THE

REPRINT

Published!or the UNIVERSITY OF PETROLEUM AND MINERALS. DHAHRAN. SAUDI ARABIAby JOHN WILEY 81 SONS LIMITED. CHICHESTER. NEW YORK. BRISBANE. TORONTO. SINGAPORE.c~

CHEMICAL INTERACTIONS INMICELLAR/POL YMER FLOODING SYSTEMS

MebIDet Sabri Celik

Department of Petroleum Engineering,University of Petroleum and Minerals.

Dhahran 3/26/. SaUlii Arabia

and

P. Somasl8ldaran

Henry Knlmb School of Mines,Columbia University,

New York, U.s.A.

~~,

. . .J~ JjjI.}l ~jl J \:-:')'~ I.a~ ~I .;,,~\d' ~J:4' ~J1..,..sI4.»;I.J

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4ioJi . . ..11. (;;: . .. !.;- Jf'

.r:-1' ':>I~..;,- J'}i~1 'r: ~ J' .;,,~\i-1 ~J ~ .;,,~\dl t.1a 1;.) ~ ..;,- .;~ IJaJ

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~;I\:Io r:: ~I U,J.JI .;,,1,) .:Ibfl ~" ~.;JI .;,,"J'\po 1;.) J" ~ ~I lJA

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A~TRAcr

In micellar flooding systems, chemical inter~tions at various interfaces playagoverning role in the displacement of oil through porous media. Since suchinter~tions can significantly influence the loss of surf~tants owing to pr~ipitation,adsorption. etc., it is important to understand the mechanisms involved and toidentify conditions that would minimize surf~tant loss from the slug. In this study,pr~ipitation behavior of long-chain sulfonates with various mono- and multiva-lent inorganics, alcohols, and polymers has been investigated with the aim ofestablishing the molecular mechanisms responsible for the system behavior.

0377-9211/86/OIOOSI-IISOl.lOC; 1986 by the Univerlity of Petroleum and Minerals

51The ArGbi411 JINnIIJ/ (M Scwrw:e -' £lIQillemnq, V~ II, N...",I

M. S. Celik and P. Somasundaran

CHEMICAL {NTERACfIONS INMI CELLAR/po L YMER FLOO DIN G SYSTEMS

INTRODUCflON factant species with mono- and multivalent inorganicscan lead to prQ:ipitation and. in turn. to loss ofsurfactant from the slug [9, 10].

Interaction between a high molecular weight acry-lamide-type polymer and surfactant species in solutioncan result both in precipitation and in the formation ofcharged complexes [11-13]. The charge characteristicsand concentrations of various species will govern thetYPe. as well as the surface activity. of the complexformed. The level and nature of these interactions can.indeed, be important in the desplacement of oilthrough porous media.

Information on the wetting characteristics of reser-voir rocks and the changes in them that occur uponreaction with various chemical species are powerfultools for investigating the performance of oil rQ:overysystems [14. 15]. In micellar Oooding systems. adsorp-tion or precipitation of surfactants or. polymers onrock surfaces can change the wettability of the rock.and consequently the oil rQ:overy from it. Thus. thenature of chemical interactions becomes important fora full understanding of wettability alterations.

In this study, the chemical interactions leading toprQ:ipitation of sulfonates have been investigated forthe NaCI. CaC12/Na dod~ylbenzenesulfonate(NaDDBS) systems. While increasing the sulfonateconcentration in NaCI solutions causes precipitation.in CaCl2 solutions it exhibits a complex behaviorinvolving pr~ipitation of calcium disulfonate followedby its redissolution. Details of this phenomenon as afunction of sulfonate concentration and ionic strengthare discussed. Other important factors. such as theeffQ:ts of alcohols and polymers on prQ:ipitation. arealso elucidated along with the governing m~hanisms.

