Nettability Literature SurveyPart 2: Nettability MeasurementWilliam G. Anderson, SPE, Conoco Inc.

5?E 13933

Summary. Many methuds have been used to measure wettabtity. This paper describes the three quantitativemethods in use to&y: contact angle, Amott method, and the U.S. Bureau of Mines (USBM) method. Theadvantages and fimitationa of alf the qualitative methods-inrblbition, microscope examination, flotation, glaasslide, relative permeability curves, capilkmy pressure curves, capiffarinretric method, displacement capillarypressure, penneabiLhy/saturation relationships, and reservoir logsare covered. Nuclear magnetic resomnce(NMR) and dye adsorption, two methods for measuring fractional wettabflily, are also discussed. Fdy, ametbud is proposed to determine whether a core haa mixed wettabilhy.

IntroductionThispaper is the second in a series of fitcrature surveyscovering the effects of nettability on core analysis. 1-3Changes in the wettabtity of cores have been shown toaffect electrical propertica, capillary pressure, waterfloodbehavior, relative penncabtity, dispersion, and simulatedEOR. For core analysis to predict tbe behavior of thereservoir, the nettability of the core must be the simeaa the nettability of the undisturbed reservoir reck.

When a drop of water is placed on a surface immersedin oil, a cuntsct angle is formed that rsngis from Oto 180[0 to 3.14 rad]. A typical oil/water/soIid system is shownin Fig. 1, where the surface energies in the system arereIated by Youngs equation, 4

am Cos 19=uo$-u. . . . . . . . . . . . . . . . . . .

(1)

where

aow = interracial energy [interracial tension (IFT)]between the oil and water,

LTO,= interfaced energy between the oil anddid,

o~, = interracial energy between the water andsolid, and

9 = contact angle, the angle of thewaterloillsolid contact line.

By convention, the contact angle, O, is me~uredtbrougb the water. Tbe interracial energy uOWis equal too, the IFT.

As sbowmin Fig. 1, when the contact angle is less than90 [1.6 rad], the surface is preferentially water-wet, andwhen it is greater than 90 [1.6 rad], the surface ispreferentially oil-wet. For almoat all pure flrdda and cleanrock or F&shed crystal surfaces, IJo, and UWhave valuessuch that O=0 [0 rad]. When compounds such as crude-oil components are adaorbed on rock surfaces, these in-tcrfacisl energica arc changed unequalfy. fMs changesOand hence the wettabtity. The farther Ok from 90 [1.6

CQwrmltsm %e~.! PemleumEm.e.=1246

rad], the greater the wetting preference for one fluid overanother. If 8 is exactly 90 [1.6 rad], neither fluidpreferentially wets the solid. As shown in Table 1, whenOis behveen O and 60 to 75 [0 and I twl.3 rad], thesystem is defined as water-wet. When 6 is between 180and 105 to 120 [3.1 and 1.8 tu 2.1 rad], the system isdefined m oil-wet. In the middle range of cnntsct angles,a systcm is neutrally or intermediately wet. The contactangfe that is chosen as the cutoff varies fmrn paper topaper.

The term am n., is sometimes called the adhesiontension, LTA5:

~A=v m Ows=aowcose. . . . . . . . . . . . . . . . ...(2)

The adh+ion tension is positive when the system is water-wet, negative when the system is oil-wet, and near zerowhen thesystem is neutmfly wet.

Mathods of Wettabitity MeasurementMaJJy different methods have been proposed for meas-uring the wettabtity of a system. 616 They include quan-titative methods-contact angles, imbibition and forceddisplacement (Amott), and USBM wettabfity methodand qualitative mcthoda-imbibition rates, micmscope ex-amination, flotation, glaas slide method, relative p&rnre-abdity curves, penneabilitykdumtion relationships,capillary pressure curves, capiflmimetric method, dis-placement capif.larypressure, reservoir logs, nuclear nrsg-netic resonance, and dye adsorption.

Although no single accepted method exists, tbrcc quan-titative methods generally are used: (1) contact-anglemeasurcnrent, (2) the Amntt 6 method (imbibition andforced displacement), and (3) the USBM method. g.17.18The contact angle mcasurea the wettsbilhy of a spcciticsu$ace, while the Amott and USBM methods measurethe average wettsbility of a core. A comparison of thewettsbllhy criteria for the three metbuds is shown in Ta-ble 1. we remaining tests in the fist are qualitative, eachwith somewhat different criteria to determine the degreeof water or oil wetness. Unfortunately, tbk leads to am-

JourmJo! PetroleumTechnology,November1986

bigrritieswhen expainrenta in the literature are compared.Many of the wettsbfity measurements arc alao imprecise,partimdzrly near neutral nettability. One, method mayshow that a core is mildly oil-wet, while soother showsthat the core is mildly water-wet. In tlrk paper, the differ-crrtmethnds of wettabfity mcaaurement are described, be-ginning with the quantitative methods. The morequalitative methods are then described, followed by themetfroda for measuring the nettability of fm@onal andmixed-wettabfity cores.

Quantitative Nettability MeasurementsContact Angle. The contact angle is the best wettabiityrncasurement method when pure fluids and artificial coresare used because there is no possibility of surfactants orother compnrmda aitcring the wettabthy. The method isalso used to determine whether a crude oil can alter wet-tabilhv and to examine the effects of temperature. ures-

F1.a. 1Wettabilitv of the oil/water/rnck svstem. 16

sure, and brine chemistrv on wettsbility~ Howe&, as tainin~ the mineral crystals is tilled with brine.discussed in more detaif _latcr, some dlff%ultiea arein-volved in applying contact-angle meaaurementa to reserv-oir cores. Marty methods of contact-angle measurementhave been used. They include the tilting plate method,sessile drops or btibbles, vertical rod method, tensiomet-ric method, cylinder method, and capillary rise method.

Descriptions of these methods can be found irr Admn-Son>4 Jobnaon and Dettre, 19 Good, 20 Neumann and0ood,21 and Popiel. 22 MO~tof these methods are nOtgenerally used in the petroleum industry, however, be-cause they are better suited for pure fluids with no ad-sorption or desorption of surfactants. Because of thesurface-active agenta in crude, a significant length of tineis needed for a contact angle to reach equ~lbrium.

The methods that aregenerally used in the petroleumindustry are the sessile drop method 21.23-25and a modif-ied form of the sessile dro method described by Iezch

!ef al. 26 and Treiber et al. 7 In both methods, the nrin-eral crystal m be tested is moumcd in a test cell compixedentirely .of inert materisls to prevent contamination. Thesessife drop method uses a single flat, polished minerafcryataf (see Fig. 1). The modified sessife drop methoduses two flat, polisbcd mineral crystals that are mountedparzllel to each other on adjustable posts, as shnvnr irrFig.2a. Becauae sandstnnea are ohcn composed primarily ofquartz and limestones of calcite, quartz or calcite crys-tals are used to simulate the pore snrfaces of the reser-voir rock. Obviously, the nettability of clays in thereservoir cannot be examined tit-h this method.

The first step in mcastuing contact angle is to cl+ theapp~ams thoroughly, because even tmce arnounta of cOn-tammants can alter the contact angle. Then the cell cOn-

DenW-gemted synthetic ~ormztion brine is used to pre-vent the introduction of foreign metal ions, which irrcon-centrations of only a few parts per million can alter the~e~bfi~.127 For the mOdifid aessile chop method, anoil drop is placed between the two crystals so fhat it con-tacts a large area of each crystal. After the oil/crystal in-terface hzs aged for a few days, the two crystals aredisplaced pamflel to each other. As shown in Fig. 2b, thisshifts the oil drop and allows brine to move over a por-tion of the surface previously covered with oil. The corr-tact angles measured in this fashion are calledwater-advancing contact inglea. A nonequilibriunr an-gle is obacrvcd inrmediately.after the drop is moved. Thkangle decreases for a day or two until a constant valueis obtained for that age of the oiUsolid irrterfacc. Theoilhnineral surface is then aged further, the water is ad-vanced again, and a new value is obtained.

