Characterization of Oil-Water Emulsion and Its Use in Enhanced Oil Recovery

Ajay Mandal,* Abhijit Samanta, Achinta Bera, and Keka Ojha

Department of Petroleum Engineering, Indian School of Mines, Dhanbad-826 004, India

Oil-in-water emulsions are important in the petroleum industry as a displacing fluid for enhanced oil recovery(EOR). To investigate the efficiency of oil-water emulsions in EOR, experiments were performed tocharacterize the emulsions in terms of their physicochemical properties and size distribution of the dispersedoil droplet in water phase. In the present study commercially available gear oil was used to prepare oil-in-water emulsions. Flooding experiments were also carried out to evaluate the effectiveness of the emulsion asdisplacing fluid for enhanced oil recovery. Substantial additional recoveries (more than 20% of original oilin place) over conventional water flooding were obtained in the present investigation.

1. Introduction

In view of increasing demand of energy of the world and, incontrast, depleting oil and gas resources, it is important toincrease the production from existing reservoirs by introducingnew technologies for EOR. These technologies will also helpin addressing challenges to recover oil from subsea, deep-seareservoirs and also from formations where the mobility of thein situ oil being recovered is significantly less than that of drivefluid used to displace the oil. One of the suggested routes is touse oil-water microemulsions as a promising drive fluid toimprove oil recovery of moderately viscous oils from theformations.1,2 This oil-water emulsion flooding technology maybe more effective than some, if not all, other chemical EORtechniques such as pure polymer flooding.3-5

Oil and water form emulsions under favorable conditions inpresence of extraneous materials.6,7 Emulsions are suspensionsof droplets (greater than 0.1 µm) of one immiscible fluiddispersed in another fluid. Their kinetic stability is a consequenceof small droplet size and the presence of an interfacial filmaround oil droplets.8 An emulsifying agent must be present toform stable oil-in-water emulsions.9 Such agents include clayparticles, added chemicals, or crude oil components likeasphaltenes, waxes, resins, and napthenic acids. These stabilizerssuppress the mechanism involved in emulsion breakdown.10

Oil-water emulsions in the presence of surface-active agentor emulsifier significantly reduces the interfacial tension betweentrapped oil and displacing fluid and stabilizes the interfaceagainst coalescence, once it is formed. The emulsions withdispersed phase drops effectively block the more permeablepaths and force the displacing fluid to flow through the unsweptregions, which increase the overall sweep and displacementefficiency, leadingtoanincreaseinoilrecovery.11-13McAuliffe14,15

carried out experiments on injection of crude oil-water emul-sions as a selective plugging agent to improve the oil recoveryin water floods. He reported that the most effective emulsionfor flooding is one in which the droplet diameters are slightlylarger than the pore-throat constriction in porous media. Bragg16

developed a method for recovering hydrocarbons from asubterranean formation by injecting an emulsion, comprisingoil and water, into the formation for enhanced recovery of oil.Austad and Strand1 observed that very low interfacial tensionsmay be reached with microemulsion systems. These micro-emulsions flow more easily through the porous medium, which

enhances the oil extraction performance. Khmbharatana et al.17

discussed the physical mechanisms of stable emulsion flows inBerea sandstone and Ottawa sand pack systems of comparabledroplet and pore sizes. Zeidani et al.18 showed that an oil-in-water emulsion was effective in sealing unconsolidated coresfor long periods of time. A number of researchers19-23 havediscussed the flow mechanics of emulsions in a porous medium.The emulsions also improve the mobility ratio by increasingthe viscosity of the displacing fluid.24-26 The success of theproposed method in field application is closely related to properslug design and injection scheme depending on properties anddistributions of fluid and rock.

The objective of the present study is to characterize the oil-in-water microemulsions and their application in enhancedrecovery of oil after conventional water flooding. The effect ofcomposition of microemulsions on additional recovery was alsostudied.

