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8/10/2019 Neuzil Wrr

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WATER RESOURCES RESEARCH, VOL. 30, NO. 2, PAGES 145-150,FEBRUARY 1994

How permeableare clays and shales?

C. E. Neuzil

U.S. Geological urvey, Reston,Virginia

Abstract. The permeabilityof argillaceousormations,although arely measured nd

poorly nderstood,s commonly criticalparametern analysesf subsurfacelow.

Datanow available suggest regularrelation betweenpermeabilityand porosity n

clays nd shales nd permeabilitieshat, even at largescales, re significantlyower

than sually ssumed.ermeabilitiesetween 0 23 and10 17 m2 havebeen btained

at porosities etween0.1 and 0.4 in both aboratory nd regional tudies.Althought is

clear hat transmissive ractures or other heterogeneities ontrol the large-scale

hydraulic ehaviorof certainargillaceousnits, he permeability f manyothers s

apparentlycale ndependent. hese esultshave significantmplicationsor

understandingluid transport rates and abnormal pressure generation in basins, and

couldprove mportant or waste isolationefforts.

Introduction

Sedimentaryunits dominated by clay are commonly the

leastpermeableparts of groundwater low systemsand, as

such, strongly affect fluid fluxes and flow patterns. The

permeabilityf argillaceous nits s thereforea parameterof

considerablemportance for analyzing groundwater flow in

sedimentaryenvironments, with particular importance at-

taching o the relation between permeability and porosity

because of the sediment compaction that accompanies

burial. Unfortunately, it is usually infeasible to measure

permeabilityor its variation in argillaceousunits and few

reliableguidelinesexist for estimating t. Specifically, there

are ew indicationsof (1) how permeability relates to poros-

ity in thesemedia and (2) how small the permeability of these

sediments can be at various scales. Researchers have used

estimates ased on a few widely cited measurements e.g.,

Magara, 1978;Neglia, 1979]or anecdotalvalues [e.g., Davis

andDe Wiest, 1966; Lambe and Whitman, 1969; Freeze and

Cherry, 1979] and have im,oked more or less arbitrary

relations etweenpermeabilityand porosity o account or

compaction, ractices that add considerableuncertainty to

analyses.As a result, clay and shale permeabilities are

amonghemostsignificant ncertaintiesn manyattemptso

quantify subsurface flow.

Datanow availableprovidenew insight nto this problem

andsuggesthat it is time/to reevaluate the treatment of clay

andshale ermeabilityn flow simulations.he relatively

low permeabilitieshese data imply at typical porosities

indicatehat fluid fluxes in some flow systems ould be

significantlymaller han analysesndicate;elucidation f

theseluxess crucial or understandinghenomenauch s

abnormalressure eneration, etroleum nd ore emplace-

ment, and deformation of sediments,and for guiding ntelli-

gent exploitationof the subsurface or purposessuch as

waste solation. his paper synthesizeshesedata and de-

scribesheir mplications.

This apersnotsubjecto U.S. copyright.ublishedn 1994 y he

Americaneophysicalnion.

Paper umber 3WR02930.

Laboratory Permeability Data

Until the 1970s, little was known about clay and shale

hydraulic properties beyond permeability data obtained as a

by-product of consolidation esting. Although attention has

been focused on the problem since then, the difficulty of

measuring small permeabilities has continued to limit the

acquisitionof data. As a result, extensivepermeabilitydata

for these materials do not exist. In addition, a large fraction

of the laboratory data that are available are not suited for

hydrogeologic pplications.Many studies sedpurifiedclays

or reconstituted sediments, eaving in question the applica-

bility of the results o natural systems.Others ailed to report

porosity or void ratio) or gaveno indicationof how the data

were obtained. Only a handfull of laboratory determinations

(1) used natural media in an undisturbedstate, (2) monitored

the reported porosity or equivalent information, and (3)

showed evidence that the measurements were made using

appropriatemethodology nd careful echnique.Laboratory

data I have found that meet these criteria, including data

obtained in a recent investigationof the Pierre Shale, are

plotted n Figure 1 with backgroundnformationgiven n

Table 1. With the exception of values for the Pierre Shale,

which are presentedhere for the first time, these data are

from published sources.