MATERIAlS

Samples of sodium doda:ylbenzenesulfonate(NaDDBS), specified to be CJOOIo pure by Lachat Che-micals, were purified by deoiling, ra:rystaUization anddesalting ta:hniques [16]. Characterization of ra:ry-stallized desalted sulfonate using p-NMR and 13C-NMR ta:hniques showed the presence of branchedchain impurities in it. The CMC of this sulfonate,"determined by a surface tension ta:hnique, was found

Enhanced oil r~overy (EOR) by micellar- floodingis one of the potential t~hniques for r~overing part ofthe unr~overed oil from some reservoirs. Micellarflooding consists basically of inj~tion of detergentsolutions that can reduce the interfacial tension bet-ween water and oil. The surfactant solution may thenbe pushed through porous media by a polymer, whichacts as a mobility control agent [1, 2]. The micellar/polymer process. however, encounters problems owingto surfactant losses at various stages. The candidatesurfactants. petroleum-based anionics, are subj~t tolosses for a number of reasons:

(1) adsorption of surfactants on reservoir rocks;(2) formation of polymer/surfactant complexes;(3) precipitation of surfactants by mono- and multi-

valent inorganic el~trolytes;(4) plugging of pores by precipitates or surfac-

tant/polymer complexes.

Evidently, it is VItal for a careful optimization of theOood to formulate conditions that will minimize theloss of surfactant or polymer from the slug.

Surfactant adsorption on reservoir rocks has beenshown in certain cases to exhibit a maximum in theregion of critical micelle concentration (CMC) [3-7].Systems in which adsorption maxima have been obser-ved include petroleum sulfonates of varying properties.If such a maximum exists, it may be of practicalimponance to employ surfactant concentrations cor-responding to the far end of the maximum ior achiev-ing minimum surfactant loss.

Reservoir fluids contain water with a wide range ofionic composition [8]. These ions may originate froma variety of sources:

(a) they may be present in the inj~ted water;(b) the reservoir connate water itself can contain

significant amount of ions;(c) during the inj~tion of slug, particularly brine,

divalent ions are exchanged from the reservoirrock itself.

Therefore, the candidate surfactants should have hightolerance to multivalent ions, as interaction of sur-

.'M~I1ar', 'surf~t', 'detergent', and 'soap' are used inter-changeably.

52 T~ Arabian Journal/or Sc~rw;~ GIld Engille~ring. Volume II. NunWr I.

M. S. Celik and P. Somasundaran

..to be 1.5 x 10-3 kmol/m3, in agreement with the valuereported in the literature [17]. .

The organic solvents used for purification were ACScertified reagents that had been distilled and driedusing molecular sieves. The NaCl, NaOH, and HCIused were also of AR grade. Certified ACS gradeNaCl. CaQz, and AIC13, purchased from the FisherScientific Company, were used for all the other sys-tems studied.

The cationic co-monomer, methacrylamidoprop-yltrimethylammonium chloride (MAPTAC) was pur-chased from the Jefferson Chemical Company as astabilized liquid. The charge density of the aminatedacrylamide (P,\MD), which has over one millionmolecular weight, was estimated to be 10 mol %. The14C-tagged monomer was purchased from the NewEngland Nuclear Corporation in powder form with a_specific activity of 0.1995 mCi/mmol, and was used asreceived. The n- propanol (ACS certified) was purchasedfrom the Fisher Scientific Company. Triply distilledwater (conductivity < 106 mhos) was used for prepara-tion of all solutions. -

minutes, or until all the precipitate had settled. Theamount pr~ipitated w~ determined by calculating thedifference between the initial and final sulfonate con-centrations. The supernatant was then analyz~ forsulfonate and calcium.

The analysis of the sulfonate was done by a two-phase titration tec:hnique using dimidium bromide anddisulphine blue as indicators [18]. 1 cc of the solutionwas diluted to 20 cc with distilled water in a 100 ccstoppered graduated cylinder. 20cc of methylene blueindicator solution and 15 cc of chloroform were addedand the cylinder was shaken vigorously. This solutionwas then titrated against 10-3 kmol/m3 HTAB(Hexatrimethylammonium bromide) until colorless (orwith a bluish tinge).

The residual calcium concentration was determinedby standard EDT A titration using Mg-EDT A andErichrome Black T, the latter being the indicator.

