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Fuel 95 (2012) 97–107

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Fuel

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Investigating the rheological properties of light crude oil and the characteristicsof its emulsions in order to improve pipeline flow

Madjid Meriem-Benziane a, Sabah A. Abdul-Wahab b,⇑, Mohamed Benaicha c, Mansour Belhadri c

a Mechanical Engineering Department, University of Chlef, P.B. 151, Chlef 02000, Algeriab College of Engineering, PO Box 33, Sultan Qaboos University, Al-Khod 123, Omanc Laboratory of Rheology, Transport and Treatment of the Complex Fluids, University of Science and Technology, Mohamed Boudiaf, B.P. 1505, El M’Naour, Oran 31000, Algeria

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 June 2011Received in revised form 10 September 2011Accepted 6 October 2011Available online 25 October 2011

Keywords:Non-NewtonianStressViscosity complexStorage modulusLoss modulus

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.10.007

⇑ Corresponding author.E-mail address: [email protected] (S.A. Abdul-W

The objective of this study was to investigate the rheological properties of the light crude oil and its emul-sions in order to obtain more knowledge about the rheological behavior of oil flow in pipelines. Theexperiments were carried out at a temperature of 20 �C by using the RS600 RheoStress (ThermoHaake,Germany). The results showed that the viscosity of the prepared emulsions varied with their water con-tents. In the case of 100% light crude oil, the study of the functional relationship demonstrated the quasi-Newtonian behavior with a moderate constant viscosity. However, for emulsions with different waterconcentrations, their rheological behaviors were described in better way by the Ostwald de Waele andthe Herschel–Bulkley models. The stability of emulsions was identified by measuring the rheologicalproperties including non-Newtonian viscosity, the elastic modulus, (G0), the loss modulus, (G00), the phaseangle (d) and the complex viscosity (g⁄). The results indicated that the rheological properties and thephysical stability of emulsions were significantly influenced by the water contents and the nature ofcrude oils.

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1. Introduction

It is known that crude oil plays an essential role in giving theenergy supply of the world among different sources of energy. Fur-thermore, in the hydrocarbon industry, the existence of stablewater-in-crude oil emulsion is considered to be undesirable aswater should be separated from oil. In general, the main parame-ters for identifying the crude oil are specific gravity (API), density(d) and sulfur content (S). According to their respective mean val-ues (i.e., 38 < API < 44, 0806 < d < 0830 and 0.2 < %S < 0.3), theAlgerian crude oil is classified as a light one. Therefore, it is neces-sary to transport the Algerian crude oil in complex installationspipelines in order to reduce its corrosive power and to avoid itsinteraction with the metal of the pipelines.

Apart from this, the high viscosity of heavy crude oils and emul-sions undoubtedly contribute to the increase of the transport billby the pipelines. The analysis of the influence of the viscosity ofoil–water emulsions is considered essential in the field of rheologyto find out ways for the development and transportation of oil[1,2]. Often, crude oil is found in mixed state in which the concen-tration of water is very consistent. Due to the complex behavior of

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ahab).

the crude oil, it is subjected to numerous difficulties during variousprocesses such as production, separation, transportation, and refin-ing. Indeed, the high concentrations of sulfur and acids in the hea-vy petroleum feed stocks has a negative impact on their transportthrough pipelines. It should be noted that the water contents in thelight oil–water mixture has a great impact on its rheologicalbehavior and its type [3].

With respect to energy point of view, the presence of water inthe oil is a factor which affects its quality and therefore the elimi-nation of water improves the calorific value of the light crude oil[4]. In addition, research on the rheological behavior of the lightcrude oil is very important, especially if someone takes into ac-count the presence of surfactants.

The characteristics of emulsions with respect to their concen-trations indicate the possibility of their structural changes. Accord-ing to Wang et al. [5], these structural changes can be addressed byintroducing the concepts of viscous modulus, elastic modulus, andrelaxation time (i.e., viscoelastic analysis). The stability of an emul-sion depends on the presence of solid particles in the films andtheir interfacial elasticity.

According to Xia et al. [6], the measurements of the dynamicviscoelasticity of expansion can be used to study the chemicalcomposition of the emulsions in order to provide more informationon the dynamics of polymer chains and their interactions with thesurfactant molecules. Also the study revealed that the compositionand the structure of the rigid layer around droplets of water in the

98 M. Meriem-Benziane et al. / Fuel 95 (2012) 97–107

emulsions are responsible for the stability and for the preventionof coalescence phenomenon [7]. According to White et al. [8], inmost of the emulsions which are stabilized by proteins, the inter-actions between the colloids present in the droplets and electro-static interactions influence the state of aggregation of droplets.According to Dan and Jing [9], the rheological behavior of heavyoil–water emulsion is non-Newtonian.

