TIMODAZ · 2018. 9. 25. · contribute to the state of the art to be done in the early phase of the...

1

WP 2 Report on the state of the art

Deliverable 2

GRS Contribution

Laboratory Experiments

on the THM Behaviour of Clay Rocks

Chun-Liang Zhang and Tilmann Rothfuchs Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH

Final Repository Safety Research Division, Braunschweig - Germany

Reporting period: 12 month

Date of issue of this report: 28/02/07

Start date of project : 01/11/06 Duration : 48 Months

TIMODAZ (Contract Number: FP6-036449)

)

3

Project co-funded by the European Commission under the Euratom Research and Training Programme on Nuclear Energy within the Sixth Framework Programme (2002-2006)

Dissemination Level PU Public RE Restricted to a group specified by the partners of the TIMODAZ project XCO Confidential, only for partners of the TIMODAZ project

GRS: Laboratory Experiments on the THM Behaviour of Clay Rocks

[TIMODAZ] Report – Deliverable 2 4 Dissemination level :RE Date of issue of this report : 28/02/07

DISTRIBUTION LIST

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[TIMODAZ] Report – Deliverable 2 5 Dissemination level : RE Date of issue: 28/02/07

Table of Contents

1 Introduction 2

2 Thermal expansion 2

3 Pore-water pressure 2

4 Swelling pressure 2

5 Swelling and shrinking 2

6 Long-term deformation 2

7 Strength 2

8 Self-sealing 2

9 THM processes 2

10 References 2


6


7

1 Introduction

To investigate the THM behaviour of clay rocks being considered as host rock for the disposal of radioactive waste, various laboratory experiments have been performed by GRS on the Callovo-Oxfordian argillites (COX) at Bure and the Opalinus clay (OPA) at Mont Terri, including thermal expansion, pore-water pressure, swelling pressure and strain, long-term deformation, strength, self-healing, and THM coupling processes. The associated reports and publications are listed in references (Zhang et al., 2002 – 2007). The main experimental findings are summarized and discussed in this paper to contribute to the state of the art to be done in the early phase of the TIMODAZ project.

2 Thermal expansion

Thermal expansion of a clay rock is strongly dependent on water saturation and porosity. On OPA-samples with a water saturation degree of ~95% and a porosity of ~16%, the thermal expansion behaviour was investigated by heating and cooling between 25 and 75°C at a rate of 7.5°C/h. The samples were sealed in rubber jackets to prevent water release and unloaded or loaded at a confining stress of 3 MPa. Thermal strains were measured by using strain gauges which were directly attached on the sample surface. Figure 1 shows the test results. The linear thermal expansion coefficients measured on the unloaded samples suggest a significant thermal anisotropy of the rock. The mean value of α┴ = 1.6·10-5 K-1 determined perpendicular to the bedding plane is about one order of magnitude higher than that of α// = 1.5·10-6 K-1 parallel to the bedding. On the loaded samples, a mean value of αm =1.5·10-5 K-1 was determined, which coincides with that value measured perpendicular to the bedding under unloaded condition.

0

20

40

60

80

0 5 10 15 20 25Time (day)

Tem

pera

ture

(°

C)

1E-7

1E-6

1E-5

1E-4

1E-3

Coe

ffici

ent o

f th

erm

al e

xpan

sion

/ co

ntra

ctio

n (

°C-1

)

temperature

expansion

α┴endface

α┴surface

α//endface

α//surface

contraction

a. tests on unloaded samples

0.00

0.04

0.08

0.12

0.16

10 20 30 40 50 60 70 80 90 100Temperature (°C)

Ther

mal

exp

ansi

on

(%)

BHED4/6A

BHED5/8A

BHED5/8BBHED5/6

Confining pressure = 3 MPa

Opalinus clay at Mont Terri

b. tests on loaded samples

Fig. 1 Measurements of thermal expansion of the Opalinus clay rock under unloaded

and loaded conditions According to Zhang et al. (2007a), the thermal expansion coefficient of a saturated clay rock can also be calculated by

φ⋅α

+φ−⋅α=α3

1 wsm )( (1)


8

where αw is the volumetric thermal expansion coefficient of water (3.4·10-4 K-1, (UPC, 2002)), αs is the linear expansion coefficient of clay solid grains (2.0·10-6 K-1, (Noynaert, 2002)), and φ is the rock porosity. For the studied rock, the calculation gives a value of αm = 1.9·10-5 K-1, which is only slightly larger than that of 1.5·10-5 K-1 measured on the not fully-saturated samples. Because the thermal expansion coefficient of the pore-water is two orders of magnitude higher than that of the solid grains, the thermal expansion of the saturated clay rock is dominated by the water expansion.