For completeness, omewidely cited data were included

in Figure 1 (dashed ines)even hough he test protocolwas

not reported.Test protocol s a critical consideratione-

causemeasurementsof small permeability are quite suscep-

tible to errors. Minutes leaks in the testing apparatus or

around the specimen n the test cell and subtle damageor

deteriorationof the specimentself are especiallydifficult o

avoid and make measured ermeabilitiesoo large; skepti-

cismof relatively arge eported alues s prudent. n view of

this, the significance f regions11 and 12 is problematical.

In contrast, various considerations ndicate that the data

shownwith solid ines n Figure are reliable. Regions1, 3,

5, and8 wereeachdefined y more hanone estingechnique

(see Table 1). Other data were obtainedusingrobustand

well-established mechanical transient (consolidation) tech-

niques region6) or with particular are devoted o the test

equipmentndproceduresregions , 4, 7, 9, and 10).

Figure 1 suggestshat a log-linear elation betweenper-

145


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146 NEUZIL' HOW PERMEABLE ARE CLAYS AND SHALES?

-16

0o8-

0.2

0.0

- I

Loghydrauliconductivity,.sl

-14 -12 -10 -8

...............

I I I I I I I

6

r' .11

I, I I , I I .... ,, I

-20 -18 -16

Logpermeability,

Figure 1. Plot of laboratory-derived permeability versus

porosity for a variety of natural argillaceousmedia. Perme-

ability is shown along he lower horizontalscale: he corre-

sponding hydraulic conductivity to water at room tempera-

ture is shown along he upper horizontal scale.The numbers

are keyed to Table 1 which provides he sourceof the data

and background information.

meability and porosity exists over an exceptionallywide

range of consolidationstates; materialsranging rom re-

cently depositedmarine clays to mildly metamorphosed

argillite are represented.Excluding egions 11 and 12, the

data fall in a bandapproximatelyhree ordersof magnitude

wide that spansporositiesexceeding .8 to less than 0.1.

Permeabilitydecreases y approximately n order of mag-

nitude with each 0.13 decrease in porosity and ranges over

eightordersof magnitude. egions 1and 12depart rom he

trend, suggestingabo'ratory-scaleermeabilitiesxceeding

10 7 m2 at porositiess ow as0.2.

The log-linear elationexhibited y the data n Figure1 is

not unexpectedor clay-richmedia.Several heoretical

expressionselatepermeabilityo porosity nd poresize;

one derived by Kozeny [1927] and modified by Carman

[1937],whichuses pecific urface reaasa measure f pore

size, appears o provideuseful nsightn the present ontext.

Although he Kozeny-Carmanquations basedon rather

restrictive ssumptionsScheidegger, 974], t predicts er-

meabilityof kaoliniteclay cakeswith reasonable ccuracy

[Olsen, 1962].The predicted elation s nearly og linear,

with a slopesimilar o that suggestedy Figure1.

The broad range n permeability t a givenporosity n

Figure1 (even f regions 1 and 12 are discounted)ould

have multiplecauses. irst, very smallpermeabilitiesre

measured at or near the limits of instrumental esolution,

which can introduce error. Second, effects of anisotropy

maybe presentn someof the data,with different erme-

abilities long ndacross epositionallanes. owever,he

bulk of the variation is attributable to differences n clay

microstructure, hichOlsen 1962] onsideredo arise rom

clusteringf clay particles. xperiments ith clay cakes

[Mitchell t al., 1965] how earlyhree rders f magnitude

variationn permeabilityecause f microstructuraliffer-

ences. omparableariations expectablen nature ecause

of the physicallyndchemicallyiverse nvironmentsn

whichargillaceousedimentsre deposited.

