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Nettability Literature Survey—Part 3:
The Effects d Nettability on the
Electrical Properties of Porous Media
WMiam G. Andarson SPE, Conoco Inc
l393f+
Summary. This paper examines the effects of wettabflity on the Archie saturation exponent and the formatiOn
factor, which are determined exper.imentally in cores. These parameters are irhportamt in the investigation of the
hydrocarbon saturation of a formation by use of resistivi~ data.obtained from well logging. The Archie
saturation exponent, n,”typically has a value of about 2 in water-wet formations and clearrcd cores, whfle in
native-stnte, non-water-wet cores and formations it is generally larger than 2. In uniformly oil-wet cores with
low brine saturations’, n can reach values of 10 or more. The exponent is ~gher in’oil-wet cores at low
saturations because a portion of the brine is trapped or isolated in dendritic fingers where it is unable to
contribute to electrical conductivity. If a cleaned water-wet core is used to measure n and the reservoir is
actually oil-wet, interstitial water, will be underestimated during Iogging. No definite conclusions can be drawn
about the effects of nettability on the formation factor. However, the wettabilky of clays in a core is fikeIy to
affect this Lmmn3eter.
Introduction
This paper is the third irra series on the effects of wetta-
bllity on core analysis. 1-3Changes in the nettability of
the core have been shown to affect electrical properties,
capillary pressure, waterflood behavior, relative permea-
bility, dispersion, tertiary recovery, irreducible water
saturation, and residual oif saturation. For core analysis
to predict the”behavior of a reservoti~ the wettabMy of
the core nmst be the sync as the w.ettabllity of the un-
disturbed reservoir rock.
In the first report, 1 the various kinds of nettability,
such as mixed wettabtity, were discussed. That paper also
detirred native-state, cleaned, nnd restored-state cores nnd
gave the procedures necessary to obtain each type. Note
that a restored-state core has been cleaned and then aged
with native crude oil and brine at reservoir temperature
until the native nettability is restored. This definition is
used in the majority of the more recent literature. Be
aware, however, “Ilratin some papers, pwticularly older
ones, the term “restored state” is used for what are ac-
tually “cleaned” cores (e..g., see Craig4).
Wettnbtity nnd saturation hktory ire irnpopant factors
in the detcrrrtjnationof the electrical rcsistivity of a porous
medhm because they control the location and distribu-
tion of fluids. The electrical resistivi~ of“acore is deter-
mined by the lengths and cross-sectional areas of the
conducting piths through the brine. Large resistivi~ is
caused .by small cross-sectional areas and long conduc-
tion paths. First, consider a 100% brine-saturated core.
The rcsistivity of tie core ismuch higher than the resistivi-
ty of an cquivgent volume of brine because the noncon-
ductive rock reduces the cross-se:tiorwd area through
which the current can flow. At the same time, the rock
increases the ‘length of the conducting paths.
CQPY@t
1986
So.f.w
f
P.tf. wm
%$..-
The resistivitv of the core is increased further by arw
hydrocarbon sat&ation in the core because hydroca;bon_s
are also nonconductive. The incrense will depend on the
saturation, nettability, and saturation history, +e factors
that control the location and distribution of the oil and
water in the reck. Irra water-wet rock, the brine occupies
the smaIl pores and forms a continuous fflm on the rock
surfaces. In an oil-wet rock,”the brine is located in the
centers of the larger pores. Thk difference in brine dis-
tribution caused by the wettabili~ becomes very impor-
tant as the brine saturation is lowered. Generally, almost
nll of the brine in the water-wet rock,remains continuous,
so the resistivity increases because of the decrease in the’
cross-sectional area that can conduct flow. In an “oil-wet
rock, a pmtion of the brine will lose electrical continuity
as the saturation is lowered, so the electrical “rgsistivity
will increase at a faster rate.
Effects ofWettabitity on Resistivity and
the Archie Saturation Exponent
The hydrocarbon saturation of a formatiori is often esti-
mated from resistivity data obtained by well logging. The
empirically determined Arch1e5 saturation equation “is
often used
s;”=
... . . . . . . . . . . . . . . . . . . . . ... ...(1)
o
where
SW = brine saturation in the porous medium,
R, = resistivity of the porous medium at
saturation
S’w,
and
R. =
resistivity of the
100
brine-saturated for-
mation.