PAMD, the cationic polymer used for pr~ipitationtests, was tagged with 14C so that the analysis could bedone easily using a Beckman LS 100 C liquid scintilla-tion counter. 1 cc of the solution was taken in ascintillation vial and 15 cc of Aqu'asoI-2 cocktail wasadded. The vial was then shaken before measuring thecounts per minute. The residual polymer concentra-tion was determined using a calibration curve that hadbeen found to be insensitive to the presence of sul-fonate. No interference by sulfonate was found.

EXPER""ENT AL PROCEDURE

T18'bidity Measurements

A Brinkman PC-600 probe colorimeter was used tomeasure the turbidity of the solutions. Equal amountsof surfactant and inorganic electrolyte solutions weremixed in a test tube, shaken for 10 seconds and thenkept at the desired temperature in a constant tem-perature bath. The electrolyte solutions were alwaysintroduced into the test tube before the surfactantsolutions. The dependence of the turbidity on thesurfactant and electrolyte concentrations was deter-mincd from the values for light transmission(i.. = 670 nm) 24 hours after mixing. No dependence ofthe solution depth on the turbidity measurements wasnoted.

Determination of CMC

Surface Tension Measurements

The Wilhelmy plate technique was used to measurethe surface tension of solutions. A sandblasted plati-num sensor plate of known width was immersed in thesolution, and the pull exerted by the solution on thesensor was directly measured with a Cahn-2<XX> micro-balance. The time dependence of the surface tensionwas monitored using an x-y recorder. The platinumsensor was first cleaned with alcoholic potassium hy-droxide followed by nitric acid, and then rinsed withtriply distilled water. A teflon container of lOcc capa-city was used, and the sensor depth in the solution wasmaintained at about 2 to 3 mm. Reproducibility of theresults was checked using duplicate samples and wasfound to be ::to.2 mN/m.

Chemical Analysis

To determine the amount and composition of theprecipitate solutions containing the surfactant and theinorganic electrolytes were placed in glass vials andmixed by shaking vigorously for about 10 seconds.The vials were left at the desired temperature for 24hours and then centrifuged at 17 (XX) rev/min for sixty

Conductivity Measurements

Specific conductivity measurements for the deter-

53The Arabian Journalfor Science and Engineering. Volume 11. Number

M. S. Celik and P. Somasundaran

mination of the CMC of sulfonate solutions wereperformed with a linear conductance/resistance meter(A. H. Thomas Company, Model 275 at l(xx) Hz).

Dye Solubilization

The dye solubilization ta:bnique [19-21] used hereis based on spectral changes around the CMC. Belowthe CMC, the dye (pinacyanol chloride) exhibits apink color owing to the formation of a highly insol-uble salt formed by reaction of the surfactant anionwith the dye cation [19]. Above the CMC, the solutionturns blue owing to the solubilization of the dye by themicelles. The tests were conducted by mixing thesurfactant solution with a 10-5 kmoljm3 dye solution.The solutions were then equilibriated in the dark fortwo hours and measure~ts were made using aHitachi Uvida:-505 spectrophotometer.

(C ) to initial (C.-i) sulfonate concentration isindicative of the fraction of sulfonate precipitated. Asthe ratio approaches one, the turbidity of the solutionsdecreases.

Addition of K, Na, and Li salts causes precipitationof sulfonate at approximately 0.2, 0.5, and 0.8kmol/m3salt concentrations, respectively. The K + ion, which

has the smallest hydrated ionic size, has the leasttolerance to precipitation of sulfonate. Also, the potas-sium salt of the sulfonate has the lowest CMC and,again, the most tendency to precipitation. It is theinteraction of the hydrated ion with the micelle that iscrucial, viz smaller hydrated ions generate greaterinteraction and produce lower CMC [22]. The repor-ted CMC order of the counterions, K + < Na + < Li, is

in accord with the above explanation. These results areimportant in micellar flooding systems, since oil reser-voirs are usually preflushed with brine prior to anychemical injection.

