Multi-geophysical Approach for the Characterization of Thermally-Induced Cracks in Granite: Discussion of Reproducibility and Persistence

Abstract

We study damage induced by low temperature variations in granite samples given their role in shallow geological reservoirs. We consider two thermal treatments, slow cooling and thermal shock, and implement a multi-geophysical approach to characterize the induced micro-scale damage. The methodology consists in monitoring elastic wave velocity and thermal conductivity as well as describing the damage by the way of Hg-porosity measurements and microscopic observations. To discuss the reproducibility of the induced damage, the same thermal protocol is performed on five samples. Our first results indicate that the thermal shock leads to a more pronounced damage. This is interpreted to be due to a larger variety of nucleated intragranular and intergranular cracks as observed by SEM and optic microscope. Yet, this more significant damage does not appear reproducible from one sample to another compared to the damage introduced by slow cooling. According to this first result, thereby, we propose a timely monitoring of elastic wave velocity, conductivity and Hg-porosity. It appears that the damage introduced by the slow cooling, unlike the thermal shock, does not present a long persistence. Indeed, after 15 days, the different properties had returned to their initial state. A time-dependence mechanism is proposed to discuss this observed process.