Asphaltene Adsorption and

Desorption From Mineral SurfacesS.T. Dubey, SPE, Shell Development Co , and M.H. Waxman, SPE, Consultant

S)% 1f+%?Summary. This paper reports results of asphaltene adsorptiomfdesorption on clay minerals, silica, and carbonates. It afso describes

the effect of adsorbed asphaltenes on rock wettabtity aod a screening pyrolysis-flanw-ionization-detection (1-FfD) test to evaluate theability of solvents to remove asphaltene from kaolin arid formation core material.

Introduction

Reservoir nettability is a major factor controllkg the location, fluiddistribtuion, and flow properties of the system. Wettabtity condi-tions affect formation capillary pressure and relative permeabilitybehavior, electrical properdes, and residual oil saturations. The wet-tabiMy of originally water-wet mineral surfaces maybe reversedby adsorption of polar organic compounds in cmde oils. The highestconcentra,don of polar organic compounds generslly is found in theheavy ends of cmdes, particularly in the asphaltene and resin frac-tions. Nettability alterations of oil-bearing formations, particular-ity those containing clay m@erds, have been amibuted to adsorption

of these compounds onto mineral surfaces. 1.4 Because of the highmolecular weights and multifunctional chzacter of asphaltenes andresins, their adsorption by specific minerals is a major element innettability changes and was therefore selected for study.

Signhlant factors that conkol adsorption of asphaltenes and resinson mineral surfaces are (1) the presence, thickness, and stabilityof water films on the mineral surface; (2) the chemical and struc-tural mture of the minerti substratq (3) asphaltene and resin con-tents of the cmdq (4) the presence of asphaltenes and resins in crudeoils in the form of colloidal micelks or ag~egates; and (5) the abifkyof the hydrocarbon fraction of the crude to stabfize these colloidal

aggregates ~, the oil or even to dissolve them into true solution.Fmther, speafic asphaltenehninerd interactions control the degreeto which such adsorption is irreversible with respect to various or-ganic solvents and, hence, may determine optimum Iaboratow core-cleaning procedures.

A varietyof physici?l measureinente, including molecular-weightdeterminations in various solvents,5 have deduced that asphaltenesassociate or form aggregates even in dilute solution.

Moschopedis et al. 6 showed that intermolecvh hydrogen bond-ing is involved in asphaltene association and is reflected in theobserved molecular weights. Vapor pressure osmometry determi-nations in nitmbenzene (6,=34.8) produce molecular weights of1,650 to 2,100 compared with 5,000 to 6,700 in benzene(e,= 2.3). These nitrobenzene-derived molecular weighs may bethe molecuku weights of the individual asphaltene particles.

The power of various solvents has been expressed in terms ofHildebrand volubility parameters, & The relation between 6 andasphaltene volubility was continued by Mitchell and Speight. 7They compared the weight of asphaltenes separated from Athabas-ca bitumen with a series of solvents (solventlbimunen volume ratioof 40: 1) with the maximum asphaltene precipitate obtained by ad-dition of an excess of n-pentane. The weight percent of precipitat-ed asphaltenes decreased linearly with increasing & Completesolubilization of tie asphahenes in the bitumen was obtained forsolvents with 828.4 calA cm-3/2.

For our initial adsorption experiments, the effect of solvent var-iation for asphaltene adsorption on the clay mineral kaolin was ex-smined. The solvent series toluene/n-dodecane at 1.75 to 1.fs2 wthvttoluene and chloroform was chosen primarily because of ifs increas-ing & The tolueneln-dodecane mixture of aromaticliliphatic sol-vents has a 6 close to, the finking value required for completesolubilization of our asphaltene sample. Chloroform is a protondonor in hydrogen-bond formation and bas a somewhat higher di-electric constant and dipole moment Omn the other two solvents.

CoPyrighl 1991 Sm{ety of Petroleum Engine.,,

Asphaltene adsorption studies were also carried out with a widevariety of other mineral and clay absorbents from toluene solution.CoUins and Melmse, 1 Clement?s and Cuiec9,10 indicated that thepresence of a thin iibn of water on the mineral surface reducesasphaltene adsorption and can affecf the kinetics of adsorption. Toestablish base cases for further work, the presence of water, as wellas resins, was excluded from all systems.

The effects of various solvents on the desorption process wereafso examined to evaluate their effectiveness in core-cleaning op-erations, Solvents were chosen on the basis of their 6 values, polarcharacter, and hydrogen-bonding capabilities.

Experimental

Asphaltene Adsorption on Kaofin From Solution. Adsorptionstudies were carried out with a tar-sand-derived asphaltene (n-pentane insoluble) sample in different solvents and kaolin clay min-eral (from J.T. Baker Chemica3 Co.) as the adsorbent. TabIe 1 givesthe asphaltene elemental analysis. Elemental analysis and X-raydiffraction (XRD) indicate that the clay is predominantly in the so-dium form, consisting of 15 % illite The Bn+auer-Emmett-Telle~@ET) surface area, with N2 gas, was 11.9*0.4 m2/g (five de-terminations). The cation-exchange capaci~, measured by Ba/Mgconductimetiic titration, was 4.572 meq/100 g.

The solvents chloroform and toluen. were analytical reagent grade@KUickmdt Co.) and contained c 400 and 200 ppm water, re-spectively, according to Karl Fisher titration. n-Dodeca.ne (AldrichChemical Co.) was 99% pure (with

TABLE 1 ELEMENTAL ANALYSIS FOR ASPHALTENES

Amount

Element (wt%) Atomic Ratio ,

c 79.35 H/c 1.21,H 8.05 N/c 0,0097,~,.

,.. ~~ 9.2 SIG 0.04340 2.26 o/c 0.021

N 0.9Ash (Ni,w 0.24

Total 100.0

ous solvents, followed by vaiuum-dryiig of the samples at 110C.P-FfD pyromm were obtained and compared witi pyrogm.m$ fromthe original core material to evaluate hydrocarbon amounts remain-ing on tie mineraf surfaces after the various extractions, giving anes-&ate of cleming efficiency.