During the transport of oil via pipeline, the stability of crude oilis particularly favored by the concentration of chemical additivessuch as surfactants which contribute to decrease the interfacialtension between the crude oil and the water [10].

The water content in the oil emulsions can be high (about 60%by volume). But, this quantity is not very important for refiningoperations. However, the effective viscosity of the emulsion de-pends largely on the volume fraction of water and the temperaturephases in thermodynamic equilibrium. In this connection, the pre-vious research work guides in developing correlations to predictthe actual values of viscosities [11–13]. With increasing tempera-ture, the stability of the oil–water emulsion decreases. For differentlevels of oil, pour points of oil–water emulsions are always lowerthan those of crude oil from the reservoir.

According to Hasan et al. [14], the modeling analysis can beused to find out certain rheological models that fit the measure-ments. Three rheological models of Newton, Hershel–Bulkly, andCasson models can be used to determine the rheologicalcharacteristics.

The objective of this work was to study the rheological proper-ties of light crude oil and the characteristics of its emulsions. Thestudy was divided into four parts. The first part was devoted to-wards experimental analysis to establish the viscosity rheogramas a function of the composition of the emulsions with differentwater concentrations (i.e., 30%, 50% and 70%) at a temperature of20 �C. The second part investigated the yield stress of the emul-sions. In the third part, different rheological models were used(e.g., Newton law, Hershel–Bulkly law and Casson law) in orderto simulate the flow behavior of the emulsions. The fourth partinvestigated the impact of the stability on the rheology ofemulsions.

Fig. 1. Samples of light crude oil after the agitation.

2. Methodology

2.1. Materials

In this study four different samples of light crude oil were usedfor the preparation of emulsions. These samples of light crude oilwere taken from different oil fields in Algeria. The origin of thesesamples was recognized by identifying them with letters A, B, Cand D. At temperature of 20 �C, the samples were characterizedby a density of 806 kg/m3 and a specific gravity (API) between 36and 44. The emulsions were prepared with different concentra-tions of water (i.e., 30%, 50% and 70%). The perfect homogenizationwas achieved by stirring the solution for 10 min with a magneticbar. The desired temperature for each test was maintained byusing a temperature controller (type DC30). The PH of the emul-sions was determined by a PH meter electrode calomel type HannaInstruments-213.

The rheological behavior of different samples was studied byexploiting the performance of the rheometer (RheoStress 600, typeZ40 DIN, Germany) which was operated at a pressure of 2.5 bar.Once, the thermodynamically stabled emulsion was obtained,and then different rheological performance of the samples wasmeasured by exploiting the rheometer as mentioned earlier. Thestudy of liquid–liquid stability and the influence of temperatureon the partitioning of coexisting phases were essential in choosingthe appropriate solvent (liquid–liquid extraction). Thus for

analyzing this physicochemical aspect, the emulsion was kept fora period of 24 h in order to settle in vials after stirring.

It was noted that the formation of emulsions light crudeoil–water (O/W) was an alternative for improving its flow in pipe-lines. Therefore, the study of their rheological properties was ofgreat importance in the petrochemical industry. These werediscussed by analyzing their stability on the basis of the volumeconcentration in water.

2.2. Rheological measurements

The samples were tested with a modular rheometer (Rheostressof Rs. RS600) that was manufactured by ThermoHaake in Karlsrule,Germany. The meter was equipped with a cone–plate and coaxialcylinder geometries. Measurements were made using the configu-ration of the cone–plate combinations having the flowing charac-teristics of diameter 60.0 mm, angle 3�, distance 0.105 mm, andsample volume of 2.0 cm3. The temperature was maintained con-stant as 20 ± 1 �C.

There were several operating test modes of the rheometer. Theywere the universal controlled rate (CR) mode, the controlled stress(CS) mode, and the oscillation (OSC) test mode. In the CS mode,shear stress was applied to a test sample by means of extremelylow inertia [14].

The resulting deformation of the sample was detected with adigital encoder that processes 106 impulses per revolution. Thisresolution made it possible to measure small yield values, strains,or shear rates. The computer controlled rheometer was switchedbetween both CS and CR modes, and was used to provide oscillat-ing stress inputs. A controlled variable lift speed was used to posi-tion the cone on the plate. The Haake software was used to controlboth the test routines and the data evaluation. The rheometer wasequipped with a cone and a plate sensor.

