THE INFLUENCE OF PRESSURE ON THE ELECTRICAL
RESISTIVITY OF CLAY-WATER SYSTEMS*
by
W. J. LA~G t
U n i v e r s i t y of Illinois
ABSTI~ACT
TH~ influence of pressure on the electrical resistivity of c l a y - w a t e r s y s t e m s was investigated a n d found to reflect the a b n o r m a l n a t u r e of adsorbed water. The resistivity of
m o n t m o r i l l o n i t e - w a t e r s y s t e m s varied w i t h pressure in a m a n n e r similar to a dilute
salt solution at pressures above 7000 psi. At pressures below this, a b n o r m a l p r e s s u r e resistivity relations resulted which were a t t r i b u t e d to the presence of a b n o r m a l w a t e r
associated with clay mineral surfaces. The pressure-induced b r e a k d o w n of the a b n o r m a l
w a t e r associated with montmorillonites w a s found to occur in a b r u p t stages as indicated
b y anomalies ill the p r e s s u r e - r e s i s t i v i t y curves.
The anomalies indicated t h a t layers of adsorbed w a t e r h a v e differing degrees of
structural bonding b y which variations of the water-orientating ability of clay minerals
m a y be compared. I n general calcium montmorillonite was f o u n d t o allow the m o s t
rigid b o n d i n g of the adsorbed w a t e r while s o d i u m montmorillonite gave rise to the m o s t
extensive development of the a b n o r m a l water.
The a b n o r m a l water w a s found to re-form with some hysteresis effects w h e n tho
pressure w a s released. An increase in t e m p e r a t u r e was f o u n d to decrease b o t h the a m o u n t
a n d the relative s t r e n g t h of the a b n o r m a l water.
INTRODUCTION
DuaING the past ten years, the complex interaction of clay mineral surfaces
with water has been under intensive investigation, l~umerous individual
investigations have provided evidence which indicates that the water adsorbed
on clay mineral surfaces, particularly montmorillonites, has physical and
thermodynamic
properties that differ from those of normal water. Such
differences were generally anticipated because of the theory of "rigid" or
"oriented" water that has evolved during the past years as a result of the
anomalous behavior of clay-water systems. The present work has been
intended further to define differences between clay-adsorbed water and
normal liquid water by resistivity and pressure techniques. The material
* Presented at the ]4th Conference, Berkeley.
t Presently with International Minerals and Chemicals Corp., Libertyville, Ill.
455
456
FIFTEENTR CONFERENCE ON CLAYS AND CLAY M_INERALS
presented in this paper is part of a Ph.D. thesis submitted to the University
of Illinois. The writer is indebted to Drs. l~alph E. Grim, Philip Low, and
Carl A. Bays for contributions to this investigation.
LITEI~ATUI~E
Of the more recent investigations, Oster and Low (1963) report that the
entropy of water adsorbed on montmorillonite surfaces was lower than pure
liquid water and that the more coherent water structure was primarily
responsible for a greater activation energy for ion movement. Yamaguehi
(1959), in a series of tests, related the development of the structural characteristies of adsorbed water to the strength and sensitivity of soils. The electricai resistivity and heat capacity of the adsorbed water were determined
to be greater than that of normal water and closely related to the strength
of the soils.
Although eonsiderable experimental evidence has indicated that there is
a substantial degree of structural arrangement within clay-adsorbed water,
there has been very little evidence from which a sound model can be made
interrelating the water structure with the clay surface and the exchangeable
cations. A number of models have been proposed in which the c-axis of an
open-structured ice-like water is parallel to the c-axis of the clay mineral
(e.g. Hendrieks and Jefferson, 1938; Macey, 1942; Barshad, 1949); however,
more recently Anderson and Hoekstra (1965) have presented evidence that
suggests that the c-axis of the adsorbed water m a y be perpendicular to the
c-axis of the clay mineral.
