Gas diffusion measurements in natural and synthetic rocks: correlations and key relationships

Gas generation is an inevitable consequence of radioactive waste storage and disposal. The rate at which gas diffuses through host rocks and overlying strata is an important consideration in safety assessment. To examine the impact of material variability, British Geological Survey developed a methodology to manufacture ‘synthetic rock’ samples from mixtures of clay, sand, and silt. Diffusion experiments were conducted on these, and natural samples of Boom Clay and Eigenbilzen Sands. Samples were tested under an isotropic stress equivalent to 400 m burial, assessing anisotropy by measuring intrinsic permeability and diffusion normal and perpendicular to bedding. Boom Clay exhibited permeability and gas diffusion anisotropy ratios of 4.5 and 1.5, respectively. A semi-log relationship between permeability and diffusivity was observed. No significant correlation to mineralogy changes was found, indicating fabric and pore morphology are more important. This was supported by a correlation between porosity and diffusivity. X-ray computed tomography analysis suggested diffusivity was inversely proportional to bioturbation features and that stress promoted micro-crack closure which may also help to explain previous data dispersion. The study confirmed the usefulness of synthetic samples in exploring complex processes, generated data relevant to natural clay systems and showed diffusivity was not highly sensitive to large changes in permeability.