Dye solubilization and surface tension measure-ments indicate that precipitation takes place onlyabove the CMC of the system (the color of the dye isblue). In cases where precipitation with monovalentinorganics occurs above the CMC, the micelles maypossibly go through various phase transformations be.fore precipitation. Sequences of molecular interactions

RFSUL 1'8 AND DISCUSSION

Sulfonate Predpitation wid. MollOyale.t Salls

The precipitation behavior of NaDDBS with va-rious monovalent salts as a function of salt concen-tration is illustrated in Figure I. The ratio of residual

0.8NaDD8S/SaltT: 25°CpH: Natural (5.5-6.6)

'j"0.6

\,)«

..I

~(..) 0.4

6, KCl. NaCl0 LiCL

0.2:

0.0' . I ,. I I I I

0.1 1.0Salt concentration (kmol/m3)

Figure J. Effect of Monovalent lnorganics on the Precipitation of NaDDBS

54 The Arabi411 Jour/lalfor SciellCe and Ellgineerillg. Volume J J. Nwnbe,. J.

M. S. Celik and P. Somasundaran

tion takes place in the absence of micelles as:

TKraft>T. K+ +R-~K~aql~K~"

leading to precipitation can be shown as follows.An anionic surfactant. potassium sulfonate (KR), inwater ionizes into:

15\

Precipitatioa;Redissolutioa in Sulfoate-Multivaient loaSystem

Unlike monovalent inorganics, multivalent ions un-dergo a complex and interesting behavior as shown inFigure 2. Again, the residual to initial concentration ofsurfactant is plotted versus total (initial) surfactantconcentration at various CaCl2 levels. The systemexhibits precipitation and most importantly redis-solution at a particular sulfonate concentration. TheC.-,/CR-i versus CR-i plots at all CaCl2 levels show aminimum which corresponds to the CMC of the sys-tem. The CMC measurements were performed by anumber of techniques inc)uding surface tension anddye solubilization. Redissolution begins with the for-mation of micelles and, at a particular sulfonate con-centration, even complete redissolution is achieved.

Chemical analysis of precipitate-containing super-natants shows that there is a certain stoichiometrybetween the sulfonate and multivalent ion, i.e.

KR~K++R- (1)where R - is the surfactant molecule and K + repre-

sents the counterion. As the concentration of sur-factant is increased. a critical point called the Kraffttemperature is reached where formation of miceUeswith bound counterions occurs:

7: < T MK + + nR M(ft-m,- (2)Krall I , ..-

where n is the number of monomers in the micelle andm is the number of K + ions attached. Further increaseof the concentration of either K + or R - results in the

precipitation of sulfonate:

(n-m)K++M(ft-ml-~(KM)ft (3)

(KM)ft~Liquid crystals~Precipitate. (4)

The micelles in the above process are in equilibriumwith the monomers.

On the other hand, if the Krafft temperature ishigher than the solution temperature. then precipita-

0.8

0.6"I

C

~'-I

\.)C 0.4

NaOO8S/CoClzpH = 5.8-6.8T = 25°CContact time =24h

CaClzI;)

0.2l .6. 4X10-4

C 1 X10-3

. 3 X10-36 5X10-30 1 X10-2.

0.0 . I I I I I I I I I I I I I I III I I I I I I I I I I c I. I I J I.J

10-4 10-3 10-2. 10-1

Total NaDDBS Concentration (kmol/m3)

. Ratio of Residual to Initial Concentration of Sulfonate as a FUllCtion of the Initial SulfonateConcentration at Different CaCiz levels

Figl4re 2

55The Arabi4n JQljmalfor Science and Engineering, Volume II. Number I.

M. S. Celik and P. Somasundaran

tion. In contrast. the following s«tion demonstratesenhancement of redissolution by NaCI addition. Sincethe evidence presented above does not support any ofthe premises except micellar solubilization, this mecha-nism is proposed as the sole mechanism for precipitateredissolution. Briefly, this involves the uptake of cal-cium ions by micelles; the dissolution of the precipitateto replenish the system with calcium ions; followed bythe formation of additional micelles with the releasedmonomers.

PrecipitatiOft,'Redissolutioa/Reprecipitatioa in theSulfonate-Monovalent/Multivalent Ion System

Redissolution of the precipitate can also be achievedwith the addition of NaC~ as shown in Figure 3. Thefraction of sulfonate precipitated is plotted versus theNaCI concentration for calcium and aluminum salts.These data prove that NaCI addition (other mono-valent salts, such as NH. CI, also generate the sameetT~t [16]) can produce redissolution of the pre-cipitate. This finding is important as it suggests thatsulfonate pr~ipitation can be controlled by the addi-tion of monovalent electrolytes.