The procedures sre similzr in the sessife drop methnd.A drop of crude oil is formed at the end of a fine capil-lary tube and brought into contact with the flat mineralsurface (see Fig. 1). The droplet is allowed to age on thesurface. The water-advancing and water-receding contactangles are measurd by usirrg the capillary tube to expandzrrd contract the volume of the cmde-oil drop. 23,24

When the crude oil containa naturaf surface-active sub-stances; the water-advancing contact angle increases aathe OiUcrystafinterface ages, approaching a iimhing valueas adsorption equilibrium is reached. To achieve this liit-ing vahre may require hmrdrcds or even tbOm~ds Ofhours of interface-aging time. Fig. 327 gives examplesof tbe change in the water-advancing contact angle as theoilholid interfke ages. This demon~es that early mcaa-

TABLE 1APPROXIMATE RELATIONBHIP BETWEEN WSTTABILITY, CONTACT ANGLE,AND THE USSM AND AMOTT WETTABIUTY INDZXES

Water-Wet Neutrally Wet Oil-WetContact angle

Minimum 00 60 to 75 105 to 1200Maximum 60 to 75 105 to 1200 160

USBM nettability index W naar 1 W near O W near -1Amott nettability index

Displacement.by-water ratio Positive Zem ZeroDisplacement-by-nil ratio Zero Zero Positive

Amoti-Harvey nettability index 0.3s/s1.0 -0.3

~ 1

i

:ECRKiTAL IWATER OIL/ CRYSTAL /l. ____.. __J

[a)

1-

.1

II

I

I1~L__.___;

(b)

I Fig. 2Contar$angle measurement. 7 I

urements can show that the system is water-wet eventhough it is actnally oil-wet at equilibrium. Because it con-tains no surfactmrts, the contsct angle for pure &cane(Curve A) did not change from zero as the interface wasaged.

Orie problem in contact-angle measurements is hyste-resis, because it is genirally found experimentally thata liquid drop on a surface cm have msny different stablecontact angles. The contact angles reported in the litera-ture sre either the water-advancing or water-~~g FOn-tact angle because these two angles are the mostreproducible. Tbe water-advancing angle cmr also bethought of as the oil-receding one. The advmrcing angle,Oad,, is measured by pulling the periphery of a drop overa surface, while the reding contact angle, @~; is meaa-ured by pushing it back. The dMference, O,~, - @,e., isthe contact-angle hysteresis snd can be greater thsn 60[1 rsd]. 19

Johnson and Dettre 19 and Adanrson4 state that thereappear to be three causes of contam-zngle hysteresis: (1)smface roughness, (2) surface heterogeneity, and(3) sur-face immobility on a macromolecular scale. To see howsurface ruughness can cause hysteresis, consider ahorizontal but rough plate. Because tie rough surface con-tains pesks and vslfeys, a liquid drop WY generslly beattached to a surface that is not horizontal. The macm-scupically observed contact sngle will not be the same asthe true contact angle on a microscopic scale. The rough-ness of the surface will 2U0w a large number of metaata-ble states of the drop to exist with different contact angles.The surface roughness wilJ generally diminish the appar-

9ent cuntsct nn e for water-wet mck and increase it foroil-wet reck. 1 ,22

1248

~ l-.* ..

Fig. 3-Approach to equilibrium contact angla.m I

Hysteresis resulting from surface heterogeneity can becaused by either heterogeneity in the rock surface com-position or differential adsorption of wettabifity-akcringcompounds. This problem is generally avoided by mcas-@g tie angle on a singfe crystal and by rigorously &an-mg the entire appsmms before measurement. Finslly,surface immobiihy can cause hysteresis by preventing thefluid motion necesssry for the contact angle to reach itaequilibrium value. For example, slow adsorption of a arw-factant from the soIid/Iiquid interface into the brdk liquidcan cause hysteresis. Some cmde oils w even form asolid film at the oil/water interface. 2s-30Tlwse Iihna sreparticularly likely to form if the crude has been exposedto oxygen, but have even been found in some anaerobiccmdes.

Even though it is possible, with great care, to get exactand reproducible contact-mrgle measurements, the ques-tion of how representative these results are of the wetta-btity of reservoir core arises. The contsct angle cannotmke into account rhe roughness, heterogeneity, and com-plex geometg of reservoir reck. First, consider the prob-lems caused by muglrness. Morrow31 bsa pointd out thatroughness and pore geumetry will intluence theoiJ/water/solid contact line and can change the apparentcontsct srrgle. On a smooth surface, the contact angle istixcd. On the sharp edges found in reservoir rock, how-ever, this condition is relaxed, and there is a wide rangeof pussible contsct angles. 31.32Morrow postulates thatmost of the oillwatcrfrock contact limes will be locatedat the sharp edges because, at these edges, the cuntact an-.gle can change without movirr~ the position of the con-tact line.

A second problem with applying contact-angle meas-urements to reservoir rocks is that the contact angle can-not take into account the heterogeneity of the reck surface.Contact angles sre measured on a single mineral crystal,while a core contains many different constituents. As dis-cussed preciously, surfactmrts in the cmde can affect thewettzbility of the sands and clays differently, causinglocalized heterogencmrs wettabifity.

A Wlrd limitation is that no information can bs gaindabout the presence or absence of permanently attachedorganic coatings on reservoir rocks. 33 These films canbe detected ordy by making other nettability measure-ments. This is particrdsrly importrurt when workirrg with~e~mr~-mte ~ores. 3435 Before the original we~bfitycan be restored, all the adsorbed materials must be re-moved, wbicli will generally leave the cure in astfonglywater-wet stste. The only way to determine whether thecleaning prccess b+ been successfid is to measure the wct-

Jo.mal of Petrol..m Technology,November1986

tabiilty of the cleaned core. If it is not strongly ivater-wet, addhional cleaning is necessay.

AIuott Method. The Amott method 6.8.36combines im.hibition and forced displacement to mcaaure the averagewetmbili~ of a core. Both reservoir core and fluids canbe used in the test. The Amott method is baaed on thefact that the wetting fluid will generally imbibe spontane-ously into the cure, dkplacing the nonwetting one. Thetatio of apontaneouaimbibition to fumed imMbition is usedtu kduCe the influence of other factors, such m Elativepermeability, viscosity, and the initial saturation of therock.

Core is prepared by centrifuging under brine until the,residwd oil saturation (ROS) is rcabhed. The Amott wet-tabilhy measurement then consists of the following fouratcps: (1) immerse the core in oil, and meamre the volumeof water displaced by the spontaneous (free) imbibitionof oil after 20 hours; (2) centrifuge the core in oil untilthe irreducible water saturation (lWS) is reached, andmeasure the total amount of water displaced, includingthe volume displaced by spontaneous imbibition; (3) im-merre the core in brine, atrd measure the volume of 01spontaneously displaced by imbibition of water after 20hours; and (4) centrifuge the core in oil until ROS isreached, atrd meaaure the total amount of oil displaced.Note that the core may be driven to IWS and ROS by flowrather than with a centrifuge. This is especially necessaryfor unconsolidated material that cannot be centrifuged.

The tmt restdta are expressed by (1) the displacement-by-oil ratio the ratio of the water volume displaced byspnntatmms oil imbibition alone, VW,Pto the total dis-placed by oil imbMion and centrifugal (forced) displace-ment, Vm,

~o=+, ..............................(3a)w,

and (2) the displacement-by-water ratio the ratio ofthe oif volume displaced by spontaneous water imbibi-tion, V..p, to the total oif volume dkplaced by itnbibi-tion and ccntritkgal (forced) displacement, Vol:

L=+. .. . . . . . . . . . . . . . . . . . . . . ..(3b)0,

. .

As shown in Table 1, preferentially water-wet coreshave a pusitive displacement-by-water ratio and a zerovalue for the displacement-by-d ratio. The displacement-by-water ratio approaches 1 aa the water wetness in-creases. Simifarly, oif-wet cores have a positivedisplacement-by-nil ratio and a zero displacement-by-water ratio. Both ratios are zero for neutrally wet cores.

