High-resolution characterization of the EDZ in the Callovo-Oxfordian Clay using hydraulic tomography and gas interference tests

Ralf Brauchler1, Rémi de La Vaissière2, Jean Croisé1, Rainer Schwarz1, Reto Thöny1, Ralph Mettier1
1 AF-Consult Schweiz AG
2 Andra, Agence nationale pour la gestion des déchets radioactifs

O 4.5 in Final repositories and underground disposal sites

15.04.2016, 11:15-11:30, Audimax A, Geb. 30.95

The Meuse / Haute Marne Underground Research Laboratory (URL) provides the location for an experiment, designed to investigate the induced fracture network around open or sealed drifts. The aim of this experiment, called the CDZ-experiment (Compression of the Damaged Zone) is to study the effects of mechanical loading and unloading on the mechanical and hydraulic properties of the EDZ (Excavation damaged zone). In the context of this experiment, a large number of gas permeability tests were performed between six closely spaced wells prior to and after the mechanical loading of the EDZ. The tests allow for the characterization and quantification of the effect of the mechanical loading on the hydraulic (pneumatic) properties of the EDZ [de La Vaissière et al., 2014].

The gas tests were first analyzed based on the pressure and flowrate data recorded solely at the source boreholes using the numerical borehole simulator Multisim, which was developed by AF-Consult Switzerland Ltd. Multisim is particularly suited for the analysis of hydraulic tests performed in low permeability media.

In a second step, the cross-hole pressure responses of the gas permeability tests were analyzed with a travel time based tomographic approach proposed by Brauchler et al. [2003]. The inversion is based on the transformation of the transient ground water flow equation into the eikonal equation using an asymptotic approach. The eikonal equation can be solved with ray tracing techniques or particle tracking methods, which allows the inversion of large data sets in a short time with relatively low computational effort (common PC). The main feature of this procedure is a travel time integral relating the square root of the peak travel time, assuming a Dirac point source at the origin, to the inverse square root of the hydraulic diffusivity.

The reconstructed three-dimensional hydraulic diffusivity distribution displays the different zones of the EDZ with a high level of detail and provides important information about the spatial distribution of hydraulic parameters within the EDZ. Particularly, the reconstructed diffusivity distribution reflects the different zones of the excavation-induced fracture network described by Armand et al. [2014], wherein the fracture network was characterized in great detail, based on drill-core logging and resin injection. The comparison of the reconstructed tomograms with the results of the single-borehole analysis shows a reasonable agreement. 

Armand, G., Leveau, F., Nussbaum, C., de La Vaissiere, R., Noiret, A., Jaeggi, D., & Righini, C. (2014). Geometry and properties of the excavation-induced fractures at the Meuse/Haute-Marne URL drifts. Rock mechanics and rock engineering, 47(1), 21-41.

Brauchler, R., Liedl, R., & Dietrich, P. (2003). A travel time based hydraulic tomographic approach. Water Resources Research, 39(12), 1370.

de La Vaissière, R., Morel, J., Noiret, A., Côte, P., Helmlinger, B., Sohrabi, R. & Nussbaum, C. (2014). Excavation-induced fractures network surrounding tunnel: properties and evolution under loading. Geological Society, London, Special Publications, 400(1), 279-291.

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