servoirs.the brine

nduce oiler dryingder water.

ic forcecreases. Theare thinnered with juste highest

Journal of Colloid and Interface Science 289 (2005) 206–217www.elsevier.com/locate/jcis

AFM study of mineral wettability with reservoir oils

K. Kumar, E. Dao, K.K. Mohanty∗

Department of Chemical Engineering, University of Houston, 4800 Calhoun Road, Houston, TX 77204-4004, USA

Received 21 July 2004; accepted 12 March 2005

Available online 16 April 2005

Abstract

Wettability plays a key role in determining fluid distributions and consequently the multiphase flow and transport in petroleum reMany crude oils have polar organic components that collect at oil–water interfaces and can adsorb onto the mineral surface iffilm breaks, rendering the medium oil-wet or mixed-wet. Mica and silica surfaces have been aged with brine and crude oils to icomponent adsorption. Bulk oil is eventually replaced by water in these experiments by washing with common solvents without evthe mineral surface. The organic deposit on the mineral surface is studied by atomic force microscopy in the tapping mode unDrying the surface during the removal of bulk oil induces artifacts; it is essential to keep the surface wet at all times before atommicroscopy or contact angle measurement. As the mean thickness of the organic deposit increases, the oil–water contact angle inorganic deposits left behind after extraction of oil by common aromatic solvents used in core studies, such as toluene and decalin,than those left behind by non-aromatic solvents, such as cyclohexane. The force of adhesion with a probe sphere for minerals agthe asphaltene fraction is similar to that of the whole oil. The force of adhesion for the minerals aged with just the resin fraction is thof all SARA (saturates, aromatics, resins, and asphaltenes) fractions. 2005 Elsevier Inc. All rights reserved.

Keywords: Wettability; Atomic force microscopy; Contact angle; Asphaltenes; Oil-wet

-portntlyed-

ightablen of

ndof

and

a-

urnlied

plein-

pturee oferi-iable

lter-iontedard

1. Introduction

Wettability plays a key role in determining fluid distributions and consequently the multiphase flow and transin petroleum reservoirs. Once thought to be predominawater-wet, it is now believed that many reservoirs are mixwet. Mixed wettability was first proposed by Salathiel[1]who suggested that some portions of the rock surface mbe oil-wet and some water-wet. Since then a consideramount of work has been done to understand the origimixed wettability by Melrose[2], Buckley et al.[3–6], andMorrow et al.[7,8]. Oil is often produced in source rocks athen migrates into originally water-wet reservoirs. Somethe polar components of crude oil, mostly asphaltenesresins, collect at the water/oil interface[9] and adsorb ontothe mineral surface if the water film breaks. Stability of w

* Corresponding author.E-mail address: mohanty@uh.edu(K.K. Mohanty).

0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2005.03.030

ter films depends on the disjoining pressure, which in tdepends on brine, oil, mineral compositions, and the appcapillary pressure[10–12].

Contact angle measurement is one of the most simand practical techniques of assessing wettability of a meral surface after exposure to crude oil and brine[11,13,14].But these measurements are macroscopic and cannot cathe microscopic mechanisms of wetting. The exact naturthe adsorbed polar oil components is still unknown. Expments with deasphalted crude oil do not produce apprecwettability change as compared with original crude oil[15];thus asphaltenes are partly responsible for wettability aation [16]. Asphaltenes are formally defined as the fractof a crude oil that precipitates when the oil sample is dilu40 to 1 withn-pentane under the conditions of a standASTM test[17]. Radke[9] and Mohammed et al.[18] have

experimentally studied the properties of the asphaltenic filmformed at the crude oil and brine interface to understand therole of interfacial microstructures in establishing the mixed-

nd In

ilityoil.

tenesity

n-tactsoftsur-ue.of

eneFMsed

romon-

urey aet-

omFM, orair-sesitebyy hy-M

ace

min-im-ndery ofith

ents casonovu

e in-wets byeri-

sil-

ithFMcesalin,s onrcefacesro-logyand

s inom-nones.layan,ter

luedgle

-Q

eticiso-h,ca-andly-

s

fromth-

es

K. Kumar et al. / Journal of Colloid a

wet state of reservoirs. Yang[19], Ese[20], and Buckley[21]have studied the effect of asphaltenes in altering wettabof mineral surfaces by varying the asphaltene content inSimple correlations based on oil composition and asphalcontent had only limited success in predicting the propenof crude oil to alter wettability[22].

Atomic force microscopy (AFM) is currently used extesively for imaging surfaces. It can be used in the conmode for hard surfaces and in the tapping mode forsurfaces. It can be used to image dry surfaces or wetfaces; tapping mode in water is a relatively new techniqAFM images have been used to confirm the depositionoil components on mineral surfaces[9,23,24]. Ese et al.[20]have studied the effect of addition of resins in asphaltsolution on aggregation at a mineral surface through Aimages. They found that the addition of resins increathe size of asphaltene aggregates. Toulhoat et al.[23] usedAFM to study the adsorption of asphaltenes extracted fdifferent crude oils and observed a significant change in ctact angles due to asphaltene adsorption. Yang et al.[14]have tried to correlate macroscopic contact angle measments to AFM images without much success. Typicallmica surface is aged with a crude oil to reproduce the wtability of a sandstone reservoir rock. The excess oil frthe surface must be removed before imaging with an Aby washing with a solvent, e.g., cyclohexane, decalintoluene. The solvent is then removed from the surface bydrying[14]. However, allowing the surfaces to dry can causome changes in the molecular arrangement of the depolayer. In this work, we try to overcome this shortcomingremoving the excess solvents by a series of progressiveldrophilic solvents (with water at the end). We use wet AF(tapping mode in water) to image the mineral–water surfwith adsorbed/deposited oil components.

