Effects of desiccation on the elastic wave velocities of clay-rocks

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International Journal ofRock Mechanics & Mining Sciences

1365-16

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Effects of desiccation on the elastic wave velocities of clay-rocks

Ahmad Ghorbani a,b,�, Maria Zamora a, Philippe Cosenza c

a Institut de Physique du Globe de Paris, CNRS and Universite Paris 7 Denis Diderot, Paris, Franceb Department of Mining and Metallurgical Engineering, Yazd University, Yazd, Iranc UMR 7619 Sisyphe, CNRS and Universite Paris 6, Pierre et Marie Curie, Paris, France

a r t i c l e i n f o

Article history:

Received 25 August 2008

Received in revised form

11 January 2009

Accepted 19 January 2009

Keywords:

P wave velocity

S wave velocity

Clay-rocks

Drying

Desiccation

Anisotropy

09/$ - see front matter Crown Copyright & 20

016/j.ijrmms.2009.01.009

esponding author at: Institut de Physique d

ite Paris 7 Denis Diderot, Paris, France.

ail addresses: ghorbani@ipgp.jussieu.fr, ah.gho

rbani).

e cite this article as: Ghorbani A, et2009), doi:10.1016/j.ijrmms.2009.01.

a b s t r a c t

Compressional (P) and shear (S) wave velocities were measured in a set of clay-rock samples subjected

to a desaturation phase during which the samples were dried at ambient temperature conditions, with a

median relative humidity equal to 37%, followed by a heating phase during which the same samples

were heated to five different temperature levels, ranging from 65 to 105 1C. The recorded data shows

that when the degree of saturation is decreased, both P and S wave velocities increase, thereby lying in

the range (0–50%). The increase in S wave velocity following desiccation, by as much as 10%, and the

associated increase in dynamic shear modulus, suggests the presence of desiccation-driven hardening,

which is commonly observed in clay soils. The existence of such a phenomenon, which is not observed

in other sedimentary rocks over such a range of saturation values, proves that the classical models (e.g.,

the Biot–Gassmann equation) used to study the effect of saturating fluids, fail to correctly assess the

influence of variations in water content on seismic velocities measured in clay-rocks.

Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction

There has been a recent and growing interest in the study ofdesiccation and rehumidification effects on the mechanicalparameters of clay-rocks, for at least two reasons. Firstly, thehydric state of these rocks controls the short-term, as well as thelong-term stability of numerous earthflows associated with clay-shale basins (e.g., [1]). Secondly, the desaturated zone close to thewalls of a deep underground repository, excavated into clay-rocks,could have a detrimental impact on its performance (e.g., [2]).Indeed, in many countries (including Belgium, Germany, France,Japan, and Switzerland), deep clay-rocks are considered to bepotential hosts for the disposal of radioactive waste material, andthe characterization of the so-called excavated damaged zone(EDZ), induced by excavation and ventilation, is of crucialimportance in assessing the performance and safety of radioactivewaste disposal concepts.

However, the effects of desiccation on the mechanical para-meters of clay-rocks are poorly understood, and the rare datarelevant to this issue seems to suggest contradictory results.Ramambasoa [3] and Su et al. [2] observed, for different clay-rocks, that Young’s modulus is multiplied by a factor of 3–4 whenthe rock is desaturated. Ramos da Silva et al. [4] mentioned thatthe rock’s uniaxial compressive strength increases significantly, bya factor of 5, at high suctions close to 10 MPa.