Amott chose an arbitraty time period of 20 hours forthe spmmmeous oil and water imbibition steps in hismetbcd. If possible, we recommend instead that the coresbe allowed to imbibe until either imbibition is completeor a time liit of 1 to 2 weeks is reached. imbibition can@e from several hour.. to more tlIan 2 months to com-plete. M If the imbibition is stop~d after a short periodof time, then the measured spontaneous itnblbition volumewill be lower than the equilibrium value for low-

Jaud of PetroleumTechnolo~,i%enber 1986

permeability samples, causing an underestimation of 60or ~~. 8.36The mcaaurcd displacement ratios wilf under-estimate tie water- or oil-wemess of the ruck. Of course,it is necessafy to choose some upper time limit to finishOre measurement in a reasonable length of time. If thecore is still itnbibmg when the time limit is reached, how-ever, then the measured spontaneous imbibition volumewill underestimate the rcaemoir wettabfity, and the AIZIOttratios should be interpreted with caution. 8.36

A number of researchers37.3s used a modification ofthe Amott nettability test called the AntomHarvey relat-ive displacement index. This procedure haa an addi-tional step in the core preparation before the testis run:the cure is centrifuged first under brine and then undercrude to reduce the plug to fWS. The displacement-by-water and dkplacenzent-by-oil ratios are then calculatedby the Atnott method. The Amott-Hawey relative dis-placement index is the displacement-by-water ratio mi-nus the displacement-by-oil ratio:

I=8W80=++. - . . . . . . . . . . . . . . . ...(4)01 w

This combmes the two ratios into a single wettabflityindex that varies from + 1 for complete water wetnessto 1 for cnmplete oif wetness. Cuiec 39 states that thesystem is water-wet when +0.3s I< 1, intermediate wetwhen 0.3

I00iVERAGE WATER SATURATION, PERCENT

*

*

10~ OIL WET LOG A,,A2=-O.5+ ~

.

0\

I00AVERAGE WATER SATURATION , PERCENT

Ii NEUTRAL LOQA,/A~= 0.02

~

wet. the area under the bfinedrive cntdlkwv messure

k/ERAGE WATER SAT URATION, \ PERCENT!

lg. 4-USBM wettcbility measurement 9: (Ibrine drive. U-oil drive) (a) untreated core, (b) cora treated with organochlo-osilanes, (c) core pretreated with oil for 324 hours at 140R brine contains 1,000 ppm sodium tripolyphosphdte.

USBM Wettabiity Index. The third quantitative test thatis used to measure tie nettability is the USBM test de-veloped by Donaldson et al. 9,17,18The USBM test olsomeasures the average nettability of the core. The test isrelatively rapid, requiring a few &ys to test four to eightplugs. A major advsntage it has over the Amott wettabfl-ity test is its sensitivity near neutral nettability. A minordisadvantage is that the USBM wettabili~ index can onlybe measured on plug-size samples because the samplesmust be.spun in a centrifuge. The USBM test comparesthe work necessary for one fluid to displace the other.Because of the favorable free-energy change, the workrequired for the wetting fluid to displace the nonwetdngfluid from the core is less than the work required for theoppsite displacement. It has been shown that the requiredwork is pro rtionsl to the area under the capillary pres-aue ~umeY,46 ~ ~ti~~ ~~~~a, wh~~ a ..~~ is ~at~~.

1250

cu~e (when the water displaces the oifj is srn~ler thanthe area under the capillary pressure curve for the reversedisplacement. In fact, if the water-wetting is stiongenough, most of the water wifl spnntmeously inddhe intothe core, and the area under the brine-drive curve willbe very small.

Before the testis mn, plugs are prepared by centrifi-gation under oil at high speed to drive them to lWS. Thispoint is denoted by the satefiaka (*) in ,Flgs. 4a though4c, which represent wettabfli~ test results in cores withthree tifferent sufface treatments. During the USBMmeasurement, a modified version of the procedure de-scribd by Hassler and Bnmner47 and Slobod et al. 4s isused to calcnlate the centrifigaf capillary pressures. (TheUSBM method uses the avefage saturations in the core. 17J.o mntrsat, the centrifugal capillary pressure cufve is

Journalof PetroleumTechnology,November1986

based ontbe saturation at the face of the core, which isczfculated from the ayerage saturation by the methodfound in Ref. 47.) In the first step of the measurement,cores are placed in brine and centrifuged at imcrementzl-ly increasing speeds until a capilla~ pressure of 10 psi[70 kpa] is reached. This step is known as the brinedrive becauae brine displaces oil from the core. At eachincremental capiflary pressure, the average saturation ofthe plug iz calculated from the volume of expelled oif.Curve I (Figs. 4a through 4c) is a pIot of capiIlary przs-aure vs. the average saturation for the brine drive.

In the second step, the core is placed in oif znd cen-trifuged. During this oil-drive step, oil dkplacea brinefrom the core. As in the first step, the capillzry pressuresznd averzge saturation are measured until a capillarypressure of 10 psi [70 kpa] is reached. In each czae, thecurves sre Iinezrly extrapolated or truncated if the lastpressure ia not exactly 10 psi [70 Wa]. The results of theoil drive are plotted as Curve II in Figs. 4a through 4c.

The USBM method uses the ratio of zreas under the.two capillary prezzure curves to czlctdate a we~btily in-dex according to l?.q. 5..

w=10g(,4, /A*), . . . . . . . . . . . . . . . . . . . . . . . . ...(5)

where,4 ~ snd A2 we the areas under the oil- and brine-drive curves, respectively. As shown in Table 1,. whenW is greater thzn zero, the core is water-wet, znd whenW is less that zero, the core is oil-wet. A wettzbilty in-dex nezr zero mezns that the cure is neutrzlly wet. Thelzrger the absolute vzlue of W, the greater the wettingpreference.

Exzmplcz of water-wet, oil-wet, znd neutrzlly wet coressre shown in Figs. 4a though 4C for an initizlly water-wet outcrop Torpedo sandstunc core. Fig. 4a shows theUSBM wettahility index of the untreated water-wet core.The srea underthe oil-drive curve is much lsrger thsnrhe zrez under fhe water-drive curve, yiehiing a wettz-bility index of 0.79. In Fig. 4b, tbe core was treated withzn organosibne compound, which rendered it oil-wet. Ikezrea under the oil-drive curve is now much smzller thsnthe mea under the water-drive curve because oif is thewetting fluid, yielding a wettabWy index of 0.51. InFig. 4c, the core was aged with crude, znd the brine wzztreated with sodium tripoIyphosphate. The core is nowneutrzlly wet, znd both of the areas zre equal, mzkingthe USBM wettabfity index zero.

A major advantzge of the USBM wettzbility test overthe Amott testis its sensitivity nenr neutrzl wettzbflity.On the other band, the USBM test cannot determinewhether a system has fractions or mixed wettzb~ity,while the Amutt test is sometimes senzitive. In somefractional- or fixed-wet systemz, both water znd oil willfiblbe frmly. 49-51me AMUti method will have pOSitiVedisplacement-by-wster znd displacement-by-d rstios, in-dicating that the system is nonuniformly wetted.

Cumbined Amutt/USBM Methud. Sharma andWunder1ich51 bzve recentfy developed a modification ofthe USBM method that ZIIOWS,the czdculation uf buth theAmott ind USBM wettzbdity indices. The procedure,shown in Fig. 5, haa tive steps: (1) initizl oil drive, (2)spontaneous (free) imbibition of brine, (3) brine drive,(4) spontanecma (free) imbibition of oil, znif (5) oil drive.Journal of PetroleumTechnology,November19s36

The zreas under the brine- znd oil-&lve curves zre usedto calcufate the USBM index, while the Amott index usesthe volumes of free and totzl water znd oil displacements.

