Re-evaluation of the experimental data of the MEGAS experiment on gas migration through Boom clay

The present report re-evaluates some of the experimental data obtained by the MEGAS project on gas migration in Boom Clay. In the MEGAS project, five In Diffusion and two Through Diffusion experiments have been performed with H2. The In Diffusion experiments allow to determine only the combination and the Through Diffusion experiments the effective diffusion coefficient R D. The In Diffusion experiments lead to values for between 1.5×10-5 m/s0.5 and 9×10-5 m/s0.5. Fitting Through Diffusion experiment H2THRD3 suggests a value of RD of around 2-3×10-10 m2 /s. Taking the extreme values of these two ranges, the apparent diffusion D should be between 5×10-12 m2 /s and 4×10-10 m2 /s. For the product R, this range is approximately between 1 to 40, clearly not in agreement with the original idea of R ≈ 0.4 (entire porosity accessible, no retardation). For comparison, gas diffusion coefficients for other gasses (methane and N2) in mudrocks at 90°C are in the range between 2 × 10-12 m2 /s and 5 × 10-11 m2 /s.