All samples were allowed at rest for 5 min after loading to allowtemperature equilibration and induced stress to relax. The linearrange at which viscoelastic properties were independent of strainrates for all samples. Storage modulus (G0), loss modulus (G00) andloss tangent (tan d) vs. frequency (x) were measured for allsamples.

3. Results and discussions

3.1. Rheological analysis of light crude oils

In this section, four samples of light crude oil as shown in Fig. 1were used to study their rheological properties by using the rhe-ometer. During experiment, the viscosity of these samples wasanalyzed with the shear rate at a temperature of 20 �C. The varia-tion of the viscosity of these four samples with the shear rate isshown in Fig. 2. Overall, it was observed that the viscosity of light

Fig. 2. Viscosity behavior for four types of light oil.

Fig. 3. Viscosity behavior of emulsions (A–D) based on 30% of light crude oil.

Fig. 4. Different emulsions (A–D) based on 30% crude oil (after shaking).

Fig. 5. Viscosity behavior of emulsions (A–D) based on 50% of light crude oil.

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crude oils increased with the shear rate. This increased in the vis-cosity showed the existence of a transition between two regions. Inthe first region, it was noticed that the viscosity increased in nonlinear form with the shear rate. This region was identified by vis-cosities in the range of 0.0012 to 0.0025 Pa s and by shear rate from5 s�1 to 50 s�1. In the second region, the viscosity of light crude oilsincreased linearly with the shear rate (for shear rate above 50 s�1).These findings showed a quasi-Newtonian rheological behavior oflight crude oils, regardless of their compositions. It was noticedthat the average value of viscosity was still matched to that ofwater at the same temperature of 20 �C.

3.2. Influence of water contents on the rheology of emulsions

The knowledge of the rheological behavior of emulsions is nec-essary for flow modeling analysis. In this study, the four samples oflight crude oil were used for the preparation of emulsions with dif-ferent water contents (30%, 50% and 70% water contents). After for-mation of emulsions, the rheological tests were performed onthem. According to the water concentrations, the rheologicalbehavior of these emulsions (oil–water) was modeled accordingto the main laws of the rheology of complex fluids. A series ofexperiments was conducted at a temperature of 20 �C to analyzethe variation of viscosity of these emulsions with the shear rate.The results of the four emulsions (A–D) with water contents of70%, 50% and 30% are shown in Figs. 3–8.

Figs. 3, 5 and 7 also illustrate the most suitable models for thebehavior of these emulsions at different water contents. Such re-sults highlighted a different rheological behavior of a Newtonianfluid. On the basis of this analysis, power laws in two or threeparameters were used for the classification of these emulsions.

3.3. Modeling

According to Hasan et al. [14], the rheological behavior of vari-ous emulsions can be described by Ostwald de Waele and Her-schel–Bulkley models that strongly explain the colloidalsuspensions of emulsions. The Ostwald de Waele model for emul-sions that are based on light crude oil and water is given by twoparameters (k and n) as shown in the following equation [14]:

g ¼ k _cn�1 ð1Þ

On the other hand, the Herschel–Bulkley model for emulsions [14]is described by three-parameters (s0, k and n) as shown in the fol-lowing equation:

g ¼ s0

_cþ k _cn�1 ð2Þ

According to these two equations, the least squares method wasused in identifying the appropriate model parameters, namely; theintrinsic viscosities (g0 and g1) and the constants n and k. Thismodeling analysis was performed to find out the variations ofparameters of the Ostwald de Waele and Herschel–Bulkley modelsat different water contents [14]. Note that the limiting viscosity andthe threshold stress of the flow are associated with the shear rateswhich are in perfect agreement with the formation of coagulum inemulsions. Furthermore, this tendency is increased with increasingthe water contents. The decrease in the limiting viscosity remainsdependent on the ratio between hydrodynamic energy and interac-tion energy between the constituent particles of the emulsion.

Fig. 7. Viscosity behavior of emulsions (A–D) based on 30% of light crude oil.

Fig. 8. Different emulsions (A–D) based on 70% light crude oil (after shaking).

Fig. 6. Different emulsions (A–D) based on 50% crude oil (after shaking).