During the initizl oil-drive step (Curve 1), the plugs aredriven to IWS. Next, the cores we immersed in water,and the vulume of water that imbibes freely is measured(Curve 2). During the brine-dive step (Curve 3), the aver-age saturation of the plug is determined from the zmountof expelled uil at each incremental capilla~ pressure.These data zre used to cslculate the area under the brine-drive curve, A2, for the USBM method. At the end ofthe brine-drive step, the plug is left at ROS. The Amottdisplacement-by-water ratiu, 5~, is the ratig of the oilvulume displaced by free brine imbibition to the totzlvolume displaced by free imbibition znd centrifugzJ dk-placement (Eq. 3a).

In the fourth step (Curve 4), the plug is immersed inoil, znd the volume uf oil thzt imbibes spontaneously ismeasured. The finzl step is the oil drive (curve 5), wherethe czpillmy pressures znd average saturations are usedto czhxdate A, for the USBM method. Eq. 5 is then usedto calculate the USBM wettzbili~ index. At the end ofthe oil drive, the plug is left at IWS. The Amott dkplace-ment-by-oil ratio, 60, is the ratio of the free oil imbibi-tion to the totzl volume displaced by free imbibkion andcentiitlzgal displacement (Eq. 3b).

There zre two advzntzges of the combined USBM/Amott method over the stzndard USBM method51: tberesolution of the USBM method is improved by account-ing for the saturation chznges that occur at zero capillarypressure, znd the Amott index is ZISOcalculated. As dis-cussed eadier, the AMOttmethod will sometimes indicatethat a system is nonuniformly wetted.

Qualitative Wettabiti,tyMeaaurementiImbibition Method. The most cummonly used qunMz-tive nettability measurement is the imbibition meth-od, 52.57because it gives a quick but rough idea of thewettsb~hy without requiring zny complicated equipment.The originzl imbibition appzrztus tested the wettabfityat mom temperature and pressure. 52

More recently, Kyre er al. 57 described a modificationof the apparatus that zllows wettzbility to be measuredat reservoir conditions. In zn imbibition test, a core atIWS is first submerged in brine undementb a graduatedcyliider, znd the rzte and zmount of oil dkplzced by brineimbbitiun zre measured. The core ii strongly watier-wetif l.zrgevolumes of brine are rzpidly imbibed, while low-er rates znd smzller volumes imply a more wezkfy water-wet core. If no water is imbibed, the core is either oil-wet or neutrzlly wet. Non-water-wet cores zre then drivento ROS znd submerged in oiI. The imbibition apparaNsis inverted, with the grzduated cylinder below the coreto measure the rate and volume of water dkplaced by uilimtdbltiom If the core imbibes oil, it is oil-wet. Thestrength of uil-wetness is Miczted by the rste and volumeuf oif imbibition. If neither oil nor water is imbibed, thecure.is neuuslly wet. Finzlly, some cores will imbibe bothwater ~d ofi. 49-s] These cores hzve either fiaCtiOnidormixed wettab~ky. One problem with the imbibitionmethod is that, in addition to wettahiity, imbMion ratezzlso depend on relative permeabfity, viscosity, IFT, purestructure, znd the initial saturation of the core. 3,10 Fre-quently, this dependence on other vzriables ia reduced by

1251

zL

:

2$2K

55#:

0 1(AvERAGEWATERSATURATION,PERCENTW

Fig. 5Combhmd Amott/USBM method.51

100

comparison of the measured imbibition rate with a refer-ence rate measured when the core is strongly water-wet.To do this, the core is cleaned by heating at 750F[400C] for 24 hours to oxidize alf of the organic materi-al, leaving the core strongly water-wet. The core is thenremturated m ita origiual oiJaatnration with a refined whiteoil having the same viscosity aa the crude oiJ, and fhereference imbibition rate is measured. Denekaa et al. 53~~~ nettability changes in terms of the relative rateof Imbibitiorx

R=

of residual oil form spherical drups in the center of thepores. If the system is intermediately wet, ,both oil andwater will be found in contact with the rock surfaces, ,~dboth can be found in the smslf pores. Finally, if the sys-tem is oil-wet, the roles of the oil and water are reversed.The ofl forms a fti around the grain surfaces and is foundin the small pores, whfle the water rests on an oil tilmor forms small spheres.

The method of quzlitstively deteruining the wettabfi-ty by microscope examination is pmticukly importantin the study of wettabil@ reversaJs, 58>m&one of thepropoacd mechanisms for EOR that occurs dining alka-fine waterfloodmg. m In ~e~e ex~tienta, a chefi~that changes the nettability is injected into the porousmedium during a waterflood, causiug a zone of wettabd-ity reversal to propagate through the core. A microscopeis used to follow wettabtity changes and to determinewhether EOR will occur by thk mechaniam.

Flotation Methods. Flotation methods are fast but workonly for strongly wetted systems. In the sinrpleat method,water, oil, arrd sand are placed in a glass bottle. The bot-tle ia shaken, and tie experimenter observes the fate oftie smd gains. 65-6s This method is recommended byAPI for determining the effects of surfactarrts on netta-bility. C If the system is strongly water-wet, clean sandgrains will settle to the bottom of the bottle. Sand grainsplaced in the oil wilI aggregate aud form small clumpsof grains surrounded by a thii layer of water. If the sys-tem is oil-wet, some of the grains can be suspended atthe oil/water interface. Oil-wet sand grains in the waterwill clump together, forming small oil globuka coatedwith sand. This flotation system is qualhative and workaonly for strongly wetfed systems.

SeVeFJ experimenters 69.70 have used more elaborateflotation tests developed in the minin industry that were

?based on liquid/liquid extraction. 22,7 In these tests, par-ticles are initially suspended in water. A second fluid,either oil or sir, is bubbled from below. The parricles thatare water-wet remain in the water, while the hydropho-bic, oil-wet particles adhere tu the oil (air) and rise to thesurface. The fraction of articles in each phaae can thenbe measured. elenrmtz% used the flotation method tomeaaure the wettsbihty of small clay particka, which cmr-not be conveniently me$suked in any other way. Untreat-ed, strongly water-wet particlea would not float. Afterexpusrue to cmde, the clay particles floated, demonatrat-irrg that their nettability had been altered:

Flotation tests based on liquid/liquid extraction appeartn divide particles intu two categoria$ stron 1 water-wet

~2~~BesidG ~~and mildly water-wet tu strongly oil-wet. ,wettabilky, flotation of a particle also depends on parti-cle size, particle density, and IFT. A small particle withlow density and high IFT might float if the contact anglewas ~eater than about300 [0.5rad]. On the other hsnd,the minimum contact angle for flotation of a large, denseparticle could bc 90 [1.6 rad]. 72,73

Glass SIide Method. Another esrly qualitative nettabil-ity meaaurcment tcduzique is the @as slide methnd, 30,mwhich assumes thst a glass surfsce is representative ofthe reservoir rock. A clean, dry, glaas microscope slideis suapmrded in a layer of cmdc oif floating on water ina transparent contsiuer ~d aged. The glaas slide is then

Journal of PetroleumTechnology, November 1986

lowered iuto the water. If tie slide is water-wet, the waterquickly displacea the oif on the slide. On the other hand,if the slide is oil-wet, a stable oil-wet tilm is formed, andthe oil is very slowly d~placed. Reisberg and Doscher30aged slides in crude oil snd found that it took up to 30days for the 6MI wettabtiiy to be reached. Cwke st al. 5sused a simple variation of the glass slide method as aquick, qrmlitative test tu screen different acidic-oil/alka-lii~water comblnationa for uae in zdkaliie waterflood-ing experiments They placed oil and water without-g fi a glass vkd and waited to see whether a stableod-wet tihn formed on the vial. This was determined bytilting the vial srrd seeing how the water and oil behavedon the previously oil-covered surface.