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The ratio of the two resistivities is called IR, the
resistivhy
index. The Archie saturation exponent, n, is
a ditiensionkss erripirical parameter that is determined
experimentally from’cpr~ plugs. The value Of
n
depends
on the fotiation but ustially has a value of about 2 for
water-wet formations and cleaned water-wet cores.
Oil-Wet vs. Water-Wet Cores. Mungan and Moore6
have puinted out tit the Archie saturation equation rn&es
three implicit assumptions: (1) the saturation/resistivity
relation is unique, so only one resistivity will ever be
measured at a given saturation; (2) n is constant for a
given porous mediw and (3) all the brtie contributes
to the flowof electric current. It has been shown that tbcse
assumptions are valid only when both the reservoir nud
core tie strongly water-wet because n depends on the dis-
trib”tio,” of the conductirtg phase .in the porous IIIed@III
and therefore depends on the wettabfily. If the nettability
is sltercd, the change in the spatial distribution of the fluids
alters the lengths and cross-sectional areas of the conduct-
ive pat@, which in turn changes the resistivity. Hence
the Archie equation isnonunique when ibe wettabili~ is
akcred because different resistiviiies iii be measured at
the same satiuition.
“Theexperiments discussed below show that n can be
a great deal higher in oil-wet than in water-wet rocks.
M0rr0w7 provides additional discussion. Because the
saturation exponent depends on the nettability,
n must
be measured at reservoir wetting conditions, o: invtild
saturations will be ob@ned from logs. For example, if
n is
measured in a cleimed water-wet core and the reser-
voir is actually ox-wet, ihe water saturation in the reser-
voir would be underestimated. Pkson and Fraser 8 cite
an example of a well in an oil-wet reservoir that produced
onfy water. Assuming a water-wet reservoir, logs in this
sape well indicated an interstitial water satiation of only
25%.
TMeeffects of tietiabtity on the Arcbie saturation ex-
ponent become more important as the brine, saturation
decreases because, in an oil-wet system, there is more
@co@ction aid isolation of globules of brine, The iso-
lated brine is surrounded by oil, w~ch acts.as an insiia-
tor snd causes this brine to be unable tu conduct a current
flow. First, consider a water-wet system initially ai a high
brine saturation. The brine is Iocatcd in the “smaflpores
and m a thin layer on the rock surfaces, whfie the &i is
located in the.center of the larger pores. All the brine ii
continuous and can conduct current. As the water satura-
tion is lowered to the irreducible water saturation (NW),
essentially all of the brine in a water-wet system remains
continuous and conductive, allowing the saturation expo-
nent to remain about 2. This continuity at all saturations
above fWS has been demonstited experimentally by
atcady:statcmistible flonds. .Thesefloods show that gener-
ally; there. is little or no trapping or isolation of any ,of
the brine by oil. 912 This implies ttiat most of the increase
in the rcsistivity is caused by the decrease in the cross-
sectional area available for conduction, not by increases
in tbe path length or brine t~pping..
In a uniformly oil-wet system, the oil is located in the
smull pores and on the rock surfaces, while .tbe brine is
located in the center of the Itiger pores. At high brine
saturations, the brine is continuous, jtit as it is in a watir-
wet system, even though its location is different. For this
1372
situation, the Arcbie resiativity/saturation “relationbehaves
as it does-in the water-wet case, with n around 2. In con-
trast to the water-wet case, however, as the brine satura-.
tion decreases, a portion of the br@eno longer contribute
to”the current flow. In some experiment, the saturation
exponent increases as soon as the brine saturation is de-
creased, while in others the brine saturation must be re-
duced to about 35% before n increases. At very low water
saturations (10 can occur.