Nettability Determination

The effwt on nettability of adsorbed asphaltenes on &r&? and Bent-hebn sandstones was measured by the U.S. Bureau of Mines(USBM) method developed by Donaldson et al. 14 Higfdy water-wet plugs were obtained by heating at 550C for 24 houfs.Asphaltene treatment was made by exuosing evacoated cores to 2. ....

surface areas were measured with either nitrogen or krypton. Com- wt% a.sphaftene in toluenc solutio~ fo~ abo~t 24 hours. The cores

plete adsorption isotherms were obtained for mog mineral$ other- were toluene-ffushed IO@ no color was visible in the effluent. Coresu+se, adsorption levels atone high asphaltene concentration (2,500 were then evacuated and dried at ambient tempsramre. For tie Bent-ppm) were ttien as monolayer coverages. beim sand GET surface 2rea=0.49 mzlg), the resulting asphaltene

adsorption level was 0.74 mglm2.

Desorption of kphaftenes From Kaolin With Vafious Solvents.A desqtion isotherm nnd hysteresis loop were obtained withtoluene solvent, fnitial equilibration was carried out with a 2,545-ppm asphalteneltoluene solution and a kaolinlsolution ratio of1:100. Si@Im equilibration and analytical procedures were usedas in the adsorption measurements. After equilibration, a knownweight of ~olution was, removed and the remainder diluted with aknown weight of toluene for the next desorption step. The proce-dure was terminated when the final equilibrium aSph21tinelt01ueneconcentration was -160 ppm.

Irreversible adso~ion effects were examined with toluene as thereference solvent. Atler reicfdng equilibrium at various asphzkene/solvent concentradons, the kaofin samples were repeatedly washedwith fresh toluene until the toluene showed negligible traces ofasphaltenes, as determined spectrophotometrically. The sampleswere subsequently vacuum-dried at 110eC and examined by P-FfD.P-Fff3 pyrogrwm were obtained for the kaolinhphaltene samples,the original kaolin mineral (blank), and the asphaltene solute. Theamount of asphaltenes irreversibly adsorbed by kaolin was obtainedfrom these data.

The effect of differmt solvent washings on irreversible adsorp-tion was examined in a sbnik manner. Initial adsorption on kao-lbt was carried out with 3,000-ppmaspbaftme intoluene/ndodecane(1.75: 1.00 wthvt) soIutioi. Subsequent washings were conductedwiti a variety of solvents, and ,the remaining %phaltene adsorbatedetermined by P-FfD methods. In some cases, Soxhlet~ extrac-tion methods were used to check the multiple washing procedure.;good agreement was obtained.

P.FID Method for Evaluation of Core Cleanfitress

The use of the P-FID method for rapid evaluation of solvent effi-ciencies in core-cleaning operations was also examined. Tbe P-FfDmethcd measures the pyrolyzable organic material in t&e sample.Details of the experimentnf method are described elsewhere. 13Multiple samples were subjected to extensive extraction with vari-

Results and Discussion

Asphaltene Adsorption From ScIhItfOn. Table 2 leeO* Bic~rdtitration results for cblorofonn, toluene, and tolueneln-dodecanemixtore solvenb with asphaltene solute. Practicaf volubility pamm-eters, cot 0, for this solvent series are propotionaf to Hfldebcand& vafues for these solventsi. e., 3(c2fh cm-3/2)=0.75 cot0+8.6. Note that these asphaltenes are completely solubilized onfyin the toluene/n-dodecane mixture at the lower limit of 8 = 8.7@J% .cm3n (f?=57.

Fig. 1 shows adsorption isotherms for the asphaltenes iu a va-rie~ of solvents on kaolin mineral as adsorbed asphaltenes vs. equi-librium asphaltene concentration.

For the th~e nonpolar solvents, asphaltene adsorption foflowedLangmuir Type 1 iiotlefms, indicadng monolayer adsorption. Max-imum aspbaftene adsorption (at the isotherm plateaus) decreasedwM increasing 6 in the order toluenelndodecane, toluene, andchloroform-29 .8, 26.4, and 23.8 mg/g, respectively.

The lineaf transform of the Langmuir equation is

cla=(cl17J+(11a&), . . . . . . . . . . . . . . . . . . . . .. . . . . . ...(1)

where a= asphaltene d.$orpdon in qydlibrium with asphaltene con-centration C, aS =saturated asphaltene adsorption at ~ompletemonolayer coverage, wd K=rati~ of rate constants of the adsorp-ticmldesorption reactions. us values are generally in good agree-ment with plateau values (Table 2).

Clementz2 repotted asphaltene adsorption studies from varioussolvents under near-anhydrous conditiom, montmorifkmite in sever-al ion forms was the adsorbent. Czamecka and Gif10tt3 describedsimifar work. Both observed the irdlucnce of solvent carriers, butunique correlation of adsorption levels on monmoflonite with 30l-vent solubfity parameters was not obtained.

As obtained from Lnngmuir Type I isotherm plateaus, the weightper unit surface ma of the adsorbed asphaltene monolayer increasedfrom 2.0 to 2.5 mglm2 with a decrease in 6 from 9.3 to 8.7

TABLE 2Adsorption OF Asphaltenes ON KAOLIN FROM NONPOLAR SOLVENTS

Ma%imum AsphalteneAdsorption Level

Per Unit Per Unit

6Weight Surface Monolayer

of Mineral Area Thickness

Solvent (Ca,vt .~~ -?J2) (mg/g) (mglm) (rim)Chloroform 9.3 23.8 25.8.. 2.0 1.757 2,4*Toluene 3.9 26.4 26.2 2.2 1.9 2.7

Toluene/ 8.7 29.8 30.1 2.5 2.2 3.0

n.dodecane(1 ,75:1.0 Wuwt)

. From isotherm ply 1,,.1..Yxlculeted mm Ihnw transform 0+ L.a.wnulr Isotherm, Ok. (clas) + (n@).Calc!a!ed from uncorrected BEl $fia.~ area of 11,9 KIzlg desi@i d mphaltaes= ; ,145 glcma* Awmms.0 edge sdsrptbn Wth planar wfaee ares. 0,9 d fold surface arm also, whalt.ne packing fraction. 0,8.

!~

-80

~ 30-

1

; 24-60

z

: 20-3.

