.-ADSORPTION FROM FLOODING SOLUTIONS IN POROUS MEDIA-

.." A STUDY OF INTERACTIONS" OF SURFACTANTS AND

POLYMER~ WITH RESERVOIR MINERALSBY

Prof. Ponisseril Somasundaran

School of Engineering and Applied ScienceColumbia University

New York, N.Y. 10027

ABSTRACT- -

This project at Columbia University, co-sponsored by--DOE,NSF and nine major oil companies, is directed towards the studyof interactions of surfactants with reservoir rock minerals,the understanding of which is important for minimizing the lossof surfactants. The scope of the present work consists ofstudies of adsorption and precipitation of surfactants undera wide variety of physicochemical conditions and elucidation ofthe mechanisms involved.

Our past work on adsorption/abstractiQp of Na-dodecyl-benzenesulfonate and Mahogany sulfonate on h~moionic Na-kao-linite, Berea sandstone, Agricultural limestone and Bedfordlimestone showed that the natute and extent of adsorption iss~rongly dependent on the type and concentration of electrolytes,pH, solid to liquid ratio, surfactant purity and solids pre-treatment. During the past. year, adsorption/abstra~tion ofsulfonates on Na-ka-blinite was extended to high salinity(up to 1 kmol/m3 NaCl) conditions. Kinetic and equi1ibriumstudies of adsorption of Na-dodecylbenzenesulfonate in thepresence of oil has been initiated. - -.-

. .-

A major emphasis, during the past years, was placed on thestudy of the phenomenon of surfactant prec:ipitation--'-"in the -

presence of multivalent ions and. redissolution and reprecipi-tation of it upon increasing the surfactant concentration. Pre-~cipitation/redissolution/reprecipitation of sulfonates in thepresence of NaCl, CaCl2 and mixtures of NaCl and CaCl2 was studied.Characterization of surfactant precipitation and c~ay u.sing .techniques such as SEM and EDXRF has also been initiated.Based on these studies as well as the study-of other systemproperties such as CMC and Kraft point solubility the mechan-ism of redissolution is formulated. The Na-dodecvlbenzenesul-fonate used for the adsorption and precipitation experiments

,"

conducted during the current per~od was purified by re-crystallization and characterized by using analytical LC,proton NMR, l3C NMR and mass spectrometry.

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Oil and air wettability of reservoir rock minerals trec.tedwith sulfonates has been correlated with the surfactantadsorption at the solid/liquid interface and the accompanyingzeta potential changes and the mechanisms of adsorption havebeen further developed using resultant correlations.

.HISTORY OF PROJECT

DOE began co-sponsoring the present project at ColumbiaUniversity on July 1,1979. The total cost for the periodJuly 1979 through-October 1981 is $442,119 of which DOE contri-bution is $247,119. During the first year emphasis was p~acedon the study of the depletion (loss) of sulfonates owing toadsorption/abstraction on reservoir rock minerals as well astheir precipitation by mono and multivalent ions. Also.asystematic investigation of the phenomenon of redissolution ofsulfonate precipitates at higher sulfonate concentrationsencountered during an earlier period was initiated.

RESULTS AND DISCUSSION -

I. Solution Chemistry of""Surfactant/Inorganic Ion System

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PRECIPITATION-REDISSOLUTION-REPRECIPITATION PHENOMENONA.Our past studies have shown that precipitation and

redissolution of s~onates upon their interaction with inorganicscan playa major role in determining tQeir .abstraction byminerals- Since inorganics are invariably present in significantamounts in reservoir fluids, the understanding of the precipi~tation-redissolution phenomenon is-. qf major interest.- .'

It was re~orted earlier (1-4) that mu1tiva1ent.ions ~uchas Ca2+ and A1 + will cause precipitation of su1fonates followedby redisso1ution at higher sulfonate concentrations. It wasfound that redisso1ution of the above precipitates can beachieved also by adding monovalent inorganics such as NaCl. Inorder to determine the nature' of interactions of sulfonate wi.thNaC1 and CaC12 in precipitation-redisso1ution'-reprecipitationsystems, precipitation in sodium dodecy1su1f9nate and sodiumdodecy1benzenesu1fonate solutions was studied in detail as afunction of CaC12 (in the range 10-4 to 10-2 kmol/m3) and NaC1(up to 2.0 kmo1/m3) concentrations at various levels of sulfon-ate (10-5 to 3x10-1 kmol/m3).

