Schramm l l (ed ) surfactants fundamentals and applications in the petrolium industry CUP (2000)

The primary focus is on applications of the principles of colloid andinterface science to surfactant applications in the petroleum industry, andincludes attention to practical processes

This book provides an introduction to the nature, occurrence, physicalproperties, propagation, and uses of surfactants in the petroleum indus-try It is aimed principally at scientists and engineers who may encounter

or use surfactants, whether in process design, petroleum production, orresearch and development

The primary focus is on applications of the principles of colloid andinterface science to surfactant applications in the petroleum industry, andincludes attention to practical processes and problems Applications ofsurfactants in the petroleum industry are of great practical importanceand are also quite diverse, since surfactants may be applied to advantagethroughout the petroleum production process: in reservoirs, in oil and gaswells, in surface processing operations, and in environmental, health andsafety applications In each case appropriate knowledge and practicesdetermine the economic and technical successes of the industrial processconcerned The book includes a comprehensive glossary, indexed andfully cross-referenced

In addition to scientists and engineers in the petroleum industry, thisbook will be of interest to senior undergraduates and graduate students inscience and engineering, and to graduate students of surfactant chemistry

LAURIERL SCHRAMMis President and CEO at the Petroleum RecoveryInstitute, and adjunct professor of chemistry at the University of Calgary

Dr Schramm received his B.Sc (Hons.) in chemistry from CarletonUniversity in 1976 and Ph.D in physical and colloid chemistry in 1980from Dalhousie University, where he studied as a Killam and NRCScholar From 1980 to 1988 he held research positions with SyncrudeCanada Ltd in its Edmonton Research Centre Since 1988 he has held aseries of positions, of progressively increasing responsibility, with thePetroleum Recovery Institute

His research interests have included many aspects of colloid andinterface science applied to the petroleum industry, including researchinto mechanisms of processes for the improved recovery of light, heavy,and bituminous crude oils, such as in situ foam, polymer or surfactantflooding, and surface hot water flotation from oil sands This research hasinvolved the formation and stability of dispersions (foams, emulsions and

surfactants in solution.

Dr Schramm has won several national awards for his research,including the Canadian Society for Chemical Engineering ± BayerAward in Industrial Practice and the Natural Sciences and EngineeringResearch Council of Canada ± Conference Board of Canada Award forBest Practices in University±Industry R & D Partnership He is a Fellow

of the Chemical Institute of Canada, a past Director of the Association ofthe Chemical Profession of Alberta, and a member of the AmericanChemical Society He has 100 scientific publications and patents in theopen literature and over 220 proprietary research reports for industry.This is his fifth book, following Emulsions: Fundamentals and Appli-cations in the Petroleum Industry, The Language of Colloid and InterfaceScience, Foams: Fundamentals and Applications in the PetroleumIndustry, and Suspensions: Fundamentals and Applications in the Petro-leum Industry

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# Cambridge University Press 2000

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First published 2000

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Library of Congress cataloguing in publication data

Surfactants: fundamentals and applications in the petroleum industry / Laurier L Schramm, editor.

p cm.

Includes index.

ISBN 0 521 64067 9

1 Surface active agents ± Industrial applications 2 Petroleum industry and trade.

I Schramm, Laurier Lincoln.

TN871.S76784 2000

665.5Ðdc21 99-15820 CIP

ISBN 0 521 64067 9 hardback

Preface vii

S URFACTANT F UNDAMENTALS

Laurier L Schramm and D Gerrard Marangoni

R.J Mikula and V.A Munoz

Laurier L Schramm and Susan M Kutay

S URFACTANTS URFACTANTS IN IN P OROUS M EDIA

Laura L Wesson and Jeffrey H Harwell

Eugene A Spinler and Bernard A Baldwin

Tor Austad and Jess Milter

Fred Wassmuth, Laurier L Schramm, Karin Mannhardt, and

Laurie Hodgins

O ILWELL , N EAR -W ELL , AND S URFACE O PERATIONS

Todd R Thomas and Ted M Wilkes

Hisham A Nasr-El-Din

Laurier L Schramm, Elaine N Stasiuk, and Mike MacKinnon

E NVIRONMENTAL , H EALTH , AND S AFETY A PPLICATIONS

Varadarajan Dwarakanath and Gary A Pope

12 Use of Surfactants for Environmental Applications 461 Merv Fingas

Larry N Britton

G LOSSARY LOSSARY AND AND I NDEXES

Laurier L Schramm

This book provides an introduction to the nature, occurrence, physicalproperties, propagation, and uses of surfactants in the petroleum indus-try The primary focus is on applications of the principles of colloid andinterface science to surfactant applications in the petroleum industry, andincludes attention to practical processes and problems Books available up

to now are either principally theoretical (such as the colloid chemistrytexts), much more general (like Rosen's Surfactants and InterfacialPhenomena, Myers' Surfactant Science and Technology, or Mittal'sSolution Chemistry of Surfactants), or else much narrower in scope (likeSmith's Surfactant Based Mobility Control) The applications of surfac-tants in the petroleum industry area are quite diverse and have a greatpractical importance The area contains a number of problems of morefundamental interest as well Surfactants may be applied to advantage inmany parts of the petroleum production process: in reservoirs, in oilwells,

in surface processing operations, and in environmental, health, and safetyapplications In each case appropriate knowledge and practices determineboth the economic and technical successes of the industrial processconcerned

In this volume, a wide range of authors' expertise and experiences arebrought together to yield the first surfactant book that focuses on theapplications of surfactants in the petroleum industry Taking advantage of

a broad range of authors' expertise allows for a variety of surfactanttechnology application areas to be highlighted in an authoritative manner.The topics chosen serve to illustrate some of the different methodologiesthat have been successfully applied Each of the chapters in this book hasbeen critically peer-reviewed and revised to meet a high scientific andeditorial standard

The target audience includes scientists and engineers who mayencounter or be able to use surfactants, whether in process design,petroleum production, or in the research and development fields It doesnot assume a knowledge of colloid chemistry, the initial emphasis beingplaced on a review of the basic concepts important to understandingsurfactants As such, it is hoped that the book will be of interest to seniorundergraduate and graduate students in science and engineering as wellsince topics such as this are not normally part of university curricula.The book provides an introduction to the field in a very applicationsoriented manner, as the focus of the book is practical rather thantheoretical The first group of chapters (1 to 3) sets out fundamental

vii

surfactant principles, including chemistry and uses Subsequent groups ofchapters address examples of industrial practice with Chapters 4±7 aimed

at the use of surfactants in reservoir oil recovery processes, Chapters 8±10covering some oilwell, near-well, and surface uses of surfactants, Chap-ters 11±13 addressing several environmental, health, and safety applica-tions, and the Glossary containing a comprehensive and fully cross-referenced dictionary of terms in the field