3 Pore-water pressure

In highly-consolidated natural clay rocks, a very significant portion of the water content is strongly bound on mineral surfaces. Only in relatively large pore spaces there may be small amounts of free water. According to the stress concept of Horseman et al. (1996), the isotropic total stress σ in a saturated clay rock can be expressed by wDweff pp +∏=+σ=σ (2) where σeff is the conventional effective stress, ΠD is the disjoining (swelling) pressure in interparticle water-films, and pw is the pressure of free pore-water. This equation implies that a) the total water-film pressure pfm is the sum of the disjoining pressure ΠD and the free pore-water pressure pw, and b) the effective stress σeff is equal to the swelling pressure ΠD, which means that the bound water-films in clay rocks are capable of bearing external loads and carry the lithostatic stress. The so-called pore-water pressure in clay rocks was examined on COX- and OPA-samples in triaxial cell under mechanical and thermal loads by measuring back-pressure at the end faces, as shown in Figure 2. In fact, this test method assumes a perfect interconnection between the free pore-water in the sample and the water in external reservoirs. This assumption is true for conventional porous media such as soils, granite, sandstone etc., but not for clay rocks without or minor amounts of free pore-water. Thus, the water back-pressure may not really be representative for the inner pore-water pressure of highly-consolidated clay samples.

clay particle bond

σ

σ

adsorbed water-film

other minerals

free pore water

σ3

σ1

specimen

T T

TT

pout

pwΠDpfm

pin

σ1 = axial total stress σ3 = radial total stress pin = back-pressure at inlet pout = back-pressure at outlet pw = free pore-water pressure ΠD = disjoining pressure in

water-films pfm = total pressure in

water-films pfm = ΠD + pw T = temperature

Fig. 2 Principle of the pore-water pressure measurement on a sample in a triaxial cell Figure 3a presents an example of the pore-water pressure measurement on a COX- sample by compaction at constant temperature. First, the sample was re-saturated by injection of synthetic pore-water at a pressure of 3 MPa under an external stress of 5


9

MPa for 30 days. Subsequently, the isotropic stress was increased from 5 to 12 MPa. This resulted in different increases of the inlet and outlet water pressures, indicating that there was no hydraulic network through the sample. According to equation (2), the local disjoining pressures in the interparticle water-films might be equal to the applied external stress and higher than the water pressures in the external reservoirs. So it was impossible for the external water to entry the pores. Obviously, the observed back-pressures are not representative for the pore-water pressure in the sample. Only when the pore spaces filled by free water are connected with external water (due to pre-existing cracks usually caused by sample coring and preparation), the back-pressure is equal to the free pore-water pressure, as shown in another test on an OPA-sample (Fig. 3b). Increasing external stresses from σ1 = σ2 = σ3 = 5 MPa to σ1 = 13 MPa and σ2 = σ3 = 12 MPa led to a same increase of the back-pressures from 4 to 11 MPa. This value should be equal to the free pore-water pressure in the sample.

0

4

8

12

4 6 8 10 12 14

Total stress (MPa)

Wat

er p

ress

ure

(M

Pa)

-1.5

-1.0

-0.5

0.0

Volu

met

ric s

trai

n (

%)

Skempton's coefficientB = 1.0

B = 0.93

inlet pressure

outlet pressure

volumetric strain

undrained bulk modulus K= 7.0 GPa

EST05684 Calovo-Oxfordian argillite at Bure

T = 33±2°C

a. test on a COX-sample

0

5

10

15

0 4 8 12 16

Stre

ss

(MPa

)

axial / radial stress

inlet pressureoutlet pressure

closed

saturation phase undrainedcosolidation

drained

pressureincreased

closed

open

BVE1-03 Opalinus clay at Mont Terri

T = 33±2°C

Time (day)

b. test on an OPA-sample

Fig. 3 Response of pore-water pressure in clay rocks to compaction In case of heating, the adsorbed water-films become thin due to thermally-induced mobilization of the bound water. Because the very low hydraulic conductivity of consolidated clay rocks does not allow the expanding pore-water to disperse adequately, an over pressure builds up in the pore-water. Figure 4 illustrates a typical heating test on a COX-sample. At σ1 = 20 MPa and σ3 = 18 MPa the sample was heated by elevating the temperature from 32 to 91°C. The reached water back-pressures at the end faces show that after a short-term impulse, the pressures at the outlet and inlet rose gradually from 0.5 to 7.8 MP and from 1.0 to 12.5 MPa, respectively. Whereas the inlet pressure maintained almost constant, the outlet pressure decreased gradually to zero, indicating that the outlet part of the sample was not fully saturated. Because of the mobilisation of the adsorbed water at the high temperature, an equilibrium between both external water reservoirs was reached at a pressure of 10.5 MPa after 35 days, which represents the actual pressure of the thermally-mobilized pore-water in the sample. The magnitude of the pore-water pressure increase due to a temperature rise is dependent on the confining stress and can be evaluated by the so-called “pore pressure temperature parameter” F which is defined as:

T

pF effm

w

∆σ∆

= (3)

where effmσ is the mean-normal effective stress, ∆T is the increment in temperature, and ∆pw is the increment in pore pressure. Based on the test data of ∆T = 59°C, ∆pw = 10.0


10

MPa, and effmσ = 8.7 MPa, the parameter F = 0.019 °C-1 is established for the COX-sample, which is close to a common value of 0.017 °C-1 for many saturated clay soils (Horseman et al., 1996). In addition to the pore-water pressure increase, suddenly heating led also to a short-term expansion, and after that, a gradual consolidation followed due to the collapse of unsaturated pores. Cooling caused a rapid drop of the pore pressure and a sudden compaction.

