Thesis: This thesis is part of the research and expert activities that IRSN is carrying out on the understanding and modeling of gas migration mechanisms in the project of the Industrial Geological Storage Center for radioactive waste (Cigeo). This work contributes to the determination and evaluation of gas migration (H2) mechanisms within a radioactive waste disposal site, through the numerical modeling of two-phase (water-gas) flow in porous media that are initially quasi-saturated, complemented by sensitivity analyses with respect to input parameters, code benchmark tests, and modeling of both in-situ and laboratory experiments. In particular, this work takes into account several physical phenomena neglected in previous studies (gas entry pressure, hysteresis of water retention curve and of relative water & gas permeability curves) : theoretical aspects, implementation in computer codes, and sensitivity analyses. A database was constructed concerning measured hydraulic properties (retention curves, water and gas permeabilities) of the various materials constituting engineered barriers and host rock (CEM I, CEM V, Bentonite, Callovo-Oxfordian "COx" claystone). For each physical phenomenon, an analysis of the experimental data was carried out towards developing and testing physically based models. The selected models are then implemented in the TOUGH2 code platform developed by the Lawrence Berkeley National Laboratory (LBNL, CA, USA), and with several Python and Matlab scripts developed especially for this thesis. After verification and validation tests, including benchmarks with other codes, experiments were modeled in order to test the impact of entry pressure and of hysteresis phenomena (through their parameters). In particular, the nitrogen gas injection experiment (PGZ1-GAS) performed in situ in the COx host rock was modeled with & without entry pressure, and the LBNL laboratory experiment (injection of non-wetting Soltrol 220 emulating CO2 injection) was modeled with & without hysteresis. In addition, preliminary results are presented concerning the modeling of these phenomena at the scale of a waste cell ("alvéole" in the French terminology). The modeling of gas migration at the waste cell scale, as well as the previous tests and sensitivity studies with respect to input parameters, reveal the importance of these phenomena (entry pressure, hysteresis) and the need to take them into account for the evaluation of gas migration in a radioactive waste disposal site.