Experimental Design for in situ Characterization of the Callovo ...

Available online at www.sciencedirect.com

Procedia Earth and Planetary Science 7 (2013) 533 – 536

Water Rock Interaction [WRI 14]

Experimental design for in situ characterization of the CallovoOxfordian pore water composition at 85°C M. Lundya*, B. Garitteb,c, Y. Lettryd, A. Vinsota b

a Andra, Centre de Meuse/Haute-Marne, F-55290 Bure, France Department of Geotechnical Engineering and Geosciences, Universitat Politècnica de Catalunya, E-08028 Barcelona, Spain c NAGRA (National Cooperative for the Disposal of Radioactive Waste), 5430 Wettingen, Switzerland d Solexperts AG, CH-8617 Mönchaltorf, Switzerland

Abstract When emplaced into deep geological disposal cells, the high-level radioactive waste packages will induce a transitory temperature increase in the surrounding geological environment. High temperatures will have an influence on the composition of water that will flow into the cells and come in contact with the disposal materials. An in situ experiment which aims at characterizing the CallovoOxfordian pore water at 85°C has been running since 2012. This paper presents the design of the experiment and the predictive thermo-hydro-mechanical (THM) modeling results. © The Authors. Authors.Published Publishedby byElsevier ElsevierB.V. B.V.Open access under CC BY-NC-ND license. © 2012 2013 The Selection and/or peer-review peer-reviewunder underresponsibility responsibilityofoftheOrganizing 2013. Selection and/or Organizingand andScientific ScientificCommittee Committee of of WRI WRI 14 14 –– 2013 Keywords : Underground Research Laboratory, in situ experiment, Callovo-Oxfordian, pore water, temperature, predictive THM modeling.

1. Introduction Since 1994, Andra (National Radioactive Waste Management Agency) has been studying the feasibility of a highlevel long-lived radioactive waste disposal in the Callovo-Oxfordian clay-rich rock. As chemistry has an influence on the processes that govern the durability of the disposal materials (metals, concrete, glass…), knowledge of the geochemical composition of pore water is required. Callovo-Oxfordian pore water composition has been characterized in situ at standard temperature [1]. However, when the radioactive waste packages will be emplaced in disposal cells, the heat emitted will cause a transitory increase of temperature in the surrounding geological environment. At the sleeve-claystone interface, the temperature is expected to reach a maximum of between 85 and 90°C after 15 years [2]. Thus, pore water will flow into the disposal cells when the temperature will be close to 85-90°C, or slightly below. As the temperature may have an influence on the water-rock equilibrium that governs pore water chemistry, the geochemical composition of Callovo-Oxfordian pore water at high temperature has to be investigated. Up to now, knowledge on the Callovo-Oxfordian pore water composition at high temperature is based on geochemical modeling and on hydrothermal alteration laboratory experiments studying the interaction of clayey rock and synthetic pore water at 80°C [3]. This paper presents the original design of an in situ experiment which aims at characterizing the Callovo-Oxfordian pore water at 85°C. The objective is to obtain information on the chemical composition of the gas and water phases in interaction with a significant volume of Callovo-Oxfordian claystone heated at 85°C.

* Corresponding author. Tel.: +33(0)329756756; fax: +33(0)329755389. E-mail address: [email protected].

1878-5220 © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013 doi:10.1016/j.proeps.2013.03.091

534

M. Lundy et al. / Procedia Earth and Planetary Science 7 (2013) 533 – 536

2. Experimental concept The experiment consists of seven vertical ascending boreholes drilled from the vault of an underground drift located at 490 m depth in the Meuse/Haute-Marne underground research laboratory (URL) (Fig. 1). The functions and characteristics of the boreholes are presented in table 1.

Fig. 1. Location of the seven boreholes in an experimental drift of the Meuse/Haute-Marne URL. Table 1. Characteristics of the boreholes Borehole

Objective

Length (m)

Diameter (mm)

EPT1201

Gas circulation and seepage water sampling

15

76.3

EPT1202-EPT1203-EPT1204-EPT1205

Heating boreholes

15.5

56

EPT1206

Thermometric borehole

18

56

EPT1207

Pore pressure borehole

14

76.3

2.1. Borehole dedicated to gas circulation and seepage water sampling The last six meters of the borehole EPT1201 were drilled using argon in order to prevent any chemical alteration of the rock (e. g. pyrite oxidation) that could then modify the pore water composition. To prevent external bacteria from developing in the borehole, the drilling tools were cleaned with chlorinated water and alcohol. Immediately after coring, the borehole equipment was installed. It was disinfected in the same way as drilling tools and through autoclaving. The equipment comprises an inflatable packer that can isolate a 5 m-long interval open to the rock where the water and gases produced by the formation are being studied. This interval was filled with argon at a pressure of 1.5 to 2 bars. Two stainless steel lines allow the circulation of the gas between the borehole and the drift where it can be sampled without modifying the total gas pressure (Fig.2). In the drift, the gas circuit is connected to an infrared spectrometer (Bruker) that allows online monitoring of CO2 and methane [4]. Precautions have been taken in order to avoid any contact between the pore water and materials that could have an influence on its chemistry, even at high temperatures: the water sampling lines are in PEEK (polyether ether ketone), the central tube in the test interval is covered with PFA (perfluoroalkoxy polymer) and the filters are in Teflon. A water-sampling module designed by Metro-Mesures/Fugro allows seepage water to be pumped out of the test interval with a 9-mL-volume syringe at a controlled flow rate. The expected seepage water production flow rate is from 10 to 100 mL/day. PH, Eh and temperature measurements are performed online without contact with the ambient air.


