Shales are extensively studied for their roles in various geosystems, including radioactive waste disposal, CO2 sequestration, underground hydrogen storage, and oil and gas extraction. In radioactive waste repositories, gas generation, arising from corrosion of metals and alloys, radiolysis of water and degradation of organic matters, presents a challenge for long-term safety due to the low permeability of clay barriers, which, while beneficial to isolate the waste, restricts gas release. This may contribute to significant gas pressure build-up and necessitates a detailed understanding of gas transport mechanisms to ensure repository safety, as excessive gas pressure could compromise the engineered barrier system (EBS) and the geological barriers, potentially affecting their structural integrity and performance in isolating radionuclides. Building on this context, this thesis investigates the hydro-mechanical and gas transport properties of Opalinus Clay (OPA) to evaluate hydro-mechanical couplings under gas migration processes. This research uses both experimental and numerical approaches to examine gas migration in OPA. A variety of experimental setups were utilized, including high-pressure oedometer and triaxial devices to apply relevant stress conditions, the latter equipped with Distributed Fibre Optic (DFO) technology to capture strain field evolution during gas injection. The study includes an extensive experimental campaign to characterize crushed OPA for backfill applications and a targeted investigation into remoulded OPA to assess its intrinsic hydro-mechanical and two-phase flow properties. Main findings reveal a pronounced interaction between gas and porewater in OPA, with fast gas pressure buildup rates leading to excess porewater pressure and undrained behaviour. Numerical modelling corroborates these results, providing insights into gas migration processes in low-permeability media and its effects on stress state evolution. The application of DFO technology enabled high-resolution spatial and temporal strain data, capturing strain field evolution that traditional methods cannot detect and further confirming the strong interaction between gas and porewater, and induced changes in stress state. Importantly, the results also suggest that OPA's hydro-mechanical behaviour was largely reversible, with no marked impact on the pore structure and permeability post-gas injection, underscoring the robustness of OPA as a geological barrier. Experiments on crushed OPA highlight its potential as a backfill material, highlighting the influence of its microstructure on the hydro-mechanical and two-phase flow behaviour. Findings suggest that while gas breakthrough is controlled by microstructural features, overall gas flow is affected by the material's porosity. These results advance our understanding of Opalinus Clay's hydro-mechanical and gas transport properties under repository-relevant conditions, offering valuable insights to improve repository designs and ensure safety.