A recurring theme in the chapters is the use of the fundamentalconcepts in combination with actual commercial process experiences toillustrate how to approach planned and unplanned surfactant occurrences

in petroleum processes It also completes a natural sequence, serving as acompanion volume to my earlier books: Emulsions: Fundamentals andApplications in the Petroleum Industry; Foams: Fundamentals andApplications in the Petroleum Industry, and Suspensions: Fundamentalsand Applications in the Petroleum Industry

Acknowledgments

I thank all the authors who contributed considerable time and effort totheir respective chapters This book was made possible through thesupport of my family, Ann Marie, Katherine and Victoria who gave methe time needed for the organization, research, and writing I am also verygrateful to Conrad Ayasse for his consistent encouragement and support.Throughout the preparation of this book many valuable suggestions weremade by colleagues, the external reviewers of individual chapters, and bythe editorial staff of Cambridge University Press, particularly SimonCapelin and Margaret Patterson

Laurier L Schramm

Calgary, AB, Canada

Surfactants and Their Solutions:

Basic Principles

Canada T2L 2A6

Calgary, AB, Canada T2N 1N4

3 St Francis Xavier University, Dept of Chemistry, PO Box 5000,

Antigonish, NS, Canada B2G 2W5

This chapter provides an introduction to the occurrence, ties and importance of surfactants as they relate to the petroleum industry With an emphasis on the definition of important terms, the importance of surfactants, their micellization and adsorption behaviours, and their interfacial properties are demonstrated It

proper-is shown how surfactants may be applied to alter interfacial properties, promote oil displacement, and stabilize or destabilize dispersions such as foams, emulsions, and suspensions Under- standing and controlling the properties of surfactant-containing solutions and dispersions has considerable practical importance since fluids that must be made to behave in a certain fashion to assist one stage of an oil production process, may require considerable modification in order to assist in another stage.

Introduction

Surfactants are widely used and find a very large number of applicationsbecause of their remarkable ability to influence the properties of surfacesand interfaces, as will be discussed below Some important applications ofsurfactants in the petroleum industry are shown in Table 1 Surfactantsmay be applied or encountered at all stages in the petroleum recovery andprocessing industry, from oilwell drilling, reservoir injection, oilwellproduction, and surface plant processes, to pipeline and seagoing trans-portation of petroleum emulsions This chapter is intended to provide anintroduction to the basic principles involved in the occurrence and uses ofsurfactants in the petroleum industry Subsequent chapters in this bookwill go into specific areas in greater detail

3

All the petroleum industry's surfactant applications or problems have

in common the same basic principles of colloid and interface science Thewidespread importance of surfactants in general, and scientific interest intheir nature and properties, have precipitated a wealth of publishedliterature on the subject Good starting points for further basic informa-tion are classic books like Rosen's Surfactants and Interfacial Phenomena[1] and Myers' Surfactant Science and Technology [2], and the many otherbooks on surfactants [3±19] Most good colloid chemistry texts containintroductory chapters on surfactants Good starting points are references[20±23], while for much more detailed treatment of advances in specificsurfactant-related areas the reader is referred to some of the chaptersavailable in specialist books [24±29] With regard to the occurrence ofrelated colloidal systems in the petroleum industry, three recent books

Table 1 Some Examples of Surfactant Applications in the Petroleum Industry Gas/Liquid Systems

Producing oilwell and well-head foams Oil flotation process froth

Distillation and fractionation tower foams Fuel oil and jet fuel tank (truck) foams Foam drilling fluid

Foam fracturing fluid Foam acidizing fluid Blocking and diverting foams Gas-mobility control foams Liquid/Liquid Systems Emulsion drilling fluids Enhanced oil recovery in situ emulsions Oil sand flotation process slurry Oil sand flotation process froths Well-head emulsions

Heavy oil pipeline emulsions Fuel oil emulsions

Asphalt emulsion Oil spill emulsions Tanker bilge emulsions Liquid/Solid Systems Reservoir wettability modifiers Reservoir fines stabilizers Tank/vessel sludge dispersants Drilling mud dispersants

describe the principles and occurrences of emulsions, foams, and sions in the petroleum industry [30±32].

Some compounds, like short-chain fatty acids, are amphiphilic or pathic, i.e., they have one part that has an affinity for nonpolar media andone part that has an affinity for polar media These molecules formoriented monolayers at interfaces and show surface activity (i.e., theylower the surface or interfacial tension of the medium in which they aredissolved) In some usage surfactants are defined as molecules capable ofassociating to form micelles These compounds are termed surfactants,amphiphiles, surface-active agents, tensides, or, in the very old literature,paraffin-chain salts The term surfactant is now probably the mostcommonly used and will be employed in this book This word has asomewhat unusual origin, it was first created and registered as a trade-mark by the General Aniline and Film Corp for their surface-activeproducts.5The company later (ca 1950) released the term to the publicdomain for others to use [33] Soaps (fatty acid salts containing at leasteight carbon atoms) are surfactants Detergents are surfactants, orsurfactant mixtures, whose solutions have cleaning properties That is,detergents alter interfacial properties so as to promote removal of a phasefrom solid surfaces

amphi-The unusual properties of aqueous surfactant solutions can beascribed to the presence of a hydrophilic head group and a hydrophobicchain (or tail) in the molecule The polar or ionic head group usuallyinteracts strongly with an aqueous environment, in which case it issolvated via dipole±dipole or ion±dipole interactions In fact, it is thenature of the polar head group which is used to divide surfactants intodifferent categories, as illustrated in Table 2 In-depth discussions ofsurfactant structure and chemistry can be found in references [1, 2, 8, 34,35]

The Hydrophobic Effect and Micelle Formation

In aqueous solution dilute concentrations of surfactant act much asnormal electrolytes, but at higher concentrations very different behaviourresults This behaviour is explained in terms of the formation of organizedaggregates of large numbers of molecules called micelles, in which the

4 A glossary of frequently encountered terms in the science and engineering of surfactants is given in the final chapter of this book.

5 For an example of one of GAF Corp's early ads promoting their trademarked surfactants, see Business Week, March 11, 1950, pp 42±43.