2R - /Ca2 +, indicating the pr~ipitate to be in the form

of calcium disulfonate [10].

Several m~hanisms have been proposed for thisintriguing phenomenon of pr«ipitate redissolutien[9. 10.23-25]. However. based on the evidence thatthe redissolution of calcium disulfonate pr~ipitateoccurs only above the CMC. the following micellarsolubilization mechanism is proposed for the pre-cipitate redissolution. Here. monovalent and multi-valent ions are represented by Na and Ca respectively.

(a) Micellllr solubilization:

nR-+mNa+~M(.-.'- (6)CaRZ(al~a2+ +2R - (7)

xCaz+ +M(.-.'-:;=CaM( ZZ)- (8)

where M(.-.)- is the surfactant miceile with anet change of (m - n). These equations merelydescribe the sequence of processes and are notto suggest that calcium or sodium free miceUeswith bound calcium ions exist in the solution. Atequilibrium. the dissolved Ca2 + ions will bedistributed over all the micelles {16].

Other m~hanisms that can possibly causeredissolution of the sulfonate include thosebased on complexation or charge of the aggre-gated miceUes or pr~ipitates.

(b) Complexation: charged-complex formation bet-ween the pr~ipitate and the surfactant mono-mer (R -) or dimer (R ~ -). etc., to yield a seriesof species:

The mechanism for this interesting effect of NaCIbecomes evident when its influence on miceUizationitself is considered. Increase in ionic strength owing toNaCI addition is known to lower the CMC. If theredissolution is due to formation of micelles andsolubilization of the precipitate by them, then suchenhanced micellization by NaCI can indeed be expec-ted to promote the precipitate redissolution. More-over, the experimentally determined CMC values coin-cide with the onset of redissolution as illustrated inFigure 3. The dependence of CMC and sulfonate

CaRl +R -:;:CaR) (9)

CaRl + Ri-:;:CaRl- (10)

(c) Micellar aggregation/dispersion: roc.harging rol-lowed by redispersion of the coagulated micel-les, the roc.harging being due to adsorption ofthe monomer on the micelle.

(d) Sol formation: charge reversal of the pr~ipitatedparticles upon adsorption of charged monomerson them can result in the formation of stablesols.

Since the precipitate redissolution is found to takeplace above the CMC, moc.hanism (b) is not operativein this system. Moc.hanism (c), on the other hand,requires formation of micelles even prior to proc.ipita-bon. If moc.hanism (d) is operative, one should expectthe addition of an electrolyte such as NaCI to retardthe redispersion of sols and thus to prevent redissolu-

56 The Arabilm JOIIrMlfor Science aIIIl EngiMtrireg. Volume II. Number

M. S. Celik and P. Somasundaran

rcdissolution on added NaCl concentration, con-sidering the activity of all species in the system, isdiscussed elsewhere [26].

The reprecipitation region, which is above tbe solu-bility of NaDDBS itself, is characterized by the forma-tion of either mixed precipitates of NaR +CaRz orNaR precipitate alone [10].

Effect of Akobols (lI-prOpa~)

Since alcohols are used as co-surfactants, the in-fluence of alcohols on precipitation/redissolutionphenomena is important in micellar flooding systems.Therefore, the precipitation behavior of theCaCl1/NaDDBS system in the presence of II-propanolhas been investigated in this study.

The precipitation behavior of NaDDBS/CaCl1(3 x 10-J kmol/mJ) at three levels of n-propanol isgiven in Figure 4. Our past work has identified theprecipitate under these concentrations of sulfonate inCaCl1 solutions (in the absence of n-propanol) to becalcium dodecylbenzenesulfonate [10]. It is evidentfrom the data in Figure 4 that 6o,.~lI-propanol reducedthe precipitation, and 1001.; and 25% II-propanol com-pletely eliminated the precipitation under all condi-tions tested. The onset of redissolution at 6% n-propanol is found to coincide with the CMC of thesystem determined using the dye solubilization tech-nique. Most interestingly, 1001.; n-propanol reduced theCMC of DDBS to a concentration below1.35 x 10-4. kmol/mJ sulfonate, which is the concentra-tion for the onset of precipitation of calcium dodecyl-benzenesulfonate in the absence of n-propanol. Thedye solubilization tests indicated the presence of micel-

les at all sulfonate concentrations above1.35 x 10-4 kmol/m3 in the presence of 10% and 2SOion-propanol In these cases. precipitation of calciumsulfonate was not observed. Evidently, the micelles cancause not only the redissolution of the precipitates butthey can also prevent the precipitation of the sulfonate.