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It was noticed from Fig. 3 that emulsions B and D were showingconstant values of viscosity, whatever the values of shear rate. Asimilar result was emerged from Fig. 5 for emulsion A and fromFig. 7 for emulsion B. Hence, these three emulsions (A, B, and D)exhibited quasi-Newtonian behavior. Moreover, Fig. 3 showed thatthe viscosity of emulsions A and C decreased with increasing shearrate. A similar result was emerged from Fig. 5 for the emulsions B–D and from Fig. 7 for the emulsions C and D. Hence, for these cases,the predicted shape of rheograms was indicating the state of reach-ing the limiting viscosity at high shear rates. Therefore, these threeemulsions exhibited a non-Newtonian behavior. In addition, it wasseen that emulsions B and D (Fig. 3), A (Fig. 5) and B (Fig. 7) wereseparated into two phases relatively after a short time when theagitation was stopped. These two immiscible phases of crude oil

and water were formed regardless of the percentage of water. Bytaking into account of the strongly pronounced polarity of waterand the saturated nature of the constituent species of oil, it canbe argued that there was no chemical reaction which might affectthe stability of coexisting phases.

On the other hand, it was found that the emulsions A and C (Fig. 3),B–D (Fig. 5), A, C, and D (Fig. 7) were unstable. Indeed, for high con-centrations, the droplets of water were formed which remained sus-pended without forming emulsions with oil. Therefore, anymeasurement of the initial viscosity was affected by the compositionof water and by the rheological behavior of such pseudo-emulsions.

The observation indicated that the model includes an initial vis-cosity that predicts that the fluids obeyed certainly Ostwald deWeale and Herschel–Bulkley models (Table 1). Indeed, these mod-els assumed that such fluids can be presented schematically, atrest, in form of rigid three-dimensional structure, capable of with-standing stresses below the yield stress where the viscosity wasmaximum at the initial starting point. Beyond the point of initialstarting viscosity, the structure changed completely and the fluidstarted to flow [14].

It was also found that the emulsions were very complex mix-tures. Therefore, modeling of their behavior required thermody-namic models which involved mixing rules (such as NRTL,UNIQUAC and DECHEMA) in order to predict their stability. How-ever, the deviation from the ideal solutions required the study ofthe activity coefficients and the partition coefficients of water insolutions containing high water contents. At infinite dilution, theemulsion behaved in a similar way to that of the light crude oil.

It was demonstrated that the rheological behavior of emulsionswas a pseudo-plastic type with a threshold stress (i.e., the values ofthe viscosity of emulsions were greater than that of the values ofNewtonian fluids). Their behavior looked like to that of a solid atlow initial viscosities. However, the rheological behavior of emul-sions was similar to a viscous fluid with a non-linear trend underthe influence of decreasing viscosity.

4. Measurement of yield stress

The yield stress can be defined as a limit stress below which thesample behaves like a solid. Under low stress, the elastic deforma-tion disappears when the applied stress is reached. The relation-ship between the elastic deformation and the applied stress islinear. However, above the values of the yield stress, the applica-tion of stress undergoes unlimited deformation to cause the fluidto flow [14–19].

For different volumes of water, the measurements of the yieldstress for crude oil samples were carried out (by using the RS600RheoStress ThermoHaake rheometer) under the controlled stressmode. Controlling stress rheometers provide the most direct tech-nique to measure yield stress. The shear stress on the sample wasgradually increased without approaching the limit yield stress. Toidentify the parameters of the rheological models used in thisstudy (i.e., Newton, Herschel–Bulkley, and Casson), the standarddeviation coefficient (R2) as a criterion was used for data consis-tency. It was noted that the previous models [14] can be expressedby the following equations:

s ¼ g _c ð3Þ

s ¼ sþ k � cn ð4Þ

s ¼ sn0 þ _cgp

� �n� �1=n

ð5Þ

Eqs. (4) and (5) reflected the apparent yield stress for emul-sions. Fig. 9 shows the variation of yield stress as function of shearrate for the rheological behavior of light crude oil (Fig. 9a) and of

Table 1Determination of rheological model parameters.

Composition of the emulsion (v/v) Samples Ostwald de Waele model Herschel–Bulkley model

k n R2 s0 n k R2

O(30%)–W(70%) Emulsion A 0.7449 0.5664 0.9990 – – – –Emulsion B 0.0008 1.255 0.9360 – – – –Emulsion C 0.2886 0.6151 0.9978 – – – –Emulsion D 0.0053 1.205 0.8765 0.000 1.134 0.00701 0.9303

O(50%)–W(50%) Emulsion A – – – 0.000 0.8872 0.03572 0.9952Emulsion B 0.0166 0.5199 0.9995 0.0013 0.9550 0.00368 1.0000Emulsion C 0.3414 0.6460 0.9999 0.1017 0.4061 0.50090 0.9999Emulsion D 0.0377 0.9161 0.9403 0.000 0.9358 0.03485 0.9462

O(70%)–W(30%) Emulsion A – – – 0.0009 0.9329 0.00859 0.9802Emulsion B 0.0025 1.1050 0.6289 0.000 0.9413 0.00483 0.9663Emulsion C 0.1640 0.3790 0.9998 0.050 0.5514 0.08610 1.0000Emulsion D 0.0297 0.6799 0.9799 0.000 0.9309 0.01154 0.9963

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various emulsions (Fig. 9b–d). The rheological behavior of lightcrude oil was described by Newton (Fig. 9a), whereas for differentemulsions, it was described by the non-Newtonian models(Fig. 9b–d).