0

10

20

30

80 90 100 110 120

Time (days)

Stre

ss

(MPa

)

20

40

60

80

100

Tem

pera

ture

(°

C)

inlet pressure

axial stress

radial stress

outlet pressure

temperature

a. pore-water pressure

0.0

0.5

1.0

1.5

2.0

80 90 100 110 120

Time (days)

Stra

in

(%)

heating cooling

axial strain

radial strain

volumetric strain

kompression

extension

b. deformation

Fig. 4 Thermally-induced pore-water pressure and compaction of the COX clay rock

4 Swelling pressure

As mentioned before, swelling pressure is a mean value of the disjoining pressures acting in interparticle water-films which are bound on surfaces of clay minerals. In highly-consolidated clay rocks, the water-films are overlapped in the narrow spaces between particles and support the full lithostatic stress (Horseman et al., 1996; Rodwell et al., 1999). Usually, the swelling pressure is measured on volume-constraint samples by re-saturation with liquid water. However, previous laboratory tests provided low values of less than 2 MPa for the OPA and COX clay rocks (Thury & Bossart, 1999; Lebon & Ghoreychi, 2000), much lower than the lithostatic stresses at the sampling depths. The reasons are unclear up to now. According to Rodwell et al. (1999), equation (2) can be extended by adding the atmospheric pressure po or the matric suction pc = po – pw cDowDoweff ppppp −∏=−+∏=−+σ=σ (4) This relationship between the disjoining (swelling) pressure ΠD, the matric suction pc and the total stress σ acting on a water-film between two clay platelets is schematically illustrated in Figure 5. The analogue model (C) demonstrates the coupling between the forces. This equation suggests that the matric suction is simply the difference between the effective stress (the swelling pressure) and the total stress. The matric suction is governed by the intermolecular strain of the pore-water. When suction is positive (pw < po), the adsorbed water-film is in tension. When suction is negative (pw > po), the water-film is in compression. When suction is zero by pw = po, the external total stress is equal


11

to the effective stress or to the swelling pressure. This means that the swelling pressure of a clay sample can be determined by measuring the total stress at zero suction, which can be achieved by increasing relative humidity of the surrounding air to the maximum of RH = 100%.

a) pc > 0 → pw < po

water-film in tension b) pc < 0 → pw > po

water-film in compression c) pc = 0 → pw = po

Deff ∏=σ=σ Fig. 5 Relationship between the swelling pressure ΠD, the matric suction pc and the

total stress σ acting on a water-film between two clay platelets In accordance to this model, a new test method was recently developed by GRS for the determination of swelling pressure of clay rocks (Zhang et al. 2004b). Figure 6 shows schematically the test principle. A clay sample is axially fixed and laterally unconstraint. The suction is applied by changing the air humidity around the sample, and the resulting swelling pressure is measured by the axial stress reaction on the rigid piston. The first test was carried out on a COX-sample, which was taken at a depth of 455 m below the surface, corresponding to an overburden stress of about 11 MPa. The sample was axially loaded to 13 MPa and then the axial strain was fixed. In the first wetting phase at RH = 100%, the axial stress was stabilized at 12 MPa. The subsequent drying caused a very quick drop of the stress tending to zero. Increasing the humidity again led the axial stress to rising. At an elevated humidity of RH = 100% the axial stress reached about 10 MPa and maintained during the wetting phase. The swelling pressure measured at RH = 100% (suction pc= 0) is practically the same as the lithostatic stress at the sampling depth. Moreover, it is also interesting to point out that the swelling pressure ΠD (= σ1 = σ1-eff) was built-up in the fixed axial direction (εa = 0) at laterally-unconstraint condition, σ2 = σ3 = 0, εr ≠ 0. This means that the swelling pressure in clay rocks might not be a scalar quantity but a second-rank tensor (Horseman et al., 1996; Rodwell et al., 1999). More experimental evidences are to be provided by means of adequate test methods for clay rocks.

axial fixed

axial fixedair inlet

air outlet

sample

0

5

10

15

20

0 10 20 30Time (day)

Axi

al s

tres

s (

MP

a)

0

20

40

60

80

100

120

Rel

ativ

e hu

mid

ity

(%)

stress reaction

EST05547-07Callovo-Oxfordian argillite at Bure

relative humidity

drying

wetting

Fig. 6 Axial swelling pressure obtained on a COX-sample

Rodwell et al. (1999)

solid surface

water film


12

5 Swelling and shrinking

At high external suction, the water adsorbed on surfaces of clay particles removes thermodynamically, and in contrast, external water can be taken up by clays at low external suction. In equilibrium the water content is correlated with the suction. Figure 7 shows the relationship between degree of water saturation and suction determined for the COX and OPA clay rocks.

