Permeability of macro-cracked argillite under

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(Gautschi, 2001; Croisé et al., 2004; Escoffier et al., 2001;. Boisson et al., 2001), ..... the downstream sample side simultaneously to free drain- ing on the upstream side ..... International Journal of Heat and Mass Transfer. 47, 3517–3531. Hsieh ...
Physics and Chemistry of the Earth 32 (2007) 667–680 www.elsevier.com/locate/pce

Permeability of macro-cracked argillite under confinement: Gas and water testing Catherine A. Davy b

a,*

, F. Skoczylas b, J.-D. Barnichon c, P. Lebon

c

a Industrial Engineering Research Team, Ecole Centrale de Lille, BP 48, F-59651 Villeneuve d’Ascq Cedex, France Laboratoire de Me´canique de Lille (LML), Ecole Centrale de Lille, BP 48, F-59651 Villeneuve d’Ascq Cedex, France c ANDRA, 1-7 rue Jean Monnet, F-92298 Chaˆtenay-Malabry Cedex, France

Received 1 August 2005; received in revised form 1 February 2006; accepted 26 February 2006 Available online 18 October 2006

Abstract Argillite is considered a privileged candidate for long term nuclear waste storage. Yet argillite rock drilling often induces surface cracks that locally modify its permeability. This phenomenon located in a so-called Excavation Damaged Zone (EDZ) is of importance since permeability increase means lesser confinement capacity of the argillite rock. Potentially influencial phenomena occur when argillite is subjected simultaneously to normal stress variations and fluid seepage. Therefore, this extensive experimental study (6 month duration) on macro-cracked Callovo-Oxfordian argillite is aimed at distinguishing the contribution to rock permeability of mechanical loading (crack opening and closing) on one part and of chemically active fluid seepage (water) on the other. Steady state gas flow tests show that permeability K mainly depends upon crack closure cc, with values on the order of 1014 m2. Permeability from transient water flow tests varies with test duration from 1018 to 1021 m2. In both test types, K also depends upon confining pressure Pc, mainly during the first three loading–unloading phases. A difference between water injection tests and gas injection tests is that the water-saturated rock sample swells. Swelling does not contribute to unload the crack zone but rather creates additional closure and pressure in the crack area. Indeed, water permeability is shown to depend upon cumulated crack closure ac, which sums up swelling and confinement-induced crack closure. Finally, this study outlines the strong effect of water upon crack closure amplitude and permeability. After a relatively short time (on the order of ten days), water flow within the crack drives the permeability back to very low values close to sound rock permeability (1021 m2). This reflects a complete self-sealing of the macro-crack, which is an important factor for nuclear waste repository safety.  2006 Elsevier Ltd. All rights reserved. Keywords: Nuclear waste storage; Excavation damaged zone; Argillite; Gas permeability; Water permeability; Permeability pulse test

1. Introduction Argillaceous rock is a privileged candidate for radioactive waste storage thanks to its extremely low permeability (Gautschi, 2001; Croise´ et al., 2004; Escoffier et al., 2001; Boisson et al., 2001), to the retention properties of constituent clayey materials (Patriarche et al., 2004), as well as for its high mechanical strength, its drillability, and its good availability in soils. In particular, Callovo-Oxfordian argillites are available in large quantities between 450 and *

Corresponding author. Tel.: +33 3 20 33 53 62; fax: +33 3 20 33 53 52. E-mail address: [email protected] (C.A. Davy).

1474-7065/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2006.02.055

550 m deep at the Bure site (Meuse, France) in the Eastern Paris Basin (Fouche et al., 2004). ANDRA is currently building there its deep-seated research laboratory on the feasibility of long-term nuclear waste storage. Preliminary to this, methodological underground research laboratories have allowed the investigation of Toarcian argillites behaviour at the Tournemire site (Southern France) (Patriarche et al., 2004) and that of softer materials, namely Opalinus clay, at Mont Terri, Switzerland (Meier et al., 2000; Croise´ et al., 2004), and Boom clay at HADES (Mol, Belgium) (Barnichon and Volckaert, 2003; Bastiaens et al., 2005). In this context, numerous research studies have been performed. Patriarche et al. (2004) have investigated water flow

