Microstructures in host and reservoir sedimentary rocks from grain to pore and grain boundary scales : interactions between microstructures, transport processes, deformation mechanisms and rheology

Host and reservoir rocks like rock salts, clay stones, shales and tight gas sandstones are of major importance for society since they are considered as the probable solution for the nuclear waste storage in deep geological formation; in addition, good understanding of their chemical, physical and mechanical properties is essential for reservoir development in the oil and gas industry. Bulk mechanical and fluid flow properties of such rocks are widely documented and investigated, however much less is known about the impact of microstructures on these properties. In the ideal case, the investigation of microstructures in such wet, fine grained and heterogeneous geomaterials requires a multi-scale and multi-dimensional approach with samples preserved in their original hydric state and stress field. Works presented in this habilitation presents developments of novel and innovative methods to access microstructures: (1) in rocksalt the grain microstructures are decorated by gamma-irradiation, (2) ion beam beam milling tools (focussed ion beam-FIB and broad ion beam-BIB) were established to prepare extremely high quality cross-sections suitable for SEM imaging down to nanometer scale and (3) cryogeny methods were deveopped both to preserve specimen from drying damages and quench fluid within pores and grain boundaries. The combination of methods (2) and (3) allow SEM-imaging of preserved wet structures on atomically smooth cross-sections at cryo-temperature. Particularly, a novel instrument allowing argon broad ion beam tomography in a cryogenic scanning electron microscope (BIB-cryo-SEM) to study representative microstructures in sedimentary rocks containing pore fluid was developed. The use of the different methods above coupled with other traditional methods to characterize the microstructure allows having new insigths in the coupling between the microstructural evolution, the poromechanical behaviour and the state of hydration of the system that occur within the studied geomateials. Results are paving towards the better calibration of constitutive laws for the estimation of chemical, physical and mechanical properties of the host and reservoir rocks