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In order to distinguish between the roles of variousproposed mechanisms for the redissolution (2-5) several relatedproperties were also studied for the same system. Precipi-tation was monitored by measurements of turbidity. Concen--tration of sulfonate and calcium in equilibrium with the pre-cipitate were determined by chemical analysis. Critical micelleconcentrations of all the solutions were determined by specificconductivity, Kraft point solubility, surface tension and dyesolubilization techniques. On the basis of the data from theabove measurements mechanism of the precipitation-redissolutionof sulfonate is developed. Precipitation in the tested saltconcentration range is found-.to be due to the formation ofcalcium disulfonates and obeys the equation

2eCa++ . cR - = Ksp -"

where Cta++ and ~- are the residual concentrations of thespecies and Ksp is the solubility product of the calcium di-sulfonate. - ~

Redissolution is observed to occur only in the presenceof micelles (Figure 1). .-

Correlation of the effects of ionic strength on red.is-solution to those of it on micellization supports the aboveobservation (Figure 2). From a practical point of view thepossibility of control of sulfonate precipitation with the h.elpof information on the relative~roles of various monovalentsalts such as NaCI and multivalent salts such as CaCl2 and ~C13i~ attractive. Studies in such multi-salt systems have thereforebeen given added emphasis now in this project. Sulfonatecharacterization o~L.the mono and multivalent preciprtates inions systems using .techniques such as SEM and EDXRF bas alsobeen initiated. Preliminary tests of calcium sulfonate pre-cipitation showed that calcium dodecylsulfonate is crystallinein nature. Calcium dodecylbenzenesulfonate, however, was foun~to be amorphous (see Figure 3). -'. .

,~CHEMICAL EQUILIBRIUM IN SURFACTANT SO~UTIONS AND THEIRROLE IN ADSORPTION AT IN~ERFACES

B.

The chemical equilibria in surfactant solutions whichdetermine the nature and concentration of various species in.solution can strongly influence the adsorption/abstraction andprecipitation behavior of surfactants. In the present investi-gation, therefore, chemical equilibria in surfactant solutionsare also being studied. Such studies in the past (1,2) bymeasuring the surface tension of surfactant solutions as afunction of surfactant concentration at pH 11.4 had enabledidentification of surfactant complexes such as dimers in th.e

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system. ..Similar measurements as.a function of concentrationat a pH of 9.3 and as a function 'of pH at three levels of oleate(7.8 x 10-6 kmol/m3, 1 x 10-5 kmol/m3 and 2.6 x 10-5 kmol/m3)

were conducted this year in order to study the acid-soapformation. -

The shape of the y vs. log total concentration curve atpH 9.3 did not suggest any premicellar association. However,a comparison of the saturation area per molecule at pH 9.3to the corresponding value obtained at. pH 1~.4 indicated theformation of complexes at pH 9.3 also. The value of Kad' theacid-soap formation constant-, for oleic acid was estimated f~ornthe present data to be 1.3 x 107. Species distribution usingthe above value of Kad and the earlier estimated value of Kd(the dirnerization-constant), 5 x 103 , has been obtained as afunction of pH. _0

Surface tension as a function of pH exhibited a minimumin the neutral pH region-and the position of minimum wa~ foundto shift to higher pH values with increase in total surfactantconcentration. This has been attributed to the maximum changesin the activities of oleate. species in this region caused bythe precipitation of the acid form. Interestingly, the dependenceof pH of minimum surface tension and that of minimum in waterwettability of hematite (as measured by flotation) on totalsurfactant concentration are found to be identical .in nature.This reveals the important role of chemical ~uilibria of sur-factants in governing interfa~al processes such as adsorptionand wetting.

II. Adsorption of Su~factants on Reservoir Minerals

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Loss of surfactant upon its contact with rese~oir mineralresul ts, in addition to. precipitation "and entrapment, alsofrom adsorption. Previous studies (1,2) ha_ve shown that the

adsorption of sodi\nt\ dodecylbenzenesulfonate (NaDDBS) and Mahogany sulfonate (MS) on Na-kao~~nite and Berea sandstone is.

a complex process dependent upon a number of system. parameterssuch as salinity, type of inorganic ions in solutioh, sur- .

factant purity, and solids.pretreatment. In addition, the iso-therms exhibited a maximum followed in some cases by a minimumin the region around critical micelle concentrations. Most

interestingly, desorption studies also showed the system tohave significant memory effects.