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and mass transport at the Tournemire site, both experimentally and numerically, in order to anticipate a potential radionuclide leak scenario. Complementary studies on the subject are Descostes and Tevissen (2004) and Buschaert et al. (2004) for Callovo-Oxfordian argillites and Charpentier et al. (2001), Ferry et al. (2002) for Toarcian argillites. Low level ion exchanges through argillite were measured as a function of pH conditions (Devivier et al., 2004) and ion type (Jacquier et al., 2004). Besides, argillite hydromechanical behaviour has been investigated. Conil et al. (2004) have proposed thermodynamics modelling of plasticity and damage. Parameters for different constitutive models have been identified (Zhang and Rothfuchs, 2004; Homand et al., 2004; Chiarelli et al., 2002; Liao et al., 1997a,b). For intact Callovo-Oxfordian argillites (viz. without noticeable micro- or macro-cracking), water flow permeability along rock bedding planes ranges from 1019 m2 down to 1021 m2 with increasing confining pressure, see Zhang and Rothfuchs (2004); it is less than an order of magnitude lower perpendicular to bedding planes. Gas permeability of dry intact rock ranges from 1017 m2 down to 1019 m2 and exhibits greater anisotropy than fully-saturated rock. Indeed, argillite saturation level is shown to influence strongly its poromechanical behaviour (Bemer et al., 2004), as well as its microstructure, particularly its amount of microcracks and pores (Montes et al., 2004). Among these, of primary interest is the effect of a storage tunnel drilling on argillite confinement capability. Indeed, argillite rock machining induces surface cracks that modify locally its permeability in a so-called Excavation Damaged Zone (EDZ) (Kolmayer et al., 2004; Davies and Bernier, 2004; Blu¨mling et al., 2005), leading to lesser confinement capability. However it is thought that stress redistributions after drilling and fluid seepage through time re-confine surface cracks and bring back partial crack closure (Meier et al., 2000). After backfilling and closure of the storage tunnel, stress redistributions are shown to re-confine surface cracks, and argillite permeability decreases (Bastiaens et al., 2005). On the long term, it is then probable that argillite permeability evolves towards a middle value between intact rock permeability and drilled cracked rock permeability. Assessing nuclear waste storage performance imposes an accurate knowledge of cracked argillite confinement capacity, especially when it is subjected simultaneously to normal stress increase (re-confinement) and fluid seepage. It is also useful to distinguish the contribution to permeability of mechanical loading (viz. crack opening and closing) on one part, and of chemically active fluid seepage (particularly water) on the other. Therefore, as initiated by ANDRA, an extensive experimental program (6 month duration) was performed at the Lille Mechanics Laboratory (LML) on macro-cracked Callovo-Oxfordian argillites to answer these questions, as will be shown in this paper. Two original testing devices were used to that purpose: one for steady state gas flow permeability tests, the other for transient water flow permeability tests. Particular emphasis is given in this paper to the experimental

approach and analysis. Comparison with available analytical or numerical models will be tackled in further work. Several methods exist to measure porous material permeability K under loading, depending on the order of magnitude of K (Skoczylas and Fleureau, 2003). They usually consist in applying an axial unidimensional steady state or transient fluid flow, corresponding to a given pressure difference, through a circular cylindrical sample placed in a conventional triaxial test rig. Whatever the method, particular attention has to be paid to calibrate pressure and flow rate measurements and to control micro-seepage, flow geometry, etc. 1.1. Steady state flow permeability tests For K P 1019 m2, steady state flow permeability tests are usually a good choice: Darcy’s law can be directly applied to derive K from imposed flow rate or pressure conditions (Skoczylas, 1996; Meziani and Skoczylas, 1999; Loosveldt et al., 2002), as follows. A circular cylindrical sample is subjected to a given fluid pressure P1 on one side, while the other side drains freely at pressure P0 < P1, see Fig. 1(a). The fluid flow is assumed unidimensional along the specimen ~ x axis. Height variations are assumed negligible compared to pressure variations. Therefore, for a perfectly fluid-saturated media, Darcy’s law is simplified as V x ¼ ðK x =lÞ

dP ðxÞ dx

ð1Þ

where Vx is the fluid average speed at given x, Kx is permeability along axis ~ x, and l is the fluid viscosity. l is given from standard tables when the fluid nature is chosen. In order to derive Kx from (1), Vx may be given by measuring volume flow rate Q = Vx/A through the specimen, where A is its sectional area. However, difficulties arise in finding and using standard flow rate meters in the required pressure range, therefore, an original ‘‘quasi-stationary’’ flow method was designed and validated, see Skoczylas (1996), Meziani and Skoczylas (1999). A brief summary is given below. A buffer reservoir is installed between the gas source and the sample, see Fig. 1(a). Gas flows steadily through the buffer reservoir and down to the sample upstream side at pressure P1, and then, down to the sample downstream side which is at atmospheric pressure P0, until the valve situated upstream of the reservoir is closed at time t = 0. Time variation Dt is recorded from pressure P1 and until upstream pressure reaches P1  DP1 with DP1 lower than P1 by two orders of magnitude. During Dt, the gas flow is assumed constant, and at an average pressure Pmean = P1  DP1/2 on the upstream side. The gas is assumed perfect and all tests are conducted in a thermically insulated room at constant temperature. From these assumptions, the average upstream volume flow rate Qmean is deduced from DP1 and Dt values using that Qmean ¼

V 1 DP 1 P mean Dt

C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

669

(a) injection via the buffer reservoir

valve

P1 (or Pmean) V1

sample

Gas holder manometer

pressure P1 (or Pmean)

pressure P0

(b) water from injection Injection pumppompe

drain Purge valve failure Disque de rupture : disc manometer Manomètr 100 Mpa 100 MPa

upstream water

circuit at P1 Circuit haute pression

buffer reservoir

differential Différentiel manometer

sample

Réservoir

Circuit basse pression

buffer reservoir

drain Purge valve

manometer Manomètre

downstream water circuit at P2

Réservoir

Fig. 1. Test principle for (a) steady state flow permeability tests; (b) transient pulse permeability tests.

