wettability literature survey- part 3

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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.

Journal of Petroleum Technology, Dec&ber 1986

1371


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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

,.

Xournalof Petroleum Technology, December 1986


3/8

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


4/8

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

JournalofPetroleum Technology, December 1986


5/8

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-

Joumdof PetroleumTechnology,Deccmber1986

.

,,

;

~~

; t.

:-

/)./:.-,.

0


6/8

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


7/8

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


8/8

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.:

wettability literature survey- part 3

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