Thermodynamic modelling of hydrogen migration in argillite for a deep geological radioactive waste repository: IRSN contribution to FORGE

Abstract

AbstractThe aim of this work is to provide an improved mathematical and physical description of two-phase flow in tight porous media in order to model hydrogen migration within a geological repository for radioactive waste as required to further assess the behaviour in time and space of such facilities. First, we introduce a general physical framework describing multicomponent and two-phase (liquid and gas) flow in porous media with capillary pressure curve and mass exchange between phases. Assuming thermodynamic equilibrium, mass exchange between phases is governed by thermodynamic principles which we briefly describe. This physical modelling is able to describe single-phase flow (only gas mixture or liquid solution) as well as two-phase flow (both liquid and gas phases are present). On the basis of this treatment we obtain a generalized formulation of Henry and Raoult–Kelvin laws. We show that, for a hydrogen–water mixture in repository conditions of pressure and suction, these relationships constitute an adequate mass equilibrium description. Second, we consider the mathematical formulation of the problem, which is mainly determined by the choice of primary variables. Contrary to existing approaches, we are interested in avoiding the need to switch primary variables during calculations. Indeed, such a switch is often proposed as a solution to treat phase appearance or disappearance. We will show that, in our physical framework, taking into account the capillary pressure and mass exchanges between phases, it is possible to work with universal primary variables to correctly describe gas phase appearance or disappearance. Simple numerical simulations are presented for validation as well as an application of this model to the FORGE benchmark on the cell scale together with a sensitivity analysis guided by physical considerations.