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u Globe de Paris, CNRS and

rbani@yahoo.fr

al. Effects of desiccation on009

On the other hand, on the basis of mercury intrusionporosimetry measurements, and observations by confocal micro-scopy, Gasc-Barbier and Tessier [5] claimed that drying causesmicrocracks generated by shrinkage of the clay fraction, andshould soften the material. These shrinkage-induced microcracksare localized primarily at the boundary between solid grains(quartz or calcite) and the clay matrix. Montes et al. [6] confirmedthese results with an environmental scanning electron micro-scope (ESEM). The hydration and/or dehydration (up to a relativehumidity of 2.5%) of swelling clay minerals, found in argillitesamples taken from the Meuse/Haute-Marne Underground Re-search Laboratory (MHM-URL) in Eastern France, lead to sig-nificant structural modifications: cracking of the surface, particleaggregation/disaggregation and opening/closing of pores and/orcracks. However, these microscopic observations were not con-firmed by macroscopic mechanical measurements on the sameclayey samples.

Clearly, there is a lack of adequate data for correct assessmentof the impact of desiccation on the mechanical properties,especially on the elastic moduli, which control the bulk compres-sibility of clay-rocks. In geophysics, an efficient and simple meansof monitoring and controlling both water content changes anddynamic elastic moduli changes consists in measuring the rock’sseismic properties (e.g., [7–9]).

In this paper, we provide new compressional (P) and shear (S)wave velocity measurements, obtained from mineralogically wellcharacterized clay-rocks during a particular desiccation sequence(drying at ambient temperature and progressive heating), as wellas fundamental insight into the manner in which desaturation islikely to impact the dynamic, elastic constants of clay-rocks.

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2. Sample characterization and experimental procedure

A set of three core samples was taken from the Callovo-Oxfordian argillite formation situated at the MHM-URL laboratoryin Eastern France. The mineralogy of this argillite formation is onaverage [10]: 30% quartz+feldspar, 40% clay minerals and 25%carbonates. The most abundant clay minerals include R0-typeinterstratified illites/smectites (piles of unorderly flakes, 50–70%smectite) in the upper part of the formation (down to a depth of490 m).

The samples were taken at different depths, from boreholeEST205, and measured 10 cm in diameter. The length of samplesvaries between 9.5 and 10 cm (Table 1). Immediately after thecompletion of drilling, in order to prevent loss of water anddamage, they were stored in confining and sealing cells named T1cells (the confining effect in a T1 cell is ensured by (a) an axialload of about 0.6 MPa obtained by a an axial spring and (b) a radialpressure achieved by an expansive mortar, e.g., [11]). After theirtransfer to our laboratory, they were embedded in a resin. Thephysical properties, specific surface and mineral composition ofthe clay fraction of these samples are given in Table 1.

In order to desaturate the samples, we follow the experimentalprocedure described by Cosenza et al. [12]. The samples weresubmitted to the following desiccation path. In the first step(referred to as the desaturation phase in the following), thesamples were dried at ambient air (relative humidity in the range(32–42%) and at room temperature, i.e. 20 1C on average). Thisphase of the sequence was considered to be completed when thesample weights did anymore (see Figs. 2a and b). This transientphase lasted less than 30 days. On the basis of spectral complexresistivity measurements, Cosenza et al. [12] observed nosignificant low-frequency spectral distortions in clay-rock sam-ples submitted to a desaturation phase. Since spectral distortion isusually related to strong textural changes or/and to a removal of‘‘bound’’ water ([13]), they concluded that the desaturation phaseinvolves mainly the removal of water from the macro andmesoporosity, and hence creates moderate textural changes.

In the second step (hereafter referred to as the heating phase),the same samples were successively heated (in an oven) up to fivedistinct temperatures: 65, 75, 85, 95 and 105 1C over a period of atleast 24 hours in order to make sure to have a homogenous andsteady temperature in the center of the oven. This step was started

Table 1Mineralogical composition, clay fractions and physical properties of the clay-rock

samples (*from [17]).