where V1 is the buffer reservoir volume. Finally, for an injection of gas, permeability along axis ~ x is deduced from mass balance equation and Darcy’s law as Kx ¼

lQmean 2LP mean A ðP 2mean  P 20 Þ

where L is the specimen length along axis ~ x. This formula is assumed valid for macro-cracked specimens, although gas flows preferably through the crack zone rather than through the whole specimen cross-sectional area A. Indeed, fluid average volume flow rate Qmean, from which Kx is deduced, is assumed identical all along ~ x, whether the specimen is macro-cracked or not. 1.2. Transient (or pulse) permeability tests For K below 1019 m2, steady state flow test duration gets extremely long (from several weeks to several months

for one measurement). Hence, transient techniques are preferred, such as pulse test or harmonic method. Pulse test, as described by Brace et al. (1968), consists in placing a circular cylindrical specimen between two fluid reservoirs at the same initial static pressure level P1. A pressure increase is then applied upstream and the fluid (liquid or gas) is let to flow while measuring the evolution of the differential pressure between upstream and downstream circuits, (P1  P2), versus time t, see Fig. 1(b). Simplified analytical methods (Skoczylas and Henry, 1995; Hsieh et al., 1981; Neuzil et al., 1981) approach this differential pressure with an exponential: (P1  P2)(t) = DP1exp(cKt) where c is a constant depending on the experimental arrangement and K is permeability. Adequately interpreted, (P1  P2)(t) measurements give very low values for K (below 1020 m2 within a few hours). Tests can be conducted either under gas or liquid flow. Compared to steady state flow tests, pulse test results have an accuracy better than 5%,

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C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

as shown using our experimental rig in Skoczylas and Fleureau (2003) on water flow experiments. Harmonic method, as developed by Jouanna and Fras (1979), consists in creating a sinusoidal pressure wave in the upstream reservoir. After transmission through the specimen, the amplitude and phase of the downstream wave are analysed and K and other poro-mechanical properties are derived. From our point of view, the harmonic technique appeared more delicate to implement than the pulse test. 2. Experimental method 2.1. Physical characteristics and preparation of argillite samples As presented in Fouche et al. (2004), plug coring was performed at the Bure site at several depths in the argillite layers. Samples used in this study are extracted from three plugs which main petro-physical characteristics are given in Table 1. In order to remain under similar stress to when it was underground, each plug was confined laterally in a concrete matrix and loaded axially by means of metallic springs. Each plug was deconditioned in our laboratory just prior to specimen coring, which was performed using a diamond cylindrical circular saw under dry conditions (no water lubrication), see Fig. 2(a). Right after coring, each specimen was preserved in a water-tight envelope until permeability tests in order not to modify their water content. Specimen dimensions are 37 mm diameter and 70 mm long so as to guarantee a (height/length) ratio of 2, which is commonly admitted as sufficient to avoid stress localisation end effects. Visual inspection of each plug showed that EST5600 is exempt from macro-cracks, whereas MSE761

Table 1 Main petro-physical characteristics of the argillite plugs, as received from ANDRA, and of permeability tests performed on them Plug no. Borehole no. Extraction date Upper extraction depth (m) Lower extraction depth (m) Cored sample quantity Permeability test type

EST5600 EST205 16/09/2000 467.47

MSE761 MSE101 – 567.72

MSE748 MSE101 14/01/1995 564.50

467.77

568.02

564.80

2

4

2

Steady state gas flow

Transient water flow

Density Water content (%) Sonic celerity Vp (m/s) CaCO3 (%)

2.43 7.4 3170

Steady state gas flow (1 test) and transient water flow (3 tests) 2.31 8.3 3146

2.35 7.3 3170

24.0

19.4

25.2

and MSE748 are much more fragile and degraded by a large amount of micro-cracks; in particular, MSE748 has broken itself in three or four parts right after de-conditioning. This phenomenon is interpreted as a crack amount evolution of MSE761 and MSE748 since their coring (in 1995). This could be due to slow saturation rate variations with time, contrarily to EST5600 which was extracted in 2000 only, see Vale`s et al. (2005). In the following, sample saturation rate influence upon permeability is investigated, yet, it has not appeared necessary to quantify saturation rates at this point. Argillite samples were macro-cracked along a diametral plane using a Brazilian splitting test, see Fig. 2(b) and (c). In order to avoid sample shattering in this phase, coring was performed along argillite bedding planes: the artificially created macro-crack, therefore, corresponds to a split between two bedding planes. Nevertheless, although we obtained macro-cracks in the required sample diametral plane, their initial opening amplitude was not controlled in order to reproduce the in situ crack opening variability. This induces disparities in crack closure measurement levels with increasing confining pressure. 2.2. Experimental arrangements 2.2.1. Test devices Two test devices have been used: (1) a steady state gas flow test apparatus, see Fig. 3 and Loosveldt et al. (2002); (2) a transient water flow test apparatus, similar to the former. In both cases, the macro-cracked argillite specimen is sealed inside a Vitton membrane and placed in a triaxial confining cell. Confining pressure Pc is held constant using a Gilson-type pump. For all of the tests, confining pressure goes up to 17 MPa (maximum value fixed in accordance with ANDRA). For gas permeability tests, gas flows through the buffer reservoir to the specimen upstream side at pressure P1, and down through the specimen to its downstream side at atmospheric pressure P0. Gas injection pressure P1 varies from 0.35 up to 1.5 MPa. Inert 99% pure Argon gas is used: its viscosity l is taken as 2.2 · 105 Pa s. For water pulse tests, water flows between an upstream buffer reservoir at pressure P1 and a downstream buffer reservoir at pressure P2. Initial saturation static pressure P0 = P1(t = 0) = P2(t = 0) is taken as 1.5 MPa. A pressure increase (or pulse) is applied at time t = 0+ on the sample upstream side with an amplitude taken as DP1 = 0.5– 0.7 MPa. Both buffer reservoirs have an identical volume V1, which is adapted to the measured permeability range: V1 = 0.665 · 103 m3. Demineralised water added with 1.75 g/l NaCl and 0.014 g/l CaCO3 is used. Its viscosity is taken as: l = 1.005 · 103 Pa s. Pressure difference between upstream and downstream reservoirs is modelled as: P1(t)  P2(t) = DP1exp(cKt). Theoretically, parameter c is a function of reservoir volumes, fluid compressibility, reservoir and water circuit pipe compressibility, argillite sample accumulation coefficient and external factors such

