SPE Member

Abstract

Surfactants are used at high temperatures as additives for steamfloods. and in high-temperature surfactant floods. Most surfactants must be above the critical micelle concentration (CMC) in order to have appropriate properties for forming foams or mobilizing oil. The CMC of a surfactant increases dramatically at high temperatures; thus. it is important to know this value at the temperature of application for economical engineering design.

The CMC's of a series of surfactants as a function of temperature in the range 77deg-392deg. F (298-473 K) have been determined at high temperatures from heats of dilution. a novel application of calorimetry. Enthalpies of dilution were determined at ambient and elevated temperatures. Surface tension measurements at ambient temperatures served to confirm the assignment of CMC from the heat of dilution data.

The CMC's of a pure surfactant were determined in distilled water, in aqueous solutions of NaBr up to 10% salinity, in n-butyl alcohol solutions up to 4% alcohol concentration, and at temperatures of 122deg. through 347deg. F (323-448 K). In general. CMC increased with an increase in temperature. The presence of electrolyte at a fixed temperature lowered the CMC and increased the sharpness of the onset of micellization. At 2120 F (373 K). the presence of n-butyl alcohol caused the CMC to be lowered but had presence of n-butyl alcohol caused the CMC to be lowered but had little effect on the sharpness of micellization. At 347deg. F (448 K). the alcohol had very little effect on the CMC. apparently because of the change in water properties at high temperatures.

The CMC's and solution behavior of three commercial surfactants were determined at 77deg. 257deg. and 392deg. F (298. 398. and 473 K), with no added salt. The CMC for these commercial surfactants also increased with temperature.

Introduction

Surfactants at high temperatures are used for enhanced oil recovery (EOR) from high-temperature reservoirs as part of chemical slugs and as additives in steamflooding to generate foams for the improvement of sweep efficiency. They are also used In industrial applications. such as cleansing, as emulsifiers for Polymerization and other reactions. and as inhibitors in boiler and steam-handling equipment. in all of these applications, knowledge of the CMC as well as the ability to calculate activity coefficients at various temperatures (e.g. for solubility, CMC, and phase behavior properties) is vital to their successful, or at least more economic. properties) is vital to their successful, or at least more economic. implementation.

This paper describes the effects of temperature. salinity' and alcohol on the CMC and solution properties of a pure surfactant, dodceyltrimethylammonium bromide, (DDTAB) and the effects of temperature on the CMC's of three commercial surfactants. At moderate temperatures, the presence of brine and alcohol is known to lower the CMC of surfactants. Aliphatic alcohols are known to partition into micellar aggregates to varying extents, depending on partition into micellar aggregates to varying extents, depending on the alkyl chain lengths of the alcohol and the surfactant, the structure of the surfactant, temperature. micellar size, and electrolyte concentration. it is believed that this partitioning is largely responsible for the change In the micellization behavior of surfactants In the presence of alcohol.

DDTAB was selected because it is stable in aqueous solution at high temperatures, can be easily purified by fractional crystallization, and has reasonable CMC values. While cationic surfactants are seldom used as EOR surfactants, Previous studies have shown that cationic and anionic surfactants with the same tail show similar trends in their behavior. Thus, what is learned about the effects of temperature, salinity, and alcohol on DDTAB can be applied to anionic surfactants. The interaction of BuOH and DDTAB at ambient temperatures has been examined by others and results of that work provided a good basis for comparison with this study.

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