Abstract:Conventional triaxial tests were performed on three sets of samples of Tournemire shale along different orientations relative to bedding (0°, 45°, and 90°). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 160 MPa, at strain rates ranging between 3 × 10−7s−1 and 3 × 10−5s−1. This allowed us to determine the entire anisotropic elastic compliance matrix as a function of confining pressure. Results show that the orientation of principal stress relative to bedding pl… Show more
“…For slow strain rate the mechanism seem to be more ductile with plastic mechanisms such as mineral reorientation and delayed fracture localization. All these mechanisms are also supporting the conclusions made in Bonnelye et al [].…”
Section: Discussionsupporting
confidence: 91%
“…At the highest confining pressure ( P c =80 MPa, Figure c), the previous trend is enhanced: wave velocities measured along raypaths with a small angle relative to bedding decrease, while those measured along raypaths with a large angle relative to bedding increase, so that we observed anisotropy reversal before failure which illustrates fully the increasing role of plastic deformation processes with increasing confinement. This was also confirmed by the shape of the axial strain‐deviatoric stress curves presented in [ Bonnelye et al , ]. When looking closer before and after failure (Figures a and b), we see that the drop of elastic wave velocities occurs along certain raypath only several hours after failure.…”
Conventional triaxial tests were performed on a series of samples of Tournemire shale along different orientations relative to bedding (0°, 90°). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 80 MPa, and at strain rates ranging between 3 × 10−7 s−1 and 3 × 10−5 s−1. During each experiment, P and S wave elastic velocities were continuously measured along many raypaths with different orientations with respect to bedding and maximum compressive stress. This extensive velocity measurement setup allowed us to highlight the presence of plastic mechanisms such as mineral reorientation during deformation. The evolution of elastic anisotropy was quantified using Thomsen's parameters which were directly inverted from measurement of elastic wave velocity. Brittle failure was preceded by a change in P wave anisotropy, due to both crack growth and mineral reorientation. Anisotropy variations were largest for samples deformed perpendicular to bedding, at the onset of rupture. Anisotropy reversal was observed at the highest confining pressures. For samples deformed parallel to bedding, the P wave anisotropy change is weaker.
“…For slow strain rate the mechanism seem to be more ductile with plastic mechanisms such as mineral reorientation and delayed fracture localization. All these mechanisms are also supporting the conclusions made in Bonnelye et al [].…”
Section: Discussionsupporting
confidence: 91%
“…At the highest confining pressure ( P c =80 MPa, Figure c), the previous trend is enhanced: wave velocities measured along raypaths with a small angle relative to bedding decrease, while those measured along raypaths with a large angle relative to bedding increase, so that we observed anisotropy reversal before failure which illustrates fully the increasing role of plastic deformation processes with increasing confinement. This was also confirmed by the shape of the axial strain‐deviatoric stress curves presented in [ Bonnelye et al , ]. When looking closer before and after failure (Figures a and b), we see that the drop of elastic wave velocities occurs along certain raypath only several hours after failure.…”
Conventional triaxial tests were performed on a series of samples of Tournemire shale along different orientations relative to bedding (0°, 90°). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 80 MPa, and at strain rates ranging between 3 × 10−7 s−1 and 3 × 10−5 s−1. During each experiment, P and S wave elastic velocities were continuously measured along many raypaths with different orientations with respect to bedding and maximum compressive stress. This extensive velocity measurement setup allowed us to highlight the presence of plastic mechanisms such as mineral reorientation during deformation. The evolution of elastic anisotropy was quantified using Thomsen's parameters which were directly inverted from measurement of elastic wave velocity. Brittle failure was preceded by a change in P wave anisotropy, due to both crack growth and mineral reorientation. Anisotropy variations were largest for samples deformed perpendicular to bedding, at the onset of rupture. Anisotropy reversal was observed at the highest confining pressures. For samples deformed parallel to bedding, the P wave anisotropy change is weaker.
“…Moreover, the rupture continued to develop until the specimen became completely destroyed. In comparison to previous studies [30,31], the failure strength in this study was found to be much larger. Figure 5.Triaxial stress–strain curve of shale under a confining pressure of 20 MPa.…”
Section: Experimental Results and Studycontrasting
The deformation and fracture characteristics of shale in the Changning-Xingwen region were experimentally studied under triaxial cyclic loading with a controlled pore-water pressure. An RLW-2000M microcomputer-controlled coal-rock rheometer was used in the State key Laboratory of coal mine disaster dynamics and control in Chongqing University. These experimental results have indicated the following. (i) The shale softened after being saturated with water, while its failure strength decreased with the increase of axial strain. (ii) A complete cyclic loading–unloading process can be divided into four stages under the coupling action of axial cyclic loading and pore-water pressure; namely the slow or accelerated increasing of strain in the loading stage, and the slow or accelerated decreasing of strain in the unloading stage. (iii) The axial plastic deformation characteristics were similar when pore-water pressures were set to 2, 6 and 10 MPa. Nevertheless, the shale softened ostensibly and fatigue damage occurred during the circulation process when the pore-water pressure was set to 14 MPa. (iv) It has been observed that the mean strain and strain amplitude under axial cyclic are positively correlated with pore-water pressure, while the elastic modulus is negatively correlated with pore-water pressure. As the cycle progresses, the trends in these parameters vary, which indicates that the deformation and elastic characteristics of shale are controlled by pore-water pressure and cyclic loading conditions. (v) Evidenced via triaxial compression tests, it was predominantly shear failure that occurred in the shale specimens. In addition, axial cyclic loading caused the shale to generate complex secondary fractures, resulting in the specimens cracking along the bedding plane due to the effect of pore-water pressure. This study provides valuable insight into the understanding of the deformation and failure mechanisms of shale under complicated stress conditions.
“…In the case of shales, the bedding planes provide such a plane of weakness, but due to the anisotropic nature of the shale matrix, it is worth investigating the failure strengths of the bedding‐parallel and bedding‐perpendicular samples separately. McLamore and Gray (), Sone and Zoback (), and Bonnelye et al () all find samples of shales to support a slightly higher maximum differential stress in the bedding‐parallel orientation than in the bedding‐perpendicular orientation over a range of confining pressures. Ambrose () found the same in Bossier shale but found no difference between the two orientations in the Vaca Muerta shale.…”
Triaxial experiments and direct fluid injection experiments have been conducted at confining pressures up to 100 MPa on Mancos shale, Whitby mudstone, Penrhyn slate, and Pennant sandstone. Experiments were conducted with sample axes lying both parallel and perpendicular to layering in the materials. During triaxial failure Penrhyn slate was stronger for samples with cleavage parallel to maximum principal stress, but the two orientations in the shales displayed similar failure stresses. Initial flaw sizes of around 40 μm were calculated from the triaxial data using the wing crack model, with the shales having shorter initial flaws than the nonshales. During direct fluid injection, breakdown was rapid, with no discernible gap between fracture initiation and breakdown. Breakdown pressure increased linearly with confining pressure but was less sensitive to confining pressure than expected from existing models. A fracture mechanics‐based model is proposed to determine the initial flaw size responsible for breakdown in injection experiments. Flaw sizes determined in this way agree reasonably with those determined from the triaxial data in the nonshales at low confining pressures. As confining pressure rises, a threshold is reached, above which the fluid injection experiments suggest a lower initial flaw length of around 10 μm. This threshold is interpreted as being due to the partial closure of flaws. In the shales an initial flaw length of around 10 μm was determined at all confining pressures, agreeing reasonably with those determined through the triaxial experiments.
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