Two factors cau cause the resistivity, and hence n, to
ike more rapidiy compared wi@the water-wet case: the
trapping of a portion of the brine by oil, and the forma-
tion of dendrites or tingera of brine. 13As stated previ-
ously, these factora decrease the cross-sectiomd area and
increase the length of the conducting paths, thereby in-
creasing electrical resistivi~. Flow vistrilizatiom and
steady-state miscible experiments demonstrate that a sig-
nificant fraction of tie nonwetting phase becomes dkcon-
nected as the nonwetting phase ‘saturation
decreases. 4,12.W~7This isolated brine is surrounded by
nonconducting oil and cannot contribute to the current
flow. As the brine saturation is reduced, the electrical
resistivim will also be increased because some of the brine
will be located in pseudo-dead-end pores,
17,18 As.
known as tingers or dendritic structures. These fingers
consist of brine that is connected to the continuous brine
in only one location. The brine cannot conduct electricity
because of the oif/water interfaces in the remainder of the
pore throats, so the length of the conducting paths is in-
creased.
,.
Note that the volume of nonconducting dendrites is not
the same as the dendritic fraction measured in steady-stnte
miscible floodhg experiments. The dendritic fraction in
a miscible experiment is a“measurement of the brine that
is continuous but does not flow. 2n a water-wet system,
this includes thetnonflowing brine located in the small
pores, as well as the brine in the fingers On the other
hand, the volume of nonconducting dendrites is a meas-
urement of the brine that is continuous but nonconduct-
ing. These two volnmes me different because the
continuous brine in the smalf pores conducts electrici~,
while the brine in the fingers does not.
I@perimental Measurements.
The experimental systems used to study the”effectsof wet-
tabiity on the saturation exponent can be divided into three
types: (1) uniformly wetted systems; (2) reservoir cores;
which may or may not have nniform nettability; and
(3) fractional and mixed-nettability systems. In the @st
set of experiments with uniforin nettability, the wetw
bility of the entire core is varied from water-wet to oil-
wet. At any given nettability, the wettabfity of the en-
tire sufface is kept as nniform as possible.
In many cases, reservoir core will not have @form
nettability. For example, the dlffefent minerals on the
rock.surface.can have different surface chemis~ and ad-
sorption propefies, possibly causing variations in wetia-
bflity. The second set of experiments discussed is for
reservoir cores. Several of these cores are native-state,
where akcrations to the r+ervoir wettabitty are mini-
mized. Finally, the third set of experiments examines the
wettab]lity effects that occur when a.core has fractions
or mixed wettabflity, where some of the rock surfaces are
,.
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TABLE 1—ARCHIE SATURATION EXPONENTS AS A
FUNCTION OF SATURATION FOR A CONDUCTING
NONWETTING PHASES
Air/NaCl SOlution
Brine
Saturation
(o/oPV) n
66.2
1.97,
65.1 1,9s
63.2 1,92
59.3 2,01
51.4 1,93
OWNaCl Solution
Srine
Saturation
(0/0Py n
64,1
2.35
63.1 2.31
60.2
2,46
55.3
2.37
50.7
2.51
43.6 1.99 44.2
2.46
39.5
2.11
40,5
2.61
23.9
4.06
36.8
2.31
30.1
7,50
34.3
4.00
28.4 8,90
.33.9
7.15
31.0 ‘ 9
strongly water-wet but tbe remainder are oil-wet. In these
experiments, the effects of “wettabllityare studkd by var-
iation of the location and tineproportion of the surfaces
tiat are water-wet vs. oil-wet.
Uniformly Wetted Systerrrs. Mungan and Moore6
studied the effects of wettabfity on resistivity using both
synthetic poiytetrafluoroethylene (teflon) @ natural
cores. They found that n could be as Klgh as 9,when the
conductive liquid was the nonw:tting phase. When it was
the wetting phase, n was around 2 in the same core. The
fluid pairs used in the teflon core were metbnnollair,
airlbrine, and oilibrine. For the methanollair case,
methanol is boQrthe wetting and ‘conducting phase and
is analogotis to the brine in a water-wet rock. The satura-
tion exponent was about 1.9, approximately what would
be measured in a water-wet resewoir cor,e with oil and
brine. This demonstrates that the location and resistivity
index of the fluids are similw in the two systems, oiU
brine/reservoir rock and air/m&thsnol/teflon, wheri the
wetting liquid is also the conducting phase.