Sample EST05-438 EST05-624 EST05-687

Depth (m) 423.16 472.60 489.51

Length (cm) 9.5 10.0 9.5

Diameter (cm) 10.0 10.0 10.0

Clay fractions*

Smectite 51% 42% 20%

Illite/smectite 4% 14% 11%

Chlorite 0% 4% 23%

Kaolinite 0% 0% 11%

Total clay minerals 55% 60% 65%

Calcite* 25% 25% 20%

Quartz* 20% 15% 15%

Porosity* 11% 10.5% 12%

Specific surface (m2 g�1)* 9 32 36

Water content (wt%) 1.64% 2.47% 3.18%

Saturation 35% 56% 59%

Rock density (g cm�3)* 2.5 2.38 2.29

Vp (ms�1, initial state, Z direction) 2800 2320 2280

Vp (ms�1, initial state, X direction) 3320 3080 3110

Vs (ms�1, initial state, Z direction) 1900 1610 1600

Vs (ms�1, initial state, X direction) 1940 1750 1780

Please cite this article as: Ghorbani A, et al. Effects of desiccation onSci (2009), doi:10.1016/j.ijrmms.2009.01.009

immediately after the end of the desaturation phase. It should benoted that P wave and S wave velocities were not recordedimmediately after removal from the oven, but at least 12 hoursafterwards, in order to obtain a thermal equilibrium, i.e., toequilibrate sample temperature and room temperature. Thisthermal equilibrium was required since the piezoelectric trans-ducers were calibrated at room temperature. The duration neededto reach a thermal equilibrium was estimated from the followingcharacteristic time t (e.g., [14]):

t ¼ L2

DT¼

L2c

l(1)

where L is the characteristic length of the experimental set-up, c

the volumetric heat capacity and l the thermal conductivity.Considering clay-rocks and the experimental apparatus: LE0.1 m;cE2�106 J m�3 K�1; lE1.5 W m�1 K�1 (e.g., [11,15]), one obtainstE4 hours. With regard to measurements in ambient air, wechose a much longer duration i.e., 12 hours, in order to make surethat a thermal equilibrium was reached in the samples. The steadystate has been checked by using an infrared radiometer (Thermo-flash LX26s).

The last temperature level (105 1C) allowed conventionalmeasurement of the initial water content on the basis of the lossof weight of the samples (e.g., [16]). During desaturationsequence, the weight of the samples is measured discretely. Atthe beginning of desaturation, we measured the weight of thesamples every hour. At the end of this stage, measurement wasperformed once a day. During the heating phase, the loss ofweight and P and S wave velocities were measured before andafter each temperature level.

For each step, the degree of saturation, Sw, is calculated from

Sw ¼rs

rw

o1� cc

(2)

where o, rs, rw, and o are, respectively, the porosity, soliddensity, water density, and water content of the samples. Theporosity was determined by mercury intrusion porosimetry [17].

The laboratory velocity measurements were carried out on thecore samples, using an ultrasonic (1 MHz) pulse transmissiontechnique (e.g, [18]) to obtain P and perpendicularly polarizedshear (S?, S||) wave velocities (Fig. 1). P and S wave velocities weremeasured using two pairs of transducers, one for P waves and theother for perpendicularly polarized S waves (S?, S||). Each pairconsisted of identical 1-MHz Panametrics transducers (V103SB forP waves and V153SB for S waves), one being used as an emitterand the other as a receiver. A Panametrics 5055PR pulse generatorwas used to deliver the input signal. The output signals weredisplayed and stored with a digital programmable AgilentDS06014A oscilloscope. Further details can be found in [19]. Theelastic velocity measurement errors were less than 2%.

Emitter and receiver transducers were mounted at oppositeends of the core samples, in the Z direction, and at the lateralsurface of the core samples, in the X direction (Fig. 1). The Z and X

directions are, respectively, parallel and orthogonal to thedirection in which the borehole was drilled (hence parallel andorthogonal to the bedding planes). A separation of 1 cm wasselected between the positions on the lateral surface at whicheach velocity measurement was made (Fig. 1). The P and S wavevelocities in the Z direction were measured at two different points,at the ends of each sample and the arithmetic average of bothmeasurements were considered.