An open-structured model of the abnormal adsorbed water has been
supported by the density measurements of Anderson and Low (1958) in
findings that the density of adsorbed water became less than that of pure
water at close proximities to montmorillonite surfaces.
The present investigation was based on the postulate that if clay-adsorbcd
water has open-structured form and is less conductive than normal water,
then pressure should induce a structural breakdown in the water which
could be detected by resistivity measurements.
EXPERIMENTAL
Apparatus
In order to measure the influence of pressure on the resistivity of clay-water
systems, a pressure chamber-resistivity cell was constructed. To produce
interpretable results, it was necessary to control or hold constant any factors
other than the structural rigidity of the adsorbed watcr that would significantly affect the resistivity of the system. The most important factor and
most difficult to control was the prevention of water leakage from the
system. The loss of water during compression would have affected the
resistivity by (1) changing the ratio of water-to-clay and electrolyte, and
THE ELECTRICAL I~ESISTIVITY OF CLAY-WATER SYSTEMS
457
(2) changing the clay fabric which would alter the tortnosity of the path for
the movement of the conducting ions.
The vessel, Fig. 1, was constructed and found to be absolutely leakproof
up to 40,000 psi in all but a few instances: I t was constructed from a No. 303
grade stainless steel rod, 8 in. long and 4 in. in diameter. The rod was bored
)N
NALL
ESSURE
ELECTROE
)RT
)RT
[ON
FIG. 1. Diagram of pressure-resistivity cell.
to make a cylinder with an inside diameter of 1.865 in. after the cylinder
walls were polished. Pistons for both ends of the cylinder were machined
from stainless steel and pressure seals were constructed from Teflon. The
pressure seals were cut in the form of discs which had a cup-shaped face so
that with increasing pressure the Teflon would press more tightly against
the cylinder walls and maintain a high-pressure seal. The system was designed
for uniaxial compression with the bottom piston remaining fixed relative to
the cylinder and the top piston being movable during compression.
The electrical connections were made by drilling ~ in. N.P.T. holes through
458
FIFTEENTH CONFERENCE ON CLAYS AND CLAY MA-NERALS
t h e cylinder walls for connection with Conax* high pressure insulated elect r i c a l connectors. The connectors were used for connection with t h e electrodes
a n d also as pressure fitting for a ceramic insulated, s h e a t h e d thermocouple.
The electrodes were p l a t i n u m discs, 0.70 em in diameter, which were soldered
to t h e l e a d wires of sufficient l e n g t h to give a n 0.60 cm separation. The
r e m a i n i n g portions of the l e a d wires were i n s u l a t e d a n d s t r e n g t h e n e d b y t h i c k
coating of e p o x y cement.
The resistivity m e a s u r e m e n t s were m a d e a t an a l t e r n a t i n g current freq u e n c y of 1000 cycles with a General R a d i o T y p e 650-A I m p e d a n c e Bridge
a n d T y p e 650-P Oscillator-Amplifier. The i n s t r u m e n t provides an a c c u r a c y
t o w i t h i n 1 ~o a n d a s e n s i t i v i t y in t h e order of a fraction of a per cent.
The h y d r a u l i c press used for t h e compression was a Riehle Model F A - 3 0 0
w i t h a c a p a c i t y of 300,000 lb. The applied force t h a t was used to calculate
t h e i n t e r n a l pressure could be controlled a n d m e a s u r e d to a n accuracy of
o.~%.
T e m p e r a t u r e m e a s u r e m e n t s during compression were m a d e w i t h an i r o n c o n s t a n t a n thermoeouple a n d a Leeds a n d N o r t h r u p p o t e n t i o m e t e r with an
ice-point reference junction.
Calibrations of Measurements
The cell c o n s t a n t of t h e p r e s s u r e - r e s i s t i v i t y cell was d e t e r m i n e d to be
a p p r o x i m a t e l y 0.36. A comparison was m a d e of the resistance of several
electrolyte solutions of v a r y i n g c o n c e n t r a t i o n in the pressure c h a m b e r with
t h e resistance values of these solutions d e t e r m i n e d b y a cell w i t h a k n o w n
constant.