The nature of the deposited organic components on aeral surface cannot be inferred from AFM topographyages. The force between two surfaces can be used to ustand the physical nature of surfaces. Before the discoveAFM, most of the surface interactions were measured wthe surface force apparatus (SFA)[25]. Recently, AFM is be-ing used extensively to get the force–distance measurembetween a tip and a surface. These force measurementbe used to calculate the surface energies using JohnKendall–Roberts (JKR) theory, Derjaguin–Muller–Topor(DMT) theory, etc.[26,27]. Van der Vegte and Hadziioanno[26], Teschke and De Souza[28], Ducker and Pashley[29],Eastman and Zhu[30], and Rabinovich and Yoon[31] havedone extensive studies on chemically specific tip–surfacteractions in other (not crude oil) systems. In this work,try to understand the nature of the deposited componenAFM force measurements with a COOH-terminated sphcal tip.

The objective of this work is to probe crude oil-treated

icon and mica surfaces by atomic force microscopy to relatemacroscopic measures of wettability with the amount andthe nature of organic deposition. We study the effect of our

terface Science 289 (2005) 206–217 207

-

d

r-

sn–

methodology by comparing the dried mineral surfaces wthe wet surfaces using contact angle goniometry and Atopography. We remove excess oil from mineral surfaby three common core-cleaning solvents (toluene, decand cyclohexane) and study the effect of these solventthe state of the mineral surfaces. We conduct AFM fomeasurements to understand the nature of mineral surtreated with the whole oil or one of its SARA (saturates, amatics, resins, and asphaltenes) fractions. Our methodois described in the next section followed by the resultsthe conclusions.

2. Methodology

2.1. Substrates

Silicon and mica were used as two different substrateour experiments. Both mica and silicon surfaces are atically smooth and well suited for AFM imaging. Silicowas used because silica is a major component of sandstBeing alumino silicate, mica has properties similar to cminerals in sandstones[32]. Mica, obtained from Ted PellInc. was in the form of slip disks of diameter 9 mm. Silicoobtained from Silicon Inc. as wafers of 20 cm in diamewith a single polish side of 2.4-µm thick SiO2 layer, was cutto small (10 mm) square plates. These substrates were gon 15-mm metal disks for AFM imaging and contact anmeasurements.

2.2. Chemicals

Distilled, deionized water was passed through Millicartridge filters to obtain water of resistivity 18.2 M�/cmand pH 7. This ultrapure water was used to prepare synthbrine. Toluene, decalin, cyclohexane, methanol andpropyl alcohol, obtained from Mallinckrodt, Sigma–Aldricand EM Science, were used without any further purifition. Two different reservoir oils, designated as Crude ACrude B, were used for wettability tests. The SARA anasis of both oils is listed inTable 1. The SARA analysis wadone using multi-dimensional SARA HPLC separation[33].It should be noted that the asphaltene content obtainedthis method is high compared to the conventional meods[17].

Table 1Properties of Crudes A and B

API (◦) Saturates Aromatics Resins Asphalten

Crude A 18 30.1 31.3 13.6 25.0Crude B 28 48.1 28.8 6.5 16.5

nd In

etiches tod inp-edeticil,

fewde-

threeen

eth-andfor

thatpeacts,thewasr.

me.voidere

halt-Me

tionselysub

inwase asn a

adssileughthe

aleenon

pletdeotacttion-pletl theustwne ofheaxedc-verysedous

singntao-ingitedse ittactobesdeb-terunding

m-alue,

egeand

hestgeserewe

208 K. Kumar et al. / Journal of Colloid a

2.3. Treatment of substrates

2.3.1. Crude oilsMica/silicon substrates were first immersed in synth

brine for 24 h. After removal from brine, the edge of tsubstrate was touched with an absorbent paper for 10remove the bulk water. The substrate was then immerseoil without further drying and aged in oil for 7 days at aproximately 80◦C. The aging was carried out in a closcontainer at an elevated temperature to enhance the kinof adsorption of crude oil components. After aging in othe substrate was rinsed by dipping it in a solvent for aminutes. Three different solvents were tested: toluene,calin, and cyclohexane. The substrate was subjected todifferent solvent washings for about 10 min each. It was sthat afterward no deposited oil was visible to naked eye.

In order to remove the excess solvent, two different mods were followed which are designated as “dry mode”“wet mode.” For the dry mode, the sample was air drieda day to evaporate the solvent[24,34]and then imaged withthe AFM. It was observed under an optical microscopeas the sample dried, there were some changes in the apance of the surface. In order to avoid these drying effewe followed another procedure called the “wet mode.” Inwet mode, the substrate after treatment with the solventrinsed with IPA followed by methanol and finally by wateCare was taken not to let the sample dry at any point of tiThe samples were scanned under water (not brine) to asalt deposition on the cantilever tip and the experiments wrepeated thrice.

