The exploitation of the geological subsurface, either to extract natural resources (water, heat, gas, useful mineral substances, etc.), or to inject/store resources (water, heat, hydrocarbons) or undesirable compounds ...
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The exploitation of the geological subsurface, either to extract natural resources (water, heat, gas, useful mineral substances, etc.), or to inject/store resources (water, heat, hydrocarbons) or undesirable compounds (industrial water and brine, radioactive waste, acid gases, CO2, etc.) requires the control of reactive transport processes in porous and fractured geological media at different time and space scales. However, to date, little is known and mastered concerning the understanding of physicochemical and kinetic mechanisms at the interfaces at the origin of mass transfers between phases (minerals - water - gas). Indeed, several key questions and scientific barriers limit the exploitation of current knowledge, relatively inconsistent and rather scattered, and prevent any generalization of theoretical approaches available to model the fate of these systems under the influence of human intrusions.Thus, in the framework of this thesis, we have studied, at the pore scale, the precipitation and dissolution processes of model minerals representative of deep geological environments: carbonates and sulfates. The objective here is to understand the physical and chemical mechanisms at the interface of the different phases (mineral / aqueous phase).To do so, we used microfluidic reactors coupled to different in-situ characterization techniques (i.e. optical imaging, confocal microscopy, etc.) to study the precipitation and carbonation mechanisms of gypsum. We have experimentally controlled some parameters that may affect the reaction rates, such as the concentration of alkaline earth minerals (Ca, Mg) and the transport mechanisms (convective / diffusive). Gypsum crystals (CaSO4.2H2O) were first precipitated in on-chip microcavities by mixing reactive aqueous solutions of CaCl2 and Na2SO4. The saturation state of the reactive fluid with respect to the calcium sulfate phases was calculated using PHREEQC software. Subsequently, the controlled injection of carbonates within the system results in a gypsum carbonation mechanism, which was monitored in real time. In the experiments, the transport was voluntarily limited to diffusion by controlling the hydrodynamics of the system (realization of adapted microfluidic designs). A two-step carbonation mechanism was demonstrated, including a gypsum dissolution phase followed by a calcium carbonate precipitation phase (calcite and vaterite).The experimental results were numerically analysed to extrapolate the real evolution of the specific surface of the minerals by image processing. This evolution was then used for thermokinetic modelling using the PHREEQC V3 software. This modelling was used to calculate the dissolution kinetic constant for gypsum, i.e. 8 × 10-12 mol.cm-2.s-1. It was shown that there was a good match between the trends observed experimentally and those obtained by modelling, opening up the possibility of future studies using the methodology proposed during this thesis (multiphase transport, consideration of convective transport phenomena, work under pressure, etc.).
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