Next, Mungan and Moore, used airlbrine OrOYbrine
as the two fluids in the teflon core. The brine is then the
conducting, nonwetig phase, bebaving ina fashion sfi-
ku to brine in an oil-wetcore. The saturation exponents
are shown in Table 1. An examination of Mungti and
Moore’s datXshows what typic611yhappens in io oil-wet
system as the brine saturation is decreased. Above a cer-
tain conducting phase saturation, the exponent n @con-
stant and near 2. Below this saturation, however, the
exponent begins to increase rapidly.
By mimovisunl examination, Mungrin and Moore found
that portions of the brine in the teflon cores started to be-
come disconnected when the brine saturation was lowered
to about 35%. This disctrmrected brine did not conduct
“electrical current because it.was completely surrounded
by tire insulating wetting phase (air or oil). Resistivity in-
creased more rapidly as tie brine saturation was lowered
below 35%. Table 1 shows”that the exponent begins to
rise as the brine saturation drops below 40%, eventually
increasing to about 9. Mungan and Moore concluded that
the Archle saturation equation was not ,vsld at low water
saturations in al oS-wet rock. ‘fliey pointed out, however,
that a vtild saturation-resistivity relationship could be em-
pirically determined if the reservoir wettabfky were pre-
.Ca
m,.,.,.,, . . . . +
+
,m —
,0
w,,,. w,.
:
1
‘o
0,2 ..3 ..4 0., m ,,8 ,0
served with
a
native-state core and reservoir fluids were
used for the measurements.
Sweeney and Jennings 1g,20measured the effects of
wettabti~ on carbonate cores. The cores were first ex-
tracted with. toluene, which left them in a neutral-
wettabfity state, as determined by the imbibition
method. 1 This method is only qualitative, so the actual
wettsbility of the core was somewhere between mildly
watkr-wet and mildly oil-wet.
The electrical resistitiky of the neutrally wet core was
measured as a functinn of water saturation. The cores were
then fred at S40“F [450”C] to remove all of the organic
rnateris.l present, rendefing the core strongly water-wet.
Note that thk caused a slight increase in the porosity be-
cause of dksociatitm of a portion of the calcium carbonate
into calcium oxide and carbon dioxide. After the new
resistivity behavior was measured, the cores were treat-
ed witi” naphthenic acids to make them oil-wet. The
resistivity bebavinr was measured again,”and the results
are shown in Fig. 1. The Archie saturation exponent, n,
is the slope of each line. Ttie saturation exponent Ofthe
water-wet cores was about 1.6, and for the neutrally yet
cores about 1.9.
There were two different types of behavior for the cores
once they had been rendered oil-wet. In some cores,, the
saturation exponent was high (about 8) even when the
brine saturation was very high, The behavior of.tbe re-
mainder of the cores was similar to the water-wet and neu-
trally wet cores until a brine saturation of about 35% was
reached. At this point,
n
increased rapidly to a v61ueof
about 12. This is similar to Mun 8n and Moore’s find-
L?
ings. Sweeney arid Jennings ‘g,z stated that the oil-wet
carbomte cores could also be separated into the same two
Journal of Petroleum Technology,
December1986 1373
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I
i
,0
I
[
I I
I
I
m—
\
.
(0
.,
g
:
.,
\
:,
‘ :J(
0
:.
~
;
4
,:
0
CORE
N .44
. .
~ ~ ~ ~TRAc D
n. ,,,,
.
.
‘,0
20 40 m w r,,
WA,,* ,.7,,.,,0., ,,.
~9. 2—Ef~ct of cleaning on the Arcfde saturation
:xponent.
grotips on the basis of pore-size distribution and petro-
graphic analysis. Unfortunately, they give no detsils.
,Rust21 compared tie saturation exponent for oil and
water in a cleaned sandstone before and after it had been
trded with an orgsnochlorosikme solution to render it
mildfy oil-wet. He found that the saturation exponent for
the clean water-wet sandstone was about 1.7, while for
the oil-wet sandstone it wss about 13.5 even at tigh brine
saturations. Unfortunately, Rust miayhave had problem:
with his experimental apparatus, indkated by saturation
exponent vuluea in the oil-wet sandstone of only 3.5 when
air and water were used.