I n order to relate t h e pressure in the c h a m b e r to t h e a p p l i e d load, it was
necessary to t a k e into account t h e friction of t h e pressure seals. F o r this
purpose, t h e compression c h a m b e r was filled w i t h a h y d r a u l i c fluid, a n d a
B o u r d o n - t y p e pressure gauge was connected to one of t h e 89in. N.P.T.
enlarged portions of t h e electrode ports. D u r i n g t h e compression of this
fluid, it was found t h a t gauge pressure became c o n s t a n t after a few seconds
for a n y given load a n d t h a t t h e r e was a linear relationship of 0.350 psi p e r
p o u n d of a p p l i e d load. The p o i n t d e v i a t i o n from linear was no g r e a t e r t h a n
1% a t pressures a b o v e 1000 psi b u t reached a m a x i m u m d e v i a t i o n of 10%
a t 500 psi, the lowest value on t h e pressure gauge. This relationship is assumed
to be t h e same for t h e compression of t h e c l a y - w a t e r systems a n d t h e error
in t h e same order of m a g n i t u d e . The pressure values of less t h a n 1000 psi
r e p o r t e d in this p a p e r are considered to be s u b s t a n t i a l l y in error w i t h respect
to t h e absolute value b u t still useful for c o m p a r a t i v e purposes. The t r u e
pressures in t h e range below 1000~psi would be higher t h a n those reported.
Sample Preparation and Testing Procedure
T h e clay samples selected for t e s t i n g were o b t a i n e d from t h e collection of
t h e Geology D e p a r t m e n t of t h e U n i v e r s i t y of Illinois. R e p r e s e n t e d are two
* Con~x Corporation, Buffalo, New York.
TIIE ELECTRICAL l~ESISTIVITY OF CLAY--WATER SYSTE~IS
459
types of calcium montmorillonite, Cheto and Aberdeen, a Wyoming sodium
montmorillonite, and a kaolinite from Redwood Falls, Minnesota. The
montmorillonites are not homoionic and reference to either sodium or calcium
type means only that these cations dominate the exchange positions. Further
information including chemical analysis of these general montmorillonite
types may be obtained from Grim and Kulbicki (1961).
I n preparation for testing, the clays were dried and ground to pass a
180-mesh sieve. Degassed distilled water was added to 120 g of clay in a
plastic bag while under vacuum in a desiccator. Sufficient water was added
to the clays to make them plastic and easily workable. The samples were
then removed from the desiccator and kneaded thoroughly in the plastic
bag for 20--30 min to obtain a uniform mixture of clay and water. :Before
placing the clay paste into the compression chamber, the inside cylinder
walls were sprayed with several coats of Fluoro Glide,* a Teflon-like dry
film lubricant to reduce friction of the pressure seals, prevent sticking of the
clay paste to the cylinder walls and to provide an electrical insulation. The
clay pastes were placed in the cylinder and the pistons were forced together
until any air trapped during the loading was driven out and the clay was
just beginning to extrude through the electrode ports. The electrodes were
then inserted and secured.
Resistance measurements were made during the compression when the
resistance became constant at a static load. This generally required only
10 to 15 see. The measurements were taken during the compression cycle
except in one test where measurements were taken both during the compression and decompression. The rate of compression and decompression when
tested was about 100 psi per minute.
The pressure-resistivity testing was done at room temperature except for
one test at an elevated temperature. In this case, the pressure chamber and
sample were heated to 45~ and placed in a styrofoam container with
expanded vermiculite packing for the complete period of testing.