~ 16- -40

:

s 12 Q TOLUENE/-DODECANE !1.,5.1.0 WIW):

0 TOLUENE- 20

A CHLOROFORM:

$4>.

;,~ 400 800 1200 1600 2000 2400 280:cz

~ EQUILIBRIUM C0NCENT8&T10N OF ASPHALTENES, ppm _

Fig. IAdsorption isotherms for asphaltenes on kaolin from different solvents.

of 1.145 g/cm3, the uncorrected monolayer thi&ess increase~from 1.75 to 2,2 nm over the same solvent sequence.

Afthough older literature 15 represents asphaltenes as large poly-nuclear aromatic systems iri flat sheets with some aliphatic sidechains, asphaltenes in solution are being presented more as colloi-daf polydispersions in size and shape comprising large flat ag.gregates with typicaf dimemions of 0,7x 15 nm, 16 withoutdiscrimimting whether these are ve~ flat spheroids or very thindisks.

On the basis of XRD studies, 15.17 the asphaltene clusters aremodeled as disk-like structures with diameters k therange of 2.2to 3.7 nm. An aggregate is composed of a number of lamellae sepa-rated by an interfamellar distance of 0.36 to 0.38 urn, with eachLamella consisting of condensed polynuclear aromatic rings and var,ious afiphatic sidechains. Following Coflins and Melrose, 1 we as-sume that asphaltene aggregates are adsorbed with the disk facesin contact with the planar faces of the clay particles; further, thatthe planar surface area of the clay is about 0,9 of the totaf clay sur-face area (excludlng edge surface area) and that the adsorbedasphalene disks have a packing density of approximately 0.8. Withthese assumptions, the corrected monolayer thicknesses increasefrom 2.4 to 3.0 w (an increase in the number of lameflae fromabout 6 to 8) with decreasing solvent volubility parameter. Alter-mtively, assuming that the average diameter of the asphaltene disksis = 3 mn, the apparent molecular weight of the asphaltene ag-gregates increases from about 11,700 to 14,600 with a change insolvent carrier tlom chforofonn to the toluenelndodecane mixture.

These results are consistent with various molecular-weight de-tmminations that suggest that asphaltenes form molecula aggregatesand that this association is influenced by the nature of the solventand the asphaltene concentration, Fmkmam et al., la in isolatingasphakenes from Athabasca bhumen, found that 20 to 21 % of thebitumen is Iow-molecular-weight (= 1,200 daltons) resinous ma-teriaf containing oxygen functiomf groups, such as hydroxyl, car-bonyl, and suffoxide groups. The presence of these small moleculesin the p~sence of n-pmtane-precipitated asphaltenes means thatthey are part .of and in dynamic equilibrium with the asphaltenemicelles. These smd polarlpolarizable molecufes must contributeto the solubdization of asphaltenes in bitumen or in the nonpolzsolvents of thk study, The major factor contdfing asphaltene aggr-egation in nonpolar solvents involves intennolecufar hydrogen bond.ing between -OH and N-f groups present in the asphaltenestructures. 6,19 Hydrogen bonding may also be a major mechanism

40 ~

m

35 -

t30

$,

$/m

.

: 25*l O

~

R .> _

: 0

3 20.

0

R 0 BEREA SANDSTONE c-109 MESHIic ,5

A DICKITE [WISCONSIN]

i0 DOLOMITE [DOLOCRON1v OTTAWA SAND [SUPER r- .32s MESH:

: 0 CALU7E [DOVER CHALKIF ,0 9 KAOLIN MINERAL

m ILLITE [BEAVERS 8ENOI

0

5,

0 1000 2000

EQUILIBRIUM ASPHALTENE CONCENTRATION, PDrn _

Fig. 2Adsorption of asphaltenes on clay and mineral swfaces from to[uene.

for adsorption of a.sphaftenes on sifica and clay minerals.Asphaltenes precipitated in tie presence of clay minerals have shmmdistinct infrared adsorptions in the 370g to 3600 and 1100 to 1000cm- 1 regions owing to the stretching vibrations of Si -OH andSiOSi groups. 12 However, if hydrogen bonding is a signifi-cant adsorptionmechanism, it cmdd influence selection of efficientsolvents for the asphaltene desorption process and eventual core-cleaning operations.

TABLE 3ADSORPTION OF ASPHALTENES ON VARIOUS CLAYSANO MINERALS FROM TOLIJENE SOLUTION

Surface Asphaltene Monolayer

Area WeightJWeight Weight/Surface AreaMineral Adsorbent (mzlg) (mglg) (mgfm2]

Illite (Beavers Bend, OK) s 39.2 1.1Kaotin mineral (J.T. Baker) 11.9 26.2 2.2Calcite (Dover chalk) 1.71 5.s 3.4Ottawa sand (Super X) 1.69 3.70 2,2Dolomite (Dolocron) 2.67 3.75 1.3Oickite (Wisconsin) 0.66 1,64 2.1Berea sandstone (> 100 mesh) 1.48 1.52 1.0Alumina (Type A, Fisher Scientific) ~ 15.9 19.2 1.2

TASLE 4-ADSORPTION OF ASPHALTENES ON VARIOUS CLAYSANO MINERALS FROM TOLUENE SOLUTION

(Based on adsorption from a.sphalteneltoluene solution, 2,500 ppm)

Surface Assumed Asphaltene Monolayer

Area WeighUWeight Weightk3urfaoe Area.

Mineral Adsorbent (mZ/g) (mglg) (mg/m2)

Alumina (Type B, Fisher Scientific) 114 174.7 1.5Calcite (Omya Carb UF) 12.3 21.3 i .7Kaolinite (Twiggs) 10.5 20.8 2.0Thuringite+ 9.1 a.i3 0.s9Chlorite 3.0 0.73 0,24

.Valuea in Ref. 13 are incarwt.. . LeaChad Wlh buffered sndl.m dlt Monlte to remove imn .X(dm and water Wuhed.

Direct comparison of our data with data of Collins and Mehoselis possible o~y for asphaltene adsorption from toluene solution ondry kaolinite. CoLiis and Melrose used two kaolinite with sur-face arias of 19.3 and 18.0 m2/g. Iangmuir &p I isotherms werealso obtained with maximum adso@ion levels of 30 and 20 mglg,respectively. These vzakes bracket mmresults (26.4 mglg); how-ever, because of the higher surface areas of their samples, adsorp-tionson asurface area basis were considerably lower, 1.55 and1,11 mg[m2.