Anionic Na stearate CH 3 (CH 2 ) 16 COO 7 Na +

Na dodecyl sulfate CH 3 (CH 2 ) 11 SO 47Na +

Na dodecyl benzene sulfonate CH 3 (CH 2 ) 11 C 6 H 4 SO 37Na +

Cationic Laurylamine hydrochloride CH 3 (CH 2 ) 11 NH 3+Cl 7

Trimethyl dodecylammonium chloride C 12 H 25 N + (CH 3 ) 3 Cl 7

Cetyl trimethylammonium bromide CH 3 (CH 2 ) 15 N + (CH 3 ) 3 Br 7

Nonionic Polyoxyethylene alcohol C n H 2n+1 (OCH 2 CH 2 ) m OH

Alkylphenol ethoxylate C 9 H 19 ÐC 6 H 4 Ð(OCH 2 CH 2 ) n OH

Polysorbate 80 HO(C 2 H 4 O) w (OC 2 H 4 ) x OH

polymethylsiloxane (CH3)3SiO((CH3)2SiO)x(CH3SiO)| ySi(CH3)3

EO = ethyleneoxy CH 2 CH 2 CH 2 O(EO) m (PO) n H

PO = propyleneoxy Zwitterionic Dodecyl betaine C 12 H 25 N + (CH 3 ) 2 CH 2 COO 7

Lauramidopropyl betaine C 11 H 23 CONH(CH 2 ) 3 N + (CH 3 ) 2 CH 2 COO 7

Cocoamido-2-hydroxy-propyl sulfobetaine C n H 2n+1 CONH(CH 2 ) 3 N + (CH 3 ) 2 CH 2 CH(OH)CH 2 SO 37

lipophilic parts of the surfactants associate in the interior of the aggregateleaving hydrophilic parts to face the aqueous medium An illustrationpresented by Hiemenz and Rajagopalan [22] is given in Figure 1 Theformation of micelles in aqueous solution is generally viewed as acompromise between the tendency for alkyl chains to avoid energeticallyunfavourable contacts with water, and the desire for the polar parts tomaintain contact with the aqueous environment.

A thermodynamic description of the process of micelle formation willinclude a description of both electrostatic and hydrophobic contributions

to the overall Gibbs energy of the system Hydrocarbons (e.g., dodecane)and water are not miscible; the limited solubility of hydrophobic species

in water can be attributed to the hydrophobic effect The hydrophobicGibbs energy (or the transfer Gibbs energy) can be defined as thedifference between the standard chemical potential of the hydrocarbonsolute in water and a hydrocarbon solvent at infinite dilution [36±40]

the Gibbs energy for the process of transferring the hydrocarbon solutefrom the hydrocarbon solvent to water In a homologous series ofhydrocarbons (e.g., the n-alcohols or the n-alkanes), the value of DG8t

generally increases in a regular fashion

where a and b are constants for a particular hydrocarbon series and ncisthe number of carbon atoms in the chain The transfer Gibbs energy, DG8t,can be divided into entropic and enthalpic contributions

where DH8tand DS8tare the enthalpy and entropy of transfer, respectively

A significant characteristic of the hydrophobic effect is that the entropyterm is dominant, i.e., the transfer of the hydrocarbon solute from thehydrocarbon solvent to water is accompanied by an increase in the Gibbstransfer energy (DG 4 0) [41] The decrease in entropy is thought to bethe result of the breakdown of the normal hydrogen-bonded structure ofwater accompanied by the formation of differently structured water, oftentermed icebergs, around the hydrocarbon chain The presence of thehydrophobic species promotes an ordering of water molecules in thevicinity of the hydrocarbon chain To minimize the large entropy effect,the ``icebergs'' tend to cluster [38], in order to reduce the number of watermolecules involved; the ``clustering'' is enthalpically favoured (i.e.,

DH 5 0), but entropically unfavourable The overall process has thetendency to bring the hydrocarbon molecules together, which is known

as the hydrophobic interaction Molecular interactions, arising from thetendency for the water molecules to regain their normal tetrahedralstructure, and the attractive dispersion forces between hydrocarbonchains, act cooperatively to remove the hydrocarbon chain from thewater ``icebergs'', leading to an association of hydrophobic chains

Due to the presence of the hydrophobic effect, surfactant moleculesadsorb at interfaces, even at low surfactant concentrations As there will

be a balance between adsorption and desorption (due to thermalmotions), the interfacial condition requires some time to establish Thesurface activity of surfactants should therefore be considered a dynamicphenomenon This can be determined by measuring surface or interfacialtensions versus time for a freshly formed surface, as will be discussedfurther below

At a specific, higher, surfactant concentration, known as the criticalmicelle concentration (cmc), molecular aggregates termed micelles areformed The cmc is a property of the surfactant and several other factors,since micellization is opposed by thermal and electrostatic forces A lowcmc is favoured by increasing the molecular mass of the lipophilic part ofthe molecule, lowering the temperature (usually), and adding electrolyte

Surfactant molar masses range from a few hundreds up to severalthousands.

The most commonly held view of a surfactant micelle is not muchdifferent than that published by Hartley in 1936 [41, 42] (see Figure 1) Atsurfactant concentrations slightly above the cmc value, surfactants tend toassociate into spherical micelles, of about 50±100 monomers, with aradius similar to that of the length of an extended hydrocarbon chain.The micellar interior, being composed essentially of hydrocarbon chains,has properties closely related to the liquid hydrocarbon

Critical Micelle Concentration

It is well known that the physico-chemical properties of surfactants varymarkedly above and below a specific surfactant concentration, the cmcvalue [2±9, 13, 14, 17, 35±47] Below the cmc value, the physico-chemicalproperties of ionic surfactants like sodium dodecylsulfate, SDS, (e.g.,conductivities, electromotive force measurements) resemble those of astrong electrolyte Above the cmc value, these properties change drama-tically, indicating a highly cooperative association process is taking place

In fact, a large number of experimental observations can be summed up in

a single statement: almost all physico-chemical properties versus tration plots for a given surfactant±solvent system will show an abruptchange in slope in a narrow concentration range (the cmc value) This isillustrated by Preston's [48] classic graph, shown in Figure 2

concen-In terms of micellar models, the cmc value has a precise definition inthe pseudo-phase separation model, in which the micelles are treated as aseparate phase The cmc value is defined, in terms of the pseudo-phasemodel, as the concentration of maximum solubility of the monomer in thatparticular solvent The pseudo-phase model has a number of short-comings; however, the concept of the cmc value, as it is described interms of this model, is very useful when discussing the association ofsurfactants into micelles It is for this reason that the cmc value is,perhaps, the most frequently measured and discussed micellar parameter[39]

Cmc values are important in virtually all of the petroleum industrysurfactant applications For example, a number of improved or enhancedoil recovery processes involve the use of surfactants including micellar,alkali/surfactant/polymer (A/S/P) and gas (hydrocarbon, N2, CO2 orsteam) flooding In these processes, surfactant must usually be present at

a concentration higher than the cmc because the greatest effect of thesurfactant, whether in interfacial tension lowering [30] or in promotingfoam stability [31], is achieved when a significant concentration ofmicelles is present The cmc is also of interest because at concentrations

above this value the adsorption of surfactant onto reservoir rock surfacesincreases very little That is, the cmc represents the solution concentra-tion of surfactant from which nearly maximum adsorption occurs.

