713
interfacial viscosity. When a demulsifier is added into the emulsion, the films change
into liquid films. The liquid films are slightly soluble as compared to solid films, and
the coalescence of droplets is enhanced.
The emulsion stability primarily depends upon the type of interfacial films.
Therefore, it is important to understand the factors which affect the stability of these
films. Some factors that are briefly discussed herein [31, 32] are:
• Polar surface-active molecules (e.g., asphaltenes, resins)
• Solids including waxes and inorganic salts (cause of corrosion)
• Droplet size distribution in the emulsion
• Temperature
• Brine concentration
• pH of brine
Heavy polar fractions such as resins, asphaltenes, organic acids (carboxylic,
naphthenic), and bases are present in varying quantities and forms in crude oil and
contribute to the stabilization of the crude oil emulsion to a varying degree.
Asphaltenes are very complex polyaromatic molecules attached with some heteroatoms or metals (N, O, S, V, Ni) such as shown in Fig. 3 [33]. These are soluble in
benzene or ethyl acetate but insoluble in low molecular weight n-alkanes.
The diffusion of fine solid particles present in the crude to the oil-water interface
can stabilize water-in-oil emulsions. Solid particles can also get electrically charged
at the interface, thereby enhancing the stability of an emulsion. Typically the diameter of particles is in the range of submicron to a few microns [34]. Wettability of
particles is defined as the degree of solids that is wetted by water or oil (when the
emulsion is present). Wettability also enhances the stability of an emulsion. The oilwet solids are waxes and asphaltenes, whereas water-wet solids are clays, sand,
CaCO 3 , CaSO 4 , etc. [35]. Binks and Lumsdon [36] report that the emulsion may be
stabilized by the presence of colloidal silica particles. These particles are charged
and appear at the interface but are destabilized at higher pH. The charge on the particles is due to modification of pH (e.g., silica has zero charge at pH 2, but it becomes
negatively charged at pH more than 2) and addition of electrolyte.
Temperature significantly affects the stability of an interface and also the
solubility of surfactant. With an increase of temperature, the viscosity of the system
reduces significantly, which helps to reduce the stability of an emulsion. Jones et al.
[37] studied the effect of temperature on the interfacial film stability and found that
it gradually decreases with an increase in temperature.
Strassner et al. [12] studied the effect of pH on interfacial film stability. The
author found that there are two types of stable films found in the oil-water emulsion,
namely, rigid and mobile. Solid (rigid) films are mainly formed by the presence of
polar molecules such as asphaltene, whereas liquid (mobile) films are formed
because of resins. According to Strassner et al. [12], the amount of demulsifier used
for coalescence may be reduced by adjusting to a proper pH. The author gives an
idea about how in low pH range (acidic region) solid films are formed by asphaltenes
which are highly stable. However, with an increase in the pH of emulsion, the rigid
films change into liquid films which are less stable. The author suggested that film
Stability and Destabilization of Water-in-Crude Oil Emulsion
interfacial viscosity. When a demulsifier is added into the emulsion, the films change
into liquid films. The liquid films are slightly soluble as compared to solid films, and
the coalescence of droplets is enhanced.
The emulsion stability primarily depends upon the type of interfacial films.
Therefore, it is important to understand the factors which affect the stability of these
films. Some factors that are briefly discussed herein [31, 32] are:
• Polar surface-active molecules (e.g., asphaltenes, resins)
• Solids including waxes and inorganic salts (cause of corrosion)
• Droplet size distribution in the emulsion
• Temperature
• Brine concentration
• pH of brine
Heavy polar fractions such as resins, asphaltenes, organic acids (carboxylic,
naphthenic), and bases are present in varying quantities and forms in crude oil and
contribute to the stabilization of the crude oil emulsion to a varying degree.
Asphaltenes are very complex polyaromatic molecules attached with some heteroatoms or metals (N, O, S, V, Ni) such as shown in Fig. 3 [33]. These are soluble in
benzene or ethyl acetate but insoluble in low molecular weight n-alkanes.
The diffusion of fine solid particles present in the crude to the oil-water interface
can stabilize water-in-oil emulsions. Solid particles can also get electrically charged
at the interface, thereby enhancing the stability of an emulsion. Typically the diameter of particles is in the range of submicron to a few microns [34]. Wettability of
particles is defined as the degree of solids that is wetted by water or oil (when the
emulsion is present). Wettability also enhances the stability of an emulsion. The oilwet solids are waxes and asphaltenes, whereas water-wet solids are clays, sand,
CaCO 3 , CaSO 4 , etc. [35]. Binks and Lumsdon [36] report that the emulsion may be
stabilized by the presence of colloidal silica particles. These particles are charged
and appear at the interface but are destabilized at higher pH. The charge on the particles is due to modification of pH (e.g., silica has zero charge at pH 2, but it becomes
negatively charged at pH more than 2) and addition of electrolyte.
Temperature significantly affects the stability of an interface and also the
solubility of surfactant. With an increase of temperature, the viscosity of the system
reduces significantly, which helps to reduce the stability of an emulsion. Jones et al.
[37] studied the effect of temperature on the interfacial film stability and found that
it gradually decreases with an increase in temperature.
Strassner et al. [12] studied the effect of pH on interfacial film stability. The
author found that there are two types of stable films found in the oil-water emulsion,
namely, rigid and mobile. Solid (rigid) films are mainly formed by the presence of
polar molecules such as asphaltene, whereas liquid (mobile) films are formed
because of resins. According to Strassner et al. [12], the amount of demulsifier used
for coalescence may be reduced by adjusting to a proper pH. The author gives an
idea about how in low pH range (acidic region) solid films are formed by asphaltenes
which are highly stable. However, with an increase in the pH of emulsion, the rigid
films change into liquid films which are less stable. The author suggested that film
Stability and Destabilization of Water-in-Crude Oil Emulsion