Modelling of Hydrogeochemical Processes in Deep Geological Systems : Application to energy-related environmental challenges

This thesis investigates coupled hydrogeochemical processes in deep geological systems through a combination of laboratory experiments and reactive transport modelling with emphasis on understanding and predicting the long-term behaviour of subsurface systems under engineered disturbances. Four case studies are presented. The first examines the impact of SO2 impurities in CO2 streams on geochemical reactions during carbon capture and storage. The results show enhanced mineral dissolution, porosity changes, and trace element mobilization under acidic conditions, emphasizing the need to consider gas purity in site assessments. The second study explores how hydraulic fracturing fluids interact with shale, using batch simulations to assess the influence of fluid composition and shale mineralogy. The findings highlight the sensitivity of geochemical pathways to small variations in pH, oxidants, and mineral content. The third and fourth studies investigate solute transport in Woodford shale using multi-tracer column experiments on both crushed and intact core samples. While the granular system is dominated by advective flow, mobile–immobile interactions through diffusion and cation exchange were also observed. In the intact sample, transport is strongly influenced by diffusion and pronounced immobile zone effects. A dual-porosity model was used to simulate the experimental data, with particular focus on cation exchange, mineral reactions, and the influence of interfacial area between mobile and immobile domains. Overall, the thesis demonstrates how fluid–rock interactions, shaped by mineralogy, fluid composition, and prevailing conditions, control porosity evolution, mineral reactions, and solute mobility in subsurface environments. These results contribute to improved modelling approaches and support more robust assessments of long-term safety in engineered geological systems. By advancing the integration of experiments with process-based modelling, the work enhances predictive capabilities essential for responsible subsurface engineering.