Cmc Measurements The general way of obtaining the cmcvalue of a surfactant micelle is to plot some physico-chemical property of

Figure 2 Illustration of the dramatic changes in physical properties that occur beyond the critical micelle concentration (From Preston [48] Copyright 1948 American Chemical Society, Washington.)

interest versus the surfactant concentration and observe the break in theplot Table 3 lists the most common cmc methods Many of these methodshave been reviewed by Shinoda [11] and Mukerjee and Mysels [49] Itshould be noted that different experimental techniques may give slightlydifferent values for the cmc of a surfactant However, Mukerjee andMysels [49], in their vast compilation of cmc values, have noted that themajority of values for a single surfactant (e.g., sodium dodecyl sulfate, orSDS, in the absence of additives) are in good agreement and the outlyingvalues are easily accounted for.

For petroleum industry processes, one tends to have a special interest

in the cmc's of practical surfactants that may be anionic, cationic, nonionic

or amphoteric The media are typically high salinity, high hardnesselectrolyte solutions, and in addition, the cmc values of interest span thefull range from ambient laboratory conditions to oil and gas reservoirconditions of temperature and pressure Irrespective of aiming forprocess development and optimization under realistic (reservoir) condi-tions of temperature and pressure, it remains common to determine cmc'sexperimentally at ambient laboratory conditions and assume that thesame hold even at elevated temperatures and pressures This can be anextremely dangerous assumption

The nature and limits of applicability of specific methods for mining critical micelle concentrations vary widely Most methods havebeen developed for a relatively small set of pure surfactants involving verydilute electrolyte solutions and only ambient temperature and pressure.The determination of cmc at elevated temperature and pressure isexperimentally much more difficult than for ambient conditions andcomparatively little work has been done in this area Most high tempera-ture cmc studies have been by conductivity measurements and havetherefore been limited to ionic surfactants For example, cmc's at up to

deter-166 8C have been reported by Evans and Wightman [50] Some work hasbeen reported using calorimetry, up to 200 8C by Noll [51], and using19F

Table 3 Some Common Cmc Methods UV/Vis, IR spectroscopy

Fluorescence spectroscopy Nuclear magnetic resonance spectroscopy Electrode potential/conductivity

Voltametry Scattering techniques Calorimetry

Surface tension Foaming

NMR, up to 180 8C by Shinoda et al [52] Some work has been reportedinvolving cmc determination by calorimetry (measuring heats of dilution

or specific heats) Archer et al [53] used flow calorimetry to determinethe cmc's of several sulfonate surfactants at up to 178 8C Noll [51]determined cmc's for dodecyltrimethylammonium bromide and commer-cial surfactants in the temperature range 25±200 8C using flow calorime-try Surface tension is the classical method for determining cmc's butmany surface tension methods are not suitable for use with aqueoussolutions at elevated temperatures Exceptions include the pendant,sessile, and captive drop methods which can be conducted with high-pressure cells [54, 55]

For any of the techniques applied it appears (Archer et al [53]) thatthe uncertainties in the experimental cmc determinations increase withincreasing temperature because at the same time the surfactant aggrega-tion number decreases and the aggregation distribution increases That is,the concentration range over which micellization occurs broadens withincreasing temperature Almost all of the elevated temperature cmcstudies have involved carefully purified surfactants (not commercialsurfactants or their formulations) in pure water or very dilute electrolytesolutions Conducting cmc determinations at elevated pressure, as well astemperature, is even more difficult and only a few studies have beenreported, mostly employing conductivity methods (La Mesa et al [56];Sugihara and Mukerjee [57]; Brun et al [58]; Kaneshina et al [59];Hamann [60]) which, again, are unsuitable for nonionic or zwitterionicsurfactants and for use where the background electrolyte concentrationsare significant

In the case where one needs to be able to determine cmc's for nonionic

or zwitterionic surfactants, in electrolyte solutions that may be veryconcentrated, and at temperatures and pressures up to those that may

be encountered in improved oil recovery operations in petroleumreservoirs, most of the established methods are not practical Onesuccessful approach to this problem has been to use elevated tempera-ture and pressure surface tension measurements involving the captivedrop technique [8] although this method is quite time-consuming.Another approach is to use dynamic foam stability measurements.Foaming effectiveness and the ease of foam formation are related tosurface tension lowering and to micelle formation, the latter of whichpromotes foam stability through surface elasticity and other mechanisms[61] Accordingly, static or dynamic foam height methods generally showthat foam height increases with surfactant concentration and thenbecomes relatively constant at concentrations greater than the cmc(Rosen and Solash [62]; Goette [63]) Using a modified Ross-Miles staticfoam height apparatus, Kashiwagi [64] determined the cmc of SDS

at 40 8C to be 7.08 mM which compared well with values attained

by conductivity (7.2 mM) and surface tension (7.2 mM) Rosen andSolash [62] also found that foam production was related to cmc usingthe Ross-Miles method at 60 8C when they assessed SDS, potassiumtetradecyl sulfonate, potassium hexadecyl sulfonate, and sodium hexa-decyl sulfate.

Morrison et al [65] describe a dynamic foam height method for theestimation of cmc's that is suitable for use at high temperatures andpressures This method is much more rapid than the surface tensionmethod, and is applicable to a wide range of surfactant classes, includingboth ionic and amphoteric (zwitterionic) surfactants The method issuitable for the estimation of cmc's, for determining the minimum cmc

as a function of temperature, for identifying the temperature at which theminimum cmc occurs, and for determining how cmc's vary with signifi-cant temperature and pressure changes The method has been used todetermine the temperature variation of cmc's for a number of commercialfoaming surfactants in aqueous solutions, for the derivation of thermo-dynamic parameters, and to establish useful correlations [55]

Cmc Values Some typical cmc values for low electrolyte centrations at room temperature are:

to the relative insensitivity of the cmc value of the surfactant to the nature

of the charged head group, cmc's show little dependence on the nature ofthe counter-ion It is mainly the valence number of the counter-ion thataffects the cmc As an example, the cmc value for Cu(DS)2 is about1.2 mM, while the cmc for SDS is about 8 mM [49, 67]

Cmc values often exhibit a weak dependence on both temperature[68±70] and pressure [59, 71], although, as shown in Figure 3, somesurfactant cmc's have been observed to increase markedly with tempera-ture above 100 8C [55, 65] The effects of added substances on the cmcare complicated and interesting, and depend greatly on whether theadditive is solubilized in the micelle, or in the intermicellar solution Theaddition of electrolytes to ionic surfactant solutions results in a well

established linear dependence of log (cmc) on the concentration of addedsalt [72±76] For nonionic micelles, electrolyte addition has little effect oncmc values When non-electrolytes are added to the micellar solution, theeffects are dependent on the nature of the additive For polar additives(e.g., n-alcohols), the cmc decreases with increasing concentration ofalcohol, while the addition of urea to micellar solutions tends to increasethe cmc, and may even inhibit micelle formation [77, 78] Nonpolaradditives tend to have little effect on the cmc [79].

