Laboratory study of mechanical and petrophysical properties of both intact and naturally fractured shale samples from the Tournemire Underground Research Laboratory

Abstract:

Understanding the mechanical and physical properties of fractured shales is of major importance in many fields such as the study of cap rock integrity or targets for radioactive waste disposal. In particular, relationships between fluid transport properties and textural anisotropy are critical. Therefore, these relations need to be investigated on real fault samples.

This study in collaboration with IRSN and Total deals with the mechanical and petrophysical characterization of core samples from the Tournemire Underground Research Laboratory in Southern France. Three boreholes crossing a major fault at different angles have been investigated. The cores retrieved were carefully sampled and give access to the three major areas defining the fault zone: 1) the intact zone, 2) the damaged zone and 3) the fault core.

The goal of the study is to perform several petrophysical measurements on the same specimens sampled at different distances from the fault core. These include multidirectional measurements of P and S waves under preserved humidity conditions, magnetic susceptibility measurements and pore size distribution characterization through the BET gas adsorption method. Some preliminary results show an evolution of the anisotropy with the distance to the fault core (anisotropy reversal).

The boreholes that were sampled were drilled to perform hydromechanical in situ testing, therefore one of our objectives is also to compare the hydromechanical properties obtained in the mesoscale experiments in situ to the petrophysical measurements in the lab.

Mechanical tests were also performed on intact samples in order to understand the mechanisms of deformation of the studied shale, depending on the orientation of the maximal applied stress with respect to bedding.