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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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