Adifferent isotherm, Langmuir Type Il, indicating multilayer.tisogtion of asphaltenes on kaolin is obtained from nitmbenzene

.solution (Fix, 1). Amhdtme Aotionlevels medwsiztiGwfivhigher for &obeoz~ne than for tb~ nonpolar solvent series. fniti~layer coverage is complete at a level of about 65 mg/g kaolin or=5.5 mglmz, amounting to an adsorbed layer thickness of about4.8 run.

Evidence of at least a pwtial degree of ionization as well as in-creasing rnolecuku dissociation (greater mmberofp articles)of

,0

m

6

. ,00 !0,0 ,500 20.. 2500,$,ALT.., ,.lL,,.,,. .mcwm,,,m, ,,,-

Fig. 3-Adsor~iorddesorp fion hysteresis for asphaltenes on

kaotin from tcduene.

asphaltene solute with increasing dielectric constant of solvents hasbeen described by Eldib20 and Penzes and Speight. 21 They found2i@ku!dY higher SpE&C COnduCt2nCZs in nitrobwem and pyi. .dine compared with those in benzene and toluenee.g., conduc-bmces of about 0.04 x10-6 and 2..5 x 10-6 S/cm for 3%a.spbaftene solution in benzene and mitio6.mzene, respectively. 20This suggesk a predominantly ionic mechanism for mukilayerasphaltene adsorption from titrobenzene solution, involving posi-tively charged basic oitmgenous aod negatively charged acidic oxy-genated groups, botk present in the asphaltene condensed-ringshucture. The initial adsorption step probably consists of preferentialinteraction between positively charged asphalrenic nitrogen groupsand negatively charged sites on the clay mineral surface. Additionaladsorption would follow owing to attraction between these posi-tive and negative charges on the asphaltene molecules themselves.

Asphaltene adsorption isotienns from toluene solution were ob-tained for other cIay and common mineral substrates (3%ble 3).Lmgmuir Type I isotherms (Fig. 2), indicadng monolayer adsorption, were observed with all Omse mineral absorbents. Adsorptiondata are summarized in Tables 3 and 4. Chlorite bad the. lowestasphaltene adsorption level, 0.24 mg/m2; however, thuringite, achlorite clay minemf with hig3 Fe + + content, adsorbed about 3.7times this amount of asphaltenes.

Desorption of Asphaltenes From Kaolin Surfaces. A desorptionisotherm was also obtained on kaolin for the asphaltenes in toluenesolution. Ffg. 3 shows the adsorption and desorption branches.Asphaltene loading on kaolii in the deso@ion process decreasedfrom a maximum of 26.0 to 16.6 mg/g with decreasein equilibri-um asphaltene concentration fmm 2,310 to 160 ppm.

This hysteresis path is opposite to the usual hysteresis relationobserved for gas and vapor adsorption on solid surfaces, where thedesorption branch is displaced to th+ left of the adsorption patlui.e., at constant adsorbate loading, the desorption partial pressureis less tian the equilibrating partial pressure required for the ad-sorption step. We suggest that sorption hysteresis for the asphaltenetoluene sobxion on kaolin reflects the asphaltene concentrating de-pendence on the average number of lamellae of the asphaltenemicelle in solution. This solution is, in turn, in equilibrium with(and determines the size. and weight of) tie adsorbed asphaltene

392 3PE Zeservoir Engineering,. A.gust 1991

TABLE 5IRREVERSIBLE ADSORPTION OF ASPHALTENES ON KAOLIN FROM TOLUENE I

Equilibrium

Concentration of Asphaltenes

in Toluene Solution

(ppm)

46

375

593

1,381

2,160

By ab$nrbmm method,. .Zy P.F[D methad,

IrreversibleEquilibriumAdsorption

Level on Kaolin

(mglg)+

17.1

25.926.8

26.326.1

AdsorptionLevel Afler Asphaltene

Prolonged Toluene RetainedWashing After Washing

(mg/g). - (%)

49.0

TABLE 6IRREVERSIBLE ADSORPTION OF ASPHALTENES ON KAOLIN AFTERREPEATED WASHINGS WITH VARIOUS SOLVENTS OR SOXHLET EXTRACTIONS

Initial Final Asphaltenes

Adsorption Adsorption Retained

Solvent (~a, h .~m -312) (mglg) (mglg) (%)

Freon 11 7.6 30.7 ~ -1oo

Tolueneln-dodecane 8.7 30.8

(1 .75:1 .0, Wtlwf)To)uene

Chloroform

Chloroform/acetone

(70/30 Vollvol)Methylene chloride

Chloroform/methanol(87/13 VOl/VOl)

Pyridine

Chloroformlmethanol

azeotrope

(76.7/21.3 VOi/VOl)f.litrobenzene

Chloroform/methanol

(70/30 Vollvo[)To[uenelmethanol

3,9

9,2

9.4

9.8

10.5

10.6

10.9

11.1

11.5

13.7

30.s29.230.630.7

30,0

30,s

29.9

30,6

30.6

30.332.8

32.8

30.629.8

30.8

30.0 .,

31.7

25,5

24,6

13.3

17.9

16.2

8.2

21.422,1

21.2

16,2

6.74.12.41.61.0

=1821.5

6.9

11.2

22.3

62.6S4.259.856.3

52.927.3

69.5

73.9

70.952,9

21.913.5

7.94.9.3.0

58.472.1

22.4

37.3.

71.9

3ze0tr0p0

(29.1 /70.9 VOUVOI)

. m&ePeae18~8:l$9s.

Now Initial adswplim on 811 kaolin samples @inept chlomfom &d chlomfmmlmethanol @ 87/13 VOI from 27,000PPm) from 3,000 mm asphaltene solution In tol.enelndodecane [1.75 :1,0 wvwi).