0

1

10

100

1000

0 20 40 60 80 100

Degree of water saturation S (%)

Suct

ion

s (

MPa

)

GRS-data for drying path

GRS-data for wetting path

UPC-data for drying path

UPC-data for wetting path

GRS-new-data, drying path

GRS-new-data, wetting path

Van Genuchten model

a. OPA clay rock

0

1

10

100

1000

0 20 40 60 80 100

Degree of water saturation S (%)Su

ctio

n s

(M

Pa)

CO-DIR, drying path

CO-DIR, wetting path

CO-TER, drying path

CO-TER, wetting path

Van Genuchten model for CO-DIR

Van Genuchten model for CO-TER

b. COX clay rock

Fig. 7 Relationship between suction and degree of water saturation for the OPA and

COX clay rocks In correspondence with the water content change, the distance or pore space between solid particles changes too, resulting in swelling or shrinking strain. The magnitude of swelling and shrinkage is determined mainly by the clay content, the density, the amount of water uptake or release, and the confining stress. Swelling and shrinking are the significant processes occurring in clay soils such as bentonite by wetting and drying. For consolidated clay rocks there are less data available. Meanwhile, several tests were performed by GRS on OPA- and COX-samples under loaded and unloaded conditions. Figure 8 shows the volumetric changes measured on unstressed samples in a desiccator by drying at a relative humidity of RH = 20% and by wetting at RH = 100%. During the drying phase, the reduction of water content (with respect to the initial weight of the sample) to 4.0% – 4.5% resulted in a shrinkage of 0.3% at a COX-sample and of 1.5% at an OPA-sample, respectively. During the following wetting phase of 6 months, the water contents of both samples increased to a same value of 4.0%. Correspondingly, the OPA-sample with a clay content of ~67% expanded up to 11.0%, which is larger than that of 6.5% observed on the COX-sample with a lower clay content of ~42%. Figure 9 presents two other tests on COX- and OPA-samples under axial loads. The initially saturated COX-samples at an axial stress of 15 MPa were exposed to dry air (RH = 25%). Consequently, the pore water evaporated from the initial content of 6.7% to 2% and a significant shrinkage of 0.3% in the load direction took place (Fig. 9a). The fluctuation of the axial strain (compression and expansion) is correlated with the change in water content. On another OPA-sample at a low axial stress of 1 MPa the swelling/shrinking behaviour was examined by wetting and drying at different temperatures between 24°C and 58°C (Fig. 9b). It is obvious that drying caused a volumetric reduction up to 1.6% and conversely wetting led to an expansion up to 2.4%. The swelling and shrinkage are well correlated to the amount of water uptake and release, and also partly reversible depending on the confining stress.


13

samples in desiccator

-4

-2

0

2

4

6

8

10

12

14

-5 -4 -3 -2 -1 0 1 2 3 4 5

Change in water content (w.%)

Volu

met

ric s

trai

n (

%)

OPA-sample

COX-sample

RH = 20%

RH = 100%

drying

wetting

Fig. 8 Swelling and shrinkage measurements on unconfined clay samples

0.8

1.0

1.2

1.4

200 250 300 350 400Time (day)

Axi

al s

trai

n

(%)

0

2

4

6

8

10W

ater

co

nte

nt

(%

)

Axial stress = 15 MPa

perpendicular to the bedding

parallel to the bedding

air drying (rH = 25%)

Callovo-Oxfordian argillite at Bure

water loss

axial loadair outlet

sample

air intlet

a. shrinkage of COX-samples

-2

-1

0

1

0 60 120 180 240 300 360

Time (day)

Stra

in

(%)

0

2

4

6

8

10

Wat

er c

onte

nt

(%)

com

pres

sion

exte

nsio

n

axial strain

radial strain

volumetric strain

water content Opalinus clay at Mont Terri

axial stress = 1 MPa

drying T=24-38°C

RH=20-50%

wetting T=24°C

RH=75-95%

drying T=58°CRH=3%

axial loadair outlet

sample

air intlet

b. swelling / shrinkage of an OPA-sample

Fig. 9 Swelling and shrinkage of the COX and OPA clay rocks under axial loads

Because of the presence of bedding structure in clay rocks, dehydration generates a heterogeneous collapse of desaturated pores and thus micro- and macro-fractures, as observed on a piece of the Opalinus clay at 105 °C (Figure 10). It is obvious that the thermally-induced fractures penetrated mostly along the bedding planes and extended with time even without further water loss after 2 days heating and drying.

t = 0 d

t = 14 d

t = 22 d

t = 32 d

Fig. 10 Thermally-induced fractures in an Opalinus clay sample at 105°C


14

6 Long-term deformation

The long-term deformation behaviour of the COX and OPA clay rocks was studied in uniaxial creep tests at different loads of 0.6 to 18 MPa and temperatures between 24°C and 90°C. Most of the tests lasted over an exceptionally long duration of 1 to 4.5 years with each creep phase of several months. The samples with natural water contents were sealed in rubber jackets and steel platens to prevent a water loss during the long testing periods. In the tests, a number of influence factors such as load level, material anisotropy, scale effect, water content and temperature were examined. Figure 11a shows the creep curves of COX-samples loaded perpendicular and parallel to the bedding plane. Over several months of the creep period, quasi - steady state creep seems to be reached beyond the transient creep. The quasi - steady state creep rates for the samples are summarised in Figure 11b as a function of the applied stresses. It can be recognised that the creep rates differ by a factor of less than 7 from one sample to another. It seams that the creep rate is a linear function of the applied stress. These creep tests performed at ambient temperature of 24°C suggest that there is no lower creep threshold, no significant scale effect and no significant anisotropy effect on the long-term deformation for the Callovo-Oxfordian argillite.

