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
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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)