Relative Permeabfity Methods. A number of quaMa-tive methuda are baaed on the effects of wettabfity on rela-tive penneabtity. However, they are all suitable ordy fordiscriminating between strongly water-wet and stronglyoil-wet cores. A smaller change in nettability- e.g.,be-tween strongly and muderstely water-wetmay not k nn-timd by these methcda. One method developd by Ehrlich~d WYg~74 is based on the rules of thumb given byCraig7 to differentiate be~een stmmgly water-wet aud~mon~y ~fi-wet ~or=. C~g, ~16,52,75 fieS ~f ~mb me

. .

aa fouows.1. Connate water saturations are usually greater than

20 to 25% PV in a water-wet rock, but less than 10%PV in an oil-wet rock.

2. Water aaturztion at which oil and water relative pcr-meabtities are equal is generally greater than 50% forwater-wet cores and less than 50% for oil-wet ones.

3. The relative permeabfity to water at floodout isgenerally less than 30% in water-wet rocks, but frnm 50to 100% in oil-wet ones.

These relative permeabtities are based on the oil per-meabtity at the comate water saturation. Examples ofrelative perrneabfity curves in strongly water-wet snd oil-wct corer taken from Craig7 are given in Fig. 6. Notethat Raza et al. 16 state that there are exceptions tn thegezieral rule that the connate water saturation is higherfor a warcr-wet rock than for an oil-wet one.

Treiber et al 27 pro~sed a secrmd quali@ve techniquefor strongly wetted rocks The method comp=es theoil/water, gas/oil, and gas/water relative permeabilkie.sand takes advantage of the fact that relative permeability

:E.;E?JF2:EP;:,:EZZ$2:E;water-wet, the relative perrneab~ky tu oil (the preferen-tially wetting phaae with respect to the gas) in the gas/oilrelative pcrmeabtity test shordd be a continuation of therelative permeabdity to the water (the wetting phzae) inthe water/Oil relative pcmzeabil~ test. 76 If significantdifferences are observed, the sample is not stronglywater-wet.

An example of the comparison of the relative perr22ea-btity curves in a strongly water-wet core taken fromOwens and Archer76 is shown in Fig. 7. The gas/oildrainsge relative perrneabfity, where the oil ia the stron-glywetting fluid, is shown as the dotted Iiues. The water/oilrelative pcrrneabfity, where tbe water is the strmrgly wet-dug fluid, ia shown aa the solid lines. Note that the waterrelative penneabi3ity, where the wetting fluid saturationis iricreasing, is a continuation of the oil relative pe2n2ea-

1253

o

CO

T

OIL

\

./

wATER

0.1 1 1 1 10 20 40 60 60

\GA2\\\\\I

WE771NGFtlXi5 SATURATION,PERCENTFORE ?PACE

i9. 7cOMpariSOfl of gasioil drainage and water/oil im.ibition relative permeabltity relationships. Torpedoandstone. 76

Thi~ demonstrates that iite core is water-wet.Ba~c~ et al. 78 developd a third wettSb~@ memu~-

ment technique that is based on unsteady-state relstive per-meability. Their method uses the capillary end effect thatoccurs when a core initially at IWS is waterflooded at aconstant, slow injection rate. The end effect is the accumtJ-Iation of wetting phase near the outlet end of the corecaused by the discontinuity between the porous medium~d the otfet pipe. 79 An ihcreased pressure drOP cmoccur because of tbia wetting fluid accumulation. Batyckyet al.s relative-permeab fitytwettabihty tests are run atvery slow flow rates, so end effects are very importantin determination of the pressure drop across the core. Incontrast, standard unsteady-state relative pertneabiltymeasurements use high flow rates to minimize the endeffect.

Batycky et a-f.determined the wettabfky by waterflcod-ing the core at very low rstesuntil the ROS WS.Vreached.The flow was stopped to alfow the fluid to redistribute,then restarted in the reverse direction. The core k water-wet if there is no change in the pressure drop after theflow reversal snd oil-wet if the pressure drop is reducedimmediately after the reversal. In a water-wet core atROS, the wetting fluid saturation will bc high through-out the cure, with no addkional water accumulation at ibeoutlet end. 7s,s0 There will be no redismibution of fluidswhen the flow is stopprG consequently, the pressure drupwill not change. On the other hand, if the core is oil-wet,capillaty fortes will cause oil (the wetting phase) to .ac-cumtdate near the outfet. The pressure drop caused by

12s4

bditv. where the wetting fluid saturation is decreasing.

%

a%.:AA *3

AAA

& AWAT&T ROCK

(NUGGET SAND]hAA0 AA0 A ~3m A : C.L0 A000 OIL-WET ROCKo 0 [SWINGER ~D1.0Ok~

CONNATE WATER SNUR.T,ON % W

~9. 8Relatiom~~ betwean connate water saturation at!urpermeabltity.

this oil accumulation is detected by stopping the flow,thereby allowing capillary forces to redistribute the oilevenly throughout the core. When flow is started in thereverse direction, the pressure drop will initially be low-er, gradually rising to its original value as the end effectis re-established on the opposite end of the core.

Permeahiity/Saturation Relationships. Two qwdiativemethods based on sir permeability and fluid saturationshave been proposed. Both methods are statistical, requirea relatively tige number of sqnples, and give only a veryrough idea of the wettabfli~. The advantage of themethods is that only routine core anafysis measurementsare required. However, the reliability of these methodsis unknown. The methods are afso limited to core sam-ples without significant fractures or vugs, in which thepore structure determines the air permeability.

~ et al. 16 proposed an empirical methud to deter-mine reservoir nettability based on connate water satu-ration and air penneabihy. To obtain the connate watersau.tration,core is obtained with an oil-based drilliig fluid,then the freshly cut cores are analyzed for their water con-tent. The cores are extracted and dried, and the air per-meabi@ is messured. A qualitative measure of thewettabtity ii obtained by plotting the comate water satu-ration vs. the sir permeability. Fig. 8 shows exanrplesof tlte plot for strongly oil-wet and smonglyw6ter-wet con-ditions. 16For the oil-wet case, the average connate watersaturation is generally relatively low. The curve is near-ly vertical and extends over only a smsll saturation inter-val. Conversely, for the water-wet reservoir, the curwe

Jourmtof PetroleumTechnology,November1986

has a gentle slope and extends over a large saturation in-terval.

Frehse81 proposed a second statistical method .b,ascdon the assumption that low-permeability core samples wiJlhave a higher wetting-phase saturation than the high-perrnmbilhy ones. For a uniformly wetted rock, the smallpores are fdled with the wetting fluid, while the largepores contain both the wetting and nonwetting fluids. Incomparison to higher-permeability samples, a low-permeability sampIe will generally have spore structurecontaining a larger number of small pores that are filledwith the wetting fluid. To determine the nettability,Frehseclassifica the routine core mralysis samples intodifferent permcab~ky ranges. The saturation distributionsfor the bighfit and lowest permeability ranges are thencompared. For example, consider a core taken with awater-based mud, where the reaiduaJ oil saturations areknown. The rcacrvoir is assumed to be oil-wet if the 10W-permeability samples haye a higher average ROS andwater-wet if the high-permeability sarnpIes have a higheroil saturation. Currently, this method appears to be theo-retical only. We are not aware of auy.tests comparing theresults of tbk method with more standard nettabilitymeasurements, such as tbe Amott nr USBM indices.

We feel that wettabflty evaluations based on air per-meabili~ and fluid saturations should not be used at pres-ent. Rara et d.s method is empirical, and it is not kuownwhether it is generally vrdid. Frebaes method hw not beentested. UntiI tbesemethods are evaluated hy comparisonwith standard wettabdity measurement, they should beconsidered mrreliable.

Grigorevs2. proposed a theoretical method for deter-mining an appwent contact angle based on the IWS andROS. The method is probably not generally valid. It isbased on a large number of unprovenasaurnptions aboutthe bebavior of the waterloillrock system. In addhion,there do not appew to be my testa comparing thk methodwith other wettabdit y measurements.

