Exploring thermo-hydro-mechanical-chemical (THMC) coupled processes during swelling of clay-sulfate rocks

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Despite the importance in geotechnical applications and geological processes, the swelling of rocks is still insufficiently understood, while its effects at the scale of engineering projects are significant. This joint research will bring together the experimental and modeling skills of French and German partners to characterize, understand and model the macroscopic effects of rock swelling from microscale processes. The planned research will focus on clay-sulfate rocks, where clay swelling and chemical swelling occur together and influence each other. The overarching goal of the proposed research is to generate experimental results that quantitatively characterize the thermal, hydraulic, mechanical and chemical (THMC) coupled swelling behavior of rocks, and translate the results into constitutive equations for numerical simulations. The research plan is structured into four tasks. The first three tasks are experimental and will be used to inform the models developed in Task 4. Task 1 includes the preparation and characterization of the samples. Both mineralogical analysis (X-ray diffraction, Rietveld method, thin section analysis) and chemical analysis (X-ray fluorescence spectrometry) will be performed. Fluid samples taken during the swelling experiments will also be analyzed (microwave plasma atomic emission spectrometry, titration). We will perform flow-through swelling experiments in Task 2 to observe how samples swell and how their permeability evolves during swelling. To understand and interpret the macroscopic swelling behavior observed in Task 2, advanced imaging of the samples at a smaller scale will be performed in Task 3. We will use X-ray computed microtomography (XRCT) and magnetic resonance imaging (MRI) to make direct observations of pore structure and fluid flow, and their changes during swelling. Moreover, chemical reactions can be tracked in space and time contemporaneous with swelling by observing changes in pore space, density and water content. These experiments will be complemented by scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and NMR cryoporometry. The results will enable us to tell where and when both fluid flow (e.g., rock matrix versus discontinuities) and chemical swelling (location and time of conversion) occur, and how they are related to mechanical (e.g., density/volume) and hydraulic (e.g., porosity, flow rate) parameters. Data gathered in Tasks 1 to 3 will be the basis for model development, calibration, and validation in Task 4. THMC constitutive laws will be deduced based on the data obtained in Tasks 2 and 3, and predicted by Fast Fourier Transform from the microstructural observations performed in Task 3. The constitutive laws will be implemented in two open-source THMC-coupled finite elements codes (Bil and OpenGeoSys) and validated on the experiments. The two numerical approaches will be compared with each other.