Our previous investigations (1-3), thus, clearly demon-strate the complex nature of abstraction processes. Elucidationof mechanisms of adsorption in such complex systems necessitatesthe use of characterized surfactants and minerals. A majoraim of the present investigation has, therefore, been to s.~udy

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the phehomenon of abstraction/ad~orption in reservoir rockminerals using purified and characterized minerals and sur-factants.

Kaolinite treated with NaCl to make it homoionic andcharacterized using techniques such as SEM and EDXRF has beenused as the solid for all our abstraction/adsorption studies.

Purification and characterization of NaDDBS have receivedmajor emphasis in our work. Sulfonates were deoiled and sub-sequently recrystallized and~purified using prep LC. Theanalysis of the products using ALC and ~NMR showed that theprep LC product obatained, at least using the columns andsolvents tested h~re, is not superior to the recrystallizedone. Since the yield of sulfonate by the recrystallizaticn-~desalting '~ocess was higher than that "obtained using the prep LCtechnique, the former technique has been adopted for the puri-fication of sulfonates. - ".

Characterization of the recrystallized-desalted product13C N}1R and Mass spectrometr~ have shown the purified NaDDBSto ccr.tain 1,2 and 3 isomeric impurities and .as much as 10%C-13 sulfonate. Most importantly such characterization studieshave led to the novel technique of the use of sulfonic acidsfor 13C NMR and Mass spectrometry. The low-solubility of su1-fonates in CDC13 and the decomposing nature 6f it upon heatinghad resulted in the limited use of these techniques for thecharacterization of su1fonates~in the past.

~

Abstraction, precipita~ion and adsorption studies in thepresence and absence of oil were continued during the currentyear, particularly ~nder high ionic strength conditiQns (0.5krnol/m3 NaCl). Both abs.t~action and adsorption (in the absenceof oil) were found to increase ~harply in t~e precipitationregion. . ~.-

- .

Abstraction isotherms of NaDDBS in the presence of n~octaneand n-hexadecane (1-5\ by volume) exhibited pronounc~d maximum.Most interestingly, the abstraction increased with increase inoil concentration but only b"elow the maximum (Figure 4). Thishas been attributed to the eru~anced adsorption of NaDDBScaused by the synergistic co-adsorption of the non-ionic oilmolecules with the ionic sulfonate species. "

-Dependence of abstraction an pH is found to be similar in

the presence and in the absence of oil; abstraction is found tomarkedly decrease with increase in pH.

:. Chemical and physical characterization of the precipit~teformed upon contacting clay with surfactant solutions duringadsorption/abstr~ction tests can be useful and important in

5

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elucidating the mechanisms involved. In the present investi-gation this is being attempted using SEM and EDXRF techniques.

Precipitate obtained from an aged sulfonate/clay system-by scanning electron microscopy showed the presence of coarsecrystalline and fine amorphous precipitates. Upon EDXRFanalysis, the coarse precipitate showed the presence of Na,Cland S and the fine precipitate showed the presence of Ca, Ai,Si in addition to Na, Cl and S (see Figure 5). The results ofSEM, EDXRF reported here are exploratory in nature and furtheranalysis of fresh and washed precipitates and of clay is inprogress at present. ~

III. Wettability of Reservoir minerals and Correlation With.- Surfactant Adsorption

Wetting characteristics of reservoir rocks and theiralterations upon micellar flooding are of importance for studyingthe performance of oil recovery systems. In micellar floodingsystems adsorption or precipitation of surfactants on rocksurface can change the wettability of the rock and therebyaffect the oil recovery. In-the present study, therefore,changes in wettability of reservoir rock minerals upon con-tacting them with surfactants is being investigated.

Wettability is usually determined by means of contact angletechniques. This technique however requires'a number of pre~treatments such as polishing aDd leaching. All these treatmentsare known to affect the surface roughness as well as the sur:fac~composition of the sample significantly. A method that appearedpromising for measuring wettability is flotation using air oroil extraction. Air flotation is done in a Hallimond cellwhere the particles which acquire hydr~phobicity ar~collectedseparately. Oil extraction, on the other hand, requires onlya separatory funnel and a high ,speed stirring mechanism. Again

. ~.-the partl.cles that are hydrophobiC;. ~re collected in the oil.."phase. Both techniques, unlike contact angle, do not requiredrastic pretre~tments such as polishing and can be ~tilizedunder variable conditions of pH, temperature, ionic strength' etc

