Hydro-mechanical response of a gas shale upon suction variations: experimental analysis and preliminary modelling

Shales are a fine-grained and laminated sedimentary geomaterials. Due to their particular features and their large worldwide presence in the Earth’s crust, they are of interest in several geo-engineering applications such as oil and gas extraction, CO2 sequestration and nuclear waste disposal. In light of the engineering applications many challenges exist related to the characterization of the mechanical behaviour of these geomaterials for which temperature, chemistry and unsaturated conditions play a key role. In particular, it is really important to study and understand how these materials behave in the partially saturated condition in which they are found. It is also relevant to know how their geomechanical proprieties vary due to the changes in water content. The study conducted in this thesis aims at improving the knowledge on the behaviour of partially water saturated shales involved in the gas extraction by analysing the change in water content and in volume upon suction under free stress conditions. Due to the peculiar features of gas shales (e.g. low porosity, low permeability and reduced dimensions of pores), the use of the vapour equilibrium technique is required to study the water retention capacity of the tested material. Advanced experiments were set up with the aim of characterizing the material from a macro scale to a micro scale. Firstly, the hydro-mechanical response of the tested gas shale was investigated looking at the swelling and shrinkage behaviour experienced due to the change in water content. Then, the microstructural response was studied upon suction variation using micro-CT analyses. The experimental results obtained in terms of volumetric deformation in free stress conditions during hydraulic loading show a significant anisotropy and irreversibility in the behaviour. Cracks opening upon water content evolution was observed at the macro scale and then studied further in details at the micro scale, paying attention also to their changes in thickness due to wetting and drying. In summary, due to the water uptake, a macroscopic expansion was observed associated with the swelling of the clay minerals inside the shale matrix and with the opening of the micro cracks. On the other hand, due to the water content reduction during drying, a decrease in volume was observed without registering closing of the openings. Experimental results obtained indicate the response of the material upon suction variation and the relevant role of partial saturation for a reliable and complete understanding in the hydro-mechanical characterization of the tested gas shale. Furthermore, a preliminary model was developed in order to reproduce the experimental results in terms of changes in water content, due to suction variations, with time. The numerical analyses were implemented in the Finite Element code COMSOL Multiphysics 5.5, allowing to define suitable values of the vapour diffusion coefficient for the tested gas shales. In particular, the variation of the tortuosity factor with the imposed total suction steps was studied through back analysis. This parameter describes the tortuosity variable for the diffusion of the water vapour trough the porous media, and the evolution with suction that was obtained seems to suggest further the importance of the role of crack aperture on the equilibration times.