No data are known to me that indicate clay or shale

permeabilities maller han shown n Figure 1, and theres

reason to believe that the minimum measured valuesare

indeed close to the minimum values which can be expected

in thesemedia.Grim [ 1968,p. 464] abulated aluesof clay

specific urface rea. t is reasonableo associatehe argest,

a theoretical maximum for montmorillonite, with the lowest

clay permeabilities.Using this value in the Kozeny-Carman

relation ieldsvalueswithinan orderof magnitudef the

minimum values in Figure 1. This argues that the data

delineateminimum lay and shalepermeabilitieseasonably

well.

Effect of Scale

Many workers expect an increase in permeability with

scale n low-permeabilitymedia [Bethke, 1989], eading hem

to discount laboratory-derived permeabilities as being oo

low. The scaledependency,usuallyattributed o fractures r

commingling f nonclaysediments, an be difficult o detect

directly but is clearly present n someargillaceous nits.The

Pierre hale,or example, hich asa 10 20m2 permeabil.

ity at laboratoryscale region 8 in Figure 1), has a regional

permeabilityf 10 16 m2 [Bredehoeftt al., 1983]. cale

dependence lso has been detected n lacustrineclay [Ru-

dolpheta ., 1991]and clay till [Keller et al., 1988]ands

implied n numerous nstanceswhen in situ permeability

measurements n shalesyield relatively large values [Davis,

1988].

Other evidence, however, shows that permeabilityscale

dependences not ubiquitous n argillaceousmediaand,

whenpresent,may affect low only at the largest egional

scales, and not at intermediate scales. I found several

instanceswhereclay and shalepermeabilities t subregional

to regional caleskilometerso hundreds f kilometers)are

consistentwith the trend shown in Figure 1. These large-

scalepermeabilitiesre plotted n Figure2 to permitcom-

parisonwith the laboratory ata; Table 2 provideshe

background nformation.

The data n Figure2 were obtainedrom nverse nalyses

of a varietyof flow systems.nverse nalysis ields sti-

mates f system roperties,enerally, y numericallyimu-

lating lowandadjustinghe valueof uncertain arameterso

obtain omputedydrauliconditionshatbestmatchhose

observed. nverse methodswork reliably for well-posed

problemsndgenerallyre heonlyway o evaluatemall

permeabilitiesat large scales.

Certain nverse nalyses ieldedonly maximum alues;

these re ndicatedy arrowsn Figure . In thecasef

regionsand , this s duesimplyo ambiguitynherentn

the analyses; rangeof permeabilitiesower han hose

indicatedive qually oodmatcheso observedonditions.

Theuncertaintyn region exists ecausenverse nalyses

sometimesncorporateignificanthangesn permeabilit

overgeologicime. Region is derived rom an inverse

analysisfgeopressuresnsedimentsf heU.S. GulfCoast

byBethke1986a].hese edimentsayhave xpelled

excessluidhroughaturalydrofracturesuringartsf

their istoryEngelder,993, . 41;Capuano,993].f

permeabilityndeedariedn imewithhepresencend

absenceof transmissiveractures,region 5 represents

time-integratedveragef the woconditions.nlyhe

unfracturedermeability,hich ustesmaller,sdirectly

comparableithheaboratoryeasurementsnFigure.


