An experimental study of localized compaction in high porosity rocks : the example of Tuffeau de Maastricht

Athanasios Papazoglou 1, 2
2 GéoMécanique
3SR - Laboratoire sols, solides, structures - risques [Grenoble]
Abstract : Given their high porosity, carbonates form important water and hydrocarbons reservoirs, and they are also suitable for other applications such as CO2 storage and nuclear waste disposal. However, localized compaction in carbonates affects the stress field and the hydromechanical properties of these rocks leading to inelastic deformation and failure with potential economic, environmental and social impacts. Previous field and experimental studies have shown that in porous carbonates, unlike sandstones, a variety of micromechanisms such as pore collapse, grain crushing, debonding, crystal plasticity and pressure solution can potentially lead to inelastic compaction. Due to the coexistence of such multiple inelastic processes and the interplay among them, the dominant micromechanism responsible for failure remains poorly understood.This doctoral thesis presents an experimental investigation into the deformation mechanisms governing the mechanical behavior and failure mode of high porosity carbonate rocks. To this end, Tuffeau de Maastricht, a bioclastic sedimentary limestone exhibiting up to 52% porosity, has been tested under dry conditions. This study focuses on how stress path, confining pressure and bedding orientation affect the onset and propagation of localized compaction. Three main experimental campaigns are conducted on cylindrical specimens of 11.5 mm diameter and 22 mm height to study the brittle-ductile transition: (i) isotropic compression, (ii) uniaxial compression, and (iii) triaxial compression tests at confining pressures ranging from 1 to 5 MPa. A systematic analysis of the anisotropic behavior of Tuffeau de Maastricht is conducted on samples cored perpendicular, oblique 45° and parallel to the bedding plane. High resolution x-ray computed tomography (CT) is used to obtain 3D images of the entire specimen under loading. The acquired images are processed and full-field measurements have been used to elucidate the mechanics of initiation and propagation of localized compaction. Porosity variations during loading are measured macroscopically as well as locally. The porosity measurements are performed over a REV, which is defined with the use of statistical tools. The systematic use of x-ray micro tomography combined with the use of advanced image analysis and Digital Image Correlation (DIC) provides a quantitative 3D information the strain field inside a sample and its evolution during a test.Two failure modes are identified, based on porosity measurements and DIC: compactive shear bands at low confining pressure, and compaction bands (perpendicular to the maximum compressive stress) at higher confinement. These bands develop at essentially constant deviator stress and propagate through the whole sample punctuated by episodic stress drops. Triaxial compression tests at much higher axial strain present three distinct phases: (1) an initial quasi-linear increase of deviator stress, followed by (2) a plateau and (3) a post-plateau hardening. The essential observation from these experiments if the occurrence of a debonding phase which converts the specimen from rock-like to sand-like. A second localization, typical of dense sand, eventually occurs for very axial strain. Additional experiments that are performed on artificially debonded specimens emphasize this destructuration phase during the plateau of deviator stress. The experimental results also highlight the strong anisotropy of the mechanical behavior of the studied material.
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Athanasios Papazoglou. An experimental study of localized compaction in high porosity rocks : the example of Tuffeau de Maastricht. Materials Science [cond-mat.mtrl-sci]. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAI100⟩. ⟨tel-02071176v1⟩

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