A numerical large-scale investigation of gas transport processes in a generic nuclear waste repository in argillaceous porous media

In this paper, we present the results of a large-scale numerical model of a generic nuclear waste repository situated in an argillaceous host rock formation. Modelling the evolution of an entire repository presents challenges due to the strong contrast in spatial and temporal scales at which the different processes take place, ranging from the centimetres to the kilometres and days to hundreds of thousands of years, respectively. From the view point of the physical processes, a further challenge originates from the different gas transport mechanisms: Gas advection as well as gas dissolution and diffusion jointly govern the efflux of gas from the repository and mitigate excess pore pressures, but there is a significant contrast between the rates of these two transport mechanisms. Using the TH2M implementation in the open-source finite element code OpenGeoSys-6, we analyse the impact of gas transport via advection (in the partially saturated zones such as backfilled drifts, shafts and desaturated host rock) as well as gas transport via diffusion (in fully water-saturated media such as the undisturbed host rock and over- and underlying formations). Finally, this work outlines and discusses possible simplifications in modelling choices, such as mechanical surrogate models, geometrical simplifications as well as the impact of discretization. The work presented in this paper was carried out within the scope of the European Joint Programme EURAD, workpackage Gas, Task 4.