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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.
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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.)