TABLE 7SOLVENT CLEANING OF SILICEOUS CORE MATERIAL,P-FID SCREENING METHOD

Solvent

NonePyridineTetrachloroethy16ne(Step 1) Toluene(Step 2) Chloroformlmethano[

azeotrope

(Step 1) Methylene chloride(Step 2) Chloroform/methanol

3ze0trope

(Step 1) Chloroform(Step 2) Chloroform/methanol

azeOtrODe

Chloroform/methanol meotrope

Pyrolyzable Hydrocarbonsby P-FID (wt%)

I TABLE 8USBM NETTABILITY TESTS,BEREA AND BENTHEIM SANDSTONES IUSBM Index, /

Treatment Berea Sands Bentheim Sands._

Heated, 550eC + 0.97 + 0.90+ 0.99

Adsorbed asphaltenes +0.12 + 0.4s+0.51

aggregates on the kaolin surface. The average number of lamellaein the asphaltene aggregate is a function of the asphaltene concen-tration io the solvent, as well as the solvent species, increasing withincreasing asphaltene concentration in a given solvent to some max-imum value. Thus, in the desorption step, the presorbed aggregatesmust reattain equilibrium withlfmse in a more dilute solution com-pared with the eriginul adsogxion step or with asphaltene CIIMWS

composed of fewer lamelfae than the more. concentrutod solutionsused in the adsorption measurements.

The desorption prucess is funber complicated by a considerabledegree of irreversible asphaltene adsorption on kaolin, even ut?erprolonged extraction with toluene, as shown iu Table 5. ToluenesoIvent was used because it is commonfy used in Dean-Stark ex-tractions for core saturation det~ination.s. On averuge, 83% ofthe asphaltene was irreversibly udsorbed with respect to toluenesolvent. This suggwts dmt the desorption isotherm (Fig. 3) mustcross the initiul portion of the adsorption cuive and intersect they axis at an asphaltene loading somewhat greater than 14 mg/g.

The tendency of the Dean-Stark extraction procedure to comrefi01igiWdbJ water-wet cores toward oiI-wet states has been describedby a number of workers.22 Ths can be expluined by (1) the abili-ty of toluene to remove water from mineral surfaces (solubili& ofwater in tolucne increases from about 0.05 to 0.38 g water/100 gtoluene with au increase in temperature from 25 to 90 C), thus per-mitting direct contact of the tolueneloil solution with dry mineral;(2) toluenes abfity to solvate and disperse asphaltene and resinag~egat~ in tbe cmde ofi and to facilitate their adsorption m thedw fi~e~ surfaces: @ (3) the inabifity of toluene to reverse theadsorpuon process sigmficantiy, at least on a representative claymineral substrate A similar conclusion was advanced byClementz,2 on the basis of water so[ubifity and solvation proper-ties of tohmw,

Table 6 shows the effectiveness of solvents in the removal ofadsorbed asphaltenes from kaofin, The solvents used, like toluemand metbylene chloride, were selected on the basis of low polari-ties and high 6 vafues. Nhrobenzcne was chosen because of its highdielectric constant and potentiid for inducing iouizudon and disper-sion mechanisms, Cbforofonnlmctha.nol mixtyres were chosenpredominantly for their high hydrogen bonding capaboities as both

proton donors and acceptors, and pyridine was chosen because ofits high dielectric constant und its capability as a proton acceptor.Of these solvents, the most efficient was the cbforoformlmetbanol~eotropc, which removed 96% of the adsorbed asphakenes, fol-lowed by pyridine, ,witb g average removal of 85.6%. Solventswith strong hydrogen-bonding capabilities are probably reguiredfor effedive rernowd of adsorbed asphaltenes because the monolay-er adsorbed from nonpolar solvents consist largely of hydrogcn-bonded asphaltene Iamellae.

P-FtD pyrogmms for uII solvent-washed asphakenelkaolin sofidsdescribed above were simifar to the pymgra.m for pureasphaltene, so no selectivity in asphaltene molecufar weights oc-curred as part of the adsorption mechunism on kaolin from nonpo-lm solvents.

Extraction Efficiency by P-FfD. Maiy speciul core ama.lyses ureknown to be affected by the wettabifity of the mineral swfaces.To obtain suitable core material for these umdyses, one approachuses various special precautions during coring and after the coresare taken to preserve the native-state wettabilky condition, Al-ternatively, cores are clwmed to a water-wet condhion, followedby aging .ti the presence of reservoir cmde und brine at in-situ tem-perature and pressure condbions (restored state).

394

fu the part of the restored-state procedure concerned with sol-vent extraction of the cores to provide a stmangly water-wet miner-A sorface, tbe P-FfD method is a rapid, inexpensive screeuinS toolto evaluate solvent extraction efflciencv that reauires smaff uOr-tions of disaggregate core material. - -

Table 7 gives P-FfD results on the cleaning of a sundstone reser-voir by different solvents >e cbtorofo~medmnol azeotrope wasthe inost effective single solvent, removing 98.9 wt% of the. cmdeoil in the core sumple. Dual solvent extraction, methylene chlo-ride followed by the cfdoroformfmethanol azeotrope, was margi-nally more effective (99.7 W% cmde oil removal).

USBM Wettabifi@ Tests. Table 8 gives the USBM wettabiIiU ti-dices, I, for asphdtene-tmated and -entreated Berea and Bentheimsandstones The I values for the untreuted sands ace typical of water-wet system, the asphaltene treatment convened them to less water-wet or neutral-wet systems. Siur observations were made byCrocker and IvfarchmB on Berea cores treated with asphaltenesfmm different crude oils. A3s0, Cuiec, 9 in his attempt to come-late rocklcmde oil interactions with wettabdi~, found a comela-tion between wettabtity and asphaltene content for a group of about20 reservoirs. Although asphaltene treatment of sandstones doesnot after them to a completely oil-wet state, Quilon-C~ treat-mcnt,24 used 6Y otberszs to make rock ofl-wet, produced coresthat gave USBM I vafues of = 0.4 (i.e., more oif-wet thanasphaltene-treated cores).

Conclusions

1. bngmuir Type I isotlenns, indicating monolayer adsorption,were obtained for the adsorptionof asphaltenes on kaolin clay min-eral from nonpolar solvents under unbydmus conditions. The weightand thickness of the adsorbed monolayer decreased to some extentwith increasing solvent power of the pardcula solvent.