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NEUZIL: HOW PERMEABLEARE CLAYS AND SHALES? 147

Table . Backgroundnformationor LaboratoryermeabilityataPlottedn Figure

Region Orienta- Number or Effective

in Formation Lithology Typeof Test* tion o Measure- Stress,

Figure (Location) (Mineralogy) (Permeant) Bedding ments MPa

Source

1 bottomdeposit bottommud illite, mechanical normal ?) 48

(North Pacific) smectite) transient,

steady flow,

and quasi-

steady flow

(seawater)

2 bottom deposit bottom mud (illite, steady low normal 19

(North Pacific) smectite) (seawater)

3 bottomdeposit bottommud illite, mechanical normal 9.) 26

(North Pacific) chlorite) transient,and

quasi-steady

flow (seawater)

4 bottom deposit bottom mud (illite, steady low normal 22

(North Pacific) chlorite) (seawater)

Pleistocene o marine and steady flow and

Recent (Quebec, lacustrineclay quasi-steady

Mississippi flow (natural

Delta, Sweden) pore water and

distilled water)

mechanical

transient

(seawater)

6 Gulf of Mexico unconsolidated

sediment;

varying

proportions of

clay, silt, and

sand

7 Sutherland Group glacial till

(Saskatchewan) (montmofillonite,

illite, kaolinite)

9t

10

1

12

Pierre Shale claystone (mixed

(central South layer,

Dakota) montmorillonite,

illite)

Lower Cretaceous clayey siltstone,

(Western clayey sandstone

Canada)

Eleana Formation argillite (quartz,

(Nevada) illite, chlorite,

kaolinite)

(Japan and mudstone, sandy

Alberta, Canada) mudstone,

silstone, shale

Upper Triassic, clay, shale

Mid-Miocene,

Lower Pliocene

(Italy)

normal (9.) approxi-

mately

600

various

mechanical normal 27

transient and

quasi-steady

flow (natural

pore water and

distilled water)

mechanical normal 85

transient, and

hydraulic parallel

transient,

steady flow

(pore water

duplicate and

distilled water)

steady flow (3.5 normal 25

and 5.8% and

sodium parallel

chloride)

hydraulic various 23

transient

approxi-

mately

250

0.04--0.4 Silva et al.

[1981]

Morin and Silva

[1984]

Silva et al.

[19811

0.04-0.3

Morin and Silva

[1984]

Tavenas et aI.

[1983]

Bryant et al.

[1975]

0.06-2.0 Keller et al.

[1989]

0.1-50

C. E. Neuzit

(unpublished

data, 1987)

0-40 Young et al.

[1964]

1.1-24.1 Lin [1978]

? (8 to 32 x 103 9. 33 Magara 1978]

mg/L sodium

chloride)

? ? 8 Neglia [1979]

*Mechanicalransient ests nclude standard onsolidationestsand similarproceduresnvolving ransientdeformation

under echanicaloads.Hydraulicransientests,whichnclude ulse nd njectionests,nvolve aryinghehydraulic

boundaryonditionsf hesamplend nalyzinghe ransientressureesponse.uasi-steadylow ests realso nown s

fallingeadests. teadylow ests re"standard"ermeabilityests. or urther iscussionf low-permeabilityesting

methodologies,eeNeuzil 1986].

t?orositystimatedromdescriptionf samples.


4/6

148 NEUZIL: HOW PERMEABLE RE CLAYSAND SHALES?

0.8

0.6

0.4

0.2

Loghydraulic onductivity, .s

-14 -2 -10 -8

I I ' i ' 1 .......... i

.....ii;;

0 -18 -16 -14

Logpermeability,2

Figure 2. Plot of large-scalepermeabilityversusporosity

for a variety of argillaceousunits derived from inverse

analysesof flow systems.Permeabilityand hydrauliccon-

ductivity scalesare as in Figure 1. Dotted lines show data

from Figure 1 to facilitate comparison.Arrows indicate

results which are upper limits for the permeability. The

numbers are keyed to Table 2.

1980;Davis, 1988]which mplied hat the permeability cale

effect in argillaceous rocks could be small.

Only two formations re representedn both Figures and

2: the SutherlandTill and the Pierre Shale. Keller et al.