2. Experimental Section

2.1. Preparation of Emulsion. In the present study gear oil(EPX 90) available in the market with sp. gravity 0.905 andkinematic viscosity 197 centistokes (cSt) at 40 °C and 17.3 cStat 100 °C was used for preparation of emulsion in distilled water.Quality of lubricating oil is often improved by adding detergent,dispersant, etc.,27,28 which enable formation of a milky emulsionwhen properly mixed with water. Oil-in-water emulsion wasprepared at 30 °C with a standard three-blade propeller. Foroil-in-water emulsion distilled water was used as the continuousphase and oil was used as the dispersed phase. The emulsionwas left to stand in a separation flask for 6 h and the bottompart was separated out. The separation flask can easily lead tothe separation of two different density fluids based on theirdensity. For preparation of emulsion, the oil-water mixtureswere stirred at different rpm and time as the characteristics ofemulsions are very much dependent on the stirring speed andtime.29

2.2. Characterization of Emulsion. Emulsions are charac-terized by analysis of their stability, drop size, rheologicalproperties, temperature effects, etc. It has been found that thestability and hence the characteristics of emulsion are very muchdependent on the stirring speed and time during formation.Surface tension measurements were carried out using an autotentiometer (model 6801ES with platinum ring) under atmo-spheric pressure by the ring method. The viscosity of theemulsion was measured in a viscometer (Brookfield DV II+),

* To whom correspondence should be addressed. E-mail:mandal_ajay@hotmail.com.

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10.1021/ie101589x 2010 American Chemical SocietyPublished on Web 11/05/2010

and a microscopic picture of the emulsion was taken byOlympus BX-60 microscope and analyzed by image processingsoftware (Olympus Microsuite Basic).

2.3. Flooding Experiments. The experimental setup (Figure1) for sand pack flooding tests consisted of four components:sand pack holder, displacement pump (Teledyne Isco), cylindersfor holding crude oil, and chemical slug and fraction collectors.The crude oil used in the flooding experiments was collectedfrom Ahmedabad oil-field (India). The oil had a total acidnumber of 0.038 mg KOH/g, gravity of 38.86° API, andviscosity of 119 mPa.s at 30 °C.

The sand pack holder was first tightly packed with sand usingbrine solution. Before use, the sand particles (40-60 meshes)were cleaned thoroughly and dried. For each test, fresh sandwas packed to ensure the same wettability for all tests. Then itwas completely saturated with water by continuous flooding.The sand pack was then completely saturated with brine solution,and absolute permeability was measured by injecting brinesolution at constant pressure of 25 psig. It was next floodedwith crude oil with an injection pressure of 500 psig toirreducible water saturation and initial oil saturation (Soi) wasmeasured by material balance. The sand pack was allowed torest for one day at this stage. It was then water-flooded with300 psig injection pressure and continued to flood until watercut reached above 95%. A substantial amount of oil wasrecovered during this water/brine flooding. The remaining oilwas recovered by different emulsions flooding at 300 psigfollowed by chase water flooding. The recovered fluids werecollected by switching the fraction collector at regular intervals.The effective permeability to oil (ko) and effective permeabilityto water (kw) were measured at irreducible water saturation (Swi)and residual oil saturation (Sor), respectively, using Darcy’s lawequation.

3. Results and Discussion

3.1. Emulsion Characteristics. Physicochemical Proper-ties of Emulsion. Very low interfacial tension between oil andwater is the primary requirement for emulsion formation.Lowering of interfacial tension recovers additional oil byreducing the capillary forces that leave the oil behind anyimmiscible displacement. This trapping is best expressed as acompetition between viscous forces which mobilize the oil, andcapillary forces which trap the oil. Figure 2 shows that thesurface tension decreases with increase in oil concentration inthe emulsion due to presence of higher surface-active agent oremulsifier in the emulsion. This reduction of surface tensionplays an important role in additional oil recovery.

Viscosity of the displacing fluid is also an important parameterin enhanced oil recovery. Increase in viscosity of the displacingfluid improves the mobility ratio, which increases the overall

displacement of oil by increasing the sweep efficiency. Figure3 compares the viscosity vs shear rate curves for emulsions withdifferent composition showing pseudoplastic behavior. Theviscosity of emulsion increases with increase in concentrationof oil in the emulsion.

