Hydro-mechanical characterization of gas shales and Opalinus Clay shale in partially saturated conditions

An experimental study on the behaviour of shales in partially water saturated conditions is presented in this thesis. The Opalinus Clay shale and gas shales from different unconventional reservoirs have been studied. Opalinus Clay is currently considered as a host rock for the disposal of radioactive waste in Switzerland; the importance of partial saturation is fundamental in predicting the short-term behaviour of the host rock during the excavation-resaturation phases, and the long-term behaviour during the propagation of gas generated by the waste. Shale gas reservoirs are partially saturated in their in-situ state due to the simultaneous presence of gas and liquids; these hydraulic conditions influence the mechanical behaviour during the drilling, fracturing, and extraction stages. The analysis of the geomaterials' behaviour considering partial saturation is a well-established practice in Soil Mechanics. However, in the case of shales several challenges arise. These challenges are mainly related to their features, which make most of the experimental and theoretical frameworks adopted for soils not suitable for providing a comprehensive understanding of the behaviour of shales. The aim of this thesis is to improve upon the existing understanding of the behaviour of partially saturated gas shales and the Opalinus Clay shale and to provide a quantitative assessment of the impact of such conditions on their mechanical behaviour. A key part of this work is dedicated to the establishment of experimental methodologies and testing protocols to highlight the most important features of the response of the tested materials under partially saturated conditions. The high water retention capacity of shales necessitates the use of the vapour equilibrium technique; this experimental technique allows for the imposition of wetting/drying processes on the tested materials and introduces suction as a key parameter when performing the hydro-mechanical characterization in partially saturated conditions. Three different testing set-ups have been developed to investigate the hydro-mechanical response of the tested shales. The obtained experimental results demonstrate that the considering of partial saturation is fundamental for a reliable and comprehensive hydro-mechanical characterization of the tested shales; despite the different origins of these materials, the main outcomes show a very similar impact of wetting and drying processes on the volumetric response and the mechanical properties. The volumetric response observed in the tests performed in free stress conditions is characterized by significant anisotropy and irreversibility. However, when a mechanical stress is applied to simulate in-situ conditions, a less pronounced volumetric response is systematically observed. The elastic response is also highly dependent on the hydraulic state of the tested materials; in particular, significant reductions (up to 50%) of Young's modulus and compressive strength are observed during wetting processes. Gas injection is also studied in the Opalinus Clay to better understand the dominant flow mechanisms and to assess the possible impact of partial saturation on the movement of gas in the host rock. Experimental results obtained with a testing device specifically developed for this purpose indicate the impact of gas injection on the deformation state of the material; this aspect clearly highlights the involved hydro-mechanical coupling.