C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

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Fig. 2. (a) Coring of permeability test specimens in EST 5600 plug; (b) Brazilian test arrangement; (c) macro-cracked samples for gas permeability tests.

Fig. 3. Experimental apparatus for steady state gas flow tests, as in Loosveldt et al. (2002), similar to the apparatus for transient water flow tests.

as room temperature variations. Argillite sample accumulation coefficient is a poromechanical parameter which characterises the fluid mass retainable inside the sample porosities in order to get there a given pressure increase,

see Dana and Skoczylas (1999). Complementarily to the theory, c was identified experimentally by Lion et al. (2004). With reservoir volumes equal to V1 = 0.665 · 103 m3, c = 2.2 · 1014 m2 s1, which is the value used

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C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

here. All water permeability tests have been performed in a temperature-controlled room at 22 C, and all water circuit pipes have been thermally insulated. Indeed, very small temperature variations (of a few Celsius degrees) induce small water movements inside the low permeability rock sample, which leads, in turn, to large pressure variations detrimental to the good test progress. 2.2.2. Measurement devices and accuracy At each confinement level, crack closure is measured using LVDT sensors placed inside the triaxial cell, in contact with the sample Vitton sleeve, in a plane perpendicular to the specimen ~ x axis, see Fig. 4(a) and (b). Measurements accuracy is improved using two pairs of diametrically opposed transducers, each pair being placed perpendicularly to the other. The LVDT arrangement is placed at any angle to the macro-crack. Crack closure cc is deduced as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cc ¼  ðd1 þ d2 Þ2 þ ðd3 þ d4 Þ2 where d1 and d2 are the displacements of diametrically opposed sensor 1 and sensor 2. d3 and d4 are the displace-

ments of diametrically opposed sensor 3 and sensor 4. d1, d2, d3 and d4 are equal to zero at t = 0 (test start). cc sign is identical to that of (d1 + d2) and (d3 + d4). cc is positive when the distance between the macro-crack surfaces decreases (sample crack closure), and cc is negative in the opposite case (sample crack opening or swelling). Confining pressure is measured using a traditional manometer with an accuracy of (1/100) MPa. For steady state gas tests, pressure in the upstream gas circuit is recorded with a manometer similar to that used for confining pressure records. For water pulse tests, (P1  P2) is given by a differential pressure gauge plugged between upstream and downstream water circuits, with an accuracy of (1/1000) MPa. 2.2.3. Test calibration procedure The LVDT arrangement requires a delicate calibration procedure. Measured displacements sum up the Nylon support ring deformations, the Vitton sleeve deformations, the effects of hydrostatic pressure upon the LVDT sensors, the argillite deformations, and finally the macro-crack openings and closures that we wish to measure only. Therefore, a test calibration procedure has been followed, twice before the testing campaign, and once after the steady state gas test series: (1) A first phase consists in measuring parasitical deformations of the Nylon support ring and Vitton sleeve, and the LVDT sensors response to hydrostatic pressure. To that purpose, tests are performed on a steel sample of identical dimensions to the argillite samples. Steel sample deformations are considered negligible compared to those expected from argillite under the confining stress range used (of up to 17 MPa). (2) Secondly, measurements are performed on a sound argillite sample extracted from EST5600 plug in order to evaluate if the rock deformation is significant and if it is measured by the LVDT arrangement. Simultaneously, the sound argillite sample is equipped with four strain gauges (two longitudinal and two transversal ones). A comparison is made between measurements from strain gauges and those from LVDT displacements corrected from deformations of the Nylon support ring, of the Vitton sleeve, and of the effects of hydrostatic pressure upon the LVDT sensors (all were evaluated during calibration step (1)).

Fig. 4. (a) LVDT arrangement: top schematic view; (b) LVDT arrangement: side photograph.

2.2.4. Testing procedure First, each macro-cracked sample is placed inside the triaxial confining cell and subjected to a small initial hydrostatic pressure Pc of 0.1–1 MPa (gas tests) or 3 MPa (water tests). Either gas or water injection permeability tests are then performed posterior to increasing hydrostatic loading. Crack closure value is recorded at each hydrostatic pressure level simultaneously to permeability measurements.