2. Asphaltene adsmpkm from tolnene solution on other claysand common minerul substrates ulso yielded Langmuir T~e Iisotherms.

3. Asphaltene adsorption from nitrobeuzene on ka.oliu followeda Langmuh Type f3 isotherm, indicating mukilayer adsorption. Iorncmechanisms are probably fbe major factors responsible for tbe ad-sorption mechanism in nhmbenzene.

4. A high degree of irreversible adsorption was observed forasphaltenes on kaolin with respect to many solvents, according toP-FfD. The most effective solvents were tbe cbloroformJmetbanolazeotmpes, followed by pyridiw toluene was particularly ineffec-tive in asphaltene removal.

5. Cldoroformlmetbanol azeotrope was the most effective simgle solvent, removing 98.9 W% of the crude oil in a siliceous ~ser-voir core sample. Duaf solvent extraction, metbylene chloridefollowed by ~e ctdorofonnlmethanol azcotrope, was marginallymore eff=tive.

6. Asphaltene treatment of water-wet sandstone cores ravultedin a partially water-wet or nmtrul water-wet system, as shown byUSBM wettubtiity indices und o&saturation-vs. -capiIk3ry-numbercurves.

Nomenclature

a = asphaltene adsorption, mglg

as = satmated asphaltene adsorption at compktemonolayer coverage, mglg

C = concentration, ppmI = wettabilily index, dimensionless

K = qtio of rate constants of adsorptionldesorptionreactions

6 = Hidebraud solubrby pununeter, ca3~. cm 3/2~r = &Je~ic ~Om@t, dimen~iO~es~

Acknowledgments

We acknowledge the management of Shell Development Co. forpermission to publish this paper. We ujso acknowl@ge the Geo-chemical Services Gronp for P-FfD measurements and J.A. Robanfor the centrifuge experiments. We dso thank the reviewers fortieir comments

SPi3 Reservoir Engineering, August 1991

References

1. CoUin.s. S.H. and Mekose, J: C.: Adsoq8ion of Asphaltenes and Waferon Reservoir Rock Minerals,,, paper SPE 11800 presented at the 1983

SPE Sydposium on OMield and Geothermal Chemistry, Denver, June,.. .,.

2. Clememz. D ,M.: Interaction of Petroleum Heavy Ends With .Mont-

morillonite, Clays & Clay Minerals (1972) 24, 312-19.3. Czarnecka, E, and Gillott, J. E.: .Forrnatio and Characterization of

Clay Complexes Wih Bifumen From Athabasca Oil Sand, Clays &

CWy Minemk (1980) 28, 197-203.4, Fritschy, 0, and Papixer, E,: Wmractions Befween a Bitumen, 1S Com-

,,S Fuel (1978) 57, 701-04.ponents and Model Fillers,5. Wihcrspo.n, PA. and W!nni ford, R. S.: The Asphahic Components

of Petroleum., Fundnmeml As.wm of Pwrol..n! Gcochmism. B.Nagy and V. CoWmbo (e&.), &evier-Science Publishing Co. ,Am-stemkn (1967). .

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7. Mitchell, D.L. and Speight, 1,0.: The Sol.bility of Asphaltenes inHydmcarben Soivene, Fuel (1973) 52, 149-52.

8. Clemenu, D. M.: Alteration of Reck Propedies by Adsorption of Pe-

troleum Heavy Ends: Implications for Enhanced Oil Recove~, paper

SPE 106S3 presented at fhc 1982 SPEIDOE Symposium on Enhanced

Oil Remvery, TuJsa, Aprit 4-7.9. Cuiec, L.: Reck/Cmde-01 Jmeractions and Weftabiiiy: .kn Atfempt

To Understand Their Interrelation, paper 13211 presented at the 1984SPE A.nnal Technical Conference and Exhibition, Houstcm, Sept.

16-19.

10, Cuiec, L.: WeUabflity and RocWCmde-OiJ Companent Jnteracdons,

paper presented at the 19S6 Jntermciety Energy Conversion Engineer-ing Conference, San Diego, Aug. 23-29.

11. Bichard, J. A.: 03 Solubfify, pap presented al be 1%9 Cdu Chem-

icaf Engineering Conference and Third Symposium on Catalysis, Ed-

monton, Oct. 19-22.

12. Waxman, M. H., Deeds, C. T., and Closmann, P, J,: Thermal Alter.

aL:Ons~p=ce ~vekTars, p.p.r SpE9510 p=$ented at the 1980SPE Annual TAnicaJ Conference snd Exhibition, Dallas, Sept. 21-24.

[3. Dubey, S. T.and W=an, M. H.:`$Asphdtine Adsoqtionmd Deso~-tion From Minemf Surfaces, paper SPE 18462 presented ac the 19S9SPE1nd. Symposium on OMieldChemistry, Houston, Feb. 8-10.

14, Donaldson, E. C., Thomas, R. D,, and Lor.nz, P, B.: Tenability De-tetition and Ifs Effect on Recove~ Efficiency, 7, .WEJ (March 1969)I ?-m.. ---

15, Yen, T. F., Emfmm,l.G., and PdEack,S.S .:` Tnv@@dcmoftheSamc-ture of Pelmleum Asr.halteIIes bv X-R., Diffraction.,, ,4mz1. Chem.(1961 )33,1587-94.: -

16, Ravey, J.C., Ducouret, G., and Espinat, D.: AsphaJtcne Macrostmc-tureby Smafl Angle NemonSc8ttering,F uei(l9S8)67, 1560-67.