RESULTS
AND DISCUSSION
Preliminary pressure-resistivity testing was carried to a maximum pressure
of 38,700 psi. Several abnormalities were observed in the pressure influence
on resistivity at pressures below 10,000 psi, and thus most of the observations
were confined to lower pressure ranges. In a clay-water system where the
type of conductance is primarily electrolytic, it would be expected that the
influence of pressure on the electrical resistivity would be indeed similar
to that of a comparable electrolytic solution. This was not found to be the
ease as may be seen in Fig. 2. The curve of the dilute NaC1 solution was plotted
from the data of K6rber (1909) in which only the relative resistance of the
solution under pressure and at atmospheric pressures are given. The resistance
values have been transposed to convenient resistivity values for the purpose
* C h e m p l a s t , Inc., E a s t N e w a r k , N e w Jersey.
16
460
F I F T E E N T t I COlgFER]gNCE ON CLAYS AND CLAY MINERALS
25~oc
20,000"
R
E
15,000-
P
S
J
I0~00-
C8
ABERDEEN
66~o
HpO
Nd
\~
WYOMING 6607o H20
\\\
\\x
ON~ \ \
SLOPE
\~
--~.
450
500
550
RESISTIVITY
600
650
OHM-CM
Fzo. 2. Pressure influence on the resistivity of a dilute electrolyte a n d m o n t m o r i l l o n i t e - w a t e r systems. D a t a for the 0.1N NaC1 curve were a d a p t e d f r o m
K 6 r b e r (1909).
THE ELECTRICALRESISTIVITYOF CLAY-WATERSYSTEMS
461
of comparing the slopes of the curves. The general configuration of the dilute
electrolyte curve shows that the effect of pressure is to decrease the resistance
at a rate becoming less at higher pressures. The explanation of this, according
to Bridgman (1931, p. 362), is that the mobility of the ions is enhanced by
reduction of the viscosity of the solution. This effect is reported to occur at
pressures up to about 30,000 psi, then there is a leveling off. A reverse effect
is reported to take place at higher pressures. The clay-water systems behaved
similar to the dilute electrolytic solutions only at pressures greater than
7000 psi. A downward projection of these slopes shows that there is an
abrupt deviation toward higher resistivity values. Low (1961) elaborates
upon the investigations which have been made and the evidence that clayadsorbed water has a higher than normal viscosity. He also relates a greater
activation energy for ion movement in clay-adsorbed water to the viscosity
in essentially the same way that Bridgman has related the resistivity of
dilute electrolytes to the viscosity of water during compression. It therefore
seems logical to attribute the abnormally large pressure influence on resistivity
in the lower portion of the curves to a pressure-induced breakdown of a
viscous and more highly ordered adsorbed water. With all other factors held
constant or nearly so, it is concluded that the water must have greater
structural characteristics in the lower pressure range, and that it is broken
down by compression and becomes like normal liquid water at higher pressures.
This is in agreement with the finding of u
(1959) and Oster and
Low (1963) that clay-adsorbed water has higher resistivity than normal
liquid water. This interpretation is in agreement with the open-structured,
quasi-crystalline concept of adsorbed water, since if this water were more
dense than normal liquid water, compression would have favored the existence
of the more dense phase and probably induced further growth. In such case,
an increase in pressure would be expected to produce an increase in resistivity.
From a closer inspection of the montmorillonite-water curves and a
comparison with a well-crystallized kaolinite, Fig. 3, it is apparent that there
is little if any interaction of the kaolinite and water. The anomalous behavior
of the pressure influence on the resistivity of the montmorillonite-water
systems would either seem to indicate that the structural breakdown of the
adsorbed water undergoes stages of transformation or that particular water
layers of differing degrees of structural rigidity are broken down at different
pressures. The writer favors the latter interpretation on the basis that it
better fits existing theory and evidence that the structural rigidity of adsorbed
water diminishes with distance from clay surfaces. The pressure at which an
anomaly occurs would therefore reflect the relative strength of bonding of
water layers, the ones at the highest pressures presumably representing
water layer closest to the clay surface. At the low pressures, particularly in
the case of the sodium montmorillonite where the magnitude of the resistivity
changes are large, there are possibly smaller anomalies within the larger
resistivity shifts which cannot be defined without greater control.