Keller, 22Licastro aid Keller, 23and Holmes24 meas-
ured the resistivity of oil-yet and “water-wet reSeI’VOir,
cores using air and brine. The restit$ are very Similar to
ttie experiments described previously, even ihough core
saturations were cfianged by evaporation, which possi-
bly caused stilnity variations. In addition, fhe fluid dis-
tribution may have differed from the distribution existing
when oil and’brine were used.
Goddard et al. 13measured the electrical resistivity of
mercury, a nonwetting fluid, as a function of saturation
in seveml sandstone plugs: Mercury was injected into the
dry core and then withdrawn while the resistively was
measured. When the mercury was withdrawn from the
sanrpk, the
resistivity ,wasat first slightly less than the
injection resistivity at the same saturation. It quicldy be-
cume.much higher than the injection resistivity as the mer-
,cmy saturation was reduced, however, andwas essentially
infinite at a residual mercury saturation. Goddard et
al.
1374 ,
I
TABLE 2—EFFECT OF CLEANING ON THE
ARCHIE SATURATION EXPONENT27
Core Number
Unextracted
Extracted
1
2.37
2.03
2
2.68
2.29
3 2.48 2.07
4 2.71 1.9t
5
2.82
2.44
6 2.21 1.91
Aversge
2.55 2.11
state that these high vahresoccur becauae a significantpart
of tie nonwetting mercury was either trapped or located
in dendrites where it could not contribute to the conduc-
tivity.
Z1erfuss smd Makha25 measvred electrical resiativity
during waterflood of ssdstone tid limestone cores and
artificial packs. Each porous medium was saturated with
an aqueous ammonium thiocyanate solution (’‘water”),
oilflooded, and then waterflooded. The wettsbili~ was
controlled by treating the porous medlurn yi~ different
concentrations of naphthetic acids. As the system became
more oil-wet, the waterflooding behavior was altered. At
the same time, the resistivity at IWS wasincreased
Reservoir Core. The experiments dkcussed previously
showed the effects of wettabili~ on the saturation expo-
netit in uniformly wetted cores. Generally, either a teflon
core was used or the entire core
was
treated with a chem-
ical to make it oil-wet. The experiments ii this section
demonstrate that nettability is SISOa major control pa-
rameter in the determination ofn in”reservoir core. While
the Archle saturation exponent will be higher than 2 in
nstive-state oil-wet cores, it will generally not reach the
very high vsfues for uniformly wetted systems. Because
of variations in tiers.i composition, many reservoir cores
will probably have
fractional
(heterogeneous) wettabfity,
msking a portion of their surface water-wet. This will
decrease the rste at which the wster becomes.discome&?d
at low water saturations and lower the saturation exponent.
In addition to measurements in uniformly wetted teflon
cores, Mungan and Moore6 measured the resistivity of
native-state re;ervoir cores that wete known to be oil-wet
and had an interstitial water saturation of 10%. The
resistivity was too high to be measured; implying that most
of the brine was discomected. ‘The cores were water-
flooded and then oilflooded to 30% brine saturation, and
values of
n
were measured. They varied from 2 to 3.5.
Of course, at this higher brine saturation, the wettabiliv
effects were reduced.
Luffel and Randa1126gave & example of a resemoir
where saturation exponents must be measured on native-
state core rather than cleuued core. The saturation expo:
pent was first determined induectly on the basis of logs
snd water saturations from core cut with an oil-based mud.
The average saturation exponent wss determined to be
2.6. The saturation exponent was then measured directly
on native-state core,. nnd amaverage value of 2.8 was ob-
tained. In contrast, measurements on cleaned plugs gave
‘g;$ ‘due.”f ‘“ly 1“8”
exarnmed the effects of cleaning on the Ar-
cbie saturation exponent of the Bradford Third sand,
which is kuown to be oi3-ivet. Six pairs of adjacent p)ugs
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were cut, and one from each set was extracted with
toluene, mnking it more water-wet. The other core was
unextrscted, and left oil-wet. Note that cores were not
preserved and probably bad wettabilhies that differed from
their native wettnbdity. In sddhion, toluene extraction may
not have removed all of the organic coating on the core
so the cleaned cores may not have been strongly
water-wet.