CapilJ~3 Pressure Curves. As far back as 1951,Calhoun suggested that tire entire capillary pressurecurve should be used to measure the nettability of thecore. Gatenby and MaradenM were the first to examinethe use of tbeareas under the capillary pressure curvesfor thk purpose. The capillary pressure curves used werethe complete drainage and imbibition curves for both pesi-tive and negative capillary pressures measured by theporous plate method. The two areai that they examinedwere the total area surrounded by the drainage and imbLbhion capillary pressure curves and the area under theoil-drive c!rrye. They found that neither of these areas cor-related well with the nettability of the cnre. However,Donaldaon et at. 9 later showed that the areas that shouldbe measured were the areas under both the oil-drive andbrine-drive curves. Tbii is the basis of the quantitativeUSBM method dkcussed earlier.

Capillarimetric Method. Johansen and Dunnirigg547 de-veloped a qimhtative wettabdity measurement that meas-ured the adhesion tension, a cos .9, in a glass capillaryNbe. In this capilkwimetric method, the top of the tubeis connected to a column filled with oil, wh]le the bottumis connected to a column filled with water (see Fig. 9).The top of the water colunru cm be raised or lowered rela-

Joumalof PetroleumTcchnoloa, Novgmber1986

v

Fig. 9CapilIarlmetric method.

tive to the oil cohunn, changing the hydrostatic head. Asthe hydrostatic head is changed, the oil/water interfaceswill rise or fall in the tube until the capillary forces bal-ance the gravitational forces:

20 cos eP,= =$(poho-p~hv+), . . . . . . . (7)

r

wherer = radius of the capillary tube,

P. = oil demi~,p ~ = water density,ho = height of the oil column above the

oil/water interface, andh~ = height of the water column above the

o-illwater interface.

Eq. 7 can be rearranged to calculate the product of aandcos 0, which Johansen and Dunning called the dkplace-ment energy (adhesion tension):.

wED=acos @= T(poho-pWhJ. . . . . . . . . . ...(8)

The dkplacement energy is positive if water wets theglass and negative if oil weta it. If one of the liquids com-pletely wets the glaas, then the contact angle is zero, cos8 ia unity, and the displacement energy is equal to the IFT.Johansen and Dunning usually changed the height of thewater column so that the interface moved over an areapreviously cnvered by oil; hence the contact angle in Eq.8 k water advancing. The capillarimetric methcd assumesthat glass is representative of the reservoir rock and there:

1255

fore is generslly only qualitative. Because this methodmeasures the product .s cm 0, the problems dkcussed inthe section on contnct angles SISOhinder this meifmd.

Displacement Capiffary Pressure. One of the earliestwettabWy measurements was the displacement capillsrypressure method, which uses the threshoId capillary pres-sure to calculate am apparent contact augle. 8894Thismethod is now used infrequently, however, because poregeometry effects can cause the calcqated contsct angleto differ greatly from the contact angle measured on a flatplate. 3 The displacement (or threshold) capilla~ pres-sure is the capillaty pressure at which nonwetting fluidwill first enter a core initially 100% satorated with thepreferentially wetting fluid. h apparent contact sngle iscdcnlatcd from the threshold capillary prssaure by mcdel-ing the mck as a straight, cylindrical capillary tube4, 10:

20Cos 8=P~=, . . . . . . . . . . . . . . . . . . . . . . . ...(9)

Tmx

where PT is the displacement capillary pressure, a is theIFT, O= is tbe apparent contsct nngle, and r- is theradius of the pure through which the nonwetting fluid be-gins to enter the core. Because the capillsry pressure need-ed to inject nonwetting fluid is reduced as the pore radiusis increased, rmax is m mfersge of the radii of the lar-gest pores in the core. Note. @it one limitation of thismethod is that it examines the wettabfity of only the lsr-gest pores. Because Eq. 9 has two unknowns, 0. ~drm=, the oofy way to solve for the apparent contact an-gle is to mske additional assumptions. It is usuallyassumed that some fluid exists that wfil completely wetthe core, so cos 8=1, and rmax csn be calculated. Thisallows the contact angle, to be computed for other fluidpairs.

Slobod and Blum~3 proposed two aenriqnantirativewet-tsbility measurements baaed on the displacement capjl-fary pressure, the wettabfity number, and the apparentcontact angle. The wettabfity number is calculated by car-ving out two dkplacement experiments-first, water byoil, and second, oil by sir. Eq. 9 for the oillwatcrlrocksystem becomes

2U0-W Cos &o.wP(o-w)r= . . . . . . . . ... . ... (lOa)

TIM.

and for the nirJoil/rock system,

2U=.-0 Cos %.-OP(.-O)T= . . . . . . . . . .. . . . . ..(lOb)

rmax

In hth equations, the radius of the pore is assumed tobe the ssme. The wettrrbiMy number, N, is determinedby solving Eq. 10 for the ratio of the cos Oterms:

cm O.-.N==

D.-OP(O-W)T. . . . . . . . . . . . ..(11)

Cos 6.-0 uo-w%m

12.56

Slobod snd Blum stated that if it were assumed that theoif is.completely wetting in the oillairlrock system, thencos .9..0 is unity. An apparent contnct angle for theoil/water system can then be computed from Eq. 11:

C.-OP[O-W)TcOs(oo_w)= = ... . . . . . . . . . . . . . .

Oo-wp(a-o)T, (12)

Slobod snd Bhuu resEzed that their nasumptions wereonfy approximately true and that the contact angle thatcould be calculated from the displacement pressure was,at best, only semiquantitative. In general, the apparentcontact angle measured from the displacement pressnfeis not cqusl to the contact angle messured on a smoothsurface because of pore geometry effects. Morrow sndMS c0workers4345 compared apparent contact ~~escomputed in sintercd teflon cores using pure fluid withthe true contact sngles measured on a smcotl teflon plate.There wss no chnnge in the apparent contact sngle whenthe tme contact angle was varied from Oto 22 [0 to 0.4rsd]. In addition, when@ was greater than 22 [0.4 radl,the apparent contact sngle wna always less thsn the truecontact angle. Fiiy, in some cases, the apparent cOn-tact angle calculated from the dkplacement pressure csnshow the wrong fluid to be the wetting phase. Positivedisplacement pressures for both fluids, particularly whenthe mre is initially 100% saturated with the other fluid,have been frequently reported in the litera-ture. 41.U.B,W,W.95-WWhen a positive displacement pre3-aure is required for both fluids, the fluid with the lowerdisplacement pressure is the preferentially wetting flqdbecause less energy is required to force. it into thecore, S3.98 &der~0n3 provides further discussion.

Reservoir Logs. Grshamw proposed a method to meas-ure the.nettability of in-situ reservoir mck with logs thatwsa baaed on rhe fact that the electrical resistivity of anoil-wet rock is hi her than that of a water-wet rock at the

fssrne saturation. In Grahams method, the formation is.injectd with brine, and resistivity logs are run. The for-mation is then injected with the same brine containing areverse wetting agent, which will change a water-wet for-nrstion to sn oil-wet one; if the formation is already oil-wet, the reverse wetting agent will not alter the wettdil-ity. After logs sre rerun, the nettability of the formationcm be determined by comparing the two resistivity meas-urements. If the formation was originally water-wet, thechange to oil-wet will increase the resistivity. If the fOr-mation was oil-wet, no change in rcsistivity will be ob-served.

Holmes and Tlppie lm proposed a second method thatcompsres lugs with core data. The saturation in a forma-tion is first measured with logs and the data convertedinto a capillsry pressure curve. Next, the capillaW pres-sure is messured in a clean water-wet core where it issssumed that the contact angle is zero, snd the two cspif-lnry pressure curves nre compared. If they agree, thereservoir is strongly water-wet. If they do not agree,Hofmes snd Tlppie model the porous medium as a seriesof strsight cylindrical capillaries and determine the ap-parent contact angle with equations similsr to those dis-cusacd in the previous section on displacement capiffarypressure. Because of the number of approximations, this

Journalof PetroleumTechnolom..November1986

aPPafent cOntactangle wfi provide only a rough estimateof tfre actual reservoir wettabdity.

hleaaurement of Fractional andMixed NettabilityInfractional wetted cores, a p@ion of the rock is stronglywater-wet, whtie the rest is strongly oif-wet. The termmixed wettabifhy was introduced by SalatMeI lol torefer to a special type of fractional wettablli~ in whichthe oil-wet surfaces form continuous paths through thefarger pores. Additional information can be found inRef. 1.