It is clear from Figure 4 that the effect of alcohol isto reduce the amount of precipitate under all thetested sulfonate concentrations. It is to be noted thateven below the CMC, n-propanol reduces the pre-cipitation of Ca(DDBS)2' The "-propanol thereforeaffects Ca(DDBS)2 precipitation not only by shiftingthe CMC but also by reducing the precipitation belowthe CMC. Even though the mechanism by whichshort-chain alcohols such as "-propanol affect theCMC has not been well established, speculations madein the past involve the effect of alcohol on the struc-ture, and hence the solvent power of water, as well asthe incorporation of alcohols between the chargedheads of sulfonate in the micelle [27]. The role of theformer effect can be estimated by examining the in-fluence of n-propanol on the precipitation below theCMC. The role of the latter effect, i.e. the incorpora-tion of alcohol molecules between the charged headsof the micelle, can be evaluated by estimating thesolubilization constant of the micelles - defined as the

amount (in moles) of the precipitate redissolved permole of sulfonate above the CMC,

K = (Ca~1 redissolved) (11)--s Ca-r-CMC' , ,

where CMC is the residual sulfonate at the onset ofredissolution and Ca -r is the residual sulfonate con-centration at which [CaRl] moles of precipitate hasredissolved.

Ks(6% "-propanol)=0.12Ks (0% "-propanol) = 0.30.

Assuming a constant micelle size, the low solubiliza-tion power of micelles for precipitates in the presenceof alcohol can be interpreted to be due to competitionof the alcohol with the precipitate for solubilization bymicelles. In this regard, a short.-chain alcohol such as"-propanol cannot be expected to get solubilizedwithin the micelle. On the other hand, incorporationof alcohols between the charged heads of the micelleis conceivable. Since redissolution of calcium sulfonateprecipitate can involve either the incorporation of theprecipitate with the calcium on the exterior of themicelle and sulfonate in the interior, or the uptake ofcalcium by the micelle and the formation of newmicelles by the released sulfonate [16] (both of which

0.8' NaOO8S I CoCtz In - Prop-.,ol

r. 25~O5-CpH858-68

3X10-lkmol/",ICOCtZ+ AI}~O6

I,....

""..:u ... . ... . I10-' 10-4 10-0 10-2

Total NGOOBS conc8n1rO11on, ". (kmol/m')

Figure 4. Effect of n-Propanol on the Precipitation/Redissolution Behavior oj'the NaDDBS/CaClz Syste.'IIS

57The Arabi411 JCXjT!I4Jlfor Science -' Engineering, Volume J J. Number J.

:-.\) 0.41-. 06 5~"-~<> 10~" - PropanolV 25~,,-P~

M. S. Celik and P. Somasundaran

involve the incorporation of Ca on the charged headof the micelle), the redissolution process can be expec-ted to be influenced by the presence of n-propanol.

1() ."'e.i'~;~==I:::;:=~~~ c ,.

~08

;- 05

~....