The mean values of the P and S wave velocities were computedfor the lateral surface of the samples, and at their ends. For a givenvalue of saturation, each sample was associated with one (mean) P

wave velocity and one (mean) S wave velocity. This choice waschecked a posteriori: the measured elastic velocities, obtained at

the elastic wave velocities of clay-rocks. Int J Rock Mech Mining

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Fig. 1. Simplified experimental configuration for velocity measurements: (a) laboratory ultrasonic pulse transmission set-up used to obtain P wave and S wave velocities,

(b) in the X direction: transducers were mounted at the lateral surface of the core samples, and (c) in the Z direction: emitter and receiver transducers (e.g., PzP0z) are placed

at opposite ends of the core samples.

Fig. 2. Variation of water saturation of: (a) the samples EST05-438 and EST05-624 and (b) the sample EST05-687 as a function of time.

A. Ghorbani et al. / International Journal of Rock Mechanics & Mining Sciences ] (]]]]) ]]]–]]] 3

different points on the three samples during the desaturation andheating phases, varied by less than 4% as a function of spatialposition. This low degree of spatial scattering also confirmed thehigh homogeneity of the selected samples.

3. Results

Figs. 2a and b show the evolution of saturation duringdesiccation phases (desaturation and heating), for each sample.The duration of the desaturation phase can be seen to be less than35 days. As shown in Figs. 2a and b, the kinetics of desaturationand the water saturation measured at the end of desaturationphase are different for the three samples. These differences are

Please cite this article as: Ghorbani A, et al. Effects of desiccation onSci (2009), doi:10.1016/j.ijrmms.2009.01.009

mostly explained by the differences in the initial water saturation(see Table 1). Moreover, contrary to sample EST05-687, samplesEST05-624 and EST05-438 began to be desaturated together at thesame time i.e., absolutely in the same hydric and thermalconditions. It should be emphasized that if the degree of samplesaturation is calculated on the basis of a drying oven heated to105 1C, this leads to an underestimation of the real saturation inargillite. Indeed, drying at 105 1C for 24 hours does not remove allof the water, especially ‘‘bound’’ water, in smectite clays.

The P and S wave velocities measured during the desaturationand heating phases, are shown in Fig. 3 as a function of watersaturation. All velocities are normalized with respect to the(mean) value measured at the initial state, i.e., before thedesaturation and heating phases (see Table 1).

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Fig. 3. Normalized Vp (a and c) and Vs (b and d), measured during desaturation and heating phases in the X and Z directions, respectively, orthogonal and parallel to the

direction in which the borehole was drilled.

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Firstly, it should be noted that the largest initial elasticvelocities (Vp or Vs) occurring in the X and Z directions areobserved for sample EST05-438 (Table 1). This can be explained bythe lower clay content (and hence lower initial water content) ofthis sample, when compared to the other samples (see Table 1),since it is known that clay content contributes to the lowering ofseismic velocities (e.g., [20]). However, the differences in initial Vp

measurements can be also due to the different initial saturations(see Table 1).

Figs. 3a and b show, respectively, the normalized velocities, Vp

and Vs, in the X direction as a function of water saturation. Thevalues of Vp, obtained on the three samples during desiccation,show similar trends: they are almost constant during thedesaturation phase, and increase significantly during heating, byas much as 8% in the case of sample EST05-687. Vs exhibits thesame trend, although the increase in its value during heating isgreater than that of Vp (up to 10% for sample EST05-687).

Figs. 3c and d show normalized values for Vp and Vs in the Z

direction versus water saturation. As observed in Fig. 3, thevariations of Vp and Vs, in both the Z and X directions, are verysimilar for all three samples. However, during the heating phase,samples EST05-624 and EST05-687 exhibited a smaller increase inVp in the Z direction than that measured in the X direction(compare Figs. 3a and c): a significant anisotropy of the parameterVp during desiccation is revealed. On the other hand, thevariations of Vp in sample EST05-438 are nearly the same in boththe X and Z directions, thus providing indirect confirmation of thelower clay content of this sample (e.g., [20]).