Nuclear Magnetic Relaxation. Brown and Fatr 102anduthers 103,lW proposed a nuclear magnetic resonancef,NMR) method for determining the fraction of the corethat is oil-wet vs. water-wet irr a core with fractional net-tability. The method rrses the nuclear magnetic thermalrelaxation time for water protons (hydrogen) in poruusmedia. To measure the relaxation time, the sample is firstexpused to a smmrgmagnetic field, which makes the nucleiof the hydrogen atoms line up with the field. The coreis then exposed to a much weaker field. The nuclear mag-netic relaxation time, which is the time it takes for thehydrogen nuclei to adjust (relax) to the new field, is meaa-ured. There are two relaxation times: relaxation of thecomponent parallel m the field is called thermal relaxa-tion, and relaxation of the component pe errdicrdar to

Tthe field is cabl transverse relaxation. 05 The ther-maf relaxation time is the time used to measure fractiOn-SI wettabifity.

For thermal relaxation to occur after the magnetic fieldis changed, Oreprutuns rnuat dissipate some of their energyto random thermal motion of the molecufes. The protonsare only lurwely coupled to their environment, so they re-quire a time on the order of seconds to adjust to the newmagnetic field, which is a verj long time for atomicproceasca.

The use of nuclear magnetic relaxation times to meas-ure wettabflity ia based on the observation that the sur-faces of the porous media can significantly reduce themeasured relaxation time. 102.105When a proton is neara surface, it cmr bccume temporarily bound to the sur-fiace,relaxing much faster than in tie bulk fluid. The wet-tabili of the surface can influence the relaxation

?dine. ~-1135 oil-wet surfaces cause ~ SmWer ~~ctionin relaxation time than water-wet surfaces.

Brown and Fatt 102 exmnincd 100% water-saturatedsand packs in which a fraction of the aand grains werewater-wet and the remainder bad been treated with an or-garmchforosilarre tu render them oil-wet. They found afinear rclatiun between the rafaxation rate and the frac-tion of oil-wet surface area. f,The relaxation rate is theinverse of the relaxation time.) The greater the fractionof oil-wet grairra, the longer the relaxation time, and theslower the relaxation rate. Krmrar et al. lM measuredrelimtion times with 100% water-saturated bead packacomposed of water-wet glass beads and non-water-wetpolymetlryhnethacrylate beads. The relaxation time in-creased heady as the fmctiun of non-water-wet beadsincreased.

Brown and Fattlm and Knmar et al. lM applied theirmethud only to sandpack and beadpacka. Deveraaux los

Journalof petroleum Technology,November1986

found that aaphaftene adsorption in sandstone cores coufdafso increase the relaxation time. In one set of experi-ments, clean smrdstone plugs were saturated with cm&oil, then aged for several days. The bulk of the uil wasremoved by flushing with cyclohexane, leaving bebinda Iilm of asphaltenes on the ruck surfaces. The plugs weresaturated with water, and the relaxation time rneasurcd.The adaorbed film increased the relaxation trme whencompared with the time for clean plugs. In another ex-periment, a plug was saturated with water and crude,aged, flushed with cyclohexmre, then saturated with water.The nuclear magnetic relaxation curve for this sample hadthree components: (1) a fast component for water in thesmall pores, (2) an intermediate component for water inthe large pures, mrd (3) a slow compmrent for water intire large pores that had been fdled with oil and coatedwith asphaltenes. However; Devercaux dld not suggestmy way to use this to measure the wettabfity.

Brown and Fatt 102also proposed a nuclear magneticrelaxation method to measure the wettabflity of reacrvuircore, which app=ently has not actually been used. Themethod compares the nuclear magnetic thermal relaxa-tion rate of the untreated core with reference measure-ments on the same core in both strongly water-wet andstrongly oil-wet states. The core is. first flushed withtohrene or hexane to displace all of the brine and oil. Af-ter vacuum drying, the core is saturated with distilledwater, and the therrnaf relaxation rate is measured. It isassumed that the preparation procedure above IWSnot al-tered the wettahility of the core. Next, the core is madestrongly water-wet by flushing with methanol and chlo-roform or by ftig at 950F [51OC] tn remove all oftbe adsorbed surface material. The nuclear magnetic ther-mal relaxation rate of the core in this water-wet iefer-ence stare is mcasrrred. FAy, the core is trcarcd withan organocbforosikine, which renders it strongly oil-wet,and the thermal relaxation rate is measured. The refer-ence relaxation rates for the core when it ia strongly water-wet and oil-wet are plotted vs. the percent of oil-wet sur-face, and a straight line is drawn between them. Assum-ing a linear relationship between fractional wettabdity andrelaxation rate, the fictional wettabiity of Orenative-stateuntreated core is then found by plotting ita relaxation rateon this straight line.

Unfornrrrately, this proposed procedure suffers fromseveral problems. First, the functional relationship be-tween relaxation rate and fractional nettability is not clear.Brown and Fatt found a linear relationship between frac-tional wettabilhy and reaction rate, whale Kumar et al.found a linear relationship using reaction time (the inverseof reaction rate). Second, aa discussed in Anderson, 1 themethnda that Bruwn and Fatt suggest to prepare the origi-md core will generally after the mtive-state wettabflity.Finally, it is not possible to tell whether the cleaningmethod haa rendered the core totally water-wet or the or-ganocblorosilane traamrent has rendered the core totallyuil-wet. Irr some cases, cure treated with an organo-cblorosikme is only neutrafly wet. I

Dye Adaurption. Holbruok and Bernard lM used the ad-sorption of metbylene blue from an aqueous solution in-jected into a cure to measure fractional nettability. Thismethod successfully mcaaured the wettabtity of fraction-

1257

ally wetted sandpacka containing mixtures of oil-wet andwater-wet sands. However, the method will probably notwork for reservoir cores mntaining krrgearnounKof clay.

Iz3 this merhod, water-covered rock surfaces areassumed to be water-wet, whfle the oil-covered ones areassumed to be oil-wet. The technique is based on the ob-servation that a rock surface covered with water will ad-sorb a lsrge nmount of methylene blue, whereas onecovered with oil will not. The dye adsorptionof the testcore is measured at ROS, where essentially all of tie wet:ting phase is continuous. 107-110 This enables the dye toadsorb on essentially W of.the water-covered, water-wetsurfac~. A reference dye adsorption measurement ismade on an adjacent core plug that is clcancd to renderit totally water-wet. The cleaned reference plug is sao+rated with brine, so the entire rock surface is water-covered. The fractional wettabM~ is then established bydividing the dye adsorption of the test core by that of the100% water-wet reference core. When tl@ method wsstested on fractionally wetted sandpacka containing mixt-ures of oil-wet and water-wet sands, a Iinear relation-ship between the fractional wettab@V and the dyeadsorption was found.

The dye adsorption test actually measures the fractionof the total surface area of the core that is contacted bythe injected water. Because of this, both oil und water mustbe present in the core when the dye adsorption is meaa-ured. The dye adsorption method mskes two additionalassumption: the water phase is continuous at ROS, sothe dye contacts all of the water-covered surfaces; andthe thin fk of oiI and water coating the mck surfacesare not affec@d by large changes in saturation. Tracerexperiments have shown that essentially elf of the wateris continuous at ROS for both water-wet snd oil-wet~re~, I10 me ~~~uption that the thin fti of@ adwater are not affected by Iarge changes in saturation seerrrareasonable because the amount of liquid in the thin filmsis very stmdl in comparison with the bulk tluida. Shankarand DuMen111exarrrincdhow dye adsorption varied withwater saturation in Berea sandstone cores. They injectedoil and brine at currstaut rates and allowed the saturationsin the core to reach equilibrium. The injection wsa thenswitched from brine to dyed brine, and the dye adsorp-tion was measured. They found that the dye adsorptionwas ahnoat conatsnt when tie water saturation wsa greaterthsn 40% PV. The dye adsorption drcreaaed at lower sanr-rations, where the water stinted to lose continuity. Thesemeasurements show that the wettabihty of a core meas-ured by dye adsorption is not dependent on the satura-tion, except possibly at water saturations near IWS.