Several experiments were conducted in order to show differentrheological parameters such as yield stress. It was observed thatthe emulsion (O/W) was difficult to stabilize, particularly for thecase of 30% of water.

Shear stress as a function of shear rate was measured and com-pared with the rheological models. The Newtonian law given by Eq.(3) where yield stress as zero and the Herschel–Bulkley (H–B) law

Fig. 9. Rheograms of light crude o

and Casson law (non Newtonian) are given by Eqs. (4) and (5)where yield stress different to zero [14].

The measurements and simulations of yield stress of the differ-ent models are shown in Table 2 for crude oil and for the emul-sions. Validation of these models was done on the basis of thecoefficient R2 that were closed to unity. Thus, the values of theyield stress showed a good agreement between experimental val-ues and those predicted by the models. Overall, the behavior of dif-ferent types of crude oils were quasi-Newtonian, whereas theemulsions exhibited non Newtonian and quasi-non Newtonianbehavior.

il (9a) and emulsions (9b–d).

Table 2Values of yield stress measurements and simulation.

Composition of the emulsion (v/v)

Samples Experimental Yield stress simulation Observation

NewtonModel

R2 H–Bmodel

R2 Cassonmodel

R2

Crude oil A 0 0 0.9973 – – – – Quasi-Newtonian fluidO100–W0% Crude oil B 0 0 0.9965 – – – – Quasi-Newtonian fluid

Crude oil C 0 0 0.9964 – – – – Quasi-Newtonian fluidCrude oilD

0 0 0.9973 – – – – Quasi-Newtonian fluid

EmulsionA

0.0002 – – 0.0113 0.9997 – – Non Newtonian fluid

O70–W30% EmulsionB

0 0.0038 0.9991 – – – – Quasi-Newtonian fluid

EmulsionC

0.0024 – – – – 0.0087 0.9994 Non Newtonian fluid

EmulsionD

0.0083 – – 0.0698 0.9968 – – Quasi-Non Newtonianfluid

EmulsionA

0 – – 0.0327 0.9997 – – Quasi-Non Newtonianfluid

O50–W50% EmulsionB

0.013 – – 0.0219 0.9986 – – Quasi-Non Newtonianfluid

EmulsionC

0.314 – – – – 0.1119 0.9903 Non Newtonian fluid

EmulsionD

0.0066 – – 0.1292 0.9982 – – Quasi-Non Newtonianfluid

EmulsionA

0.206 – – – 1.437 0.9875 Non Newtonian fluid

O30–W70% EmulsionB

0 0.0023 0.9892 – – – – Newtonian fluid

EmulsionC

0.0827 – – – – 0.9303 0.9075 Non Newtonian fluid

EmulsionD

0 – – 0.0716 0.9988 – – Quasi-Non Newtonianfluid

102 M. Meriem-Benziane et al. / Fuel 95 (2012) 97–107

5. Rheological characteristics

5.1. Oscillatory viscometry

The elastic and viscous responses of viscoelastic systems werequantified by undertaking dynamic oscillatory measurements.These measurements were based on the application of a sinusoidalstrain (of frequency, x) to the viscoelastic system and the mea-surement of the corresponding stress (Eq. (6)). For viscoelastic sys-tems, the stress and strain were out of phase with every other. Thephase angle shift (d) was measured in terms of the time shift (Dt)between the amplitudes of the oscillating stress (r0) and the strain(c0).

d ¼ xDt ð6Þ

For complete analysis, the components of complex viscositywere taking into account as shown in the following equations:

g0 ¼ G00=x ð7Þ

g00 ¼ G0=x ð8Þ

In this sense, the concept of the complex viscosity was introducedwhich is defined by the following equations:

g� ¼ G�=x ¼ g0 � ig00 ð9Þ

where the rheological parameters (g⁄ and g⁄) are definedaccording to the phase angle d [5–20]. The values of G0 are de-scribed by the following equations:

G0 ¼ G� cos d ð10Þ

G0 ¼ G� sin d ð11Þ

G⁄(x) is the complex shear modulus, according to the angularvelocity is given by the following equation:

G� ¼ G0 þ iG00 ð12Þ

where G0 is the elastic modulus, which reflects the elastic re-sponse, whereas G00 is the corresponding loss modulus expressingthe viscous response. There were two regimes: Viscous (if G00 > G0)and Elastic (if G0 > G00).

