Journal of Saudi Chemical Society (2012) 16, 333–337

King Saud University

Journal of Saudi Chemical Society

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Water pH and surfactant addition effects on the stability

of an Algerian crude oil emulsion

Mortada Daaou a,*, Dalila Bendedouch b

a Department of Industrial Chemistry, Faculty of Sciences, University of Sciences and Technology of Oran, P.O. Box 1505,El m’naouer, Oran 31000, Algeriab Laboratory of Macromolecular Physical Chemistry, Faculty of Sciences, University of Oran (Es-senia), Oran 31000, Algeria

Received 25 January 2011; accepted 29 May 2011Available online 1 July 2011

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KEYWORDS

Emulsion stability;

Test-bottle;

Optical microscopy;

pH;

Crude oil;

Surfactant

Corresponding author. Tel.

-mail address: daaou_morta

19-6103 ª 2011 King Saud

sevier B.V. All rights reserve

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i:10.1016/j.jscs.2011.05.015

Production and h

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Abstract In many oil production sites water injection is used as a piston to push the crude out of

the well. As the age of the field progresses, the ratio of water to oil produced increases. Agitation of

a water and crude oil mixture may give stable water-in-oil emulsion in which the water remains dis-

persed for a long period of time. These emulsions can cause severe problems in production and

transport processes since they normally possess high stability and viscosity. The most important

water properties which may contribute to the emulsion stability include pH and additive content.

In this study, we report on the effect of both, water pH and the presence of surfactant molecules

(anionic, cationic or non-ionic) on the stability of an Algerian crude oil (Haoudh el Hamra well)

aqueous emulsion prepared by a mechanical agitation procedure. The stability was followed by

the test-bottle method to measure the resolved water separated from the emulsion, and optical

microscopy to visualize the dispersed water droplets in the oil phase. The results of the effects of

varying the aqueous-phase pH suggest that the neutral medium is more efficient than acidic or basic

environment for stabilizing the emulsions. The addition of non-ionic surfactants has a better poten-

tial to improve crude oil emulsion stability with respect to both cationic and anionic surfactants

which do not show any improvement in the oil/water phase compatibility.ª 2011 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

50444965.

o.fr (M. Daaou).

y. Production and hosting by

Saud University.

lsevier

1. Introduction

Water in oil emulsion formation occurs at many stages in theproduction and treatment of crude oil. Water is used as a pis-ton to push the crude out of the well during the process of oil

production and to remove species such as chloride salts whichpoison refinery catalysts and enhance corrosion problems dur-ing refinery treatment. Agitation of a water and crude oil mix-

ture may yield stable water-in-oil emulsion in which waterremains dispersed for a long period of time. These emulsionscan cause severe problems in production, transport and

334 M. Daaou, D. Bendedouch

treatment processes since they normally possess high stabilityand viscosity. Water can form gas hydrates and is also respon-sible for corrosion problems. Thus, for other reasons too, it is

of interest to dehydrate the crude oils. An understanding of thestabilization mechanism and knowledge of the factors whichaffect this stabilization is a major concern of several authors

(Mingyuan et al., 1992; McLean and Kilpatrick, 1997b;Gafonova and Yarranton, 2001; Li et al., 2002; Havre andSjoblom, 2003; Ese and Kilpatrick, 2004). It is shown (McLean

and Kilpatrick, 1997a,b; Yarranton et al., 2000) that the stabil-ity of water in crude oil emulsion depends mainly on a rigidprotective film encapsulating the water droplets. It is believedthat this interfacial film is composed predominantly by natural

surfactants contained in crude oil i.e. asphaltenes, resins andfatty acids (Sjoblom et al., 1992; Spiecker and Kilpatrick,2004). These substances may accumulate at the water–oil inter-

face and hinder the droplets to coalesce and phase separate.Among these components, asphaltenes are believed to be themajor ones stabilizing the emulsion. Indeed, most of the time,

the other two components, cannot alone produce stable emul-sions (Lee, 1999). Moreover, they may in some cases associateto asphaltene molecules and affect emulsion stability. In par-

ticular, resins could solubilize asphaltenes in oil by the waypreventing them from associating in order to form the interfa-cial film and, therefore, lower the emulsion stability. Theasphaltenes represent the heaviest and most polar fraction of

the crude oil and their representative molecule contains mainlya sheet of more or less condensed aromatic rings with aliphaticside chains and various functional groups (Strausz et al., 1992;

Daaou et al., 2006; Bouhadda et al., 2007; Daaou et al., 2009).With these structural characteristics, asphaltenes exhibit sur-face activity and act as natural emulsifiers (see Fig. 1; Taylor,

1992; Sheu et al., 1995; Schildbreg et al., 1995; McLean andKilpatrick, 1997a,b; Sztukowski et al., 2003). However, they

Figure 1 Mechanism of emulsion stabilization: asphaltenic thin

film formation at the oil/water interface (McLean and Kilpatrick,

1997a,b).