Figure 3 Temperature variation of the critical micelle concentrations of three amphoteric surfactants in 2.1% total dissolved solids brine solu- tions (From Stasiuk and Schramm [55] Copyright 1996 Academic Press, New York.)

The Krafft Point

The solubilities of micelle-forming surfactants show a strong increaseabove a certain temperature, termed the Krafft point (Tk) This isexplained by the fact that the single surfactant molecules have limitedsolubility whereas the micelles are very soluble Referring to the illustra-tion from Shinoda [11] in Figure 4, below the Krafft point the solubility ofthe surfactant is too low for micellization so solubility alone determinesthe surfactant monomer concentration As temperature increases thesolubility increases until at Tkthe cmc is reached At this temperature arelatively large amount of surfactant can be dispersed in micelles andsolubility increases greatly Above the Krafft point maximum reduction insurface or interfacial tension occurs at the cmc because the cmc thendetermines the surfactant monomer concentration Krafft points for anumber of surfactants are listed in references [1, 80]

Nonionic surfactants do not exhibit Krafft points Instead, the solubility

of nonionic surfactants decreases with increasing temperature, and thesesurfactants may begin to lose their surface active properties above atransition temperature referred to as the cloud point This occurs becauseabove the cloud point a surfactant rich phase of swollen micellesseparates, and the transition is usually accompanied by a marked increase

in dispersion turbidity

Figure 4 Example of a ``phase behaviour'' diagram for a surfactant in aqueous solution, showing the cmc and Krafft points (From Shinoda et al [11] Copyright 1963 Academic Press, New York.)

Numerous methods have been developed for the quantitative tion of each class of surfactant The analysis of commercial surfactants isgreatly complicated by the fact that these products are mixtures They areoften comprised of a range of molar mass structures of a given structuralclass, may contain surface-active impurities, are sometimes intentionallyformulated to contain several different surfactants, and are often supplieddissolved in mixed organic solvents or complex aqueous salt solutions.Each of these components has the potential to interfere with a givenanalytical method Therefore surfactant assays may well have to bepreceded by surfactant separation techniques Both the separation andassay techniques can be highly specific to a given surfactant/solutionsystem This makes any substantial treatment beyond the scope of thepresent chapter Good starting points can be found in the several books onsurfactant analysis [81±86] The characterization and analysis of surfactantdemulsifiers is discussed in Chapter 2 of this book Table 4 shows sometypical kinds of analysis methods that are applied to the differentsurfactant classes

determina-Table 4 Typical Methods of Surfactant Analysis

Anionic

alkyl sulfates and sulfonates Two-phase or surfactant-electrode monitored

titration petroleum and lignin sulfonates Column or gel permeation chromatography phosphate esters Potentiometric titration

sulfosuccinate esters Gravimetric or titration methods

carboxylates Potentiometric titration or two-phase titration Nonionic

carboxybetaines Low pH two-phase titration, gravimetric analysis,

or potentiometric titration

There are a number of reviews available for surfactants in specificindustries [87], and for specific surfactant classes References [81±90]discuss methods for the determination of anionic surfactants, which areprobably the most commonly encountered in the petroleum industry.Most of these latter methods are applicable only to the determination ofsulfate- and sulfonate-functional surfactants Probably the most commonanalysis method for anionic surfactants is Epton's two-phase titrationmethod [91, 92] or one of its variations [93, 94] Related, single-phasetitrations can be performed and monitored by either surface tension [95]

or surfactant-sensitive electrode [84, 85, 96±98] measurements veld and Faber [99] discuss adaptation of the titration method to oleicphase samples

Grons-Surfactants and Surface Tension

In two-phase dispersions, a thin intermediate region or boundary, known

as the interface, lies between the two phases The physical properties ofthe interface can be very important in all kinds of petroleum recovery andprocessing operations Whether in a well, a reservoir or a surfaceprocessing operation, one tends to encounter large interfacial areasexposed to many kinds of chemical reactions In addition, many petro-leum industry processes involve colloidal dispersions, such as foams,emulsions, and suspensions, all of which contain large interfacial areas;the properties of these interfaces may also play a large role in determiningthe properties of the dispersions themselves In fact, even a modestsurface energy per unit area can become a considerable total surfaceenergy Suppose we wish to make a foam by dispersion of gas bubbles intowater For a constant gas volume fraction the total surface area producedincreases as the bubble size produced decreases Since there is a freeenergy associated with surface area, this increases as well with decreasingbubble size The energy has to be added to the system to achieve thedispersion of small bubbles If this amount of energy cannot be provided,say through mechanical energy input, then another alternative is to usesurfactant chemistry to lower the interfacial free energy, or interfacialtension The addition of a small quantity of a surfactant to the water,possibly a few tenths of a percent, would significantly lower the surfacetension and significantly lower the amount of mechanical energy neededfor foam formation For examples of this simple calculation for foams andemulsions, see references [61] and [100] respectively

The origin of surface tension may be visualized by considering themolecules in a liquid The attractive van der Waals forces betweenmolecules are felt equally by all molecules except those in the interfacialregion This imbalance pulls the latter molecules towards the interior ofthe liquid The contracting force at the surface is known as the surface

tension Since the surface has a tendency to contract spontaneously inorder to minimize the surface area, bubbles of gas tend to adopt aspherical shape: this reduces the total surface free energy For emulsions

of two immiscible liquids a similar situation applies to the droplets of one

of the liquids, except that it may not be so immediately obvious whichliquid will form the droplets There will still be an imbalance ofintermolecular force resulting in an interfacial tension, and the interfacewill adopt a configuration that minimizes the interfacial free energy.Physically, surface tension may be thought of as the sum of the contract-ing forces acting parallel to the surface or interface This point of viewdefines surface or interfacial tension (g), as the contracting force per unitlength around a surface Another way to think about surface tension is thatarea expansion of a surface requires energy Since the work required toexpand a surface against contracting forces is equal to the increase insurface free energy accompanying this expansion, surface tension mayalso be expressed as energy per unit area

There are many methods available for the measurement of surface andinterfacial tensions Details of these experimental techniques and theirlimitations are available in several good reviews [101±104] Table 5 showssome of the methods that are used in petroleum recovery processresearch A particular requirement of reservoir oil recovery processresearch is that measurements be made under actual reservoir conditions

of temperature and pressure The pendant and sessile drop methods arethe most commonly used where high temperature/pressure conditions arerequired Examples are discussed by McCaffery [105] and DePhilippis et

al [106] These standard techniques can be difficult to apply to themeasurement of extremely low interfacial tensions (51 to 10 mN/m).For ultra-low tensions two approaches are being used For moderatetemperatures and low pressures the most common method is that

of the spinning drop, especially for microemulsion research [107] Forelevated temperatures and pressures a captive drop method has beendeveloped by Schramm et al [108], which can measure tensions as low as0.001 mN/m at up to 200 8C and 10,000 psi In all surface and interfacialtension work it should be appreciated that when solutions, rather thanpure liquids, are involved appreciable changes can occur with time at thesurfaces and interfaces, so that techniques capable of dynamic measure-ments tend to be the most useful