Various viscoelastic parameters can be obtained from theamplitudes and the phase angle. These parameters include thecomplex modulus (G⁄), the elastic (G0) and the viscous components(G00) of the complex modulus and the tan d [20].

Measurements were undertaken to obtain these parameters forrecognizing the linear viscoelastic region. Constant strain ampli-tude in this region was then chosen for the oscillatory measure-ments. The stress amplitude was obtained as a function ofvarying oscillatory frequency, which was consequently used forobtaining the storage modulus (G0) and the loss modulus (G00) ofthe system. The extent of network formation was determined fromthese measurements. This means if the system was more struc-tured, then it showed more elastic response to shear.

In the case of oscillatory rheological measurements of storagemodulus and loss modulus, the emulsion system was identifiedas strongly, weakly or non-associated. With last category, the sys-tem behaved fundamentally like liquid. Values for d were also ob-tained which gave information about the nature of the viscoelasticresponse of the emulsion system. In elastic networks, the phase an-gle shift (d) was 0� while in purely viscous liquids it was 90�. Forviscoelastic systems, the phase angle shift ranged between 0�and 90�. The closer the phase angle shift to 0�, the more the emul-sion system displayed an elastic response to the shear stress. Thus,more gel-like network was developed [21].

The data related to the results of the emulsions are shown inFigs. 10–15. Figs. 10, 12 and 14 describe the variations of the com-plex viscosity, the elastic modulus and the viscous modulus withthe angular velocity. The phase angle as a function of oscillatoryfrequency for the emulsions are shown in Figs. 11, 13 and 15.

Fig. 10. Variations of the complex viscosity, the elastic modulus, and the viscous emulsions (a–d) with the angular velocity (30% crude oil).

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The reason for taking these measurements was to identify howemulsion stability functionally varied with the system composi-tion. This step was necessary in order to find out the main condi-tions for creating the stable emulsions.

Physically, stable emulsions were characterized by two condi-tions: (i) G0 > G00 and (ii) both G0 and G00 are independent of fre-quency [21]. The emulsion was then subjected to shear of theoscillations for finding the viscoelastic response. The fluid shearstresses of different frequencies were applied on the scale of0.70–6.3 rad/s. The relative resulting frequency sweep curves tothe rheological parameters were studied as a function of watercontent in the next section.

Fig. 11. Phase angle as a function of oscillatory frequency for different emulsions(30% crude oil).

5.2. Characterization of emulsions according to their crude oil content

5.2.1. Emulsions at 30% crude oilThe dynamic mechanical spectra and the steady flow curves of

emulsion B (30% crude oil) is presented in Fig. 10b which shows thestorage and loss modulus (G0 and G00) as a function of the frequencyof oscillatory deformation. Over all, it was seen that G0 > G00. Themeasured frequency range and both modulus were dependent onthe phase angle that was below 45� (Fig. 11). Over the entire fre-quency interval, it was seen that the rheological behavior of emul-sions was like an elastic fluid. Furthermore, the complex viscosity(Fig. 11) decreased with increasing frequency and exhibited aplateau region in the frequency range [20,21]. Emulsions gave aweaker gel structure as evidenced by G0 > G00 and both modulusincreased with frequency [22].

It was recognized that the rheological parameters (viscoelasticbehavior, together with shear-thinning behavior and the observa-tion of the yield stress) were indicating the presence of elastic net-work structure. In the case of polymer solutions, the viscoelasticbehavior occurred due to the extensive entanglement phenomena.In the case of emulsions, such rheological behavior occurred due toa network which was created between dispersed phase droplets[20–22].

Fig. 12. Variations of the complex viscosity, the elastic modulus and the viscous emulsions (a–d) with the angular velocity (50% crude Oil).

Fig. 13. Phase angle as a function of oscillatory frequency for different emulsions(50% crude oil).

104 M. Meriem-Benziane et al. / Fuel 95 (2012) 97–107

As shown in Fig. 10a and d, the emulsions A and D with 30%crude oil, a liquid-like behavior was observed at lower frequencies(G00 > G0), and a solid-like behavior was observed at higher frequen-cies (G0 > G00). It was noted that the phase angle (d) was less than45� which predicted an elastic behavior for all studied frequencies(Fig. 11). This was true except for the frequency x 2 ½0:5; 6�; inwhich the emulsion deviated from this general viscous behavior.In this emulsion, the plateau region was not appeared and thecomplex viscosity (Fig. 11) decreased with increasing frequency(Fig. 10). In the regime of emulsion, both G0and G00 followed thegeneral Maxwell model.