2.3.2. SARA fractionsThe SARA fractions (saturates/aromatics/resins/asp

enes) were obtained for the crude oil A using an ASTtechnique[17]. In order to deposit SARA fractions onto thsurface of the substrates, we first dissolved these fracin toluene at 70◦C. The solutions were prepared separatfor resins, aromatics, and asphaltenes. Mica and siliconstrates were aged in these individual SARA solutionsthe same way as aging in the crude oil. Cyclohexanenot used as a solvent for these SARA fractions becausphaltenes do not dissolve completely in cyclohexane eveelevated temperatures.

2.4. Contact angles

A contact angle goniometer was used to measure thevancing and receding water/oil contact angles by the sedrop method. All the contact angles were measured throthe water phase. The advancing contact angle refers tointerface advancing in the direction of the oil. An opticglass cell, filled with the aqueous phase, was placed betwa light source and a microscope. A horizontal mica/silic

plate was placed inside the aqueous phase. A U-shaped stainless steel needle attached to a motor-driven syringe was useto introduce an oil droplet at the bottom surface of the plate.

terface Science 289 (2005) 206–217

s

e

r-

-

-t

-

Decane was used as the probe oil. The volume of oil drowas increased until the contact line moved outward. A viframe grabber captured the image of the drop and conangles were measured by fitting the drop shape to a soluof the Young–Laplace equation[11]. The contact angle measured was the receding angle. The volume of the oil drowas later decreased using the motor driven syringe untiformation of a neck. The angle formed by the droplet jbefore the detachment from the tip of the syringe is knoas the advancing angle. On further reducing the volumthe oil droplet, the neck got thinner and finally broke. Tcontact angle at that moment, known as secondary reladvancing angle by Freer et al.[9], was measured. The seondary relaxed advancing contact angle was found to beclose to the advancing angle. The oil volume was increaor decreased slowly in order to reduce the effect of viscforces.

2.5. AFM images

The topography of the sample surfaces was imaged uan AFM (Nanoscope III A, manufactured by Veeco, SaBarbara, CA). The AFM was mounted on a vibration islation table. All the topography images were obtained usthe tapping mode in air or in water. This mode is better suover the contact mode for studying soft samples becauexerts very little lateral force as compared to the conmode. The probes used were aluminum coated silicon prfor tapping in air and Olympus oxide-sharp silicon nitriprobes for tapping in fluid. Both kinds of probes were otained from Veeco. The drive frequency for imaging in wawas around 8 kHz whereas that for imaging in air was aro300 kHz. A fluid cell, also from Veeco, was used for imagin water.

The AFM images are typically quantified by three nubers at the microscopic scale: mean roughness, RMS vandz scale. The mean roughness,Ra of an AFM image isdefined as[35]

(1)Ra =∑N

i=1 |hi − h|N

,

wherehi denotes the elevation of a data pointi, h is themean elevation, andN is the number of data points in thimage. The image RMS,Rq is the root mean square averaof the height deviations taken from the mean data planeis expressed as[35]

(2)Rq =√∑N

i=1 |hi − h|2N

.

Thez scale gives the vertical distance between the higand the lowest point in the image. The topography imacan give an idea of the deposited film thickness if this an area with no adsorption. To get a film thickness,

-

dscratched away some of the organic coating from the siliconsurface and then imaged the scratched sample at the bound-ary of the deposited layer adjacent to the bare silicon surface.

nd In

gedechsof

o aas a

eracrce

onsple,edstip

ncecon-plere-ingbeaks

herceissing

entssur-gies

nd

act

od-ereter,as-ad alater

cts,lmthece.hile

t in-ithingof

lowre-

mpletip

tlersionaredts inter-

/sili-dif-r-A.

ater

t wasthe

K. Kumar et al. / Journal of Colloid a

The silicon surface is very hard and does not get damawhen the adsorbed/deposited oil layer is scraped. This tnique cannot be applied on the mica surface because it isand gets scratched very easily.

2.6. Force measurements

2.6.1. BackgroundIn AFM force measurements, a tip is brought close t

surface and the deflection of the cantilever is measuredfunction of tip–sample distance as shown inFig. 1. At largetip–sample separations (1), there are no detectable inttion forces. As the separation distance decreases (2), fosuch as electrostatic, van der Waals, specific interactietc. come into play. As the tip is brought closer to the samat some point (3) the gradient of the attractive force excethe spring constant of the cantilever and the cantileverjumps into contact with the sample (4). This vertical distabetween (3) and (4) gives us a measure of the jump totact force. From position (4) to (5), the tip and the samremain in contact with each other until the tip starts totract. As the tip retracts, at a particular point (6) the sprconstant exceeds the gradient of the force of adhesiontween the tip and the sample and the tip suddenly breaway from the sample to its equilibrium position (7). Tvalue of the spring force at position (6) is called the foof adhesion or pull-off force or jump-off contact force. Thforce of adhesion can be related to the surface energy uthe JKR theory of adhesion mechanics as[26,27]

(3)Fadh= (3/2)πRWabc,

whereWabc is the work of adhesion to pull the tip off thsample,R is the radius of curvature of the tip, a represethe cantilever tip, b represents the medium, and c is theface.Wabccan be expressed as a function of surface enerasWabc= Wab + Wbc − Wac, whereWab is the tip surfacefree energy in equilibrium with the medium,Wbc is the sam-ple surface free energy in equilibrium with the medium, a

Fig. 1. A typical cantilever displacement vs separation distance plot as ob-tained by AFM.

terface Science 289 (2005) 206–217 209

-t

-s,

-

Wac is the interfacial free energy of the tip–sample continterface.