In comparison of the sodium and calcium montmorillonite curves, the
462
FZ~T~ENTH CONFERENCE ON CLAYS AND CLAY MINERALS
.3Z_
KAOLINITE
35% H20
SCALE XS
~
Ca
~,BERDEEN
66~ H20
Na
WYOMING
66c7oH20
Zr
s~o
s~o
5~o
RESISTIVITY
Fro.
3.
Comparison
5~o
66o
620
OHM-CM
of presst[re-resistivity
curves
of Na-montmorillonite
(Wyoming), Ca-montmorillonite (Aberdeen), and kaolinite, l%oman numerals
identify anomalies.
i n t e r p r e t a t i o n of t h e anomalies suggests t h a t t h e first few molecular layers
of w a t e r on calcium m o n t m o r i l l o n i t e are m o r e s t r o n g l y held t h a n those of
sodium m o n t m o r i l l o n i t e b u t t h a t t h e l a t t e r p e r m i t s more extensive g r o w t h
of s t r u c t u r a l w a t e r a t distances f u r t h e r from t h e clay surfaces. This is in agreem e n t w i t h conclusions of Grim (1962, p. 66), b a s e d on a comparison of t h e
plastic b e h a v i o r of t h e two m o n t m o r i l ! o n i t e types.
T~]~ ELECTRICALRESISTIVIT~rOF CLAY--WATERSYSTEMS
463
Minor differences in the location, configuration and separation of anomalies
IV and V in the two clays may provide a clue for the elucidation of the nature
of the interaction between the two cations and the quasi-crystalline water.
The general slope of the pressure-resistivity curves of the montmorillonites
in the low pressure ranges can be explained by the existence of water with
an abnormal degree of ordering. The configuration of the anomalies, however,
requires additional consideration. I f the anomalies indicate a phase transition
from more ordered water to less ordered water, then it would be expected
that there would be a decrease in resistivity at the beginning of the anomalies
rather than a period where the resistivity remains relatively constant. A
possible explanation of this is that at the pressure at which the anomaly
begins, a slight cooling occurs as a result of the absorption of heat that would
accompany the increase in entropy. The effect of such cooling would be to
increase the resistivity or in this case counteract the reduction in resistivity
associated with the pressure increase.
Thermal Relationships
The effect of temperature on the resistivity of clay-adsorbed water has been
investigated by Anderson and Low (1958) and Yamaguchi (1959). These
authors present evidence that the structural characteristics of the adsorbed
water diminish with an increase in temperature. I n order to examine the
effect of a substantial temperature increase on the pressure-resistivity
relationship of a clay-water system, a duplicate sample of the Wyoming
clay was prepared and tested at an elevated temperature of 45~ Figure 4
shows, in addition to the general trend of the resistivity to lower values, that
the temperature increase has caused a steepening of the slope of the curve
or a lessening of the pressure influence on the resistivity. This is exactly what
would be expected if the ordering of the adsorbed water was substantially
reduced by the temperature increase and therefore became less susceptible
to pressure breakdown. The anomalies are reduced in magnitude and occur
at lower pressure intervals and thus further indicate a less rigid water
structure.
Comparison of Calcium Montmorillonites
In comparison of the Cheto and Aberdeen clays, Fig. 5, there is a distinct
similarity of the curves, as would be expected, since both clays have a
similar calcium to sodium ratio. There are, however, minor differences which
may be related to a variation in the distribution of the exchangeable cations
or minor structural differences in the clay lattices.
The test with the Cheto clay is the only test in which resistivity measurements were taken during gradual decompression. A hysteresis effect appears
in the occurrence of the anomalies which suggests that the structural
redevelopment of the innermost water layer does not occur alone but simultaneously with the second layer at a lower pressure. The lowest two anomalies
464
F ~ T ] ~ E ~ T ~ CO/~IFERE~CE ON CLAYS AND CLAY ~r
7000
"o
"bo
",.
~~
".. ~
6.000
i
~
9.
