4-dimensional studies of fluid-rock interaction

Successful management of hydrocarbon reservoirs, geothermal energy extraction sites, radioactive waste and CO2 storage sites depends on a detailed knowledge of fluid transport properties, porosity and permeability. Amongst deformation processes, fluid-rock interaction plays an important role in controlling the petrophysical properties of a rock. The presence of fluids in the rocks induce chemical and physical changes in compositions and texture, affecting porosity and permeability, hence influencing dynamic transport properties and fluid flow. Fluid-rock interaction processes have been deeply investigated in nature and in numerous experimental and numerical modelling studies. However, these studies lack a spatio-temporal characterization of the dynamic evolution of porosity and reaction microfabrics. There is no clear understanding of the spatio-temporal evolution of these properties in three dimensions, and how this evolution affects fluid percolation in the rock. Computed X-ray micro-Tomography (μCT) was applied to investigate these processes in three dimensions and observe their evolution in time (4DμCT). The combination of μCT with 2D analytical techniques (e.g. scanning electron microscope, SEM, electron microcrobe, EMPA, electron backscatter diffraction, EBSD) furthermore enables the extrapolation of the information gained from 2D analyses to the 3rd an 4th dimension (4D μCT). The thesis investigates two different categories of fluid-rock interaction processes, by using 4DμCT to monitor the evolution of mineral reactions (in the first case) and porosity (second case) in relation to strain and time. In the first case study, natural rock samples were analysed. The samples show a compositional change along a strain gradient from olivinic metagabbros to omphacite-garnet bearing eclogites in a ductile shear zone. Synchroton-based x-ray microtomography (sμCT) was applied to document the 3D evolution of garnets along the strain gradient (which represent the 4th dimension). The 3D spatial arrangement of garnet microfabrics can help determine the deformation history and the extent of fluid-rock interaction active during deformation. Results from the sμCT show that in the low strain domain, garnets form a large and well interconnected cluster that develops throughout the entire sample and garnet coronas never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnets reflect a deformational origin of the microfabrics. EBSD analyses reveal that garnets do not show evidence for crystal plasticity, but rather they highlight evidence for minor fracturing, neo-nucleation and overgrowth, which points to a mechanical disintegration of the garnet coronas during strain localisation. In the second case study, pressure-solution processes were investigated using NaCl as rock-analogue, to monitor the evolution of porosity and pore connectivity in four dimensions, providing a time-resolved characterization of the processes. NaCl samples were uniaxially compacted and μCT scans were taken at regular interval times to characterize the evolution of grain morphologies, pore space and macro-connectivity of the samples. Different uniaxial loads, as well as different bulk sample compositions (phyllosilicates and/or glass beads) were used to investigate their effect on the process. Greater uniaxial loads, and the presence of phyllosilicates within the deforming NaCl columns were found to enhance pressure-solution processes. The pore space becomes highly disconnected in the presence of phyllosilicates, with important implications for fluid percolation and dynamic transport properties. Mean strain rates, calculated from volumetric Digital Image Correlation (3D-DIC) analyses, were found to be higher where phyllosilicates were located. The combination of μCT with volumetric DIC and SEM imaging proved to be an efficient analytical method for investigating the dynamic behaviour of porosity and permeability during ongoing pressure-solution processes. The results showed that fluid-rock interaction critically modifies the rocks at the pore/grain scale, with important consequences on dynamic fluid transport properties. The combination of μCT with classical 2D techniques provided a better understanding on the dynamic evolution of transport properties and fluid percolation during fluid-rock interaction processes, allowing the characterization in three dimensions of reaction microfabrics and porosity.