[1989] ound that laboratory ests and inverse estimates

yielded comparablevalues of permeability for the Suther.

landTill (compareegion in Figure1 and egion in Figure

2). However, the inverseestimates pply o relativelysmall

volumeshavingmaximumdimensions f tens of meters.n

the case of the Pierre Shale, as already noted, basinwide

analysisBredehoeftt al., 1983]yieldeda relativelyarge

permeabilityf 10 16m2 ( 0 9 ms-l); this esult pplieso

an area with dimensions of hundreds of kilometers. A more

recenttudy fa fewkm portionf hebasinNeuziI,993]

shows, owever, hat a muchsmaller alue region3, Figure

2) applies here. Permeability scale dependenceexists,but

apparentlyonly at greater han kilometer scale. This points

to the presenceof transmissiveractures hat are separated

by distances f kilometersor more. Note also that many f

the laboratory values for the Pierre Shale are higher than he

inverseestimate compare egion 8, Figure 1 and region ,

Figure 2), probably because of deterioration of the core

samplesbefore laboratory testing [Neuzil, 993].

The agreement between laboratory and inverse values in

Figures I and 2 is striking and providesexplicit evidence or

scale independence of permeability in argillaceous media.

Significant fracture permeability apparently is absent in

these materials, which includea highly ithified, ow porosity

unit that one would expect to be prone to fracturing region

7 in Figure 2). This result extends earlier evidence [Brace,

Implications

The increasing speed of digital computers has stimulated

efforts to analyze subsurface fluid flow using numerical

simulation. For example, significant effort has been devoted

to analyzing paleoflow in sedimentary basins becauseof ts

relevance to economic minerals and waste disposal. Re.

cently, we have seen studies that have simulated paleof10w

Table 2. Background Information for Inverse Permeability Estimates Plotted in Figure 2

Region Vertical and

in Formation Lithology Type of Orientation Horizontal

Figure 2 (Location) (Mineralogy) Analysis to Bedding Dimensions Source

1 (Barbados clay, calcareous transient flow various 1 and 15 km Screaton et al.

Accretionary mudstone [ 1990]

Ridge complex)

2 Sutherland Group glacial till transient flow normal and 1 km Neuzil [1993]

(Central South layer,

Dakota) montmorillonite,

illite)

claystone, shale transient flow normal 0.5 and > 100 km* Colorado Group

and Upper

Manville Shales

(Alberta)

5 Gulf of Mexico

6* Pierre, Cafiile,

Graneros

Shales (Denver

Basin)

7 (Siberia)

clay, shale transient flow normal

claystone and shale steady state flow normal

"argillaceous transient flow normal (?)

rock"

0 and >300 km

3 and 800 km

Corbet and

Bethke [1992]

Bethke [1986a]

Belitz and

Bredehoeft

[1988]

Nesterov and

Ushatinskii as

reportedby

Brace [1980]

*Porosityangestimatedrom epthndhicknessfsedimentsT.F. Corbet,r.,Sandiaationalaboratory,ersona

communication, 1993, and K. Belitz, Dartmouth College, personalcommunication, 1993).


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NEUZIL: HOW PERMEABLE ARE CLAYS AND SHALES'? 149

regimesrivenby compactionBethke, 985;Harrison nd

Summa,1991], ectonicdeformation Deming et al., 1990:

Ge and Garven, 1992], topographic elief [Garven, 1985,

1989;Bethke, 1986b,Senger and Fogg, 1987], and erosion

[Sengert al., 1987]. uch nalysesre nherentlyncertain

becausehe systemhydraulicproperties n the geologicpast

mustbe estimated.Permeabilitiesor rangesof permeability

assumedor argillaceous nits, as n the studiescited above,

arecommonlyetween 0 9 to 10 16 m2 (hydraulicon-

ductivityf 10 12 o 10 9 ms 1) forporositiesesshan .4.

Figures and2 suggesthat argillaceousormations anbe

muchesspermeable,ithvaluesn therange f 10 23 o

1017m2 (10 16 o 10 10ms 1) for hesame orosityange.