The interaction forces in water between chemically mified spherical tips and the oil-treated surfaces in water wmeasured using the AFM. Cantilevers with 1-µm diameCOOH-terminated borosilicate spherical tips (from Novcan Technologies Inc.) were used. These cantilevers hnominal spring constant of 0.32 N/m. When the sphericatip is brought close to the sample, there are issues of wdrainage from the thin water film and when the tip retrawe have water suction into the film. The dynamics of the fiis quite complicated to model. As an alternative, we useadhesion force or pull-off force to characterize the surfaForce measurements were done in the contact mode. Wmeasuring the interaction forces, the whole tip was kepside water in the fluid cell. Moreover, the tip was wetted wwater before it was placed on AFM in order to avoid trappof any air bubble at the tip or capillary effects. The forceadhesion also depends onz scan rate[36], which was takento be 0.199 Hz in our experiments. The scan rate is keptin order to minimize the effects of viscosity. Force measuments were also attempted when both the tip and the sasurface were dry but the force of adhesion between theand the mineral surface was very large. Capella and Die[36] have also observed that the attractive and the adheforces become almost 10 times smaller in water as compto that in air. Another disadvantage of force measuremenair is that meniscus forces need to be included in the inpretation[36].

3. Results and discussion

3.1. Contact angles

3.1.1. Dry modeWater/decane contact angles were measured for mica

con substrates treated with Crude A and washed withferent samples.Fig. 2 shows a typical drop of decane surounded by water under a mica plate treated with CrudeAll the contact angles were measured through the wphase.Fig. 3shows the contact angles measured[24,34] forsamples treated in the dry mode (i.e., the excess solvenremoved by drying in open air). The contact angles are

Fig. 2. A decane drop surrounded by water on a mica sample treated withCrude A.

nd In

rent

dif-

at it

hreee Amper-theal-

ntaclin,sig

-wete difatedare

lin,

dif-

tactica

ana-thn.

ss oflit-an

sol-mplennedeneeffi-

nts,pre-ions

s for-ane.hnessMS.

thetionstal-ore

ibletheye to

210 K. Kumar et al. / Journal of Colloid a

Fig. 3. Contact angles for mica aged in Crude A and washed with diffesolvents (dry mode).

Fig. 4. Contact angles for minerals aged in Crude A and washed withferent solvents (wet mode).

highest for the cyclohexane-washed surface implying this the most oil-wet.

3.1.2. Wet modeThe advancing and receding contact angles for the t

differently treated substrates in the wet mode for Crudare shown inFig. 4. The error bars represent the maximuand the minimum contact angles recorded in repeat eximents. The wet mode contact angles are smaller thancorresponding dry mode angles, indicating that dryingters the surface. Both the advancing and receding coangles are the lowest for toluene, in the middle for decaand the highest for cyclohexane. These contact anglesnify that the cyclohexane-washed samples are more oilthan toluene- and decalin-washed samples. However, thference in contact angles for the toluene- and decalin-tresamples is not significant. The contact angles with micaslightly lower than those for silica for toluene and deca

but for cyclohexane they are higher.Fig. 5shows the contactangles for samples treated in the wet mode for Crude B. Thecontact angle is the highest for cyclohexane, a similar trend

terface Science 289 (2005) 206–217

t

-

-

Fig. 5. Contact angles for minerals aged in Crude B and washed withferent solvents (wet mode).

as observed in the case of Crude A. For Crude B, the conangles were lower for the silicon substrate than for the msubstrate for all the three solvents.

3.2. AFM images of substrate

The most important requirement for a substrate to belyzed by AFM is that it should be molecularly smooth. Bomica and silicon used for this study satisfy this criterioThe mica surface was found to have a mean roughne0.125 nm which is quite close to the value reported in theerature[14]. The silicon surface was found to have a meroughness of 0.176 nm.

3.2.1. Dry imaging in airThe oil-aged mica substrate was first washed with a

vent, the excess solvent was removed by letting the sadry in open air for 24 h, and then the substrate was scausing the tapping mode in air. It was observed that toluand decalin removed the oil from the samples moreciently than cyclohexane.Fig. 6shows the 50×50 µm scansfor the mica surface treated with the three different solverespectively. The white regions in the height images resent the peaks of deposited materials and the dark regrepresent the bare mica surface or valleys. The statisticthese images are summarized inTable 2. The mean roughness and RMS was found to be the lowest for cyclohexThe toluene-washed sample had the highest mean rougwhereas the decalin-washed sample had the highest RA lot of bare mica could be seen in the images fortoluene- and decalin-treated samples and so the deviafrom the mean height are high. For all the cases, cryslike deposits form on the surface. These deposits are mdistinct in the phase images (not shown here). A possexplanation could be that when the solvents evaporateleave behind asphaltene-type of material which crystalliz

form these structures. The solvent evaporation is an experi-mental technique, not typical of the oil reservoir conditions;therefore we developed the wet mode in this work.