The changes in the saturation expnnent are shown in
Table 2. Jrteach case, extraction significinily lowered the
Archle satnmtion exponent. Fig. 2 is a plotof the resistivi-
V index vs. the brine saturation for one core pxir. The
saturation exponent, n, istie slopeof the lines. It is bigher
for the unextmcted core amdappears to be constant. Moore
measured the reiistivity of the .unextracted core only for
brine satiations gxeater than 35%; therefore, it is possible
that the saturation exponent increases rapidly at lower
brine saturations, as obse~ed by Sweeney and
Jer@ngs 19,20and Mung& and Moore. 6 Moore’s27 xnd
Luffel and Randall’s26 experiments are particularly im-
por@t because they demonstrate that cleaning a core can
alter the saturation exponent.
Trantham and Clampitt2s
measured a saturation expo-
nent of 3.1 on plugs from h-e strongly oikwet North Bur-
bank reservoir. The plugs were cleaned nnd resaturated
with brine and oil before measurement of the saturation
exponent. Cleaning thk core apparently dld not affect the
wettabllity; the.plugs remained strongly oil-wet even af-
ter cleaning. Trantham and Clnmpitt proposed that the oil-
wetness.
of this reservoir is a result ‘of a coating of
chsmosite clay rather than the more common adsorption
of surfactants from the cmde. This may explain why the
saturation exponent is very high even after cleaning.
The differences in the ‘saturation exponent for native-
state vs. clcsned core by Mungan and Moore, 6 LuffeIl
and Rxndxll,26 and Richardson et al. ‘g show that the ex-
ponent should be measured on native-state or restored-
state core, where alterations to”the reservoir wettabilky
am minimized. Note that it is not known whether the cores
used in Refs. 6 nnd 26 had uniform or fractional netta-
bility because both types of nettability are possible in
reservoirs.
Fractional and Mixed-Wet Systems. Additional w.ettn-
b~ity effects can occur when a system has nonuniform
nettability (either fractional or mixed), where portions
of the surface are strongly water-wet, wh~e the remainder
are strongly oil-wet. Sa.latfiel 30 irkrodticed the term
mixed nettability for a special type of fractional wetta-
bifity in which the oil-wet surfaces form continuous paths
through the larger pores. The smaller pores remain water-
wet and contain no oil. Note tlat the main distinction be-
tween mixed and fractional nettability is that the latter
does not imply either specific locations for the oil-wet and
water-wet surfaces or continuous oti-wet paths.
Fractional Wettabil@.
The only researchers who have
exmnined the effects of fractional wettab]litj are
Schmid31 and Morgan and Puson.32 Morgan and Pws&
made fractionally wetted bead packs by treating apor-
tion of the beads with an organochloroiilane solution to
render them miklly oil-wet. The”remainder of the beads
were untreated andhence water-wet. Witba variation in
the proportion of oil-wet snd water-wet beads, resistivi-
ty measurements could tie made as the proportion of oil-
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.
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;
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oil-wet deoosits would not be formed in the small water-
ffled Pnre;, allowing them to remiin water-wet.
Because the small pores are water-wet, the electrical
behavior of rnixe&wettab~hy core will probably be differ-
ent from the behavior ii?uniformly oil-wet systems. The
Archie saturation exponent will not reach the very high
values that can occur in uniformly wetted systems. In-
stead, it seems reasonable to expect that the electrical be-
havior of mixed-wettability cores wiff be similar to
water-wet ones because the small pores and clay pat-
cles are water-wet and filled with water in both caaes.
As the brine saturation in a mixed-nettability core is re-
duced, the water in these areas will remain connected and
conduct electricity. This will alfow the”saturation expo-
nent to behave as it would in.a water-wet core, remain-
ing constant even at low brine saturations.
Even though the behavior of mixed-wet and water-wet
cores will be sinilar, however, this does not imply that
measurements on cleaned water-wet core are applicable
to reservoir systems. Unless the reservoir is known to be
sfrongly water-wet, the saturation exponent should be
measured on native- or restored-state core. There are
several reasons why core with the reservoir wettab]lity
is necessary. First, it appears that tie surfactants in some
cmdelbrinelreservoir rock systems can difise through
a water-film, making the entire rock surface uniformfy
oil-wet. 1 Second, if a core has mixed wetta.blity, the pre-
cise numerical value of the ArchIe saturation exponent
will
probably differ ,fiom that of
a“
water-wet core because
0’*’ ‘arge “i’-wetF’.