17, Dictie, 1, P., Htier, M, N., and Yen, T. F,:`` ~=tion MicroXopich-vestigatios ontie Natiof P~oleum Asp~tees, V,3. Colloid In-

f@Ct~Ci. (1967)29,475-84.1S. Fmkman, Z.etal.: OwgenC oqOm&inA tiabmcaAphdtene,

Energy & Ruek (19PQ) 4,263-70.19, Igmsi&, T., ~usz, O. P., a"d Mont80me~, D. S.:`` Oxygen Dism"-

bution and Hydrogen Bonding in Afhabasca Asphaltene,-, Fuel (1977)56, 359-65,

.$hdla 1. 2fubey, Ssenior resesrch

chemist at Shell

Development Co.

in Houston, has

been Involved with

EOR and wettablll-

ty studies in reser-

voir mechanisms

research and In an-

alytical method de-

Dubey Waxman vilopment. Sheholds a BS degree

from the U. of Bombay, an MS degree from Memorial U. ofNewfoundland, St. Johns, and a PhD degree from the U. ofCalgaty, all in chemistry, and an MS degree in chemical en-gineering from the U. of Tulsa. Monroe H. Waxman, former-ly a senior research assoolate at the Bellalre Research Center

of Shell Development Co., retired In Feb. 19S8 after workingfor Shell since 1952. During 1961-62, he spent 1 year as ex-change scientist at the KoninkHjke/SheO E& PLaboratorIumIn The Netherlands. His Interests are In wettabllity, elec-trochemlstryof shaly ssnds, geochemistry, andtranspoffproperfles related to earth processes. He holds a BS degreefrom Long Island U. and a PhD degree In physical chemistryfrom the Polvtechnlc Inst. of Brooklvn.

20, Etdib, LA.: The ?mlvation, Ionic and Elecrrophoretic Properties ofColWidaf Asphalrmes in Petroleum, presemed at the 1962 Meeting

of the DIV. of Petroleum Chemistry, American Chernicaf SW., Washing-

ton, DC, March 20-29.

21, Pmrzes, S. and SWight, J.G.: LEIectricaJ Conduc!ivilies of Bitumen

Fractions in Nonaqueous Solvents, Fuel (1974) 53, 192-97.22. CWm, P,L. ad Anderson, W. G.: kCore Cleaning for Re$forationof

Native Weuabilhy,S PEFEt Maxhl 988) 131-3 ETram., AfME, ?J3523. Crccker, M, E. and Marchin, L. M.: .. Weuabilhy and Adsorption Char-

aclerislics of Crude-Oil Asphaltene and Polar Fractions, f,JPT(April1988) 470-74.

24. Uer, R. K.: sStearato Chromic Chloride, Id. & E.g. C&m. (1954)46. No. 4. 766-6!7.

2S. Tiffm, D.L. and Ydlig, W. F,: Effects of Mobile Water on Muldple-Contacl Miscible Gas Displacements.,, SPEI (1... 1983) 447-55.

SI Metric Conversion Factors

Cd X 4,184 E03 = kff12 x 9,29f)3M* E02 = mzF (F32)/l.8 = .~in. x 2.54* E-01 = cm

Tonvm!m factor I* exact. SPE32E

O,lglnal SPE nmn,cdpt rece!ved tar review Feb. 8,1989. PaPec Wcmted to, Pbllcaf loOct. 23

Solvent

ChloroformTolueneToluenel

n-dodecane(1.75:1.0 W/M)

wADSORPTION OF ASPHALTENESON KAOLIN

FROM NONPOLAR SOLVENTS

6112 -3/2

cal cm

9.38.98.7

Maximum AsphalteneAdsormion Level

Per unitweight

of mineralmglg

23.8() 25.8(2)26.4 26.229.8 30.1

W&t

area ~mglm

2.02.22.5

MonolayerThickness

nm

1.75(3) 2.4(4]1.9 2.72.2 3.0

) From isotherm plateau level.(2)

Calculated from linear transform of Langmuir isotherm.1

:=:+a a, a,K

where a is the asphaltene adsorption in equilibrium with asph$kene concentration ca, is thesaturated asphaltene adsorption at complete monolayer coverage and K is the ratio of rateconstants of the adsorfxionldesorption reactions.

(3)Calculated from ncorrcc~ed Bnsrface area of 11.9 m21&density ofasPhahenes equalt01.14.5 g/cc.

(4)A=me~ o edge ad~O~tiOn wjth p]aar 3UrfaCe area eqUal tO 0.9 Of tOtd surface area; also.asphaltene packing fraction e~ual to 0.8.

32blcQADSORPTION OF ASPHALTENES ON KAOLIN

FROM NITROBENZENE

EquilibriumAsphaltene

No. Concentration

ppm

1 14112 10873 7504 5115 3626 151

AsphalteneAdsorption Level

Mineral Mineral SurfaceWeight Basis Area Ba;is

mglg mgJm

91.3 7.771.7 6.064.4 5.456.g 4.849.9 4.233.4 2.8

Thickness ofAbsorbed Layer

nm

6.75.34.74.23.72.5

ADSORPTION OF ASPHALTENES ON VARIOUS CLAYS ANDMINERALS FROM TOLUENE SOLUTION

Asphaltene MonolayerNo. Mineral

Adsorbent ~~~/21 WtAVt() I Wt/Surface.AreaI mvg mglg I mglmz

1 Illite 37.0 39.? 1.12 Kaolin 11.9 26.2 2.2

Mineral3 Calcite 1.71 5.8 3.4(Dover Chalk)4 Ottawa Sand 1.69 3.70 2.2.s Dofomite 2.87 3.75 1.3(Dolocron)6 Dickite 0.88 1.84 2.17 Berea Sandstone 1.48 1.s2 1.08 Alumina 1s.9 19.2 1,2

(Type A)())MoolaYer ~eight~~b(ainedfrom linear transform of Langmuir

Type I adsorption isotherm,(2) BET surface areas using ei!her nitrogen or kryptcm.

ADSORPTION OF ASPHALTENES ON VARIOUS CLAYS ANDMINERALS FROM TOLUENE SOLUTION

(Based on Adsorption from Asphallenell%lee Solution, 2500 ppm)

m21g mglg mglmz

Alumina (Type B) 114 174.7 1.1Calcite 12.3 21.3 2.2

(Omya)K&liriiti 10.s

(TWiggs)20.8 3.4

Thuringite() 9.1 8.13 2.2Chlorite(l) 3.0 0.73 1.3

, I 1

(t) Leached ~ith buffered sodium dithionite 10 HItOW irOIt OMeaand water washed.

