In situ heating test in Callovo-Oxfordian clay-stone: measurement and interpretation

Document available in extended abstract form only. To study the thermo -hydro-mechanical effects of the early thermal phase on the clay host rock of a deep repository, Andra has performed a new in-situ heating test called TED experiment. This experiment is the second one being carried out in the Meuse/Haute-Marne Underground Research Laboratory The aim of the TED experiment is to measure the evolution of the temperature; deformation and pore pressure fields around several heaters and to back-analyse the thermo-hydro-mechanical properties of the rock. The TED experiment was also designed to estimate the overpressure generated by heat in the zero flux plan between several heaters and to study the evolution of the damaged zone due to heat. Analysis of the experimental results will help in calibrating numerical models which will be applied to the disposal cell cases. The test set-up consists of three boreholes containing the heaters and twenty one instrumented observation boreholes. Each heater is 4 m long and may generate a power of 1500 W. The distance between each heater is about 2.6 m, which is close to the ratio of the disposal cell geometry concept. The surrounding boreholes were strategically located to follow the anisotropic THM behavior of the clay-stone. There are twelve pore pressure measurement boreholes (a total of eighteen piezometers), nine temperature measurement boreholes (108 temperature sensors) and 2 strain measurement boreholes. In order to optimize the inverse problem analysis, special attention has been paid to the reduction of uncertainties regarding the sensors location in the boreholes. Possible sensors location errors were indeed found to be a problematic issue for analysis and parameter determination in the previous thermal experiment ([1]). The central heater was activated on January 25, 2010 starting with a relatively low heating power of 150 W, then the heating power was increased to 300 W and finally to 600 W. Each step was about four months long. After one year the two surrounding heaters were activated and the same heating load was applied. Before each heating step, permeability tests were performed in all pressure measurement boreholes in order to investigate the impact of heat on the rock permeability. After two years of heating, the measurements of the temperature field provide clear evidence of thermal anisotropy. The pore pressures measured (Figure 1) also confirm the anisotropy of permeability and stiffness. The in situ permeability measurements do not show any changes as a function of temperature. The correlation between the measured temperature and pore pressure and the results obtained by modeling is very good ([2], [3], [4], [5]). Thermal conductivity and heat capacity values were determined on the basis of the back-analysis of the in situ measurements ([2], [3], [4]). They are very similar to the values taking from samples which then increase the viability of those parameters. The successful reproduction of the temporary instantaneous pore pressure decrease after increasing power by numerical modeling, confirms the mechanical anisotropy in Callovo-Oxfordian clay-stone [3]. TED experiment results show the capability of current models to satisfactorily predict the evolution of temperature and pore pressure in the far-field of disposal cells. The THM behavior will also be analyzed during the cooling phase. Cooling will start once the excess pore water pressure generated by heating will be drained in most of the installed piezometers. (authors)