0.0

0.4

0.8

1.2

1.6

0 200 400 600 800 1000Time (days)

Axi

al s

trai

n

(%)

EST05582-03-II EST05671-02-=EST05671-03-= EST05582-02-IIEST05630-02-=

5

8

12

15

1818

stress (MPa):

parallel to bedding

perpendicular to bedding


a. creep curve

1E-11

1E-10

1E-09

1 10 100Stress (MPa)

Cre

ep r

ate

(1

/s)

EST05481-01-II(D100/L200)

EST05582-01-II(D100/200)

EST05582-02-II(D45/L90)

EST05582-03-II(D45/L90)

EST05630-01-II(D100/L200)

EST05630-02-=(D45/L90)

EST05671-01-II(D100/L180)

EST05671-02-=(D45/L90)

EST05671-03-=(D45/L90)

EST05677-01-II(D100/L200)

EST05751-01-II(D100/L200)

dε/dt = A·(σ/σo)σo = 1 MPa

A=3.1E-11 1/s

A=1.4E-11 1/s

A=7.5E-12 1/s

A=4.3E-12 1/s

b. creep rate

Fig. 11 Long-term creep of the COX clay rock as a function of applied loads Figure 12 illustrates the temperature dependence of the long-term deformation obtained on COX-samples at different temperatures between 24 and 90°C. The strain rates shown in Fig. 12b were measured within the last time interval at each temperature level. The following phenomena can be recognised: − A sudden increase of the temperature led to an expansion for a short time, and then

the creep continued with a higher rate. But due to possible dispersion of thermally-mobilized pore-water towards the annular space between the sample and the jacket at 50 - 60°C, the creep slowed.

− In contrast to heating, cooling from 60 to 40°C resulted in a contraction, but no

further creep appeared at the lowered temperature over 3 months. This indicates that the creep depends on the thermal loading path.


15

− Further increasing the temperature step by step from 40 to 90 °C produced a repeat of the short-term expansion and a progressive compression, but the strain rates are slightly lower than before.

− The strain rate of the clay rock with more carbonates (sample EST05481 from the

upper layer A of the COX formation) is about one order of magnitude lower than that with less carbonates (the other samples from the lower layer B).

0.0

0.2

0.4

0.6

0.8

0 100 200 300 400 500 600 700 800 900Time (day)

Axi

al s

trai

n (

%)

20

40

60

80

100

120

Tem

pera

ture

(°

C)

EST05684Callovo-Oxfordian argillite at Bure

2428

40

50

axial stress = 0.74 MPa

temperature

axial strain60

70

80

40

60

90

water content = 6.88%

a. creep curve

1E-12

1E-11

1E-10

1E-09

20 30 40 50 60 70 80 90 100Temperature (°C)

Cre

ep ra

te

(s-1

)

EST05684 at 0.74 MPa MSE00837 at 1.0 MPa

EST05671 at 13.8 MPa EST05481 at 13.8 MPa

heating path 1 heating path 2

cooling 60 40°C

b. creep rate Fig. 12 Long-term creep of the Callovo-Oxfordian argillite at elevated temperatures The strain acceleration at elevated temperature might result from two mechanisms: a) the thermally-induced reduction of the pore-water viscosity results in a reduction of the shear resistance between particles, and b) the release of thermally-mobilized pore-water causes collapse of the de-saturated pores. Up to now, deformation mechanisms in clay rocks are still not well understood. But the laboratory observations suggest that interparticle water-films adsorbed in clay rocks play a key role in the deformation. Physically-based models for the long-term deformation of clay rocks are to be developed.

7 Strength

The stress / strain relation and strength of the COX and OPA clay rocks was determined on samples in triaxial compression tests at different confining stresses and temperatures. In addition to the strain measurement, ultrasonic wave velocity was measured to detect damage of the samples. Figure 13 shows a typical example of the tests. The deviatoric stressing leads to linear axial compression and radial extension until onset of dilatancy (damage). During stressing, the pre-existing pores and micro-fissures are compacted, allowing ultrasonic waves to travel more quickly. Just before the onset of dilatancy, the wave velocity reaches the maximum. Beyond this point, the wave velocity decreases with increasing stress until failure, indicating the development of micro-fractures on a large scale. The fractures appear mostly along the bedding planes, as observed on the OPA-samples, whose bedding planes were inclined to the axial loading direction with angles of 30 - 40° (Fig. 13b). The dilatant strength of the OPA clay rock reaches ~ 80% of the peak strength.


16

0

5

10

15

20

25

-0.5 0.0 0.5 1.0Strain (%)

Dev

iato

ric s

tres

s (

MPa

)

3300

3320

3340

3360

Wav

e ve

loci

ty

(m/s

)

wave velocity

axial strainradial volumetric

onset of dilatancy

temperature = 20°C

lateral stress = 8 MPa

failure

Opalinus clay at Mont Terri

a. triaxial tests

b. samples after failure

Fig. 13 Stress / strain behaviour and failure pattern of the OPA-samples

In Figure 14, the stress / strain curves obtained at different lateral stresses of 1 to 8 MPa and at elevated temperatures between 20 and 116°C are summarized. It can be seen that the strength increases with the confining stress. The failure strains are limited to 1%. Under undrained conditions and comparable lateral stress of about 3 MPa, the clay rock becomes more ductile and weaker with increasing temperature.