ABERDEEN GG~o H2O
SCALE i
5,000
;
'~
~
--CHETO 59~7oH20
"...
'
SCALE
2
p
-
"
DUPLICATE
SA
~.~
k
r
R
o
E
i
~.
~
;
~~DECOMPRESSIDN>
~
%9
~'r
"...
~,~
":.DIE
2,000"
I ~
"%"o "fr
,,ooo-
~,
i
:SCALE '
. . . . .
O
RSES~. . . . . . .
86OOHM_ C~470
8,0
BBO
t0Lo. 4. Comparison of pressure-resistivity curves of Ca-mon~morillonites, Aberdeen
and Cheto.
T H E ELECTRICAL I~ESISTIVITY OF C L A Y - W A T E R SYSTEMS
465
s
\
5,000-
4,000"
S
U
WYOMZNG 66~~
45~
SCALE I
I
"~ WYOMING 66~~ H20
18~
SCALE 2
2,000-
1,000-
SCALEI ~ 0
3~0
3~0
3bO
SCALE2 530
540
550
560
45o
570
RESISTIVITY
4io
4~o
.4~o
4~o
4~o
580
OHM-CM
590
600
610
620
Fro. 5. T h e effect of t e m p e r a t u r e on t h e p r e s s u r e - r e s i s t i v i t y c u r v e of N a - m o n t morillonite.
466
FIFTEENTH CONFERENCE ON CLAYS A N D CLAY IV[INERALS
appear to have magnified, and suggest an improved water structure. A possible
explanation is that when melted during compression the water could become
more uniformly distributed with respect to the clay surfaces and then be
in a better position to find the appropriate structural site upon decompression.
A similar effect in relation to the redevelopment of oriented water has been
noted by Hiller (1964). His investigation of bentonite suspension indicated
that a transition from gel to sol state occurred gradually during compression
to 560 kg/em~ (about 8000 psi) and t h a t the gel structure reformed with some
time lag during decompression.
Reliability of Data
The interpretation of the resistivity data obtained requires that the clay
fabric remained unchanged throughout the pressure testing. The clay and
water, after thorough kneading, would have had a random fabric at the
beginning of the testing. In order to determine that the fabric was not
altered during the testing, cubes with 2.0 cm dimensions were cut from the
center of each sample. The resistance was measured across the three axes
and the samples were found to be electrically isotropie. This is further
substantiated by the fact that the pressure-resistivity curve plotted for the
decompression of the Cheto clay closely approached the compression curve.
The resistivity of electrolytic solutions is particularly sensitive to temperature variations. I t was therefore necessary to determine the magnitude of
temperature increase that accompanied the compression of the clay-water
systems. Separate compression tests were run on the montmorillonite-water
systems in which the temperature was measured b y a thermocouple penetrating 189in. into the sample. A temperature increase in the order of 0.9 ~ to
1.4~ was found to occur during compression to 8000 psi and about twice
this amount at 16,000 psi. The instrumentation and thermocouple employed
were only sensitive to detect temperature variations of about 0.5~ and greater.
No indication was observed of a slight cooling as was postulated in the
previous section; however, such thermal reactions would not be expected
to be within the sensitivity of the measurements. The temperature variations
that were observed, however, could be expected to contribute to the slope
of the pressure-resistivity curves b y producing a resistivity variation of
about 2.49% per degree centigrade change according to Jackson (1964, p.
231). However, since an equivalent temperature rise was observed during
compression from 0 to 8000 psi and from 8000 to 16,000 psi, this could not
account for differences in the character of the pressure-resistivity curves
within the two ranges.
A factor that would contribute somewhat to the resistivity decrease with
increasing pressure would be the increasing concentration of ions per unit
volume with the compression of the solvent. This effect, although probably
slight, would tend to be more pronounced in the portion of the clay-water
curves representing the destruction of the open-structured, more compressible,
THE ELECTRICAL RESISTMTY OF CLAY-WATER SYSTEMS
467
adsorbed water. I t would therefore tend to accentuate the differences in the
pressure-resistivity characteristics of clay-adsorbed water and normal liquid
water.
