Étude de l'évolution hydromécanique d'un carbonate après altération chimique. Application des méthodes de corrélation d'images 2D et 3D à la mesure des champs locaux de déformation lors d'essais mécaniques à différentes échelles.

摘要

In order to face the challenge of reducing greenhouse effect gases emission, many solutions have been considered. One of the most studied is the long term geological CO2 storage in deep saline aquifer. These geological structures can be found in sedimentary basins and are defined by there thickness (several hundred meters) and their geographic spread (for instance, the whole Paris basin for the Dogger formation). The second main interest of these geological formations is that the carbonated rocks composing the reservoir are very porous (porosity ranging from 10 to 30%) and present a large enough permeability for an easy injection of fluids. However, the mechanical behavior and long term evolution of these reservoirs after the CO2 injection in the brine are not well established. To this aim, the mechanical and flow properties of these rocks are studied in order to develop predictive evolution models to insure the integrity and the storage capability of the reservoir. The experimental work of this thesis is based on a retarded acid treatment protocol developed and used at IFPEN allowing a homogeneous dissolution of the samples. The goal is to study the evolutions of the hydromechanical properties of the Lavoux limestone with the alteration. The description of the flow properties of the sample has been coupled to a microstructural characterization. The pertinent parameters are the porosity and the permeability at the intact and altered states. The permeability evolution has analyzed by an image substraction technique using microtomograph RX images, which allows the visualization of fine particles in the porous network. These particles have also been revealed by laser diffraction on the flush effluents. The microstructural study of the rock after alteration has allow the characterization of the dissolution mechanisms of our rock. The mechanical behavior has been studied by trixial tests performed at three states of alteration and several confining pressures. These tests demonstrate a transition from brittle to ductile failure mechanism induced by the alteration. In order to better understand this transition, we have also performed uniaxial tests coupled with in-situ image acquisition i) with a classical electromechanical press, CCD captors and optical microscopy for the larger scale and ii) with a miniaturised uniaxial rig in a SEM chamber for the micro scale. The digital image correlation technique has been used during the mechanical loading at several scales, ranging from that of the sample to the microstructural scale. The computation of the deformation fields has shown the different patterns of damage or failure associated with different alteration states. A miniaturized triaxial cell, fitting inside a microtomograph RX, has been specifically developed at IFPEN for this study. This cell allows us to work at various confining pressures (we used 5 and 10 MPa) with volume imaging in in-situ conditions during the mechanical loading inside the tomograph. We have thus identified for three alteration states, three failure modes, namely i) the development of a shear band and brittle rupture for the intact sample, ii) a narrow shear compaction band in a three time altered sample and finally iii) a wideshear compaction band in a six time altered sample. Our set of results on mechanical and flow properties (and more specifically, the variation of permeability and the presence of fines particles in the porous network) would tend to demonstrate that alterations greatly affect the microporosity of the limestone, with strong effects on the shear-compactant behavior of the rock.