(2)BET surface ~reaS usin8 either nitrogen 01 krypton.

s

~PE 18462

IRREVERSIBLE Adsorption OF ASPHALTENESON KAOLIN FROM TOLUENE

Irreversible Adsorption PercentEquilibrium Equilibrium Level After Asphaltene

Cone. cd Asphaltenes Adsorption Proton ed Toluene Retainedin Toluene Solution Level on Kaolin \$ashin

fAfler Washing

ppm mg/g ( ] ) mglg ( )46 17.1 19.0

375 25.9 19.3 74.5593 26.8 21.8 81.3

1381 26.3 23.1 87.82160 26.1 23.0 88.1

(1)By Absorbance method

2) By P-FID method

T-fRREVERSIBLE ADSORPTION OF ASPHALTENES OS KAOLIN

AFTER REPEATED \VASHINGS WITH VARIOUSSOLVENTS OR SOXHLET EXTRACTIONS

8 [ni:ial Final Percent1,2 -312 Adsorption Adsorption Asphaltenes

No. Solvent cat cm melt mele Retained

1 Freon 11

2 Tolueneln-Dodecane[1.75:1.0, Wlhm)

3 Taluene

4 Chloroform

s Chloroform/Acetone(70/30 Vol)6 Methylene Chloride

7 Chloroforml!vtethanol(87/1 3 VOl)

8 Pyridme

9 ChhwofmmlkfethanolAzeotmpe(78.7 /21.3 vol)

10 Nitrobenzene

11 Chloroform/Methanol(70/30 Vol)

12 ToluenelMethanolAzeo!ro e(29. U7{.9 VO])

a) By repeated washingsb)

Soxhlet exactions

7.6 30.7 31.4

8.7 30.K

8.9 30.8 25.529.2 24.6

92 30.6 18.330.7 17.9

16.2

9.4 30.0 8.2

9.8 30.8 21.429.9 22. I

21.2

10.5 30.6 16.2

10.6 30.6 6.74.1

30.3 2.4

10.9 32.8 1.632.8 1.0

11.1 3J.K -1829.8 21.5

30.830.0

11.5 6.911.2

13.7 31.7 22.8

-1oo

82.884.2

59.8S8.3S2.9

27,3

69.573.970.9

52.9

21.913.5

7.9

4.9a3.ob

58.472.1

22.437.3

71.9

Note: Initial adsorption on all kaofin samples except NoF.. 4 and 7 from 3000 Dpmasphaltene solution in tolueneln-dodecane (1.75:1.0 wtlw). . .

Initial adsorption on kaolin for samples NOS.4 and 7 from 27,000 ppm asphaltenesolution in lolueneln-dodeca~e (1.75: LO wlwt).

SOLVEXT CLEANING OF SILICEOUS CORE MATERfAL -P-FID SCREENING METHoD

\Veight Percent Pyrolyzable \Veigh! Percent HydrocaSolvent Hydrocarbons by P-FfD Removed by Extracti

< 100C 400-750C TotalNone 0.525 0.099 0.624 ---Pyridine 0.038 0.008 0.046 92.6Tetrachloroelhyl~ne 0.050 0.023 0.073 88.3

(1) Tofuene 0.013 0.022 0.035 94.4(2) C!!ClJCH30H azeotrope 0.002 0.002 0.004 99.4

(1) ,Methylene Chloride 0.08 .0.02 0.10 84.0(L) CHCIJCH30H azeotrope 0.001 0.001 0.002 99.7

(1) CHC13 0.002 0.006 0.008 98,7(2) CHCIJCH,OH azecmope 0.004 0.003 0.007 98,9

CHCIJCH30H azeotrope 0.004 0.003 0.007 98.9 _

Table 10

tiSBM WEITABILITY TESTS - BEREA AND BENTHEIM SANDSTONES

Treatme-! USBM Index W

Berea Sands Bentheim Sands

Heated, 550C +0.97 +0.90+0.99

Adsorbed Asphaltenes +0.12 +0.43+0.51

00 1

r I , , 1 t

o 10 20 30 40 50 60

TITRANT (n-OODECANE) TO ASPHALTENE RATIO, mllg -

fig. l-Blcfurd titmtion test for nphdtwrw In rronpolw Mtwnt9.

w

~~ 30-

28-

24-

20-

16-

12- EITOLUENE/n-DODECANE (1.75:1.0 W/W)0 TOLUENE

a-A CHLOFtOiXOWf

4 Q NITROBENZENE

-fr t , t ,

0 400 800 1200 1600 2000 2400 28

EQUILIBRIUM CONCENTRATION OF ASPHALTENES, ppm -

Ftg. 2-Admrptlon Ieotherms for lsphrdteneeon keolhr from different solvents.

SPE 184(jz

EITOLUENEln-DODECANE (1.75:1.0 (wIw)I@TOLUENEA CHLOROFORM

400 800 1200 1600 2000 2400 2s00

c (mg/1000g) +

Fig. 3Llneer trarreformof the LengmulredeorptlonIeothermefor eephetteneeon keolhrfrom nonpolar8olvent8.

60

lm

l*

0jQ---.0

00 BEREA SANDSTONE (>100 MESH)A DICKITE (WISCONSIN)q DOLOMITE (DOLOCRON)V OTTAWA SAND (SUPER x >325 MESH)C) CALCITE (DOVER CHALK)l KAOLIN MINERALM ILLITE (BEAVERS BEND)0 ALUMINA

~

1000 2000

EQUILIBRIUM ASPHALTENE CONCENTRATION, ppm +

Fig. 4-AdwQtlon of asphaltenes on clay and mineral wrfacee fmm toluene.

El

o 500 1000 1500 2000 2500

ASPHALTENE EQUILIBRIUM CONCENTRATION, ppm +

Fig. 5-Adsorption.desorption hyetaraala for aaphaltenaa on kaolin from toluene.

61

\

BRINE DRIVE Ab /

c

I I 10 0.2 0.4 0.6 0.8 1

AVERAGE WATER SATURATION

UNTREATED BENTHEIM

10

5

65Il.Illu

ii02go0.

$a-JdE

5-5

-lo

F

BRINE DRIVE Ab /

t 1 t I t I 1

B... . . ..n

I 10 0.2 0.4 0.6 o.8 1

AVERAGE WATER SATURATION

ASPHALTENE TREATED BENTHElM

Fig. 6USBM teets, Bentheim sandstones.

62