Impact of Swelling on the Permeability and Mechanical Properties of the Callovo-Oxfordian Claystone
Authors: Grgic D. , Giraud A. , Moumni M. , de La Vaissière R. , Talandier J. 
Published: September 2022

ABSTRACT:
This study presents complex series of laboratory permeability tests in which both steady state and pulse decay methods were applied on core samples of the weakly permeable Callovo-Oxfordian claystone from Bure (France). The focus was to explain why permeability values obtained in laboratory with the steady state method are lower than those obtained with the pulse decay method and those obtained in-situ, and to assess the impact of these measurements on the mechanical properties. The intrinsic permeability values measured with the steady flow method are homogeneous and range between 1.2×10−21 and 7.5×10−21 m2. They are significantly lower than those obtained from the pulse decay method (~10−20 m2) and from in-situ measurements. This could be explained by an additional hydration of the clay minerals during steady state flow and the associated swelling, which decrease the interparticle porosity and slow down the water flow. The steady flow permeability tests induce a significant damage of the material which is mainly due to the opening of the bedding planes of swelling clay minerals (smectites). An accurate assessment of the permeability of a claystone with the steady state method requires a chemically equilibrated water and experiments with short run time.

1. INTRODUCTION
The Callovo-Oxfordian (COx) claystone in Meuse/Haute-Marne (France) has been chosen to host the Underground Research Laboratory (URL), located 500 meters underground in Bure and managed by ANDRA (National Agency for the Management of Radioactive Waste), for a feasibility study of a deep geological repository of radioactive waste due to its very low permeability. In the particular case of radioactive waste disposal management, knowledge of permeability is very important because it controls fluids transfers.
COx claystone is a very low permeable rock whose permeability has been widely studied in the past two decades. Permeability measurements on core samples have been performed parallel and perpendicular to the stratigraphy and over the entire thickness of the argillaceous formation in French laboratories with different techniques: interpretation of water profiles from water sorption/desorption isotherms, diffusion tests with tritiated water, tests in triaxial cell under steady (stationary/permanent) or transient (pulse decay test) flow regime. In-situ results were also obtained through a series of tests (short-term hydraulic/packer tests, long-term pressure monitoring, long term pumping tests with argon) carried out in deep boreholes from the surface and short boreholes in the Meuse/Haute-Marne Laboratory. In-situ measurements have the great advantage to be carried out under hydraulic and mechanical conditions relatively close to natural conditions, but difficult to control (Delay et al., 2006). Laboratory measurements on core samples allow the control of a wide range of experimental conditions (e.g., pressures, stresses, temperature…) despite the difficulties related to sampling and conditioning.