Of course, argillaceous units mapped at large scales fre-

quentlyncorporateelatively ermeableubunitsnd heir

permeabilityanbe correspondinglyigh.Nevertheless,s

Figure shows, hey apparently anbe dominated y their

low-permeabilityomponents. his probably s mostoften

true for flow normal to stratification, which is controlled by

the eastpermeablehorizons.

Adoptionof the relatively ow permeabilitiesndicatedby

Figures and2 wouldresult n reinterpretation f some low

regimes.n a topographically riven low systemmediated

by eakagehrougha shale,simulations sing oweredshale

permeabilitiest basinscale hundreds f kilometers) ould

indicate reduced rates of fluid transport throughout the

system. henomena ssociatedwith advective luid trans-

port,suchas petroleum ccumulation nd eraplacementf

ores,wouldrequire more time or differentmechanismshan

originallyndicated.

In geologicallyactive areas, such as the foreland basin

undergoingectoniccompression nalyzedby Ge and Gar-

ven 1992], he permeabilityof argillaceous ediments an

determine he degree to which flow is affected by the

geologic ctivity. Rock deformation caused by tectonic

compression,or example, ends o perturb luidpressures;

smallpermeabilitycauses he perturbations o persist and

accumulateo significant evels. A variety of geological

processesan similarlyaffect fluid pressure Neuzil, 1986].

Becausehe data presented here suggestpermeabilities

lower han usuallyassumed, hey also imply that abnormal

pressures aused by such geologic orcing may occur more

readilyhanpresently upposed. his s consistent ith the

widespread ccurrenceof features suchas thrust faulting

andmultiplegeneration racturing, which are thought to

require igh luid pressures Engelder, 1990].

The permeability ata presentedhere also have mplica-

tions or subsurfacewaste isolation, In a broad sense, the

datacan aid characterization of fluid fluxes, which is neces-

sary or assessinghe risk of advecting oxic waste rom a

repositoryo the biosphere. igures1 and2, however,have

a direct mplicationor repository iting.The absence f

secondaryermeabilityn some rgillaceousormationsmay

makehemviable epository enues.

I haveskirted uestions bout he applicability f Darcy's

law n argillaceous edia. Uncertaintyattaches o flow at

moderateo low hydraulicgradientsn clayeymaterials'n

particular, arious nvestigators ave described o-called

thresholdradients elow which clay behaves s if it is

impermeableNeuzil,1986].Laboratoryestsusehydraulic

gradientshat are too large o allowdefinitiveestsof such

non-Darcianlow models. However, the agreementbetween

laboratorynd nverse ermeabilitystimatesvidentn

Figures 1 and 2, with the latter based on flow at moderate to

low gradients, rgues gainst he existence f any significant

non-Darcianeffects n the media represented.

Conclusions

Much remains o be learned about the hydraulic properties

of clays and shales.For example, we are unable to predict

how and where heterogeneity due to depositionalarchitec-

ture or fracturing)will affect arge-scalepermeability n these

media. For the present,when explicit data are not available,

flow analysesmust considera range of permeability values

for argillaceous units. Unless there is evidence to the con-

trary, permeability values as low as those indicated in

Figures 1 and 2 should be considered possible even at

regional scales. Values lower than shown, however, proba-

bly need not be considered.Because aboratorypermeabil-

itiesat similar orositiesan vary by a factorof 10 ,

stratification in argillaceous sediments may create perme-

ability anisotropyof a similarmagnitude.These conclusions

probably apply to a wide variety of clayey deposits, nas-

much as clayey siltstone and even clayey sandstone are

represented n the data.

Acknowledgments.wish o thankGrantGarven,Ward Sanford,

Dave Rudolph,Craig Bethke, and two anonymous eviewers or

their helpful commentson this paper.

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(Received April 30, 1993; revised October 2, 1993;

accepted October 19, 1993.)

neuzil wrr

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