do not operate as individual molecules, but through an aggre-gated state (McLean and Kilpatrick, 1997a). The interfacialfilm is not monomolecular but is constituted by asphaltene

aggregates which accumulate at the surface of the water drop-lets. Consequently, the propensity of asphaltenes to flocculatein crude oil enhances the water-in-oil emulsion stability

(Sztukowski et al., 2003). This is the case of the Algerian crudeoil from the Haoudh El Hamra well in the Hassi Messaoud oilfield whose production is continuously perturbed by the aggre-

gation and deposition of asphaltenes (Daaou et al., 2009).Therefore, it appeared of some interest to study the stabilityof aqueous emulsions containing this particular oil.

The interfacial film and hence the emulsion stability can be

affected by many factors like water-pH and additive content(Schramm, 1992; Poteau et al., 2005; Fortuny et al., 2007). Lit-erature reports a multitude of pH and chemical addition effects

on crude oil/water systems which show different behaviors fordifferent oil origins (Strassner, 1968; Pathak and Kumar, 1995;McLean and Kilpatrick, 1997a; Goldszal et al., 2002). For in-

stance, Poteau et al.(2005), show that pH has a strong influenceon interfacial properties of asphaltenes at the Venezuelancrude oil/water interface and that at high or low pH, asphalt-

enes functional groups become charged, enhancing their sur-face activity. Recently, Fortuny et al. (2007) studied theeffects of salinity, temperature, water content and pH on thestability of crude oil emulsions upon microwave treatment

and found that the demulsification process was achieved withhigh efficiencies for emulsions containing high water contentsexcept when high pH and salt contents were simultaneously

involved.Emulsion destabilization occurs in several steps (Graham

et al., 2008): flocculation, coalescence, and finally phase sepa-

ration or sedimentation. The flocculation of water droplets inwater-in-oil emulsions involves their aggregation in order toform clusters. During the coalescence step, the clusters fuse

to form larger droplets, this process can eventually lead tophase separation or sedimentation under the influence of grav-ity. Emulsion stability may be determined by a number ofmethods, such as the bottle test and electrical methods (Wang

and Alvarado, 2009). The most common method in the lab-scale is the simple bottle test: varied amounts of potentialdemulsifiers are added into a series of tubes or bottles contain-

ing each the same amount of an emulsion to be broken (Mat,2006). More recently, Moradi et al. (2011) studied the effect ofsalinity on water-in-crude oil emulsion through the measure of

droplet-size distribution visualized by an optical microscopymethod. They indicate that emulsions are more stable at lowerionic strength of the aqueous phase, a result consistent withthose obtained by electrorheology and bottle tests (Wang

et al., 2010).The present work investigates the influence of both water

pH and the addition of surfactants (cationic, anionic and

non ionic) on the stability of a model emulsion made of40 vol.% of the Algerian Haoudh El Hamra crude oil and60 vol.% of water. It is designed as an attempt to contribute

to the understanding of this water-in-oil emulsion behaviorin order to provide ultimately technologic responses to someoil production problems specific to this crude. Sodium dodecyl

sulfate (SDS), dodecyl triphenylphosphonium bromide(DTPB) and decyl-tri-oxyethylene alcohol (C10E3) are selectedsince they are well known compounds representative of eachsurfactant type. The optical microscopy is used to confirm

Water pH and surfactant addition effects on the stability of an Algerian crude oil emulsion 335

the water-in-oil emulsion type and the bottle test method isemployed to evaluate the emulsion stability.

2. Experimental

2.1. Chemicals

The oil is from Haoudh el Hamra well of the Algerian Hassi-Messaoud field. Its general specifications are given in Table 1

as determined by the Refinery Company laboratory.The different chemicals used in this work were purchased

from Sigma products Company with a nominal purity of 98%.

2.2. Emulsion preparation

2.2.1. Water pH effect

The aqueous solutions of different pH were prepared by mix-ing adequate amounts of NH4OH in demineralized distilledwater for basic solutions (pH: 8–13), whereas acidic medium

(pH: 2–6) was prepared with HCl in water. The pH value foreach solution was controlled by a digital pH meter – ModelIN-112 apparatus.

Sixty milliliters of the aqueous phase at different pH wasadded to 40 ml of the crude oil. The mixture was then homog-enized by mechanical agitation (Ultra Turrax T 25 basic, IKA-

Werke) at 2500 rpm during 15 min at 20 �C.

2.2.2. Surfactant addition effect

Sixty milliliters of brine water containing 3.5 wt% of NaCl and

100 ppm of one of the surfactants were added to 40 ml of crudeoil and homogenized as described in the previous section. Theoperation was repeated for each surfactant. An aliquot of each

sample was immediately poured into a 10 ml graduated bottlewhich was left to stand at 20 �C up to 24 h until water sepa-rated (Daaou et al., 2005a,b).

