An experimental study on Boom Clay –one of thepotential deep clay formationsin Belgium for the geological disposal of long-living and heat-emitting radioactive waste–has been performed using controlled volume-rate gas injection tests on high-pressure oedometer cells. These experiments intend to analyse the influence of the gas injection rate, the stress state and deformation history, the anisotropic flow conditions (flows parallel and orthogonal to bedding) on the gas migration process. Results of selected tests on two sample orientations are presented, paying particular attention to sample volume changes on gas injection and dissipation stages. The gas pressurisation process at constant vertical stress acts as an unloading stage, which may induce expansion and degradation on the clay that have important consequences on gas migration properties.