4-dimensional studies of fluid-rock interaction
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Date
03/07/2017Author
Macente, Alice
Metadata
Abstract
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.