~«

\.i 04

Polymel'..surfactant Intenctio- in Bulk Solutioa

In micellar flooding systems, surfactant/polymerinteractions can cause precipitation of surfactant orpolymer, leading to their loss and phase separationproblems. These interactions can become even morecomplex when various inorganics are present in oilreservoirs. It has been shown that the interaction ofoppositely charged surfactants and polymers can resultin the formation of a surface active complex thatexhibits precipitation in certain concentration ranges[11-13, 28, 29]. This precipitate has also been observedto redissolve above a particular surfactant concentra-tion. While precipitation is proposed to take placeowing to head-to-head adsorption of surfactant ontothe polymer, redisolution is suggested to occur by atail-to-tail adsorption of a second layer of surfactantions [11-13]. On the other hand, we have shown thatthe interaction between sulfonate and multivalent ionscan similarly result in precipitation followed by itsredissolution [9, 10]. Unlike the case of polymer sys-tems, the mechanism of precipitate redissolution inthe above system was postulated to be due to micellarsolubilization of the precipitate. In view of thesedifferences in mechanism, it was considered importantto examine the interactions of surfactants and systemscontaining both polymers and multivalent inorganics.Chemical analysis of supernatants for calcium, poly-mer, and sulfonate under precipitation/redissolutionconditions has been carried out in otder to understandthe nature of the interactions.

NoOOBS I PAMO/NoCI500mq/kq PAMO

NqCt conc r I-C)(kmol/m') -

.. to 5;oIronote 0 ~02

1 ~~ :~:: ~. .PAMO 0 ZS 0 PAMO 10-' ZS

a PAMO 10-' 50

0.010-0 10-4 10-' 10-2

Total NoOOBS conClftlroloon !klna4/m')

Figure 5. Precipitation Behavior of DDBS and PAMD as aFunction of Sulfonate Concentration

PAMD/NaDDBS Precipitation

PAMD, being cationic, can be expected to interactelectrostatically with the anionic sulfonate in the bulksolution. In fact, during the preparation of solutionscontaining both PAMD and NaDDS, precipitationwas found to occur under certain conditions. Visualexamination clearly indicated the fibrous gel-likestructure of the precipitate, as opposed to the needle-like crystalline surfactant precipitate. Thi~ observationled to a series of tests to study the precipitationbehavior of the PAMD/sulfonate system.

mer took place upon increasing the sulfonate concen-tration and, most interestingly, the precipitate redissol-ved above a certain surfactant concentration. Thisphenomenon of the redissolution of the polymer/surfactant complex appears to be similar to theredissolution of multivalent ion/sulfonate precipitatesas discussed before.

Since the above tests were conducted without NaCl,it was difficult to distinguish the specific interactions ofsulfonate with the polymer from that owing to thechanges in ionic strength. Therefore, similar data weregenerated under controlled ionic strength conditionsusing 10-1 kmol/m3 NaCi. as illustrated in Figure S. Itis seen that both precipitation and redissolution occur,but at a reduced level under these conditions. Data attwo different temperatures also follow the same trend.These results indicate that complex formation betweenthe polymer and the surfactant can be reduced byincreasing the temperature. Similar results have been

reported by Schwuger [29].

The interaction of the polymer with the surfactantleading to precipitation can be represented as:

p." +nR~PR. (12)

K,p=[P."] [R-]" (13)

log [P;"'] = log K,p- n log [R -]. (14)

On the basis of the above, it is possible to obtaip areaction constant for the precipitation by plotting log[R -] versus log [p."]. It should be noted that such avalue can be considered only as an apparent solubilityproduct, since a number of other charged complexessuch as PR(.-%\" and PR(Y-.'- where x<n and Y>n% Ymay exist in the aqueous phase. Recognizing such aproblem, we have obtained a value of Ksp = 1.4 X 10-10

Results obtained for the precipitation behavior ofPAMD upon increasing the NaDDBS concentrationare shown in Figure 5. It is evident from this figurethat precipitation of both the surfactant and the poly-

58 The ArabiOll Journalfor Science and Engineering. Volume I J. Number J.

M. S. C elik and P. Somasundaran

Figure 6.

at the interface. The decrease in iJy/ologcT' on theother hand, suggests that monomer activity in the bulkdoes not increase in the presence of PAMD in amanner similar to that in the absence of iL This is alsoconceivable, since part of the added sulfonate is beingused by the polymer to form the polymer/surfactantcomplex.

The CMC or the PAMD/sulfonate system, as detec-ted by the dye solubilization to:hnique, coincides withthe onset of redissolution of the precipitate. Since it isknown that dye solubilization can occur much beforeconventional micelle formation [11], the dye solubili-zation observed at the onset of redissolution cannot beconsidered conclusive evidence of micelle formation. Itis to be noted that solubilization tests using hydro-carbons (octane and dodecane) show no measurablePAMD solubility, indicating that the polymer ishydrophilic in nature.