0

5

10

15

20

25

-0.6 -0.2 0.2 0.6 1

Dev

iato

ric s

tres

s (

MPa

) 88

55

22

11

Radial strain (%) Axial strain (%)

lateral stress in MPa strain rate = 1E-7 1/stemperature = 20°C

strain rate = 2E-8 1/s

a . at different lateral stresses

0

5

10

15

20

25

-0.4 0 0.4 0.8 1.2

Dev

iato

ric s

tres

s (

MPa

)

Radial strain (%) Axial strain (%)

95/3.3

116/3.3

20/2

41/3.1

69/3.1

temperature (°C) / lateral stress (MPa)

20/2

41/3.169/3.1

Opalinus clay at Mont Terristrain rate = 1·10-7 s-1

b. at different temperatures Fig. 14 Stress / strain curves obtained on the OPA- samples at different confining

stresses and temperatures

8 Self-sealing

Due to the high swelling capacity and visco-plasticity of the studied clay rocks, cracks and fractures existing in the EDZ may be re-sealed under the combined impact of confining stress and re-hydration. This aspect was investigated on damaged samples of the COX and OPA clay rocks by measurement of gas permeability under re-consolidation and re-saturation conditions expected in the EDZ after repository closure. Figure 15 compares the gas permeaiblities determined on highly-damaged samples of the Callovo-Oxfordian argillite before and after water re-saturation. Under a confining stress of 1.5 MPa, the permeabilities of the damaged samples decreased with time from 10-16 m2 to 10-17 m2 over two months. After injection of synthetic pore-water over 5


17

months, a very low gas permeability of 10-21 m2 was measured, which is 4 - 5 orders of magnitude lower than before saturation.

sample 1

sample 2

1E-24

1E-22

1E-20

1E-18

1E-16

1E-14

0 10 20 30 40 50 60 70 80Time (day)

Gas

per

mea

bilit

y

(m2 )


damaged state

re-sealed state

Test conditions:confining stress = 1.5 MPa

injection pressure = 0.75 barbefore water injection

Test conditions:confining stress = 2.4 MPainjection pressure = 20 bar

after water injection

sample 1

sample 2

Figure 15 Self-sealing tests on damaged COX-samples Figure 16 presents the test results on a hollow COX-sample, from which the main findings can be summarized as follows: − By increasing the confining stress the gas permeability measured in radial direction

decreases exponentially from 10-15 m2 at 1 MPa to 10-21 m2 at 28 MPa. The permeability values determined during the unloading path are about two orders of magnitude lower than those obtained during the previous loading path. This may be due to a pre-consolidation effect.

− The gas permeability obtained in axial direction behaves similar to the radial

permeability. But the axial permeability is 1 - 2 orders of magnitude higher than the radial one at stresses below 10 MPa. The hydraulic anisotropy vanishes with increasing the confining stress.

1E-23

1E-21

1E-19

1E-17

1E-15

1E-13

0 5 10 15 20 25 30

Confining stress (MPa)

Perm

eabi

lity

(m

2 )

loading path

unloading path

jacket

gas inflow

gas outflow

porous medium

seal

K = 10-15.σ-5.3

K = 10-15.σ-3.3

Sample EST17284: D/d/L=79/20/103mmCallovo-Oxfordian argillite at Bure

Injection pressure = 0.17 MPa

a. permeability measured in radial direction

1E-21

1E-19

1E-17

1E-15

1E-13

0 5 10 15 20 25 30

Confining stress (MPa)

Perm

eabi

lity

(m

2 ) loading pathjacket

gas inflow

gas outflow

porous medium

seal

K = 10-14.σ-4

Sample EST17284: D/d/L=79/20/103mmCallovo-Oxfordian argillite at Bure

Injection pressure = 0.17 MPa

b. permeability measured in axial direction

Figure 16 Radial and axial permeability measured on a hollow COX-sample


18

Additionally, a large self-sealing test was conducted on a big OPA-sample of 260 mm diameter and 600 mm length. Due to the coring and long-term storage even under a confiment, significant cracks developed on the surface and probably penetrated through the sample, as shown in Figure 17. The test was performed in three phases: 1) measurement of the gas permeability at different confining stresses; 2) re-saturation by injecting synthetic formation water into the sample; and 3) determination of gas entry pressure and permeability after the re-saturation.

end faces

before testing

0

5

10

15

20

0 20 40 60 80 100 120

Time (day)

Stre

ss

(MPa

)

1E-21

1E-20

1E-19

1E-18

1E-17

1E-16

1E-15

1E-14

1E-13

1E-12

Perm

eabi

lity

(m

2 )

3

6

9

12

15

18

19axial

stress

lateral stress

gas permeability

6

3

Self-sealing test on a large Opalinus clay sample

(D=260mm, L=600mm)

water injection phase gas

permeability

Figure 17 Large self-sealing test on a big damaged sample of the Opalinus clay

The following conclusions are drawn: − Increasing the axial stress from 3 to 19 MPa at a lateral stress of 3 MPa led to a

reduction of the gas permeability from 10-13 m2 to 10-14 m2 and the subsequent increment of the lateral stress from 3 to 18 MPa resulted in a strong decrease of the permeability to 10-19 m2, five orders of magnitude lower than before.