Please cite this article as: Ghorbani A, et al. Effects of desiccation onSci (2009), doi:10.1016/j.ijrmms.2009.01.009

4. Discussion

The results presented above show two remarkable features,which were not observed in other sedimentary formations(e.g., sandstones and limestones) subjected to low mechanicalpressures and to saturation paths involving the same narrowrange of water content(e.g., [21–23]): when the degree ofsaturation decreases, and lies in the range 0–50%. In the case ofsamples EST05-624 and EST05-687, Vp increases with two distincttrends, depending on the two main textural directions in which itis measured, whereas Vs increases significantly, by as much as 10%.

The first feature (i.e., increase of Vp depending on the two maintextural directions) is clearly related to the water content insidethe samples, since such a trend is not observed with Vs, which istheoretically less sensitive to water content changes. The Callovo-Oxfordian argillite formation studied in this paper is transverselyisotropic, and the measured permeability parallel to the beddingplanes is significantly higher than that measured perpendicularlyto the same bedding planes (e.g., [24]: in this formation, theconnectivity of pore volumes filled with water is higher in the X

direction (i.e., parallel to the bedding planes) than in the Z

direction. During the desiccation process, the removal of water inthe direction parallel to the bedding layers is enhanced, due to ahigher level of pore connectivity. Both of the poro-elasticparameters, and hence Vs measured in the same direction, arethus more sensitive to changes in water content.

The second feature should be also discussed, since a 10%increase in Vs is in practice rarely observed for such small changes

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in water content (e.g., [21]). For this purpose, two processes needto be considered: the first is an increase in the shear modulus, m,and the second is a decrease in density r

DVs

Vs¼

1

2

Dmm�Drr

� �(3)

The relative density change, Dr/r, can be estimated if the removalof water mass changes DM and the volumetric strain, DV/V areboth measured during desiccation:

Drr¼DM

M�DV

V(4)

Since desiccation induces volumetric shrinkage (DVo0) (e.g.,[25]), a hypothetical decrease in r associated with desiccationshould be associated only with the removal of water (DMo0). Theterm DM/M can be easily estimated from the values given inFigs. 2a and b, and an upper bound of �3% has been calculated.Consequently, in our experiments, the relative change in densityDr/r should be less than –3% (Eq. (4)), leading to a change in Vs ofless than 1.5% (Eq. (3)). Since the observed increase in Vs is fargreater (10%), this effect must be accounted for by an increasein m corresponding to an increase in the rigidity of the samples(i.e., hardening). It should be noted that this hardening effectassociated with a decrease of water saturation is not taken intoaccount in the classical models (e.g., the Biot–Gassmann equation)used in 4D seismic reservoir monitoring (e.g., [21,23,26]). All theconventional pretrophysical theories assume a constant shearmodulus, i.e., independent of water saturation.

The desiccation-driven hardening is characterized by twoessential features. Firstly, this phenomenon is clearly associatedwith the clay matrix. In particular, it was not observed in ‘‘clean’’sandstones with the same range of water content at lowmechanical pressures (e.g., [21–23]), whereas it seems to havebeen enhanced in sample EST05-687, which had a higher claycontent (see Table 1 and Fig. 3d). It is thus concluded that thiseffect is related to the removal of water from microscopic pores inor between the clay aggregates. Secondly, as shown in Figs. 3and 4, desiccation-driven hardening does not induce significantadditional anisotropy to the elastic wave velocities, since theinitial degree of anisotropy remained unchanged during thedesiccation process.

It should be noted that the first point suggests that clay-rockswould show a hygro-mechanical behaviour similar to that ofclayey soils, since in both cases clay is the main solid phaseconstituent. Indeed, many experimental studies indicate that

Fig. 4. Anisotropic ratios measured during desaturation and heating phases: (a) Vp in th

divided to Vs in the Z direction (VsX/VsZ).