Another factor that must be considered as a potential source of error in the
experimental results is the possibility of significant variations in the cell
constant during compression. Undoubtedly, some variation did occur during
compression as a result of changing of the cell dimensions and compression
of the Teflon seals. The effects of the shortening of the cell length would be
minimized as a result of having the electrodes positioned as remote as possible
from the movable piston. The amount of compression of the Teflon seals
and its effect on the ce]l constant could not be determined but it is not
considered to have significantly altered the results. H a m a n n (1957, p. 122)
reports a change in the cell constant of a cell comprised entirely of Teflon
with platinum electrodes of only about 6% at 12,000 arm (176,400 psi) and
a major contribution to this error results from a 2% contractual phase
change in the Teflon at 5000 atm. Since the cell used in the present investigation had Teflon only as pressure seals and the testing pressures never
approached 5000 atm, significant changes in the cell constant from Teflon
compression would not be expected. The lack of variation in the pressureresistivity curve of kaolinite (Fig. 3) provides a reasonable assurance that
the cell constant was not noticeably altered within the pressure ranges of
these tests.
A possible error in the resistivity measurements could have resulted from
small amounts of air being trapped at the electrode-clay paste contact.
Although precautions were taken to avoid this it is likely that small amounts
of air remained. Any such error could be significant only in the very low
pressure range because the air would have become dissolved within the
first few hundred psi of applied pressure. I t is also unlikely that such a
phenomenon could account for the differences in the first two anomalies of
the heated and unheated Wyoming montmorillonite, the calcium montmorillonite pastes and the absence of the anomalies in the kaolinite paste.
The most significant factor supporting the reliability of the experimental
results is the fact that the pressure-resistivity curve could be reproduced with
reasonable accuracy for a given duplicate sample (Fig. 4).
SUMMARY AND CONCLUSIONS
The pressure-resistivity investigation of clay-water systems has provided
further evidence of the abnormal state of water adsorbed on the surface of
montmorillonites. The abnormally large pressure influence on the resistivity
of clay-water systems at pressures below 7000 psi compared to that of
comparable dilute electrolytes is interpreted to reflect the presence of water
of a quasi-crystalline state. Anomalies in the pressure-resistivity curves are
believed to be caused by a structural breakdown of particular water layers
and appear to provide a means by which orienting characteristics of different
468
FIFTEENTll CONFERENCE ON CLAYS AND CLAY MINERALS
clays m a y be compared and studied. As other a u t h o r s have indicated, t h er e
is strong evidence t h a t the rigidity a n d a m o u n t of clay-adsorbed w a t e r
decreases substantially a t e l e v a t e d t e m p e r a t u r e s . There was evidence t h a t
the w a t e r structure, collapsed b y pressure, re-forms when t h e pressure is
released.
T h e results of this in v e s ti g a ti o n are considered to h a v e bearing u p o n t h e
s e d i m e n t a t i o n an d loss of w a t e r of clay sediments during c o m p a c t i o n since
h y d r o s t a t i c load or weight of o v e r b u r d e n m a y affect the c l a y - w a t e r interaction. F r o m a n engineering standpoint, t h e effect of pressure an d t e m p e r a t u r e
should be considered a factor in d e t e r m i n i n g t h e s t r e n g t h an d sensitivity of
soils as well as shearing forces. I n a similar m a n n e r , t h e loading of c o n t i n e n t a l
shelf sediments b y continuous deposition could cause a melting of t h e c l a y adsorbed water, a r e d u c t io n in s t r e n g t h and give rise to t u r b i d i t y currents
w i t h o u t a sudden triggering mechanism.
T h e effect of pressure upon m o n t m o r i l l o n i t i c reservoir rocks could be to
alter t h e size an d s t r e n g t h of h y d r a t i o n envelopes surrounding clay particles
and t h er efo re alter t h e reservoir permeability.
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S
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