Part of the Eurotom Fate Of Repository GasEs (FORGE) project, the FORGE benchmark modelling examined the transport of gas in a theoretical repository within a simple geology, with the objective of improving the understanding of gas migration modelling at the repository-scale to support performance assessments. Nine different groups with different codes and modelling approaches, including Geofirma representing NWMO, contributed to at least one scale of the benchmark modelling exercise. T2GGM, a modified version of TOUGH2 v2.0 with optional gas generation model, was used by Geofirma to model the three defined scales: cell, module and repository scales. A 2D radial model was developed for the cell-scale, and 3D models were developed for the module and repository scales. The greatest challenge in the module and repository scales was the inclusion of small features of the benchmark within a grid of tractable size. To obtain a working model, the interfaces were upscaled with adjacent elements, cells were converted from cylindrical to rectangular shapes of equivalent cross-sectional area, and grid discretization was nested and unstructured.
At all three scales, model results compared well to those produced by other modelling groups. For the FORGE benchmark geosphere and repository design, the repository design was effective at minimizing gas flow out of the repository, even with interfaces surrounding bentonite seals (note there is no EDZ surrounding the bentonite seals in the drifts or shaft). Important model sensitivities include: (1) the characterization of the seals, both in their capacity for resaturation (two-phase flow curves, particularly at the saturated end of the curve) and in the presence of an EDZ around these seals; and (2) the dissolved gas diffusion coefficient, as dissolution and diffusion of dissolved gas was the only transport pathway to the aquifer at the repository-scale. At the cell-scale, an increase in the dissolved gas diffusion coefficient resulted in dissolution and diffusion of gas into the host rock, rather than advective flow out of the cell.
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