C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

More precisely, for gas permeability tests, confining pressure Pc is applied in gradual 1 MPa steps from 1 MPa to 11 MPa, and in gradual 2 MPa steps from 11 MPa to 17 MPa. From Pc = 11 MPa, the sample is subjected to several loading–unloading cycles. Permeability measurements are performed after each unloading and reloading phase. For water permeability tests, the experimental procedure is designed so as to apply an initial continuous fluid flow through the macro-crack, and then to superimpose an additional pulse flow for permeability measurements. This procedure is detailed as follows: (1) Low initial confinement at 0.3 MPa. (2) Increase in water pressure on the downstream sample side simultaneously to free draining on the upstream side. (3) When water flows out of the upstream sample side, viz. when water flows through the whole specimen and fills its macro-crack, confining pressure is progressively increased from 0.5 to 3 MPa. (4) Both sample sides are subjected simultaneously to initial water saturation pressure P0 = P1(t = 0) = P2(t = 0) = 1.5 MPa. (5) A pressure increase (or pulse) DP1 is applied at time t = 0+ on the sample upstream side with an amplitude limited to 0.7 MPa; pressure variation (P1  P2) versus time t is recorded. (6) Confining pressure is increased in gradual levels: 3, 5, 11, 13 and 17 MPa, with possible unloading at each level down to 3 MPa, almost immediately followed by a reloading phase; a confining pressure level may be kept for several hours or days in order to measure water permeability evolution versus time at constant Pc (swelling effect). Permeability is measured at each confining pressure level, either right after loading, or after several hours at a given Pc, or right after unloading.

3. Results and discussion

673

Fig. 5. Gas injection: confining pressure Pc versus crack closure cc for Sample 1. Three successive phases (1)–(3) are identified.

(1) First loading of up to 3 MPa; Pc grows slowly versus cc. This is interpreted as the macro-crack closing progressively with hardly any argillite compression. (2) First loading from 3 to 17 MPa and following unloading; Pc versus cc evolves with a higher slope than during phase (1). Not being reproduced during later loading phases, this loading phase is thought to correspond to the breakage of the weakest macro-crack asperities. (3) Second and subsequent loading–unloading phases. The evolution of Pc versus cc is linear and almost identical from one loading to the other. This is interpreted as a typical elastic crack behaviour.

3.1. Test calibration Sound argillite contraction under confinement of up to 17 MPa is shown to be (2/100) mm at most, with a good correlation between values given by the LVDT arrangement and those by strain gauges. Although this contraction value is small compared with crack closure values which vary from (10/100) to (80/100) mm, it was corrected in all results presented below. All values given for crack closure cc in the following account for crack closure (or opening) only. Sound argillite contraction has been removed, just like all parasitical effects have been.

3.2. Steady state regime gas tests The evolutions of all measured parameters (Pc, cc, K) are analysed for Samples 1–3 successively, with the main results being exemplified by Sample 1. Fig. 5 shows the evolution of confining pressure Pc versus crack closure cc up to 17 MPa for Sample 1. Three successive phases are identified:

In brief, the evolution of crack closure cc subjected to Pc is irreversible. It becomes linear after a couple loading– unloading cycles. cc amounts to roughly (1/10) mm closure in total when subjected to Pc of up to 17 MPa. Most of the closure corresponds to the first two loading–unloading cycles: (1/100) mm closure only is obtained after three cycles. Permeability measurements can be plotted versus confining pressure Pc or versus crack closure cc, see Fig. 6 for Sample 1. Fig. 6(a) shows that identical K values correspond to several Pc values, either obtained during loading or unloading cycles, whereas a single cc value corresponds to a given K value, see Fig. 6(b). Crack closure cc is a relevant, biunivocal parameter for K, contrarily to Pc. A lesser correlation between K and cc is obtained on four unloading points at 0.5 MPa, see Fig. 6(b): this is interpreted as an experimental artifact. At such a low confinement level, the actual crack closure cc is possibly smaller than the measured one, which may be due to a stick-slip phenomenon in the LVDT sensors. This was observed once, during a very limited time, for a single sample only

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C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

20

Pc (MPa)

Sample 1

Sample 2

Sample 3

15

(3)

10

(2) 5

(1) 0 0

5

10

15

20

25

30

35

Crack closure cc (1/100 mm) Fig. 7. Gas injection – confining pressure Pc versus crack closure cc for Samples 1–3.

Fig. 6. Gas injection – permeability K versus (a) confining pressure Pc; (b) crack closure cc, both for Sample 1.

(Sample 1), and at very low confinement level, therefore, the test was considered globally valid. Finally, it is remarkable that K may be fully interpreted as a function of crack closure cc. Being extracted simultaneously from the same EST5600 plug as Sample 1, Sample 2 has an identical saturation rate. As a consequence, the hydro-mechanical behaviour of both samples is markedly parallel. First, both samples have parallel (cc, Pc) evolutions, see Fig. 7, with identical slopes during elastic loading–unloading phase (3), although they have very different crack closure amplitudes. The initial macrocrack thickness, given by the Brazilian test, varies widely from one sample to the other, hence, crack closure amplitudes vary widely. Fig. 8(a) and Table 2 also show that K is on the same order of magnitude as for Sample 1 (1014 m2) at Pc = 10 MPa or greater, or during any load-