Too correlate the emulsion data with literature values, therelative viscosity, the ratios of the emulsion viscosity (µr) tothat of the continuous phase, at shear rate of 46 s-1 werecalculated and plotted against oil volume fraction (Figure 4).The data have been correlated with Mooney’s equation:30

where the parameter am is between 1.35 and 1.91. Figure 4shows that the model (am ) 1.5) deviates uniformly from theentire range of the experimental data. This may be due to thefact that Mooney’s model was developed for Newtonian fluidswhereas the emulsions under study exhibit non-Newtonianbehavior. This discrepancy is corrected by incorporating a factorin Mooney’s equation as follows:

Figure 1. Schematic diagram of crude oil flooding system.

Figure 2. Variation of surface tension with the quality of oil-water emulsionat 30 °C.

Figure 3. Variation of viscosity with the quality of oil-water emulsion at30 °C: 9, 5% oil; b, 10% oil; 2, 20% oil; 1, 30% oil.

µr ) exp[ 2.5�1 - am�] (1)

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where A is constant and the above model (eq 2) fits theexperimental data well for the value of A equals 0.85 as shownin Figure 4. The experimental results are found to satisfy themodified correlation adequately (with goodness of fit equal to0.995).

Microscopic picture of a typical oil-in-water emulsion isshown in Figure 5. The figure shows the size distribution ofdispersed oil droplets. Various analytical distribution functionswere tested by statistical software (SAS) for their adequacy inrepresenting the observed droplet size distributions. It was foundthat the logarithmic normal distribution provided the mostreasonable fitting. The probability function for logarithmicnormal distribution f(db) of oil droplet diameter (db) is givenby the expression

where, µdbis the sample mean and σdb

2 is the sample variance asdefined by eqs 4 and 5, respectively.

and

where n stands for number of drops of oil in the emulsion and147 numbers of data have been considered for the distribution.Figure 6 shows the frequency distribution curve of oil dropletsize as shown in Figure 5. The size distributions of the oil dropletsize were found to deviate from a normal distribution withskewness toward the larger droplet sizes. Figure 7 shows atypical Lognormal probability plot with mean and standarddeviation 1.816 and 1.414, respectively. The size distributionof the oil droplets was found uniform throughout the entireemulsion.

3.2. Flooding Tests. To evaluate the performance of emul-sion flooding, four sets of flooding experiments were conductedwith sandpack in a horizontal orientation. These experimentsinvestigated the displacement of crude oil by water floodingand subsequent emulsion flooding. The emulsion floodingprocess involves a complex interplay of several mechanisms.31

The overall oil recovery by an emulsion flooding is dependenton so many process parameters, viz. composition of injectedemulsion, drop size distribution of the injected emulsion,emulsion size to be injected, absolute permeability of reservoirrock, viscosity of oil being displaced, etc.

Prior to emulsion injection, steady state absolute permeabilitywas measured for each core plug. The details of these threesystems (porosity, Φ, and permeability, k) are given in Table1. Since in the present work, all the experiments were carriedout in sandpack with higher porosity (∼ 37%) and permeability(441-478 mD), the water flood recovers almost 50% of theoriginal oil in place. During water flooding as the water-cutreaches above 95%, it was subsequently flooded with differentemulsion slugs followed by chase water. The recovery of oiland water-cut with pore volume injected for the four differentsystems are presented in Figures 8 through 11. The curve ofwaterflood oil recovery shows an early breakthrough and

Figure 4. Comparison of relative viscosity of oil-water emulsion withMooney’s Model (1951) at 46 s-1 shear rate: 9, experimental data; O,Mooney’s correlation; ∆, modified Mooney’s correlation.

µr ) A + exp[ 2.5�1 - am�] for 0.05 e � e 0.3 (2)

f(db) )1

dbσdb√2π

exp{-12(log db - µdb

σdb)2} (3)

µdb) 1

n ∑i)1

n

dbi(4)

σdb

2 ) 1n - 1 ∑

i)1

n

(dbi- µdb

)2 (5)

Figure 5. Microscopic picture (magnification 430×) of oil-in-water emulsion(5% oil + 95% water).

Figure 6. Size-range histogram and density distribution of injected emulsion:emulsion: 5% oil + 95% water; sample data: 147.