K. Kumar et al. / Journal of Colloid and Interface Science 289 (2005) 206–217 211

(a) (b) (c)

lin-wa

withd aencin. Threas

t onee ofr oiec-

ceor-

am-ater.am-whem todryin

eentheila

Fig. 6. Height images of (a) toluene-washed mica surface, (b) deca

Table 2Roughness analysis of the samples imaged in dry mode

Mean roughness (nm) RMS (nm) z scale (nm)

Toluene 94 128 1157Decalin 74 150 1645Cyclohexane 56 112 1847

An attempt was made to relate the contact anglesmean roughness of the sample but we could not finmonotonic correlation.Fig. 7 shows the graph of dry modcontact angles vs mean surface roughness. The advacontact angles do not change with the mean roughnessreceding contact angles decrease sharply and then incwith the mean roughness. Yang et al.[14] also tried to relatethe mean roughness with contact angles. They found thaof the oils they used had a semi-logarithmic dependenccontact angle on mean surface roughness, but the othedid not show any such dependence. We will show in Stions3.2.2and3.3 that monotonic trends exist if the surfais not dried. Drying alters the surface distribution of theganic deposits.

3.2.2. Imaging in waterAfter removing the excess solvent by drying, these s

ples were placed in water for a day and then imaged in wWhen a small drop of water was introduced on the sples, there appeared to be some changes on the sampleviewed under an optical microscope. These images seehave different topographical features as compared to thescans.Fig. 8 shows the toluene-treated sample imagedwater. Compared toFig. 6, the height (and width) of thepeaks are smaller. A lot of small peaks (white dots) are swhen imaged in water as compared to imaging in air;mean roughness and RMS decrease substantially. A sim

change is observed in the case of the decalin-washed sampleThe large crystal-like structure (Fig. 6b) seems to get smallerin size. However, for the case of cyclohexane the peaks get

shed mica surface, and (c) cyclohexane-washed mica surface aged in Crude A.

gee

l

n

r

Fig. 7. Advancing and receding contact angles vs mean roughness (drymode).

.

Fig. 8. Toluene-washed sample imaged in water.

nd In

seethe

airbeeas

alin

tter

im-ith

r oftin-wet

ater.e ex-ed

thede-

yingin

212 K. Kumar et al. / Journal of Colloid a

Table 3Roughness analysis of images viewed in water (for Crude A)

Mean roughness (nm) RMS (nm) z scale (nm)

Toluene 38 87 1874Decalin 97 134 2908Cyclohexane 100 139 1200

larger. The images in water do not have sharp peaks asin the height images scanned in air, i.e., the peaks ofdeposited material were blunt. Buckley and Lord[24] alsofound similar differences in the images when scanned inand water. The roughness analysis of these images hassummarized inTable 3. The mean roughness and RMS wfound to be the highest for cyclohexane followed by decand toluene unlike the dry mode.

3.2.3. Wet mode

(c)

Fig. 9. (a) Toluene-washed mica sample, (b) decalin-washed mica sample

terface Science 289 (2005) 206–217

n

n

in Section2. For each sample, 2 scans of sizes 1× 1 and5 × 5 µm were done at four different spots to have a beunderstanding of the surface; only the 1× 1 µm scans areshown here.Fig. 9a shows the toluene-treated sampleaged in the wet mode. The deposition is very uniform wpeaks of height of approximately 60 nm and diameteapproximately 25 nm though the adsorption is not conuous. Heights of the peaks are substantially lower in thismode as compared to the dry mode imaged in air or wThis suggests that drying the solvent leaves behind somtra material. This extra deposition might be the solubilizmaterial which is left behind during the evaporation ofsolvent. It is also possible that because of capillarity, theposited material may concentrate at few spots during drand this might be the reason for the large non-uniformitythe dried samples.

Fig. 9b shows the decalin-treated sample. The adsorptionost

h a

In order to overcome the effects of drying the solvent,

we followed a different procedure (wet mode) as detailedin this case is not as uniform as in the case of toluene. Mof the peaks were approximately 40 nm in diameter wit

(a) (b)

, and (c) cyclohexane-washed mica sample imaged in the wet mode (for Crude A).

nd In

mode

mplem-he-

Thethate ofiblegater itsorbsol-s disThis

deentof

ili-f thee to

atedog-theeemthe

allersili-cheswersili-dingad-t forthegh-mpleles.ffer-a

e di-hedthes hasnoreri-andhe

am-and

eneeight,by-d B,wetnglesquedif-not

.

de-eragehat) be-sitede oftech-

ne-heition.sed

sits.ntlys wasde-are

mode

K. Kumar et al. / Journal of Colloid a

Table 4Roughness analysis of mica and silicon samples imaged in the wet(for Crude A)

Mean roughness (nm) RMS (nm) z scale (nm)

Mica–Crude AToluene 5 4 55Decalin 7 5 79Cyclohexane 24 18 258

Silicon–Crude AToluene 3 2 52Decalin 4 3 48Cyclohexane 16 12 124

few peaks of about 100 nm. The cyclohexane-treated sa(Fig. 9c) showed deposits of approximately 100 nm in diaeter.Table 4lists the statistics of the deposited material. Tmean roughness, RMS, andz scale are the highest for cyclohexane, followed by those for decalin and toluene.roughness analysis and the AFM images both indicatetoluene and decalin are more effective in dissolving somthe oil molecules that deposit than cyclohexane. A possexplanation could be that asphaltenes molecules/aggreare often surrounded by resin molecules, which rendesolubility in oil. When these asphaltenes molecules adson to the mineral surface and are subjected to differentvents, the resin molecules surrounding these asphaltenesolve better in toluene and decalin than in cyclohexane.may be because of the aromatic nature of toluene andcalin. So the amount of the adsorbed/deposited componmay be inversely related to the oil solubilizing capacitythe solvents.