When surfactant molecules adsorb at an interface they provide anexpanding force acting against the normal interfacial tension Thus,surfactants tend to lower interfacial tension This is illustrated by thegeneral Gibbs adsorption equation for a binary, isothermal systemcontaining excess electrolyte:

Gs= 7(1/RT)(dg/d ln Cs) (4)

Capillary rise [ & [ ` `, need y = 0 ` Wilhelmy plate [ & [ ` [, need to know g `

du Nouy ring [ ` [ ` `, pure liquids only ` Drop weight [ ` [ [ `, need y = 0 [ Drop volume [ ` [ [ `, need y = 0 [ Pendant drop [ [ [ [ ` [ Sessile drop [ [ [ [ [ [ Oscillating jet [ [ [ ` ` ` Spinning drop [ & [ [ ` ` Captive drop [ [ [ [ `, forces y = 0 [ Maximum bubble pressure [ & [ ` ` ` Surface laser light scattering [ [ [ & ` [ Tilting plate [ & ` ` [ `

where Gsis the surface excess of surfactant (mol/cm2), Csis the solutionconcentration of the surfactant (M), and g may be either surface orinterfacial tension (mN/m) This equation can be applied to dilutesurfactant solutions where the surface curvature is not great and wherethe adsorbed film can be considered to be a monolayer The packingdensity of surfactant in a monolayer at the interface can be calculated asfollows According to equation 4, the surface excess in a tightly packedmonolayer is related to the slope of the linear portion of a plot of surfacetension versus the logarithm of solution concentration From this, the areaper adsorbed molecule (aS) can be calculated from

Surface Elasticity

As surfactant adsorbs at an interface the interfacial tension decreases (atleast up to the cmc), a phenomenon termed the Gibbs effect If asurfactant stabilized film undergoes a sudden expansion, the immediatelyexpanded portion of the film must have a lower degree of surfactantadsorption than unexpanded portions because the surface area hasincreased This causes an increased local surface tension which producesimmediate contraction of the surface The surface is coupled, by viscousforces, to the underlying liquid layers Thus, the contraction of the surfaceinduces liquid flow, in the near-surface region, from the low tensionregion to the high tension region The transport of bulk liquid due tosurface tension gradients is termed the Marangoni effect [27] In foams,the Gibbs±Marangoni effect provides a resisting force to the thinning ofliquid films

The Gibbs±Marangoni effect only persists until the surfactant tion equilibrium is re-established in the surface, a process that may takeplace within seconds or over a period of hours For bulk liquids and inthick films this can take place quite quickly, however, in thin films theremay not be enough surfactant in the extended surface region to re-establish the equilibrium quickly, requiring diffusion from other parts ofthe film The restoring processes are then the movement of surfactantalong the interface from a region of low surface tension to one of high

adsorp-surface tension, and the movement of surfactant from the thin film intothe now depleted surface region Thus the Gibbs±Marangoni effectprovides a force to counteract film rupture in foams.

Many surfactant solutions show dynamic surface tension behaviour.That is, some time is required to establish the equilibrium surface tension.After the surface area of a solution is suddenly increased or decreased(locally), the adsorbed surfactant layer at the interface requires some time

to restore its equilibrium surface concentration by diffusion of surfactantfrom, or to, the bulk liquid (see Figure 5, [109]) At the same time, sincesurface tension gradients are now in effect, Gibbs±Marangoni forces act

in opposition to the initial disturbance The dissipation of surface tensiongradients, to achieve equilibrium, embodies the interface with a finiteelasticity This explains why some substances that lower surface tension

do not stabilize foams [21]; they do not have the required rate of approach

to equilibrium after a surface expansion or contraction In other words,they do not have the requisite surface elasticity

At equilibrium, the surface elasticity, or surface dilational elasticity,

EG, is defined [21, 110] by

where g is the surface tension and A is the geometric area of the surface.This is related to the compressibility of the surface film, K, by K = 1/EG

EGis a thermodynamic property, termed the Gibbs surface elasticity This

is the elasticity that is determined by isothermal equilibrium ments, such as the spreading pressure±area method [21] EG occurs invery thin films where the number of molecules is so low that thesurfactant cannot restore the equilibrium surface concentration afterdeformation An illustration is given in [61]

The elasticity determined from nonequilibrium dynamic ments depends upon the stresses applied to a particular system, isgenerally larger in magnitude than EG, and is termed the Marangonisurface elasticity, EM[21, 111] For foams it is this dynamic property that

measure-is of most interest Surface elasticity measures the resmeasure-istance againstcreation of surface tension gradients and of the rate at which suchgradients disappear once the system is again left to itself [112] TheMarangoni elasticity can be determined experimentally from dynamicsurface tension measurements that involve known surface area changes,such as the maximum bubble pressure method [113, 115] Although suchmeasurements include some contribution from surface dilational viscosity[112, 114] the results are frequently simply referred to in terms of surfaceelasticities

Numerous studies have examined the relation between EGor EMandfoam stability [111, 112, 115] From low bulk surfactant concentrations,

Figure 5 Illustration of the Gibbs±Marangoni effect in a thin liquid film Reaction of a liquid film to a surface disturbance (a) Low surfactant concentration yields only low differential tension in film The thin film is poorly stabilized (b) Intermediate surfactant concentration yields a strong Gibbs±Marangoni effect which restores the film to its original thickness The thin film is stabilized (c) High surfactant concentration (4cmc) yields a differential tension which relaxes too quickly due to diffusion of surfactant The thinner film is easily ruptured (From Pugh [109] Copyright 1996 Elsevier, Amsterdam.)

the Gibbs elasticity increases with an increase in surfactant concentrationuntil a maximum in elasticity is reached, after which the Gibbs elasticitydecreases Surfactant concentrations above the cmc lie well beyond thismaximum elasticity region Lucassen-Reynders [112] cautions that thereexists no direct relationship between elasticity and foam stability,although Schramm and Green [113] have found a useful correlation forfoams flowing in porous media Additional factors, such as film thicknessand adsorption behaviour, have to be taken into account Nevertheless,the ability of a surfactant to reduce surface tension and contribute tosurface elasticity are among the most important features of foamstabilization This partially explains why some surfactants will act topromote foaming while others reduce foam stability (foam breakers ordefoamers), and still others prevent foam formation in the first place(foam preventatives, foam inhibitors).

