also the micelles formed in acid solutions were smaller than for the salt. Furthermore the surface and interfacial tensions were found to increase with increasing pH. Increasing pH ionises the carboxylic acid head group thereby making the surfactant more hydrophilic. The effect of an increase in pH can be counterbalanced by increasing the electrolyte concentration. Measurements of ultralow interfacial tensions at different salinities and temperatures have been made using a spinning drop apparatus. Three phase microemulsion systems were studied as a function of temperature and pH. It was observed that the presence of ethylene oxide (EO) moiety in the surfactant molecule made the surfactants less sensitive to salinity than anionic surfactants. In addition, the carboxylic ionic head group made the surfactant more stable to temperature than simple EO nonionic surfactants. Thus these surfactants are more robust than either simple anionic or nonionic surfactants and thus these materials have potential in the field of surfactant flooding for tertiary oil recovery. INTRODUCTION Low interfacial tension, high solubilization and acceptable adsorption are considered to be the important design parameters in optimising microemulsion systems for recovering trapped oil from the petroleum reservoirs [ 1 ]. Huh [ 2 ] empirically established a correlation between the solubilisation parameter and interfacial tension. It has been established that the interfacial tensions required to mobilise oil ganglia and to obtain sufficient oil recovery fall in the range of 10Ð2 to 10Ð4 mN/m [ 3 ]. Evidence shows [ 2 ] that the best conditions to generate such low interfacial tensions occur when a carefully designed surfactant rich phase (WinsorÕs type-III) exists in equilibrium with excess oil and water phases. The optimum middle phase, as defined by Huh [ 2 ] and Bourrel et al. [ 4 ], is the one in which the equal volumes of oil and water are solubilised. Many oil reservoirs targeted for surfactant flooding contain a medium to high salinity brine especially in the North Sea [ 5 ]. Previously, ethoxylated petroleum sulfonates were used in order to overcome the high salinity problems [ 6 ]. These surfactants are expensive, unavailable on a commercial basis and some exhibit hydrolytic decomposition at medium to high temperatures [ 7 ]. Therefore the aim of this work is to look for the surfactants which are not only commercially available but also sustain effectivness at high temperature and high salinity and show good recovery capability for improved oil recovery. Recently, ethoxylated carboxylated surfactants have received much attention because of their chemical and thermal stability, a good tolerance to mono and divalent ions, low losses in the medium through retention in the pores of the reservoir rocks [ 7 ] and [ 8 ]. As primary surfactants, alkyl carboxylated surfactants have exhibited moderate potential for use in enhanced oil recovery [ 9 ]. Another advantage is that the oil recovered by using these surfactants are free of emulsion [ 10 ]. At low degree of ethoxylation, these surfactants have demonstrated the formation of microemulsion in the absence of co-surfactants [ 11 ]. In this work, we have investigated a homologous series of commercially available surfactants, alkyl ether carboxylic acids, containing a mixture of C13 to C15 as a straight alkyl chain with different numbers of oxyethylene units. It has been shown [ 12 ] that these surfactants are very sensitive to pH and the hydrophilicity of the surfactant increases with the increase of the degree of ionisation. The critical micelle concentration is higher for the salts than for the acids. The surfactant micelles formed in the acid aqueous solution are smaller than for the salts. The purpose of this paper is to investigate the effect of pH on the micelle (aggregate) size, and hence on the phase behaviour of alkyl ether carboxylic acids in oil and water systems. 1 MATERIALS The Alkyl Ether Carboxylic Acid (AECA) surfactants were the Atlas G series, supplied by ICI Surfactants, Wilton, UK and largely consisted of an anionic surfactant (C13H27C15H31-O-(CH2CH2O)n-COOH) with a small amount of unconverted nonionic surfactant. In these structures n = 2.5, 4, and 7. The hydrocarbon (octane) was supplied by the Fluka Chemi (...truncated)


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A. Hussain, P. F. Luckham, T. F. Tadros. Phase Behaviour of Ph-Dependent Microemulsions At High Temperature and Salinities, Oil & Gas Science and Technology, 1997, pp. 228-231, Volume 52, Issue 2, DOI: 10.2516/ogst:1997024