2.3. Emulsion stability determination

The stability was monitored by measuring the volume of sep-arated water from the emulsion by the test bottle method. In-

deed, the lower the emulsion stability, the larger the waterpartition. The amount of separated water is directly correlatedto the instability of the emulsion since destruction of the inter-

facial film and subsequent water droplet coalescence is thedeterminant step in the demulsification process (McLean andKilpatrick, 1997a).

2.4. Water droplet dispersion visualization

An Axiostar optical microscope (Carl Zeiss) using a 40· objec-tive with 0.45 lm resolution is used to visualize dispersed water

Table 1 General characteristics of Hassi-Messaoud crude oil.

Density ðd204 Þ 0.83

Total acid number (mg KOH/1 g) 0.28

Asphaltenes (wt%) 0.60

Resins (wt%) 6.50

Resins/asphaltenes 10.80

droplets in the oil phase to confirm the existence of water in oil(w/o) type emulsion (Balinov et al., 1994; Nandi et al., 2003;Zhang et al., 2009; Moradi et al., 2011). The sample images

corresponding to a stable emulsion are taken with a digitalSony camera.

3. Results and discussion

3.1. Effect of water pH

The emulsion samples left to stand are observed and the resultsare depicted in Fig. 2a. Obviously, aqueous-phase pH affects

the emulsion stability. For acidic medium, the separated watervolume varies from 3 vol.% to 36 vol.% as water pH variesfrom 1 to 6. The largest volume (36%) is reached at pH = 6.

In the basic medium (pH= 8–13), separated water is com-prised between 2 vol.% and 15 vol.%. The least stable emul-sion occurs for weakly acid environment. At pH = 7, theemulsion is the most stable since no water separates and the

w/o emulsion type is confirmed by optical microscopy whichvisualizes the dispersion of water droplets in the oil phase(Fig. 2b). The system is also relatively stable at very low and

moderately basic pH. These results are comparable to thoseobtained by Strassner (1968) who studied crude oil emulsionsat different pH values and found that Venezuelan crude oil

Figure 2 (a) Separated water volume for various water pH. (b)

Water droplet dispersion in the crude oil at pH = 7 (optical

microscopy).

Figure 3 pH of separated water vs. emulsified water pH.

18

82 85

0

1020

3040

50

6070

8090

100

C10E3 DTPB SDSSurfactants

Sepa

rate

d w

ater

(%vo

l.)

Figure 4 Effect of surfactant type on emulsion stability after 24 h.

Figure 5 Surfactant type effect on kinetic stability of surfactant

in brine water/crude oil emulsion.

336 M. Daaou, D. Bendedouch

emulsions at pH < 6 are highly stable, while those at pH> 10exhibit low stability or are highly unstable although at

pH = 13 the emulsion was very stable.The pH effect on emulsion stability is usually attributed to

ionization of polar groups of surface active components which

induce sufficient electrostatic repulsive interactions to breakdown the interfacial film cohesion (McLean and Kilpatrick,1997a).

The residual aqueous-phase pH after demulsification isplotted as a function of the corresponding initial pH inFig. 3. The plot shows that it is consistently less acidic whenthe initial pH is acidic and less basic when the initial pH is

basic. This suggests that the native surface active compoundsin the crude adsorbed at the oil/water interface containstrongly basic as well as weakly to strongly acidic functional

groups. Indeed, the total acid number of this Algerian crudeoil, as given on Table 1 classes it among the less acidic crudes.Depending on the pH, the functional groups may dissociate

into the aqueous phase upon emulsification giving rise to largesurface charge densities and subsequent internal repulsion in

the film that destroy the film structure (McLean and Kilpatrick,1997a,b).

3.2. Effect of surfactant addition

Fig. 4 is a plot of the amounts of separated water against addedsurfactant type (anionic, cationic and non-ionic) 24 h after the

sample preparation. It is noticed that the maximum stability isobserved for the emulsion containing the non-ionic surfactant,whereas the ionic surfactants confer lesser stability to the

emulsion.Fig. 5 shows the kinetic emulsion stability for each of the

three surfactants. After 24 h, separated water is equal to 85,

82 and 18 vol.% for added DTPB, SDS and C10E3, respec-tively. These observations confirm that the emulsion whichcontains the non-ionic surfactant remains the most stable

one. In the case of the ionic surfactants the polar head groupsprobably prevent or inhibit the formation of the associatedasphaltene film at the oil/water interface leading to importantemulsion instability.

4. Conclusion

The instability of this Algerian crude oil-in-water emulsion is

enhanced either by water pH induced ionization of the polargroups of the surface active components or the presence ofelectrically charged ionic surfactant heads imbedded in the

interfacial film. Both circumstances induce sufficient electro-static repulsive interactions to break down the interfacial filmcohesion. This observation may be straightforwardly valued in

an efficient demulsification process.

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