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Title: Failure of anisotropic shales under triaxial stress conditions
Author: Ambrose, Jasmin
ISNI:       0000 0004 5362 1791
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Shales are highly anisotropic in their mechanical behaviour. The strength of anisotropic shales depends not only on the magnitude of the principal stresses, but also on the bedding plane orientations relative to the principal stresses. In this study, the failure of shales are investigated using triaxial compression and extension tests, while the role of intermediate stress (sigma2) on the strength of anisotropic shale is evaluated using data from new triaxial extension tests, as well as data from the literature. Triaxial compression and extension experiments were made on two organic-rich shales, at different confining stresses and bedding angles (Beta). Examination of post-failure computed tomography (CT) and thin section images for high strength anisotropy shale show that, for large and small values of Beta, the fracture plane follows the angle that is predicted by the Coulomb’s failure criterion for an isotropic material. In the range of angles of roughly 35deg. 2 were fit more closely by the Pariseau model, whereas both shales that were a better fit with the JPW model had SAR < 2. Pariseau’s model is also more robust and accurate than Jaeger’s model when using a reduced numbers of data (i.e., data collected at fewer confining stresses and/or fewer angles). Finally, both the JPW model and Pariseau’s model was applied in the true-triaxial stress regime, in which sigma1 > sigma2 > sigma3. When analysed with Mogi’s experimental data on Chichibu Schist, both models could predict failure under true-triaxial stress conditions. Mogi’s data and the triaxial extension experiments for the two shales shows that an increase in the intermediate stress sigma2 increases the intact rock strength, whereas weak plane failure depends not only on intermediate stress sigma2, but also on bedding plane angle Beta and foliation direction (omega).
Supervisor: Zimmerman, Robert W. Sponsor: Schlumberger
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available