Modelling of reactive gas transport in unsaturated soil. A coupled thermo-hydro-chemical-mechanical approach

This thesis presents the development of a reactive gas transport equation under coupled framework of thermal, hydraulic, chemical and mechanical (THCM) behaviour of variably saturated soil. The capabilities of theoretical and numerical modelling of THCM processes have been advanced by the successful implementation of various aspects of the new addition. The previously developed THCM model at the Geoenvironmental Research Centre (GRC) has been extended to include the multicomponent gas transport modelling coupled with chemical/geochemical processes. The mechanisms of advection and diffusion have been considered to define the transport of multicomponent gas and chemicals in respective phases as well as exchange via dissolution and exsolution. The governing mass transfer process is subjected to homogeneous and heterogeneous geochemical reactions under equilibrium condition. Numerical solutions of the governing flow and deformation equations have been achieved by employing finite element method for spatial discretisation and finite difference method for temporal discretisation. Advanced geochemical features of gas-chemical interactions have been incorporated in the transport model, COMPASS by coupling with the geochemical model PHREEQC. A sequential non-iterative approach has been adopted to couple the transport processes and geochemical interactions. Verification of various aspects of the developed gas transfer equation has been commenced via a number of simulation exercises. Good agreement between the results have been achieved which suggests accurate and successful implementation of the theoretical and numerical formulation. The model has been implemented to investigate the gas transport processes in variably saturated compacted clay buffers via a number of conceptual simulation scenarios which are representative of high level nuclear waste disposal. Simulation of gas migration through saturated buffer has been intended to investigate the maximum pressure development as well as the dominant flow mechanisms. Demonstration of the modelling capability in context of reactive gas transport has been carried out considering long term isothermal flow of hydrogen through unsaturated clay buffer. The conclusions drawn from the discussions of simulation results has favoured the understanding of some of the key issues associated with gas generation and migration in compacted porous media, particularly, as a case of high level nuclear waste disposal