The variation of the storage and the loss modulus (G0,G00) withfrequency for the emulsions, is presented by emulsion C (30% crudeoil) as shown in Fig. 10c. It was observed that G0 and G00 values wereincreased at high frequency where phase angle (d) was below 64�(Fig. 11), and slightly dependent on frequency. It was concludedthat the fluid was viscous for the entire frequency interval. In thisemulsion, the plateau region was not appeared and the complexviscosity (Fig. 11) was decreased with increasing frequency. Itwas noted that dilute solutions are characterized by G00 > G0 whereboth parameters showed dependency with the frequency of mea-surement [22].

5.2.2. Emulsion at 50% of crude oilThe variation of the storage and the loss modulus (G0, G00) with

frequency for the emulsions, is presented by emulsions B and C(50% crude oil) as shown in Fig. 12b and c. The storage modulus(G0) was always higher than the loss modulus (G00) within theexperimental frequency range where the phase angle (d) was be-low 45� (Fig. 13) and it was slightly dependent on frequency. Itwas concluded that the fluid was elastic for the entire frequencyinterval and the complex viscosity (Fig. 14) was decreased withincreasing frequency. These two emulsions were stabilized andthus the plateau region was attributed to the formation of a gel-like network. It was observed that the values of G0 were slightly in-creased at high frequency due to strong interactions among thedroplets which contributed to the elastic modulus [5,20,21].

As shown in Fig. 12a and d, the emulsions A and D (30% crudeoil), a liquid-like behavior was observed at lower frequencies(G00 > G0) and a solid-like behavior was observed at higher frequen-cies (G0 > G00). For these samples, it was noted that the phase angle

Fig. 14. Variations of the complex viscosity, the elastic modulus, and the viscous emulsions (a–d) with the angular velocity (70% crude oil).

Fig. 15. Phase angle as a function of oscillatory frequency for different emulsions(70% crude oil).

M. Meriem-Benziane et al. / Fuel 95 (2012) 97–107 105

(d) was generally less than 45� which predicted an elastic behaviorfor all studied frequencies (Fig. 13). This was true except for emul-sion A with frequency x 2 ½0:5; 2� and for emulsion D with fre-quency x 2 ½0:5; 5 where they deviated to adopt the generalviscous behavior. In these emulsions, the plateau region was notappeared and the complex viscosity (Fig. 13) was increased withincreasing frequency. In the regime of emulsions, the storage mod-ulus (G0), and the loss modulus (G00) followed the general Maxwellmodel.

5.2.3. Emulsions at 70% of crude oilAs shown in Fig. 14c and d, the emulsions C and D (70% crude

oil), a liquid-like behavior was observed at lower frequencies

(G00 > G0) and a solid-like behavior was observed at higher frequen-cies (G0 > G00). The crossover frequency (where G0 = G00) was found tobe at the range of the initial frequency. For these samples, it wasnoted that the phase angle (d) was less than 45� (Fig. 15) whichpredicted an elastic behavior for all studied frequencies. However,for a frequency of x 2 ½0:5; 1:50�, the emulsions deviated to adoptthe general viscous behavior [20–22]. In these emulsions, the pla-teau region was not appeared and the complex viscosity (Fig. 14cand d) was increased with increasing frequency. In the regime ofemulsions, G0 and G00 followed the general Maxwell model.

Fig. 14a and b show, for emulsions A and B with 70% crude oil,the evolution of the storage and loss modulus (G0 and G00, respec-tively) of various emulsions as a function of the frequency of oscil-latory deformation. These last exhibited G0 > G00 throughout themeasured frequency range and both modulus are dependent of fre-quency. We remark that the phase angle is globally less than 35�(Fig. 15), thereby affirming that the fluid is viscous for the entirefrequency domain. We note that the complex viscosity g⁄,(Fig. 14), increased with increasing frequency, and the plateau re-gion is normally attributed to the formation of a gel-like network,and both modulus (G0 and G00) increased with frequency.

Identification of the emulsion compositions which showed anelastic response to shear, for all frequencies, gave an indicationto find the essential compositions for producing physically stabledemulsions. This implied that the compositions, that were necessaryto support the physical stability of an emulsion, were establishedby using the following criteria: G0 > G00; where both G0 and G00 wereindependent on frequency.