Although the fractionally wetted sandpack that wereused by Holbrook and Bernard did not contain clays, theyretilzed that clays would strongly affect dye adsorptionin reservoir cores. This occurs becanse the surface areaaud dye adsorption capacities of clays are much largerha those of sad g~ns, 111,112f+olbrook ~d Be~~dstated that their test would measure the fraction of the claysurface drat was water-wet irr a reservoir core. However,they had problems when measuring the dye adsorptionsnd fractional wettabiMy of cores containing a significantamount of montnrorillouite. Irreversible chsnges in theclay structnre of the water-wet reference core reauftingfrom extraction and drying caused them to cuhxdate rela-tive water wettabtitiea that were greater tbau lCO%. Note,

1258

however, that it maybe possible to use supercriticsl dry-ing to avoid these problems. L13

Mixed Wettabfity. At the current time, there is no sin-gle nettability test that wifl determine whether a core hasSslathiels lol mixed wettabiity. It ap- possible, how-ever, to make this determination by examining the reardtaof (1) a glass slide nettability test, (2) a waterflood ofthe native-state core, and (3) several waterflood ofrestored-state coma that were aged with different brinesaturations. As discussed in the introductory paper, 1 ina mixed-wettabdity core, the oil-wet rock surfaces formcontinuous paths throughout the large pores, while thesmaller pures remain water-wet. Mined wettabilhy canoccur in a rock if the crnde forms a thick oil-wet layeron the smface only in those places where it is in dirwtcontact. This can be tested with the glxas slide method,with half the slide in crude and the other half iu brine.Quartz or calcite cryatala could also be used to give a sur-fxce more representative of the reservoir. The core mayhave mixed wettabtity if the haffof the slide aged in cru&forms a thick, oil-wet layer, while the half aged in briueremains water-wet. If the entire sfide remains water-wetor becomes oil-wet, the core will probably have a uui-form wettsbdity.

The second measurement to iudicate mixed wettabiMyis a waterflood of the native-state core. If the core hasmixed wettabilky, oil will be produced down to a verylow ROS as many PVs of water sre injected. Uniform-wettabili~ cores wiR generally have a shorter durationof production andlor a larger ROS. 6,7,114,115Finally, ,aseries of waterflood in restored-state cures cambe usedto confirm rhe mixed wearability of the core. A series ofcores is cleaned, saturated in brine, oilflonded with cmdeto dfierent brine saturations, then aged to restore its origi-nal wettabtity. Salathiel fonud that the recuve~ fmm hisrestored-state nrixed-wettabdity cores had a maximum ata pmticuka value of the brine saturation during aging.When the water aahmmionwas lower than this value, someof the arnidl pores becsme oil-wet, lowering recoveg.Conversely, at larger water sahmations, the oil patiwaysthrough the core became discontinuous.

Two other measurements that will sometimes help indetermining whether a core has mixed wettsbifity are im-bibkion meaaurementa aud capillary pressure behavior. 3Spontaneous (free) imbidti,on of both oil snd water h@been reported for some cures with iiactional or mixed wet-tabiity. 49-51These corez will have positive diaplacement-by-water aud displacement-by-uil ratios. Another indicakxof mixed wettabtity is a comparison of Oil-displacing-brine capillary pressure measured on native-state plugsva. meaauremenk on the ssnre plugs after they have beenclca.ned aud rendered water-wet. In some mixed-wetplugs, the native-state capillary pressure curve wilf crossover the cleaned curve m the capillary pressure ia in-creased. 116118Fticr discussion can be found in Ref. 3.

In summary, if the oil forms thick, oil-wet tilms onlyon thoac pm-tioui of the glass afidewith which it ia iu directcontac~ if the core can be flooded down to very low oilsaturation, yet still produce small smouuta of ofi tid ifthe oil recove~ from a rcatored-state core has a nraxi-mum at a sp.%itlc brine saturation during its aging peri-od, then the core Iiiely haa mixed wettabil@. ImbMion

Journalof PetroleumTechnology,November1986

and capillssy pressure mesanrementa can also help deter-mine whether a core-has mixed nettability.

Conclusions1. Three quantitative wettabdity measurements me in

use today contact angle, the Amott method, and theUSBM me~od. The contact angle measures the wetta-bflity of crude and brine on a polished mineral surface.It is the best method to use when pure fluids and artificialcores are used. It is also used to examine the effects onnettability of experimental conditions, such m pressure,temperature, and brine chemistry. The USBM and Amottmethods measure the average wettabil~ of core. Theyare superior to the contact-angle method when the wetta-bflity of native- or restored-state core is measured. Theyalso must be used to determine whether a core has beencleaned completely. The USBM method appsass to be su-perior to the Amott method, which is insensitive mar neu-tral wettab~lty. A modification of the USBM method,developed by Sharnra and Wunderlich, 57 allows the cal-culation of both the USBM and Amott nettability indices.

2. A Iarge.number of qualitative wettabfity measure-ment methods are available. The imbibition method is themost widely used because it is fast, does not require anycomplicated equipment, mrd gives RJJidea of the averagewettabfi~ of the core. Tbe microscope examinationmethod is otlen used.in flow visualization studies. Final-ly, wettabilhy measurement methods based on relativepermeabdity curves are often used when these data areavailable.

3. Two methods have been developed to measure thefractional wettabtity: the NMR method and the dye ad-sorption method. Neither method is widely used today.

4. There is no mefhod m determine whefher a core hasmixed wettabfity. However, it appesra that it maybe pGs-sible to make such determination by examining the re-sults of a glass slide nettability test, a waterflood of thenative-state core with many PVs of water, several water-flood of restored-state cores that were aged with differ-ent brine saturations, and imbibition and capillary pressuremeasurements.

Nomenclature,4~ = area under the oil-drive centrifigrd

capillary pressure curve, USBM method,42 =area under the brine~tike cenfrifigal

capillary pressure curve, USBM methodED = displacement energy (adhesion tension)

g = acceleration of gravityho = height of the oil column above the

Oil/water inter@ceh. = height of the water column above the

Oil/water interfaceI = Amots-Harvey relative displacement index

~ = initi~ imbibition ~k of ~ core just after itis submerged in a fluid

fir, = initial imbibition rate of a core after it iscleaned and rendered strongly water-wet

N = wettabfity number, Eq. 11P, = capilla~ pressure

r = capillary tube ra~us

rmm = computed equivalent circular radius of thelargest pores in a core, Eq. 9

R = relative rate of imbibition, Eq. 6VW = volume of oil displaced by sponfsneous

imbibition of water, Amott methodVo, = toti volume of oil displaced, Amott

methodvWSp= volume of water displaced by spontaneous

imbibition, of oil, Amen method Vw = total volume of water dkplaced, Amen

methodW = USBM nettability index

60 = displacement-by-oil ratio, Amen method8~ = displacement-by-water ratio, Amott method

@= contact anglep. = oil densityP w = water density

~.=~

aA = adhesion tension0., = interracial energy between the oil and solidLT.. = interfaced energy between the oil and

waterf7w = intefiacid energy between the water and

solid

Subscriptsa = apparent

adv = advsncinga-o = sir-oil0-w = oil-waterrec = receding

T = fhmahold

Acknow[sdgmentsI am grateful to Jeff Meyers for his many helpful sugges-tions and comments. I also thank the management ofCormco Inc. for permission to publish this paper.

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