Please cite this article as: Ghorbani A, et al. Effects of desiccation onSci (2009), doi:10.1016/j.ijrmms.2009.01.009

desiccation leads to the consolidation of clayey soils underconditions of negative pore-water pressure (i.e., suction)(e.g., [27]). The dominant mechanism involved in structuraldevelopment during desiccation is the growth and aggregationof soil particles, to form a stiffer structure. In the case of thepresent study, it would be safe to assume that, on the basis of theobservations of Montes et al. [6], and by analogy with clayey soils,the desiccation of clay-rocks also leads to the aggregation of clayparticles/aggregates and/or to the reduction of intra-aggregatespaces. These microscopic processes, which coexist with destruc-turing processes (microcracks generated by shrinkage of the clayfraction at the clay–quartz boundaries), were observed by Gasc-Barbier and Tessier [5]. They are believed to dominate otherprocesses, and to explain the observed macroscopic behaviour(i.e., hardening).

With regard to the second feature of the desiccation-drivenhardening described above, it may be reasonable to assume thatmicrostructural isomorphic modifications are involved, ratherthan reorientations of microstructural units, since the initialanisotropy is not significantly enhanced (Fig. 4). If this hypothesisis correct, the reduction of intra-aggregate spaces, which donot imply strong aggregate reorientation, could play a significantrole: this explanation should be checked, by means of ESEMobservations.

Nevertheless, it is intuitively understandable that the desicca-tion-driven hardening observed in the case of our experimentswould not necessarily occur for other thermal and mechanicalsequences, i.e., for sequences differing from our experimentalprotocol, which was based on moderate temperature levels andthe absence of mechanical loading. Indeed, sample EST-624,which was heated to a temperature of 150 1C for a period of oneweek, finally revealed macrocracks parallel to the bedding planes.The creation of such macrocracks was associated with a drasticdecrease (by as much as 15%) in the velocity of the elastic waves.During this very strong heating phase, the microscale destructur-ing processes, induced by differential thermal expansion of thedifferent components, dominated the effects induced by previoushardening microprocesses (e.g., aggregation of clay particles/aggregates and/or reduction of intra-aggregate spaces). Thisdestructuring process may be also associated with the cooling ofthe sample after their removal from the oven. Indeed, this coolinggenerates stress relief and a high tensile stress (e.g., [28]), whichmay go beyond the low tensile strength of the argillaceous rock,typically in the range of 2–10 MPa (e.g., [29]). This resulthighlights the need to investigate other thermo-mechanical

e X direction divided to Vp in the Z direction (VpX/VpZ) and (b) Vs in the X direction

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processes in order to fully characterize the observed desiccation-driven hardening.

5. Implications and concluding statements

Our experiments have demonstrated desiccation-driven hard-ening of clay-rock samples: the dynamic shear modulus increaseswhen the degree of saturation decreases in the range 0–50%. Thisremarkable feature, which has not been previously observed inother sedimentary formations, proves that classical models(e.g., the Biot–Gassmann equation), used to study the effect ofsaturating fluids, fail to account for the influence on seismicvelocity of variations in water content. In other words, in the caseof clay-rocks subjected to very low confining pressures and highdesiccation conditions (high ventilation rates), new models whichexplicitly account for textural changes in clay will need to bedeveloped.

Moreover, our study has shown that the S wave velocity, Vs, is akey property for studying the impact of desiccation on elasticconstants: contrary to the P wave velocity, Vs provides significantinsight into the understanding of textural changes related todesiccation. This point supports the current interest in thedevelopment and application of new geophysical methods forthe in situ measurement of S wave velocities.

Acknowledgements

The research work described in this paper was supported bythe GDR FORPRO. The authors wish to thank the French ANDRA(Agence Nationale de Gestion des Dechets Radioactifs) forproviding clay-rock samples from the MHM-URL laboratory.

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