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channel flow which causes much lower oil recovery. After waterstarts to produce the water-cut sharply increases above 90% ineach case. The crude oil used for flooding experiments wasviscous enough (119 mPa.s) at the test temperature. So the longtransient production during waterflood may be due to theunfavorable mobility ratio between the injected water and crudeoil. During injection of emulsion slug, the water-cut declinesgradually, and then again reaches to 100% at the end of flooding.To compare the efficiency of the four systems, same volumes(0.5 PV) of slugs having different composition were injected.A comparative picture of oil recovery is shown in Figure 12.During the waterflooding of the viscous oil only about 15% ofthe injected water enters the lower-permeability zone.32 After

the high-permeability zone is flooded out, significant oil stillremains in the low-permeability zone. With the emulsion floodmore flow is routed to the low-permeability zones of thesandpack. The emulsified oil droplets are also captured in thehigh-permeable zone and reduce its permeability, which in termincreases the sweep efficiency. On the other hand, sinceemulsions are more viscous than the constituent oil, it signifi-cantly reduces the mobility ratio, which plays an important rolein the displacement of heavy oil by higher viscosity W/Oemulsion phase.33,34 Emulsions with higher percentage of oilresult in better additional recovery due to significant decreasein interfacial tension between oil and water and improvementof mobility ratio. The detailed results of the different systemsare given in Table 1.

Conclusion

Based upon this investigation, the following conclusions canbe drawn: (1) Characterization of emulsion shows that it follows

Table 1. Flooding Results of the Different Systems

permeability, k (Darcy) saturation, % PV

sand packsample porosity kw (Sw ) 1) ko (Swi) kw (Sor) design of chemical slug for flooding

recovery of oil afterwater flooding at 95%water cut (% OOIP)

additionalrecovery

(% OOIP) Swi Soi Sor

sample 1 37.72% 0.441193 0.00198 0.02902 0.5 PV 5% emulsion + chase water 49.5714 20.714 14.64 85.36 25.36sample 2 36.80% 0.47797 0.002786 0.027809 0.5 PV 10% emulsion + chase water 50.5294 20.941 15.04 84.96 24.25sample 3 36.80% 0.47797 0.002778 0.02987 0.5 PV 20% emulsion + chase water 51.542 21.678 13.75 86.25 22.87sample 4 37.72% 0.441193 0.002567 0.02880 0.5 PV 30% emulsion + chase water 50.7143 23.144 14.63 85.37 22.92

Figure 7. Lognormal P-P plot of drop size.

Figure 8. Production performance of emulsion (5% oil) flooding: 9, % oilrecovery; b, % water cut.

Figure 9. Production performance of emulsion (10% oil) flooding: 9, %oil recovery; b, % water cut.

Figure 10. Production performance of emulsion (20% oil) flooding: 9, %oil recovery; b, % water cut.

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pseudoplastic behavior and drop size of dispersed oil phasefollows log-normal distribution. (2) Dilute oil-water emulsionsare shown to aid in the displacement of viscous oils bydecreasing the mobility of the aqueous displacing phase andreducing interfacial tension between oil and water. (3) Afterwater starts to produce the water-cut sharply increases above90% in each case due higher permeability of the sandpack andunfavorable mobility ratio between the injected water and thecrude oil. (4) The flooding behavior with water and subsequentemulsions were found to be identical; only a little higherrecovery was observed with higher emulsion concentration. (5)Recovery efficiency is more than 20% of original oil in placeover the conventional water flooding, which may be due toreduced interfacial tension, improved mobility ratio, andincreased sweep efficiency caused by the injected emulsions.

Acknowledgment

Financial support through projects from Council of Scientificand Industrial Research (CSIR, 424/07) and University GrantCommission [37-203/2009 (SR)] are gratefully acknowledged.

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Figure 11. Production performance of emulsion (30% oil) flooding: 9, %oil recovery; b, % water cut.

Figure 12. Comparative production performance of emulsion flooding: O,emulsion with 5% oil; ), emulsion with 10% oil; 0, emulsion with 20%oil; ∆, emulsion with 30% oil.

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ReceiVed for reView July 26, 2010ReVised manuscript receiVed October 20, 2010

Accepted October 21, 2010

IE101589X

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