ing–unloading phase after the first three ones. Again K versus cc relationship is biunivocal, see Fig. 8(b), whereas it is not biunivocal as a function of Pc due to an irreversible loading–unloading behaviour during the first three cycles, see Fig. 8(a). Sample 3 is extracted from a different MSE761 plug, which was initially, even before sample coring, visibly damaged and micro-cracked, see Subsection 2.1. Microcracking is a typical consequence of partial argilite desaturation, see Montes et al. (2004). Hence, saturation rate variations may account for hydro-mechanical features which have not been observed previously. In particular, Sample 3 is much more deformable than Samples 1 and 2, see Fig. 7: its crack closure cc after the first three cycles is approximately 50% higher than for Sample 1. This deformability is interpreted as being due to the main artificial macro-crack closure, but also to the closure of the network of micro-cracks, which are both measured as cc by the LVDT arrangement. Yet, after three cycles, the elastic behaviour for all three samples produces a similar slope on the cc versus Pc diagram, although it is slightly lower for Sample 3. Fig. 8(a) and (b) and Table 2 show that Sample 3 permeability is much lower than for Samples 1 and 2 whatever the confinement or crack closure level. For example, K is 22 times lower for Sample 3 than for Sample 2 at t = 0. Nevertheless, this is not possibly a consequence of sample micro-cracking. Indeed, micro-cracking increases K by increasing fluid flow through the specimen, whereas micro-cracked Sample 3 has lower K. Rather, K value is strongly influenced by the initial macro-crack width and profile. Indeed, visual inspection showed that Sample 3 initial macro-crack width is much smaller than for Samples 1 and 2, see Fig. 2(c). And the thinner the macro-crack is, the smaller the fluid flow is, hence, the smaller K is. Moreover, for Samples 1 and 2, the fluid flow is closer to that between

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3.3. Further analysis of steady state gas flow tests

K (10-14 m2) 50

Sample 1

40

Sample 2 Sample 3

loading

30

20

Pc range in which permeabilities are similar

10

0 0

5

10

15

20

Pc (MPa)

All experimental data relating permeability to crack closure may be fitted to a single relationship independent of the sample. Yet, gas test results exhibit permeability values ranging from 1013 to 1016 m2, see Table 2, and these results are highly sensitive to the crack closure amplitude. Let K0 be the initial sample permeability as measured with the test rig and K its permeability under loading. Let also ac be the crack closure variation (cc  cc0) between the initially measured value cc0 and the value under loading cc. Normalizing permeability K by K0 and substracting cc0 from cc enables to account for the initial macro-crack width and profile. Tests performed on Samples 1–3 show that ratio K/K0 has a one-to-one, biunivocal, relationship with crack amplitude ac, whether during opening or closing cycles, and independently of the sample, see Fig. 9: it is, therefore, an argillite characteristic directly usable in the context of numerical modelling validation.

K (10-14 m2) 50

3.4. Water injection tests Sample 1

In this phase, the experimental context is very different from gas permeability tests, mainly because water is not a neutral fluid towards argillite. Unlike Argon gas, water may interact chemically with argillite and induce decohesion and swelling. These phenomena will make test results dependent on fluid injection and flow duration. Our aim here is to evaluate their actual effect. Moreover, gas flow tests allow objective conclusions on confinement and crack closure effects upon permeability. On the opposite, water flow tests introduce additional parameters. Those parameters are difficult to quantify although they certainly are closer to the in situ storage tunnel reality, where de-saturation and re-saturation processes may occur.

40

Sample 2 Sample 3

30

loading

20

10

0

0

5

10

15

20

25

30

35

Crack closure cc (1/100 mm) Fig. 8. Gas injection – (a) permeability K versus Pc; (b) permeability K versus crack closure cc, both for Samples 1–3.

K/K0 (relative permeability)

1

Sample 1 0.8

Sample 2

Table 2 Summary of steady state gas flow test results Sample no.

Plug no.

Crack closure amplitude (mm)

Initial permeability K0 (m2)

Final permeability Kf (m2)

0.6

1 2 3

EST5600 EST5600 MSE761

0.11 0.31 0.27

17 · 1014 44 · 1014 2.1 · 1014

0.47 · 1014 1.6 · 1014 0.05 · 1014

0.4

Sample 3

0.2

two argillite planes (at sample scale) rather than to the flow along a smaller, more tortuous, crack. Apart from these remarks, Sample 3 has a similar behaviour to Samples 1 and 2. For instance, Fig. 8(b) displays a biunivocal relationship between K and cc for all of the three samples.

0 0

5

10

15

20

25

30

Crack closure variation (1/100 mm) Fig. 9. Gas injection: variation of KK0 ratio with crack closure variation ac.

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Table 3 Summary of water pulse test results Sample no.

Plug no.

Max crack closure (mm)

Max positive crack closure (mm)

Swelling (mm)

K0 (m2)

Kf (m2)

Test duration (days)