Schramm et al [116] discuss some of the factors that must beconsidered in the selection of practical foam-forming surfactants forpetroleum recovery processes Kerner [117] describes several hundreddifferent formulations for foam inhibitors and foam breakers

Surface Rheology

Surface rheology deals with the dynamic behaviour of a surface inresponse to the stress that is placed on the surface Two types of viscositiesare defined within the interface, a shear viscosity and a dilational viscosity.For a surfactant monolayer, the surface shear viscosity is analogous to thethree dimensional shear viscosity: the rate of yielding of a layer of fluiddue to an applied shear stress The surface dilational viscosity expressesthe magnitude of the viscous forces during a rate expansion of a surfaceelement A surfactant monolayer can be expanded or compressed over awide area range Thus, the dynamic surface tension experienced during arate dependent surface expansion is the resultant of the surface dilationalviscosity, the surface shear viscosity, and elastic forces Often, thecontributions of shear and/or the dilational viscosities are neglectedduring stress measurements of surface expansions Isolating interfacialviscosity effects is rather difficult The interface is connected to thesubstrate on either side of it and so are the interfacial viscosities coupled

to the bulk viscosities Therefore, it becomes laborious to determinepurely interfacial viscosities without the influence of the surroundings

A high interfacial viscosity can contribute to emulsion stability byreducing the rate of droplet coalescence [118±121] This is therefore aproperty that one may wish to enhance in the formulation of a desirableemulsion For example, oilfield water-in-oil (W/O) emulsions may bestabilized by the presence of a protective film around the water droplets

Such a film can be formed from the asphaltene and resin fractions of thecrude oil As drops approach each other the rate of oil film drainage will

be determined, in part, by the interfacial viscosity which, if high enough,will significantly retard the final stage of film drainage and possibly evenprovide a viscoelastic barrier to coalescence More detailed descriptionsare given in references [121±123] On the other hand, in an enhanced oilrecovery process one will generally desire low interfacial viscosity so thatonce the oil is emulsified and displaced from pores within which it wastrapped, the same emulsion drops can later coalesce into an oil bankwhich can be displaced from the reservoir [30] Wasan et al [124] foundsuch a correlation between oil droplet coalescence rate and interfacialviscosity

As bubbles in a foam approach each other, the thinning of the filmsbetween the bubbles, and their resistance to rupture, are thought to be ofgreat importance to the ultimate stability of the foam Thus, a highinterfacial viscosity can promote foam stability by lowering the filmdrainage rate and retarding the rate of bubble coalescence [125] Fastdraining films may reach their equilibrium film thickness in a matter ofseconds or minutes due to low surface viscosity, while slow draining filmsmay require hours due to their high surface viscosity Bulk viscosity andsurface viscosity, thus, do not normally contribute a direct stabilizingforce to a foam film, but act as resistances to the thinning and ruptureprocesses The bulk viscosity will most influence the thinning of thickfilms, while the surface viscosity will be dominant during the thinning ofthin films

The presence of mixed surfactant adsorption seems to be a factor inobtaining films with very viscous surfaces [27] For example, in somecases, the addition of a small amount of nonionic surfactant to a solution

of anionic surfactant can enhance foam stability due to the formation of aviscous surface layer; possibly a liquid crystalline surface phase inequilibrium with a bulk isotropic solution phase [21, 126] To the extentthat viscosity and surface viscosity influence emulsion and foam stabilityone would predict that stability would vary according to the effect oftemperature on the viscosity Thus, some petroleum industry processesexhibit serious foaming problems at low process temperatures, whichdisappear at higher temperatures [21]

Adamson [110] illustrates some techniques for measuring surface shearviscosity Further details on the principles, measurement and applicationsare given in references [127±130] for emulsions, and in reference [131]for foams It should be noted that many experimental studies deal with theinterfacial viscosities between bulk phases rather than on droplets or thinfilms themselves

Surfactants and Surface Curvature

Surface tension causes a pressure difference to exist across a curvedsurface, with the greatest pressure being on the inside of a bubble Thepressure difference across an interface between one phase (A), havingpressure pA, and another phase (B), having pressure pB, for sphericalbubbles of radius R, is given by:

This is the Young±Laplace equation It illustrates the facts that Dp varieswith the radius, and that the pressure inside a bubble exceeds thatoutside If the interface had a more complex geometry, then the twoprincipal radii of curvature, R1and R2, would be used,

Dp = pA7 pB= g(1/R1+ 1/R2) (8)The Young±Laplace equation forms the basis for some important meth-ods for measuring surface and interfacial tensions, such as the pendantand sessile drop methods, the spinning drop method, and the maximumbubble pressure method [101±103, 107] Liquid flow in response to thepressure difference expressed by equations 7 or 8 is known as Laplaceflow, or capillary flow

Detergency and the Displacement of Oil Detergencyinvolves the action of surfactants to alter interfacial properties so as topromote removal of a phase from solid surfaces Obviously, wetting agentsare used, and usually those that rapidly diffuse and adsorb at appropriateinterfaces are most effective In this section we will consider a petroleumindustry example

When a drop of oil in water comes into contact with a solid surface theoil may form a bead on the surface or it may spread and form a film Aliquid having a strong affinity for the solid will seek to maximize its contact(interfacial area) and form a film A liquid with much weaker affinity mayform into a bead This affinity is termed the wettability Since there can bedegrees of spreading another quantity is needed (see Figure 6, [132]) Thecontact angle, y, in an oil±water±solid system is defined as the angle,measured through the aqueous phase, that is formed at the junction of thethree phases Whereas interfacial tension is defined for the boundarybetween two phases, the contact angle is defined for a three-phasejunction

If the interfacial forces acting along the perimeter of the drop arerepresented by the interfacial tensions, then an equilibrium force balanceequation can be written as,

gW/Ocos y = gS/O7 gS/W (9)

where the subscripts refer to water, W, oil, O, and solid, S This is Young'sequation The solid is completely water-wetted if y = 0 and only partiallywetted otherwise This equation is frequently used to describe wettingphenomena, so two practical points should be remembered In theory,complete non-wetting by water would mean that y = 1808 but this is notseen in practice Also, values of y 5 908 are often considered to represent

``water-wetting'' while values of y 4 908 are considered to represent water-wetting'' This is a rather arbitrary assignment based on correlationwith the visual appearance of drops on surfaces

``non-In primary oil recovery from underground reservoirs, the capillaryforces described by the Young and Young±Laplace equations are respon-sible for retaining much of the oil (residual oil) in parts of the porestructure in the rock or sand It is these same forces that any secondary orenhanced (tertiary) oil recovery process strategies are intended to over-come [26, 29, 30, 133] The relative oil and water saturations depend uponthe distribution of pore sizes in the rock The capillary pressure, Pc, in apore is given by,

where R is the pore radius, and at some height h above the free watertable, Pcis fixed at Drh (Dr is the density difference between the phases).Therefore, as the interfacial tension and contact angle are also fixed, and ifthe rock is essentially water-wetting (low y), the smaller pores will tend to

Figure 6 Illustration of spreading, beading, and the contact angle in

a solid/liquid/liquid system (From Shaw [132] Copyright 1992 Butterworth±Heinemann, Oxford, UK.)

have more water in them (less oil) than larger pores, as illustrated inFigure 7 [134].