Richardson et
al. 2
prowded an example of the differ- .
ence in resi
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as the nettability of the core is &tired. ‘Sweeney and
je.~g~ 19,20found hat tie fom’tion factor ~as ~hangqd
after the nettability was altered. Because their chemical
treatment with naphthcnic acids was drastic enough that
other core propert ies such as the porosity were nlso al-
tered, it is unclear whether nettability effects were dem-
onstrated. They suggested that the naphthenic ‘acidsused
to”render the core oil-wet may have partially sealed off
pore$that had previously contributed to current flow.
Rust21 compared sandstone cores that were either
cleaned (water-wet) or cleaned and treated with nR or-
ganochlorosilane solution (mildly oil-yet). He found no
significant difference in the formation factors. Mungmi
and Moore 6 found no effects on the formation factor in
their teflon cores. They realiied, however, that tbk was
not a conclusive test
because of the uniform composition
and wettabiky of the teflon. They went on to state, “How-
ever, because natural cores contain clays, a chmge in core
nettability nlways brings about other changes, such as clay
swelliig and dispersion, ion exchange, effective porosity
change, and surface conductance variations, and these will
affect the measured vslues of R. and FE. Thus preser-
vation of the natursl core nettability is always pmdent. ”
Conclusions
1. The Archie
saturation
exponeat, n, is
almost in-
dependent of the wettabflity when the brine saturation is
sufficiently high that the brine is continuous.
2. Nettability effects become very important when the
brine saturation is lowered. In genersl, essentially all the
brine in a uniformly water-wet core remains continuous
and electrically conducting as the brine saturation is
lowered to the irreducible saturation. The Archie satura-
tion exponent has a vnlue of about 2 in water-wet forma-
tions sod cleaned water-wet core.
3. The Archie safurafion exponent can reach
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25. Zietiss, H. and MaUha,A.: “Re@ding fheReiadonshipBefween
tie Fomtkm Resiwivity Index and the Oil Recovery Mechanism
During WaterRoodiDg Procedures,,. Erd61 und Kohle-Erdgas-
Pemochenie (1967) 20,549-52. E@ish transfaficmavailable from
the John Cr.,. , Library, translation m. 68-15700.
26. Luffel, D.L md RandJ.U,R.V. : ;’Coii Hmdfirig and Measure-
ment TechniquesforObtini”g ReliableReservoir Chmacreristics,>7
paper SPE 1642-G presented at tbe 1960SPEFonyiticm EvahM-
tm Symp.mium, Housbm, Nov. 21-22.
27, Moore, J.: c‘Laborator$ Determined Electric Logging Pmeters
of the Bradford Third Sand,27
Producep Monlhly
(March 195S)
22, No, 5, 30-39,
28, Tradmm, J.C, andClampiti, R.L. ‘aDetmminatio”ofOifSamratim
AfterWaterfloodim ina“ Oil-Wet Reservoir-The Nmfb Burbmk
Unit, Tract 97 P+,’, JPT (May 1977) 491-500 .
29. Richardson, J.G., Perkins, F.M., and Omba, J.S.: “Diffmencss
i“ the Behavior of Fresh md Aged East Texas Woodbine Cores, SS
TmI.s, , A2ME (1955) 204, S6-91
30. Watiel, R.A.: ’’OflRaveNby Sh#am FUmDtimgein Mtid-
Nettability Rocks,,. JPT(Oct, 1973) 1216.24; Tmns. , AJME, 255;
31. Scbmid, C.: ..The Wettabili~y’ofPctrole.m Rocks andRes.lWof
Experitnepfs to Study the Effects of Variaions in Wetfabiliw,of
,-
COre SamP1es,,, Erdal und KohlXTmm,, SP~,
FifthAnnuafLoggingSymposium,Midland,TK (May 13-1S, 19@)
Sec. B.
1378
33, Sm ~on, B.F.:
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