2 3 4 5

MSE761 MSE761 MSE748 MSE748

0.03 0.06 0.05 0.06

0.07 0.11 0.06 0.16

0.03 0.07 0.04 0.18

9.1 · 1018 2.8 · 1018 6 · 1020 1.5 · 1019

3 · 1021 3 · 1021 7.9 · 1021 1 · 1021

4 6 5 21

Results summarized in Table 3 mainly show that macrocracked argillite water permeability evolves rapidly towards that of the sound rock as measured by GDR FORPRO research program, see GDR-FORPRO (2003). Such permeability levels (from 1020 to 1021 m2) have not allowed us to use steady state flow tests as initially planned, due to prohibitive test durations. In particular, a first sample (Sample 1) required 13 days steady state water testing of up to 13 MPa confining pressure, although it exhibited a permeability on the order of 1018 m2 only. Our available time would have allowed only a couple of tests, at best, so it was decided to switch to a pulse testing procedure. Four samples were tested in transient water flow conditions, using a test arrangement designed by our laboratory in the framework of GDR FORPRO research group (GDR-FORPRO, 2003). For each of the four samples tested, at each confinement level Pc, pressure difference values were numerically approximated using an exponential function of time as explained in 2.2.4: P1(t)  P2(t) = DP1exp(cKt), with a correlation generally higher than 97%, from which K values were deduced. Figs. 10(a) and (b) display Pc evolution versus crack closure for Samples 5 and 2. As indicated in Fig. 10(a), and unlike the gas tests, crack closure may take negative values when Pc is held constant, which is due to sample swelling. Swelling may be of two origins: (1) located in the fractured zone, and/or (2) global (bulk) argillite rock swelling. For all of the tests, visual a posteriori inspection of water permeability samples showed very limited water penetration in the argillite sample bulk. Therefore, measured swelling is assumed mainly limited to the macro-crack zone. At the test beginning, Sample 2 displays crack closure at constant confining pressure (at 3 MPa), whereas later constant Pc phases correspond to swelling only. During the last loading–unloading phases, Sample 2 tends to stiffen with increasing Pc. Sample 5 testing lasted the longest of the four tests, see Table 3. It was, indeed, subjected to a long lasting 5 MPa confinement, which led to a large amplitude swelling (of about 15/100 mm) following a much smaller one at 3 MPa, see Fig. 10(a). Subsequent behaviour evolves towards significant crack closure at 13 and 17 MPa confinement. These observations show that argillite reactivity towards water corresponds to a complex phenomenon. Our four tests do not allow conclusions to be made regarding systematic swelling (or crack closure), although swelling appears to be dominant. Further research would identify parameters influencing phases of swelling or contraction, which may follow each other on a single sample

Pc (MPa)

20

8

9

closure

15

7

6

chronological order of points

10

5 swelling

4

5

3 2

1

0 0 -15

-10

-5 0 Crack closure (1/100 mm)

5

10

Pc (MPa) 18

9

swelling

chronological order of points

16 14

crack closure

6

12

8

4

10 8 6

7

10 4

3

5 0

2

-6

-4

-2

2 1

0

2

4

Crack closure (1/100 mm)

Fig. 10. Water injection tests – confining pressure Pc versus crack closure for (a) Sample 5; (b) Sample 2.

(such as on Sample 5). Those parameters may be associated with injected water minerality and argillite chemical composition. In Table 3, maximum crack closure adds up cc values due to confinement increase or decrease to those due to swelling. cc values are positive when Pc increases or at constant Pc, and they are then interpreted as crack closure.

C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

Their sum is given in the third column of Table 3 as maximum positive crack closure. When unloading, negative cc values are measured, and they are interpreted as crack opening. Swelling corresponds to negative cc values at constant Pc only. No swelling is assumed to occur during Pc increase or decrease phases, due to their short duration (on the order of several minutes only) compared to the whole test duration, and also compared to constant Pc loading duration (on the order of several hours or days). Swelling is reported positive for comparison purposes in the fourth column of Table 3. Our four tests show that swelling grows regularly with test duration. Moreover, swelling and maximum positive crack closure are on the same order of magnitude, whatever the test duration. Both contribute similarly to the macro-crack width evolution, viz. swelling is not negligible. Maximum crack closure has positive values only, which is interpreted as crack edges getting globally closer. Maximum crack closure does not perfectly correspond to the difference between maximum positive crack closure and swelling, due to irreversibilities during confinement loading–unloading phases and possible test arrangement hysteresis. In terms of permeability evolution versus crack closure cc, water tests do not display any clear trend, unlike the gas permeability tests, see Fig. 11(a) and (b). Further analysis of this point is proposed in Section 3.5. Several water permeability levels correspond to a given confining pressure Pc, see Fig. 12(a) and (b). For example, K varies by two orders of magnitude at Pc = 5 MPa for Sample 2. K versus Pc evolution is not satisfactorily described using a least-squares sense polynomial fit. Fig. 12(b) also gives the chronological order of experimental points for Sample 2. After several (two to three) loading–unloading cycles, K remains on the order of 1020–1021 m2 whatever the confinement level. Similar behaviour is observed with Samples 3 and 4. Therefore, two successive phases are identified in these tests: (1) during

677

the first two (or three) Pc loading–unloading phases, K decreases by several orders of magnitude with increasing Pc; (2) during later Pc loading–unloading phases, K still varies with increasing Pc although it remains on the same order of magnitude. Finally, water permeability K is plotted versus time t, see Fig. 13(a) and (b). A drastic drop in K is observed after only about two (Sample 2) to ten days (Sample 5). For Sample 2, the initial K value is 100 times higher than for Sample 5 and K looses two orders of magnitude within two days time. When K values for Sample 2 reach initial K values for Sample 5 (on the order of 1019 m2), K evolution with time slows down drastically. K evolves towards an asymptote on the order of 1020–1021 m2, although displaying small variations due to confining pressure variations (or due to measurement inaccuracies at such low K values). This asymptote corresponds to sound argillite water permeability levels, see GDR-FORPRO (2003). 3.5. Further analysis of water permeability tests: comparison with gas tests An interesting comparison is between Table 2 for steady state gas test results and Table 3 for pulse water test results. First, gas permeability levels (from 1016 to 1013 m2) are several orders of magnitude higher than water permeability (from 1021 to 1017 m2) although maximum crack closure levels are similar. Water undoubtably induces lower argillite permeability, even when initially macro-cracked. With gas, the ratio between initial and final permeability is on the same order of magnitude from one test to the other (it varies from 27 for Sample 2 to 42 for Sample 3), whereas, with water, the same ratio varies from 10 (Sample 4) to 3000 (Sample 2). Moreover, low permeability values appear from the test start. This is interpreted as a kinetic water action upon argillite, which acts in the opposite K (10-18 m2)