The presence of a network structure was indicated by the mea-surements (i.e., G0 > G00) where both G0 and G00 were dependent onfrequency, particularly for the case of emulsion B (30%, 50% and70% crude oil). However, when G0 was lower or equal to G00 andboth were functionally dependent on frequency, then this was an

106 M. Meriem-Benziane et al. / Fuel 95 (2012) 97–107

indication of viscous liquid-like systems with little network struc-ture. Such behavior was seen in the curves of different emulsions.In the case of emulsion D (30%, 50% and 70% crude oil), it was dif-ficult for the network formation to occur because the dispersedphase droplets were too far from each other and was not capableto interact.

On the other hand, the data for emulsion A (70% crude oil),emulsion B (30%, 50%, and 70% crude oil) and emulsion C (50%crude oil) indicated that both G0 and G00 were dependent on fre-quency. This was observed with the entire frequency range whereG0 > G00. It was noticed that emulsion B was more stabled physicallythan the other emulsions of different percentages.

6. Impact of the stability on the rheology of emulsions

The rheological behavior, discussed in previous section, wasused in this section as a mean to predict the compositions that pro-vided physically stabled emulsions. To this effect, it was identifiedthat physically stabled emulsion was formed when the systemshowed an elastic gel network [23–25,17,26]. According to Ross-Murphy [27], the dilute solutions are characterized by G00 > G0 andboth parameters showed a noticeable dependency upon frequencyof measurement. In this case, the phase angle (d) was higher than45� and the plateau region was not appeared.

Furthermore, weak gels were characterized by G0 > G00 and bothparameters showed little dependency upon frequency. In this casethe phase angle (d) was less than 45� and the plateau region wasappeared. It was also noticed that a liquid-like behavior was ob-served at lower frequencies (G00 > G0), whereas a solid-like behaviorwas observed at higher frequencies (G0 > G00). The crossover fre-quency (G0 = G00) was appeared in the frequency range and in thiscase, the plateau region was not appeared. In the regime of emul-sions, both G0 and G00 followed the general Maxwell model. Thus thefollowing criteria were used to distinguish physically between sta-ble and unstable emulsions. An emulsion was considered stable ifG0 > G00 and both G0 and G00 were independent of frequency. Underthese conditions, the emulsion was stabilized and thus the plateauregion was normally attributed to the formation of a gel-like net-work. On the other hand, an emulsion was considered physicallyunstable if G00 > G0 and both G0 and G00 were independent upon fre-quency. Under these conditions, the emulsion exhibited the prop-erties of a dilute solution.

Viscoelastic properties were influenced by the geological originof the crude oil and the percentage of its water. The storage mod-ulus (G0), the loss modulus (G00), the phase angle (d) and the com-plex viscosity (g⁄) depend on the frequency of different crudeoils at different percentages of water [25,17,26,27].

7. Conclusions

The rheograms of s ¼ f ð _cÞ clearly established that crude oilexhibited a quasi-Newtonian rheological behavior regardless ofits genesis. The results of different emulsions were calibrated bythe rheological models and it was found that they were obeyingthe power law with a weak behavior index and a yield stress.Depending on the oil field, the rheological behavior was distin-guished as followed:

(1) Herschel–Bulkley model for emulsions of 50% and 70% crudeoil.

(2) Ostwald de Waele model for emulsion of 30% crude oil.

The behavior of the viscoelastic emulsions was analysed by thefrequency sweeps of the emulsions. In the first part of the study,the results showed that G0 and G00 were dependent on the geological

origin of the crude oils. Also, both G0 and G00 were found to be in-creased with the percentage of the water. However, the viscoelasticbehavior was mainly governed by the water emulsion content. At30%, 50% and 70% for emulsion B, and 50% for emulsion C and 30%for emulsion A, a weak gel structural network was observed(G0 > G00) due to the droplet flocculation. For this case, both moduluswere dependent on the frequency, and the phase angle (d) was be-low 45�, and a plateau region exhibited in the frequency range. Itwas concluded that emulsions were incomplete stabled. For the vis-cosity complex (g⁄), two cases were existed. The first case showedthat the viscosity complex decreased with increasing frequency ofemulsions based on 30% crude oil. On the other hand, the secondcase indicated that the viscosity complex increased with increasingfrequency of emulsions based on 70% crude oil.

The second part of the study was dealt with emulsions thatwere classified as physically unstable (G00 > G0), where G0 and G00

were dependent upon frequency and the emulsions exhibited theproperties of a dilute solution. In this case, the plateau regionwas not appeared.

Further, the experimental results indicated that the rheologicalproperties and the physical stability of studied emulsions were sig-nificantly influenced by their water contents and the nature of thecrude oils. In general, water addition was found to produce a pro-gressive increase in the values of viscous and viscoelasticparameters.

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