Primary production from an oil reservoir, using only energy inherent

in the reservoir, will only recover up to about 15% of the original place (OOIP) In secondary oil recovery, flooding the reservoir with water(waterflooding) can produce an additional 15% or so of the oil originally inplace After waterflooding some 70% of the original oil-in-place stillremains trapped in the reservoir rock pores In a water-wet reservoir thisresidual oil is left in the form of oil ganglia trapped in the smaller poreswhere the viscous forces of the driving waterflood could not completelyovercome the capillary forces holding the oil in place

oil-in-In tertiary, or enhanced, oil recovery one generally attempts to reducethe capillary forces restraining the oil and/or alter viscosity of thedisplacing fluid in order to modify the viscous forces being applied todrive oil out of the pores The ratio of viscous forces to capillary forcesactually correlates well with residual oil saturation and is termed thecapillary number One formulation of the capillary number is:

Figure 8; beginning after even the most efficient waterflooding, when

Ncis about 1076and the residual oil saturation is still around 45% [135].Suppose that one wished to design a tertiary recovery process so thatadditional oil would be recovered, reducing the oil saturation to around25% A residual oil saturation of 25% requires increasing the capillarynumber to about 5 6 1073 This could be done by raising the viscousforces, i.e., viscosity and velocity, but practical limitations on the size ofpumps and the need to avoid inducing fractures in the reservoir preventone from using these factors to achieve the needed orders of magnitudeincrease But, by adding a suitable surfactant to the water one can readilydecrease the interfacial tension from say 20 mN/m to 4 6 1073mN/m,increasing the capillary number to the desired 5 6 1073 Substitution ofthese interfacial tensions into the above capillary pressure equation showsthat with the reduced interfacial tension oil will be recovered from smallerpores down to R' = 0.0002R A more detailed treatment of this topic isgiven in Chapter 6 of this volume

In some systems the addition of a fourth component to an oil/water/surfactant system can cause the interfacial tension to drop to near-zerovalues, ca 1073to 1074mN/m, allowing spontaneous emulsification to

Figure 8 Correlation between residual oil saturation reduction and the capillary number (From Taylor and Hawkins [135] Copyright 1990 Petroleum Recovery Institute, Calgary, AB.)

very small drop sizes, ca 10 nm or smaller The droplets can be so smallthat they scatter little light and the emulsions appear to be transparentand are termed microemulsions Unlike coarse emulsions, microemul-sions may be thermodynamically stable Microemulsions can be used in

an enhanced oil recovery process The much lower interfacial tensionsproduced increase the oil displacement from reservoir rock by increasingthe capillary number The micelles present also help to solubilize the oildroplets, hence this process is sometimes referred to as micellar flooding.The emulsions can be formulated to have moderately high viscositieswhich help to achieve a more uniform displacement front in the reservoir;this gives improved sweep efficiency, see Figure 9 [136] Thus, there are a

Figure 9 Oil displacement, with good sweep efficiency, in a reservoir (From Ling et al [136] Copyright 1987 Royal Society of Chemistry, Cambridge.)

number of factors that can be adjusted using a microemulsion system forenhanced oil recovery.

Surfactants and Surface Potential

Most substances acquire a surface electric charge when brought intocontact with a polar medium such as water For crude oil/aqueous systemsthe charge could be due to the ionization of surface acid functionalities.For gas/aqueous systems the charge could be due to the adsorption ofsurfactant ions For porous rock or suspensions, the charge couldoriginate from the diffusion of counter-ions away from the surface of amineral whose internal crystal structure carries an opposite charge due toisomorphic substitution (in clays for example) In a practical petroleumprocess situation the nature and degree of surface charging is morecomplicated than in these examples, and surfactant adsorption may cause

a surface electric charge to increase, decrease, or not significantly change

at all For example, the bitumen±aqueous interface can become tively charged in alkaline aqueous solutions due to the ionization ofsurface carboxylic acid groups, the adsorption of natural surfactantspresent in the bitumen, and the adsorption of charged mineral solids[139±141] The degree of such negative charging is very important to thesuccess of in situ oil sands bitumen recovery processes, and surface oilsands separation processes, such as the hot water flotation process (seereferences [142, 143], and Chapter 10 of this volume)

nega-The presence of a surface charge influences the distribution of nearbyions in the polar medium Ions of opposite charge (counter-ions) areattracted to the surface while those of like charge (co-ions) are repelled

An electric double layer, which is diffuse because of mixing caused bythermal motion, is thus formed The electric double layer (EDL) consists

of the charged surface and a neutralizing excess of counter-ions over ions, distributed near the surface (see Figure 10) The EDL can beviewed as being comprised of two layers, (i) an inner layer that mayinclude adsorbed ions, and (ii) a diffuse layer where ions are distributedaccording to the influence of electrical forces and thermal motion.Taking the surface electric potential to be c8, and applying the Gouy±Chapman approximation, the electric potential c at a distance x from thesurface is approximately

Thus c depends on surface electric potential and the solution ioniccomposition (through k) 1/k is called the double layer thickness and forwater at 25 8C is given by:

k = 3.288pI(nm71) (13)

where I is the ionic strength, given by I = (1/2) Sicizi2, where ci isconcentration of ions and ziis charge number of ions For 1:1 electrolyte,1/k = 1 nm for I = 1071M and 10 nm for I = 1073M.

Also, an inner layer exists because ions are not really point charges and

an ion can only approach a surface to the extent allowed by its hydrationsphere The Stern model specifically incorporates a layer of specificallyadsorbed ions bounded by a plane known as the Stern plane (see Figures

10 and 11) In this case the potential changes from c8 at the surface, toc(d) at the Stern plane, to c = 0 in bulk solution

An indication of the surface potential can be obtained throughelectrokinetic measurements Electrokinetic motion occurs when themobile part of the electric double layer is sheared away from the innerlayer (charged surface) Of the four types of electrokinetic measurements,electrophoresis, electro-osmosis, streaming potential, and sedimentationpotential, the first finds the most use in industrial practise In electro-phoresis, an electric field is applied to a sample causing charged dispersedspecies, and any attached material or liquid, to move towards the

Figure 10 The electric double layer around a charged species in aqueous solution The left view shows the change in charge density around the charged species The right view shows the distribution of ions (Courtesy L.A Ravina, Zeta-Meter Inc., Staunton, VA.)