CONCLUSIONS

and n = 1.2. This value or n can be taken as anindication or 1: 1 binding or PAMD with sulfonate rorpra:ipitation.

Redissolution oCthe multivalent ion/sulfonate preci-pitates, as discussed before, occurred only in the pre-SCIx:e or micelles. In order to investigate the role or themiccUization in the present case, the surface tension ofPAMD/NaDDBS solutions was measured (see Figure6). The point of complete redissolution is also ind~a-ted in Figure 6. In the preseIx:e or PAMD, the surfacetension or NaDDBS is reduced significantly up toabout 10-3 kmoi/m3 NaDDBS. Also, the slope or thesurface tension versus log (concentration) curve isreduced in the presence of PAMD. Most importantly,the onset of rcdissolution is below the CMC of thesurfactant. This behavior is different from that obser-ved in multivalent ion/sulfonate systems, whereredissolution was found to occur only in the presenceor micelles.

Since the rcdissolution of the polymer/surfactantcomplex takes place apparently below the CMC, itmust be due to a complexation process such as:

PR.+aR -~PR::;:.. (IS)

Such a reaction can result from the chain-chaininteraction or a surfactant molecule with another sur-factant that is already ela:trostat~ally bound to thepolymer. Interaction of doubly-charged dimers (whicharc expected to form at high surfactant coIx:entrations)with the polymer can also result in soluble complexes.

The surface-tension lowering of sulfonate solutionsowing to the addition of PAMD to the system iscaused either by the 'salting out' or the surfactant orby the adsorption of the polymer-surfactant complex

Chemical interactions in micellar/polymer floodingsystems are extremely complex when considering allspecies and variables simultaneously. It was the aim ofthis study to identify some major problems responsiblefor surfactant loss in porous media. The major findingscan be summarized as follows:

(1) The interaction of sul(onates with dissolvedmonovalent inorganics shows that increasingeither the surfactant or the salt concentrationcauses sulfonate pr~ipitatioD.

(2) Sulfonate precipitation by multivalent ionsundergoes redissolution upon increasing either

59Thl ArabiGII J~rJIalfor Sci-.,e -- £",iMffiIrg. Volul9lt II. Nu-.r I.

M. S. Celik and P. Somasundaran

the sulfonate or the monovalent salt concen-tration. At higher sulfonate or monovalent saltlevels, the surfactant reprecipitates.

(3) Based on chemical analysis, the sulfonatep~ipitate in the pr~ipitation and redissolutionregions is found to be calcium disulfonate. Inthe repr~ipitation region, the pr~ipitate can beeither sodium sulfonate alone or a co-precipitateof sodium and calcium sulfonates.

(4) The precipitate redissolution occurs only in thepresence of micelles. This involves incorporationof calcium ions by micelles, followed by theredissolution of the pr~ipitate to replenish thesolution with calcium ions and the formation ofnew micelles by the released sulfonate.

(5) The alcohol n-propanol not only reduces thepr~ipitation but also shifts the CMC to a lowersulfonate concentration. Solubilization con-stants show a d~rease with addition of alco-hol, indicating competition of alcohol with CaR 2-pr~ipitate species for the micelle.

(6) Pr~ipitation/redissolution of polymer-sulfon-ate complexes shows pr~ipitation to take placeowing to adsorption of the charged sulfonateonto the polymer. Redissolution of the pr~ipi-tate, on the other hand, is suggested to occurby adsorption of a second layer with a reverseorientation. The enhancing role of NaCI addi-tion on sulfonate pr~ipitate redissolution is inter-preted to be due to a salting-out etT~t.

ACKNOWLEDGMENTS

The support of the Department of Energy (DE-AC19- 79BC 10082), National Science Foundation (78-11776), Amoco Production, Chevron Oil FieldResearch, Exxon Research and Engineering, GulfResearch and Development. Marathon Oil, MobilResearch and Development, Shell Development.Texaco, and Union Oil is gratefuUy acknowledged.Mr. V. A. Jamaludin's assistance in typing themanuscript is appreciated.

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61The Arabian JOIIrnalfor Science alld Engineering. Volume I I. Number I.