A similar set of experiments was carried out with scon as the substrate. The height and the diameter opeaks in the toluene-treated silicon surface (not shown duspace considerations) are very similar to the toluene-tremica surface (approximately 25 nm diameter). The topraphy of the decalin-treated silicon surface is close totoluene-treated sample except for a few peaks which sto connect to form bigger oil patches. When compared todecalin-washed mica surface, the grain size is much sm(almost half) in this case. For the cyclohexane-treatedcon surface, the height and diameter of deposited oil patare higher than the other two silicon surfaces, but lothan those of cyclohexane-treated mica. All the threecon samples show lower adsorption than the corresponmica surfaces but they follow the same trend (i.e., thesorption being the highest for cyclohexane and the lowestoluene).Table 4summarizes the roughness analysis forsilicon samples. Similar to mica surfaces, the mean rouness and RMS are highest for cyclohexane-washed safollowed by those for decalin and toluene-washed samp

The same set of experiments was repeated with a dient crude oil, Crude B.Fig. 10a shows the height image of

mica sample aged in Crude B and treated with toluene (wetmode). The sample exhibits uniform adsorption except for afew peaks which are shown as white patches.Fig. 10b shows

terface Science 289 (2005) 206–217 213

s

-

-s

a decalin-washed sample. The deposited materials havameters varying from 25 to 75 nm. The cyclohexane-wassample (Fig. 10c) shows adsorption of large clusters andpeaks are larger. The roughness analysis of these figurebeen summarized inTable 5. Similar to Crude A, the mearoughness, RMS, andz scale were found to be highest fcyclohexane followed by decalin and toluene. These expments were also performed with silicon as the substratethe trends observed were very similar to that of mica. Tresults are also summarized inTable 5.

For both Crude A and Crude B, it was seen that the dieter and the height of the deposited patches on both micasilicon surfaces are higher for cyclohexane than for toluand decalin. Also the roughness parameters (average hRMS, andz scale) are highest for cyclohexane, followeddecalin and toluene.Figs. 11 and 12show the contact angles as a function of surface roughness for Crudes A anrespectively. As the mean roughness of the images inmode increases, the advancing and receding contact aincrease, for both oils and both minerals. However, a unicorrelation could not be developed. Also the hysteresis (ference between advancing and receding angles) couldbe directly related to the mean roughness of the sample

3.3. Silicon scratch technique

The roughness analysis is not a good measure of theposited amount because roughness is defined as the avdifference from the mean height. It is actually possible tthe tip never sees the substrate (mica/silicon in this casecause there might be a continuous covering of the depoorganics on the substrate. In order to get the actual valuthe thickness of deposited materials, we use the scratchnique as described in Section2.

Fig. 13shows the scratched silicon image for the toluetreated silicon sample for Crude A. The top right of timage shows the scratched area which has no deposSection analysis at different spots on the image was uto get an average value of the thickness of the oil depoTable 6shows the average thickness for the three differetreated samples. The average thickness of the oil patchehighest for the cyclohexane-washed sample, followed bycalin and toluene, which implies that toluene and decalin

Table 5Roughness analysis of the mica and silicon samples imaged in the wet(for Crude B)

Mean roughness (nm) RMS (nm) z scale (nm)

Mica–Crude BToluene 8 5 99Decalin 9 6 102Cyclohexane 21 13 212

Silicon–Crude BToluene 6 4 68

Decalin 8 5 113Cyclohexane 15 11 138

214 K. Kumar et al. / Journal of Colloid and Interface Science 289 (2005) 206–217

(a) (b)

(c)

mple or Crude B).

hanl by

av-conntaccor-eanesd to

rces.les

d astact-andenur-

d astoandcalinave

Fig. 10. (a) Toluene-washed mica sample, (b) decalin-washed mica sa

more effective in dissolving the deposited oil molecules tcyclohexane. The state of adsorption is shown very welTables 4 and 6together.

Fig. 14shows the contact angles as a function of theerage height of the deposited films obtained form the siliscratch technique. Both the advancing and receding coangles increase almost linearly with average height. Therelation with average height is better than that with the mroughness shown inFig. 12. It was also observed that thslopes of the lines inFig. 14are approximately equal. Thuthe hysteresis of the contact angles could not be relatethe average height of the deposited film.

3.4. Force measurements

A large number of force measurements were conductedusing a 1-µm COOH-terminated borosilicate particle at sev-

, and (c) cyclohexane-washed mica sample imaged in the wet mode (f

t

eral spots in order to get an estimate of the adhesive foFig. 15shows the force of adhesion for six treated sampand a COOH-terminated mica surface, which was usea reference. The force of adhesion or the jump-off conforce has been modeled by Maugis[36] as being proportional to the surface energy of the sample. Van der VegteHadziioannou[26] measured the force of attraction betwethe COOH surface and COOH tip in ethanol and the sface energy was found to be 4.5 mJ/m2. We carried out asimilar experiment and the surface energy was calculate3.7 mJ/m2 using Eq.(3). The force of adhesion was foundbe the highest for decalin. This is observed for both micasilicon surfaces. The deposited materials in case of demay have characteristics similar to that of resins, which h

the highest force of adhesion among the different SARAfractions (shown in the next section).