Measurements of Na+, Ca2+, Mg2+, K+, Sr2+, Li+, Cl-, SO42-, F-, Br-, I-, acetate, and formate concentrations are performed online with a Metrohm ion chromatograph [5]. Once extracted from the borehole, the temperature of the water sample will decrease, however this sample will evolve as a closed system. Considering that, mineral precipitations are not likely to occur.

Fig. 2. Schematic representation of the instrumentation for the borehole EPT1201.

2.2. Heating boreholes The four heating boreholes (EPT1202, EPT1203, EPT1204 and EPT1205) are placed around the borehole EPT1201. The last six meters of each borehole constitute an interval isolated by a high temperature packer. The heating boreholes were drilled using the same precautions as for borehole EPT1201 to prevent oxidation of the rock and external bacteria from developing in the boreholes. Then the space between the central tube and the rock was filled with argon at a pressure close to 1.5 bar. Once this space will have been filled with pore water, these intervals will be used to heat the rock volume by circulating hot water in their central tube (Fig. 3 (a)). Temperature controlled heater modules with water tanks are located in the drift. 2.3. Thermometric and pore pressure boreholes Two additional boreholes are located outside the zone defined by the four heating boreholes: borehole EPT1206 contains 12 temperature sensors distributed between 2 and 18 m from the vault and borehole EPT1207 contains 3 isolated temperature and pore pressure measurement intervals at 7.5, 11 and 14 m from the vault. 3. Predictive THM modeling Predictive thermo-hydro-mechanical (THM) computations were done to design the configuration of the boreholes and to estimate the sequence of the heating phase. In undisturbed conditions, at the depth of the test interval, the Callovo-Oxfordian argillaceous rock is at a temperature of about 22°C. 2D plane strain THM computations have been conducted with the objective of reaching 85±5°C in the rock volume of 10 cm around the walls of borehole EPT1201. Computations showed that, assuming the application of a temperature of 95°C in the heating boreholes, the inter-axis distance required between EPT1201 and the heating boreholes was 0.42 m (Fig. 3 (b)). The increase of temperature in the rock is also believed to generate hydro-mechanical disturbances. Pore water pressure increases are likely to occur and were shown to depend on the heating rate. Thus, in order to minimize these disturbances and to facilitate their

535

536


management, the heating ramp should last at least several months. Moreover, the results of THM modeling allowed asserting the interest of having two additional boreholes dedicated to temperature and pore water pressure observation in the volume of rock surrounding the heating boreholes. It also helped to decide on their location and on the importance of increasing the number of temperature sensors in the 7 boreholes (35 sensors have been installed).

Fig. 3. (a) Schematic representation of the instrumentation for a heating borehole; (b) Influence of the inter-axis distance between EPT1201 and the heating boreholes on the temperature obtained at EPT1201 walls 100 days after the application of a 10 days heating ramp.

4. Further steps 250 days after the drilling of the boreholes, the experiment is currently in its first phase, which aims at acquiring stabilized parameters on the gas and water composition at ambient temperature in borehole EPT1201. The second phase will consist in heating the claystone formation. Before that, further THM modelling taking into account the real configuration of the seven boreholes will be carried out in order to precise the planning of the heating phase. References [1] Vinsot A, Mettler S, Wechner S. In-situ characterization of the Callovo-Oxfordian pore water composition. Phys Chem Earth 2008; 33: S75S86. [2] Andra. Dossier 2005 argile – Evolution phénoménologique du stockage géologique. Available at www.andra.fr. 2005. [3] Beaucaire C, Tertre E, Ferrage E, Grenut B, Pronier S, Madé B. A thermodynamic model for the prediction of pore water composition of clayey rock at 25 and 80◦C – Comparison with results from hydrothermal alteration experiments. Chem Geol 2012; accepted. [4] Cailteau C, Pironon J, De Donato P, Vinsot A, Fierz T, Garnier C, Barrès O. FT-IR metrology aspects for on-line monitoring of CO2 and CH4 in underground laboratory conditions. Anal Methods 2011; 3: 877 [5] Lundy M, Vinsot A. On line ion chromatography for the in situ characterization of the Callovo-Oxfordian pore water composition. Fourth International Meeting on Clays In Natural And Engineered Barriers For Radioactive Waste Confinement, Nantes, France, 2010, pp 365-6.