K (10-19 m2) 2

1.6 1.4 1.2

1.5

1 1

0.8 0.6

0.5

0.4 0.2 0 -15

-10

-5 0 Crack closure (1/100 mm)

5

10

0 -6

-4

-2

0

Crack closure (1/100 mm)

Fig. 11. Water injection tests – permeability K versus crack closure for (a) Sample 5; (b) Sample 2.

2

4

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C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

10

K (10-19 m2) 10

K (10-19 m2) 1

1

0.1 0.1

0.01 0.01

Pc (MPa) 0.001 0

5

10

15

20

0.001

0

4

8

K (10-19 m2) 100

12 Time (days)

16

20

24

K (10-19 m2)

1 100

2

chronological order of points

3 10

10

4 7

5

1

6

10 14

1

9

8 11

12

0.1

0.1

13 0.01 0

4

8

12

16

20

Pc (MPa)

0.01

0

1

2 3 Time (days)

4

5

Fig. 12. Water injection tests – permeability K versus confining pressure Pc for (a) Sample 5; (b) Sample 2, with the chronological order of experimental points.

Fig. 13. Water injection tests – permeability K versus time t for (a) Sample 5; (b) Sample 2.

direction to the pulse water flow. It is also remarkable that final water permeability levels are equivalent to those measured on sound argillite by GDR-FORPRO research group (between 1020 and 1021 m2), see GDR-FORPRO (2003), and are independent of confining pressure variations. An interpretation is that the sample macro-crack gets filled with time with dissolved or degraded argillite sludge, so that its presence is no longer detectable. Therefore, the flow and permeability control by crack closure measured with gas is no longer observed for water permeability tests, and there is no particular relationship between water permeability and crack closure cc, see Fig. 11. On a practical point of view, these observations are reassuring in the sense that they mean a quick argillite self-healing whenever cracked as soon as it is subjected to a water flow. This is all the more true as we tested macro-cracked samples,

which have bigger cracks than expected in situ micro-cracking (due to storage tunnel drilling). As no clear trend is noticeable from raw water test results, see Fig. 11(a) and (b), we propose to observe K vs. cc plots differently, with the idea that swelling is similar to a confinement-induced crack closure. As explained in Section 3.4, measured swelling displacements are assumed limited to the macro-crack zone. Hence, swelling does not mean any pressure release in the crack zone, but, rather, the opposite happens. Let cumulated crack closure ac be the sum of crack closure due to confinement and crack closure due to water-induced swelling: negative cc values corresponding to swelling are switched to positive values, whereas negative cc values due to unloading phases remain negative. Figs. 14(a) and (b) show that ac is strongly related to argillite water permeability K for Sample 2 and 5. It is also the case for Samples 1, 3 and 4.

C.A. Davy et al. / Physics and Chemistry of the Earth 32 (2007) 667–680

1.6

K (10 -1 9 m 2 )

1.4 1.2 1 0.8 0.6 0.4 0.2 0 -10

0

10

20

30

40

Cumulated crack closure ac (1/100 mm) K (10-18 m2 ) 2

679

must indeed be pointed out that after a short injection period (2–10 days), for all of the four samples tested, water permeability drops down to values consistent with sound argillite as measured in the GDR FORPRO framework (1021 m2), and it is independent of confining pressure variations. This reflects a complete self-sealing of the macrocrack, which is an important factor for nuclear waste repository safety in the context of storage tunnel drilling, backfilling and long term closure. Results also indicate that, unlike the gas tests, flow duration drives permeability variations more than crack closure. A second major difference from gas tests is that the sample subjected to a water flow swells. Sample swelling due to water induces additional crack closure. Swelling does not contribute to unload the crack zone but rather creates additional closure and pressure in the crack area. As a result the relationship between K/K0 and crack closure is different and harder to interpret than that for gas tests. Argillite water permeability K is more strongly linked to cumulated crack closure ac (crack closure due to confinement summed up with crack closure due to water-induced swelling) than to crack closure cc only. Acknowledgments

1.5

The authors are grateful to ANDRA (French Agency for Nuclear Waste Management) for funding this research program under Contract No. 027245.

1

References

0.5

0 -2

0

2

4

6

8

10

Cumulated crack closure ac (1/100 mm) Fig. 14. Water injection tests – permeability K versus cumulated crack closure ac for (a) Sample 5; (b) Sample 2.

4. Conclusion Permanent gas flow tests show that permeability K depends mainly upon crack closure cc, with values on the order of 1013–1016 m2. K also largely depends on Pc during the first two to three loading–unloading phases. Water permeability tests lead to very different results. First, measured permeability values vary from 1018 to 1021 m2, which is 2–8 orders of magnitude smaller than gas permeability test results. Lower values for K with water rather than with gas are obtained almost from the test beginning, after only a few hours of water injection. This means that the physico-chemical phenomena governing macro-crack self-sealing have extremely rapid kinetics. It

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