)

(for

faces.

ntofe ofonethatplese.as

sinsap-

pher-

K. Kumar et al. / Journal of Colloid and Interface Science 289 (2005) 206–217 215

Fig. 11. Contact angle vs mean surface roughness of organic deposits onmica.

Fig. 12. Contact angle vs mean surface roughness of organic deposits onsilicon.

Table 6Average height of silicon substrates treated with Crude A

Silicon Average height (nm

Toluene-treated sample 10Decalin-treated sample 30Cyclohexane-treated sample 70

Fig. 14. Contact angle vs average height of deposited film on siliconCrude A).

Fig. 15. Force of adhesion for the Crude A-treated, solvent-washed sur

3.5. SARA fractions

Fig. 16 shows the force of adhesion for the differeSARA fractions (solubilized in toluene) along with thatthe Crude A (washed subsequently in toluene). The forcadhesion for the case of the whole oil is very close to theobtained from the asphaltene solution. It can be inferredthe deposited material left behind when the cores/samare washed with toluene is primarily asphaltenic in natur

Another important thing observed was that the force wthe largest for the case of resins. It is believed that reare more liquid like than asphaltenes. So, when the tipproaches the surface, resins engulf a larger area of the s

Fig. 13. Scratched silicon sample which has been treated with Crude A andtoluene.

ical tip than the other fractions and thus have a larger force ofadhesion. This may be also due to the high compressibilityof resin films deposited[20]. Asphaltenes are envisioned as

nd In

tact.endhe-conenesb-

orce, th

mallrce

sited

asde-, Bues

wewasngleand

ode.ucesre-

laryfacey of

ugh, th

e is aounaceerv

ions, bu

ilizerhindde-ueneuresigherde-

osits

ralsthem-

ged

ratethe

untt forsin

n-id

mesea-

d or-on-

oiluchhind

as-en

ustes,

rate

mthenk

216 K. Kumar et al. / Journal of Colloid a

Fig. 16. Force of adhesion for different SARA fractions.

crystalline and thus have a relatively smaller area of conThe force of adhesion for aromatics is the least. The tris similar for mica and silicon surfaces. The force of adsion for asphaltenes was similar for both mica and silisubstrates. This might imply that the deposited asphalton both mica and silicon are very similar. It was also oserved that resins have the highest jump to contact f(not shown). For the case of asphaltenes and aromaticsjump to contact force was negligible. For asphaltenes, slong range repulsive forces were also observed. AFM fomeasurements give us insights into the nature of depomaterials.

3.6. Discussion

Many parameters were varied in this work, suchmethodology (dry mode, wet mode), solvents (toluene,calin, cyclohexane), mineral (mica, silicon), oil (Crudes Aand SARA fractions). Many characterization techniqwere used, such as contact angle goniometry, dry andAFM topography, and adhesion force measurement. Itseen that in the dry mode, the water–decane contact awere much higher than those in wet mode. The diameterheight of deposited peaks were also higher in the dry mFrom these observations, we conclude that drying introdartifacts to the solid surface. Additional organic material pcipitate out of the solvent as it is evaporated. The capilforces redistribute the organic materials on the solid surduring drying. We recommend the wet mode for the studthese surfaces.

It was observed that as the mean thickness and the roness of the deposited organic layer on minerals increasedoil–water contact angle increased. The scratch techniquprudent way to measure the mean thickness. As the amof organic deposit increases, the hydrophilicity of the surfdecreases and the contact angle increases. This was obsfor both minerals and both oils. We looked for a correlatbetween the roughness and the contact angle hysteresidid not find one.

The organic deposits left behind after extraction withtoluene and decalin are thinner than those for cyclohexaneThis was observed for both oils and for both mica and sil-

terface Science 289 (2005) 206–217

e

t

s

-e

t

ed

t

icon substrates. The aromatic solvents are better solubof asphaltenes than cyclohexane. Thus deposits left bewere thinner. The force of adhesion was higher forcalin and cyclohexane washed surfaces than for the tolwashed surface. This indicates that the chemical natof these surfaces are different. The resins showed a hforce of adhesion than asphaltene, indicating that theposits for decalin may have more resins than the depfor toluene.

It was also seen that the force of adhesion for mineaged with just the asphaltene fraction is similar to that ofwhole oil, implying that asphaltenes were the major coponent of the deposited material that is left on the oil-amica/silicon surface after toluene treatment.

The amount of organic deposit on the silicon substwas smaller than that on the mica. The topography andnature of the deposition were not very different. The amoand the topography of the organic deposits were differenthe different oil. This is because the composition of the reand asphaltene fractions is different for different oils.

4. Conclusions

• Drying the surface during the removal of bulk oil iduces organic precipitation modifying the solid–liqusurface. It is essential to keep the surface wet at all tibefore atomic force microscopy or contact angle msurement.

• As the mean thickness and roughness of the adsorbeganic deposit on minerals increases, the oil–water ctact angle increases.

• The organic deposits left behind after extraction ofby common aromatic solvents used in core studies, sas toluene and decalin, are thinner than those left beby non-aromatic solvents, such as cyclohexane.

• The force of adhesion for minerals aged with just thephaltene fraction is similar to that of the whole oil whwashed with toluene.