− Decreasing the stresses to an isotropic state of 6 and 3 MPa increased the

permeability to 6·10-17 m2 and 3·10-16 m2 respectively. − During the following water injection at a pressure of 1.5 MPa over more than one

month, no water outflow was observed. − After the water re-saturation, gas was injected again at a pressure of 0.5 MPa, but

no gas outflow was measured over two weeks. Increasing the gas pressure to 0.7 MPa reopened some of the cracks and a low gas permeability of 1·10-18 m2 was measured. Increasing the confining stress from 3 to 6 MPa reduced the permeability to 5·10-21 m2, which is the same as that of the intact rock. The permeability values measured after water re-saturation were two to three orders of magnitude lower than before.

All these experimental findings provide a strong evidence for the high self-sealing potential of the damaged clay rocks.


19

9 THM processes

In order to support the HE-D heating experiment performed in the Mont Terri URL, heating tests were also conducted in the GRS laboratory on big samples of 260 mm diameter and 700 mm length. The in-situ experiment was simulated by heating the sample at its top to 80°C and cooling down at a confining stress of ~8 MPa and a pore-water pressure of 0 MPa at the heater/sample interface and a water pressure of 1 MPa at the bottom. THM processes in the sample were observed by monitoring temperature, pore-water pressure, and axial/radial strains with instruments installed outside and inside the sample, as shown in Figure 18. Four test intervals (P1 to P4) for measuring the pore-water pressure in the sample were sealed with mini-packers of 10 mm diameter and 30 mm length. The temperature in the sample was measured with four sensors (T1 to T4) installed at different levels along the central axis.

Heater

piston

jacket

sinter filter

sample

extenso-meter

heater 1

water pressurept = 0 MPa

heater 2

triaxial cell

heater 3

thermal isolation

water pressurepb = 1 MPa

heater 4

15

30

45

60

2

P3

T4

P4

P2

P1

T3

T2

T1

To

P

h cm

70

0

a. test layout

b. installation of sensors

c. GRS-mini-packers

d. temperature sensors

Fig. 18 Test layout and instrumentation of a heating test on a large OPA-sample The test was conducted in consecutive stages of re-saturation, re-consolidation, heating and cooling, as shown in Figure 19a. The sample was re-saturated by injecting synthetic pore water at a pressure of 1 MPa into the end faces and the test intervals at σ1 = 3.5 MPa, σ3 = 3.0 MPa, and at a temperature of 30°C over 18 days. After adjusting the water pressure at the upper heater/sample interface to zero and to 1 MPa at the bottom, the sample was compressed by increasing the stresses to σ1 = 8.5 MPa and σ3 = 8.0 MPa over 15 days. Under the applied HM boundary conditions, the sample was heated at the top by increasing the temperature from 30 to 55 and 80°C for 24 days, while the temperatures at the other boundaries were kept constant at 30°C. Finally, the temperature was decreased to 30°C for 3 days. Figure 19b shows the temperature measured in the sample. Heating caused a short-term thermal transient phase followed by a steady phase. The temperature decreased with the distance to the heater. The subsequent cooling led to a quick decrease of the temperature.


20

0

2

4

6

8

10

0 10 20 30 40 50 60Time (day)

Stre

ss /

pres

sure

(MPa

)

20

40

60

80

100

Tem

pera

ture

(°C

)

σ1= 3.5 MPaσ3= 3.0 MPa

σ1= 8.5 MPa σ3= 8.0 MPa

Temperature T = 30°C

T= 80°C

Pt= 0 MPaPb= 1 MPa

55°C

re-saturation consolidation heating cooling

Stress

Pressure

30°C

a. boundary conditions

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60Time (day)

Tem

pera

ture

(°C

)

test data

modelT4 = 76°C

T3 = 52°C

T2 = 40°C

T1 = 35°C

Tb = 30°C

Heater Th = 80°C

Initial T = 30°C

To = 31.5°C

To

heater Th

T3

T2

T1

T40

Tb

(cm)

2

70

45

30

15

b. temperature

0

1

2

3

4

5

6

0 10 20 30 40 50 60Time (day)

Wat

er p

ress

ure

(M

Pa)

test: Pt test: Pb

test: P3 test: P2

test: P1 model: P1

model: P2 model: P3

loading

heating

cooling

water injection Pt= 0 MPa

Pb= 1 MPa

Pt

P4

heater

P3

P2

P1

0

(cm)

45

30

70

60

15

Pb

Model THM2: E=3300 MPak=2E-20 m2

° sv=1.5E-6 K-1

c. pore-water pressure

-0.8

-0.4

0.0

0.4

0.8

1.2

0 10 20 30 40 50 60Time (day)