Tests performed using l~estones (the Agricultural andBedford limestones) Berea sandstone and kaolinite with Mahoganysulfonate-AA indicate good correlation between wettability asdetermined by flotation and adsorption density. Both aoirflota~ion and adsorption exhibit maxima in the same concentrationrange. Zeta potential of particles measured under the sameconditions, however, shows a continuous increase in its negativevalue with sulfonate concentration. Correlation of these datasuggests that sulfonate monomers adsorb in increasing amountsbelow as well as above the critical micelle concentration ~itha reverse electrostatic exclusion of micelles. Therefore, the

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decrease observed in measured surface exce~s can be possiblydue to micellar exclusion and precipitate redissolution. Oilextraction exhibited also a maximUm with increase in surfactantconcentration. The maximum however ~ed at lower surfactantconcentrations than that of air flotation.

Both air flotation and oil extra'ction appears to bepromising techniques for the measurement of wettability ofreservoir rocks. Adsorption, wettability and zeta potentialtogether can yield more insight into the system behavior thananyone technique alone. In the present study a multifacet-approach will therefore be continued. .

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IV. Flow Distributions in Consolidated SandstoneUtilizing a Tomographic Technique*

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A complete understanding of the interactions of ~a~twith reservoir rock minerals demands also the k~ow-ledge of oil displacement in the field.

Computed tomography, which is a non-destructive and a non-invasive technique, is very effective for studying the complexmechanism of oil displacement. in the field of Enhanced OilRecovery. Using computed tomography (CT), it is possible tostudy the transient flow distributions as oil displaced byaqueous phases in consolidated cores without interrupting oraffecting the recovery process. -' .

In the present investigation,computed tomography is beingused to observe the dynamic displacement of oil from Berea.core. The technique has been calibrated to yield quantitativepore composition data over t.he cross section of the core witha resolution of app~oximate1y 10-6m2. The observation of dis-placement of oil wit~ brine solution (1M KI) and wit~po1y(acry1amide) solution, indicates that there exists a signifi-cant distribution in core porosity. This is_linked to a strati-fication in the sandstone. The flowing polymer/oil interface"-exhibits finger to a much less degre"e than does the brine-oil .'

interface. ~ '

Further work using computer tomography is in progress atpre£:ent. The experimen~al data from tomographic examination isexpected to yield interesting information regarding oil dis-placement.

* Done under the supervision of Prof. C.C. Gryte.

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ACKN OWLEDGEMENT S

The author is thankful to Dr.;R.P. Ananthapadmanabhan, Ms.S. Ayral, Mr. M.S. Celik, Mr. P. Chandar, Dr. E.D. Manev, Ms. S.V.Sazdanova-Maneva and Dr. R.V. Viswanathan for their contri-bution to this work. The author wishes to acknowledge the -

support of DOE, NSF and Amoco Production Company, Chevron OilField Research Company, Exxon Research and Engineering Company,Gulf Research and Development Company, Marathon Oil Company,Mobil Research and Development Company, Shell Development Company,Texaco Inc. and Union Oil Company of California.

REFERENCES

Somasundaran, .P., -Adsorption from Flooding solutions inPorous Media, II Annual Report submitted to NSF and a Con-sortium of Supporting Industrial Organizations, ColumbiaUniversity, New York, July 1978.

1.

2. Soma~undaran, P., "Adsorption from Flooding Solutions inPorous Media," Annual Report submitted to DOE, NSFand a Consortium of Supporting Industrial Organizations,Columbia University, New'York, July 1979..

Celik, M., Goyal, A., Manev, E. and'Somasundaran, P.,..The Role of Surfactant Precipitation a~d Redissolutionin the Adsorption of Sulfonate on Mineral$,. SPE Paper8263, Presented at the 54th Annual SPE Meeting, LasVegas, 1979. ~

3.

4. Ce1ik, M.S., Somasundaran, P., and Ivanov, I.B., "On theSolubility and Precipitation of Dodecy1benzenesu!fonateSalts in Oil Recovery Systems," presented at the' 3rdInternational Conference on Surface. and Colloid Science,August 20-25,1979, Stockholm, Sweden.

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O'Brien, E.F. and Wiers, G.H.,..:."precipitation Boundariesin Ca1cium-A1kylbenzenesu1fonate Systems," preprints,48th National ColI. Symp., 220 (1974). .~ I

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