• The force of adhesion for the minerals aged with jthe resin fraction is the highest of all SARA (saturataromatics, resins, and asphaltenes) fractions.

• The amount of organic deposits on the silicon substis lower than that on mica.

Acknowledgments

This work was partially supported by the funding frothe Texas Higher Education Coordinating Board andNPTO of the US DOE (DE-FC26-01BC15186). We thaDr. K.K. Bissada for the SARA analysis.

.References

[1] R.A. Salathiel, J. Pet. Technol. (October 1973) 1216.

nd In

ep-

al-

.ce.

llas,

999)

En-63,

ov-

ups

ds

. 33

78

ill,

ng.

7.

5thling,

K. Kumar et al. / Journal of Colloid a

[2] J.C. Melrose, in: 57th Annual Fall Tech. Conf., New Orleans, LA, Stember 1982, Paper SPE 10971, p. 1.

[3] J.S. Buckley, Y. Liu, J. Pet. Sci. Eng. 20 (1998) 155.[4] J.S. Buckley, Curr. Opin. Colloid Interface Sci. 6 (2001) 191.[5] Y. Liu, J.S. Buckley, Soc. Pet. Eng. Form. Eval. (March 1997) 5.[6] J.S. Buckley, Soc. Pet. Eng. Adv. Technol. Ser. 3 (1995) 53–59.[7] N.R. Morrow, J. Pet. Technol. (December 1990) 1476–1484.[8] P.P. Jadhunandan, N.R. Morrow, in: 66th Annual Fall Tech. Conf., D

las, TX, October 1991, Paper SPE 22597, p. 111.[9] E.M. Freer, T. Svitova, C.J. Radke, J. Pet. Sci. Eng. 39 (2003) 137

[10] K.K. Mohanty, H.T. Davis, L.E. Scriven, in: D.O. Shah (Ed.), SurfaPhenomena in Enhanced Oil Recovery, Plenum, New York, 1981

[11] G.J. Hirasaki, Soc. Pet. Form. Eval. (June 1991) 217.[12] S. Basu, M.M. Sharma, J. Colloid Interface Sci. 181 (1996) 443.[13] J.S. Buckley, C. Bousseau, Y. Liu, in: SPE Annual Tech. Conf., Da

TX, September 1996, Paper SPE 30765, p. 341.[14] S.Y. Yang, G.J. Hirasaki, S. Basu, R. Vaidya, J. Pet. Sci. Eng. 24 (1

63.[15] J.S. Buckley, N.R. Morrow, in: SPE/DOE Seventh Symposium on

hanced Oil Recovery, Tulsa, OK, April 1990, Paper SPE/DOE 202p. 871.

[16] R.S.H. Al-Maamari, J.S. Buckley, in: SPE/DOE Improved Oil Recery Symposium, Tulsa, OK, April 2000, Paper SPE 59292, p. 1.

[17] ASTMD2007-80, Standard Test Method for Characterization Gro

in Rubber Extender and Processing Oils by the Clay–Gel AdsorptionChromatographic Method, ASTM, 1980.

terface Science 289 (2005) 206–217 217

[18] R.A. Mohammed, A.I. Bailey, P.F. Luckham, S.E. Taylor, ColloiSurf. A 80 (1993) 223.

[19] S.-Y. Yang, G.J. Hirasaki, S. Basu, R. Vaidya, J. Pet. Sci. Eng(2002) 203.

[20] M.-H. Ese, J. Sjoblom, J. Djuve, R. Pugh, Colloid Polym. Sci. 2(2000) 532.

[21] J.S. Buckley, G.J. Hirasaki, Y. Liu, S. Von Drasek, J.-X. Wang, B. GPet. Sci. Technol. 16 (3&4) (1998) 251.

[22] L. Cuiec, in: Proceedings of 21st Intersociety Energy Conversion EConf., San Diego, 1986.

[23] H. Toulhoat, C. Prayer, G. Rouquet, Colloids Surf. A 91 (1994) 26[24] J.S. Buckley, D.L. Lord, J. Pet. Sci. Eng. 39 (2003) 261.[25] C. Drummond, J. Israelachvili, J. Pet. Sci. Eng. 33 (2002) 123.[26] E.W. Van der Vegte, G. Hadziioannou, Langmuir 13 (1997) 4357.[27] J. Schneider, W. Barger, G.U. Lee, Langmuir 19 (2003) 1899.[28] O. Teschke, E.F. De Souza, Langmuir 19 (2003) 5357.[29] W.A. Ducker, R.M. Pashley, Langmuir 8 (1992) 109.[30] T. Eastman, D.-M. Zhu, Langmuir 12 (1996) 2859.[31] Y.I. Rabinovich, R.-H. Yoon, Langmuir 10 (1994) 1903.[32] L. Liu, J.S. Buckley, J. Pet. Sci. Eng. 24 (1999) 75.[33] K.K. (Adry) Bissada, E.B. Szymczyk, D.G. Nolte, presented at the

International Conference on Petroleum Phase Behaviour and FouAlberta, Canada, June 13–17, 2004. Article in preparation.

[34] D.L. Lord, J.S. Buckley, J. Pet. Sci. Eng. 206 (2002) 531.

[35] Digital Instruments, Command Reference Manual, 2001.[36] B. Capella, G. Dietler, Surf. Sci. Rep. 34 (1999) 1.