Rad

ial s

trai

n (

%)

water injection

com

pres

sion

exte

nsio

n

loading

Pt= 0 MPa

axial strain

radial strain

volumetric strain

re-saturation consolidation heating cooling

loading

heating

cooling

d. deformation

Fig. 19 THM processes observed in a large OPA-sample during a heating test Increasing the confining stress generated different responses of the pore-water pressures in the test intervals, as shown in Fig. 19c. The different responses might be caused by some disturbances resulting from the instrumentation. Due to the drained conditions, the pore pressure dissipated progressively. The heating produced only a slight increase of the pore pressure because of the drained conditions. Due to the pre-existing cracks oriented mainly along the sample axis, the radial strain change is much more significant than the axial one (Fig. 19d). The water injection pressure of 1 MPa could reopen some of the cracks at the confining stress of 3 MPa. The cracks were closed again by compression up to a stress of about 8 MPa. The heating induced a rapid expansion, followed by a gradual consolidation. The subsequent cooling resulted in a rapid compaction which was remained unchanged. The heating test was numerically simulated by using CODE-BRIGHT and some key parameters were estimated from this as average properties of the clay rock: − Thermal conductivity λ = 1.7 W·m-1·K-1 − Specific heat capacity C = 800 J·kg-1·K-1 − Linear thermal expansion coefficient of the solid grains αs = 1.5·10-6 K-1 − Volumetric thermal expansion coefficient of the pore water αw = 3.4·10-4 K-1 − Linear thermal expansion coefficient of the rock mass αm = 1.7·10-5 K-1 − Elastic modulus E = 6680 MPa − Poisson’s ratio ν = 0.33 − Intrinsic permeability k = 2·10-20 m2.


21

These parameters were also applied for modelling the in-situ heating experiment. The calculated temperature, pore-water pressure and deformation are well comparable with the results of the field measurements (Zhang et al. 2007a).


22

10 References

Horseman, S.T., Higgo, J.J.W., Alexander, J., Harrington, J.F. (1996): Water, Gas and Solute Movement through Argillaceous Media. Report CC-96/1, 1996.

Lebon, P., Ghoreychi, M. (2000): French Underground Research Laboratory of

Meuse/Haute-Marne: THM Aspects of Argillite Formation. EUROCK2000, Aachen, 27-31, March 2000.

Noynaert, L. (editor, 2000): Heat and radiation effects on the near field of a HLW or

spent fuel repository in a clay formation (CERBERUS Project). EUR 19125EN, Contract No F14W-CT95-0008.

Rodwell, W.R., Harris, A.W., Horseman, S.T., Lalieux, P., Müller, W., Ortiz Amaya,

L., Pruess, K. (1999): Gas Migration and Two-Phase Flow through Engineered and Geological Barriers for a Deep Repository for Radioactive Waste. A Joint EC/NEA Status Report, European Commission, 1999, EUR 19122 EN.

Thury, M., Bossart, P. (1999): Mont Terri Rock Laboratory, Results of the

Hydrogeological, Geochemical and Geotechnical Experiments performed in 1996 and 1997. Geological Report Nr. 23, 1999, Bern.

UPC (2002): CODE-BRIGHT, A 3-D program for thermo-hydro-mechanical analysis in

geological media, 2002. Zhang, C.L., Dittrich, J., Müller, J., Rothfuchs, T. (2002): Experimental Study of the

Hydromechanical Behaviour of the Callovo-Oxford Argillites – Part of the MODEX-REP Project. December 2002, GRS-187, ISBN3-931995-55-0.

Zhang, C.L., Rothfuchs, T. (2004a): Experimental Study of Hydromechanical

Behaviour of the Callovo-Oxford Argillites. Special Issue: Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, Reims, France, 2002. Applied Clay Science 26 (2004) 325-336.

Zhang, C.L., Rothfuchs, T., Moog, H., Dittrich, J., Müller, J. (2004b): Thermo-Hydro-

Mechanical and Geochemical Behaviour of the Callovo-Oxford Argillite and the Opalinus Clay. June 2004, GRS-202, ISBN3-931995-69-0.

Zhang, C.L., Rothfuchs, T., Jockwer, N., Kröhn, K.-P., Miehe, R., Moog, H. (2004c):

Results of Laboratory Investigations on Clays, Proc. International Conference on Radioactive Waste Disposal - DisTec 2004, Berlin 26-28 April 2004.

Zhang, C.L., Rothfuchs, T., Su, K., Hoteit, N. (2005): Experimental Study of the

Thermo-Hydro-Mechanical Behaviour of Indurated Clays, 2nd International Meeting of Clays in Natural and Engineered Barriers for Radioactive Waste confinement, Tours, March 14-18, 2005.


23

Zhang, C.L., Wieczorek, K., Rothfuchs, T. (2006a): Experimental study on damage and self-sealing of indurated clays, Poster presentation on the NF-PRO-Third Workshop, 14-16 Nov. 2006, Spain.

Zhang, C.L., Rothfuchs, T., Jockwer, N., Wieczorek, K., Dittrich, J., Müller, J.,

Hartwig, L., Komischke, M. (2007a): Thermal Effects on the Opalinus Clay – A Joint Heating Experiment of ANDRA and GRS at the Mont Terri URL (HE-D Projekt), GRS – Final Report, February 2007.

Zhang, C.L., Rothfuchs, T. (2007b): Moisture Effects on Argillaceous Rocks.

Proceedings of the 2nd International Conference “Mechanics of Unsaturated Soils,” Weimar, Germany, 7th – 9th March, 2007.


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