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When cracks propagate in nature it is rarely in perfectly isotropic material, and in some cases, anisotropy can make a strong difference in terms of wave propagation or strength. For that purpose, it is crucial to study fracture propagation in anisotropic material and to address questions such as : what will be the influence of anisotropy on deformation mechanisms? On the pattern of deformation? Which parameter could be used to quantify crack propagation?In order to try to answer these questions, we performed a set of experiments, at the microscale, aimed at deforming uniaxially samples of anisotropic shales. A hole (0.9mm) was drilled on cylindrical samples (Φ=8mm, l=16mm) with a flattened surface in order to create artificial stress concentration. Three samples were prepared with different bedding orientations with respect to the loading axis (0°, 45°, 90°). Digital images are then taken around the hole during the experiments using an optical set up with a pixel resolution of 0.55µm and a magnification factor of 10. The full strain tensor is then extracted with Digital Image Correlation (DIC) method. These experiments allowed us to see fracture initiation and propagation with different patterns depending on the bedding orientation. From our experiments we then extracted mechanical parameters such as anisotropic fracture toughness.We also performed some calculations within the elastic domain in order to understand strain patterns by solving the problem of infinite plane under uniaxial loading with different bedding orientations assuming a transversely isotropic material.Our study allows to understand how some complex structures that are encountered both in the lab (discontinuous fractures often seen in shales) and in natural faults (such as anastomosed faults) develop. Our understanding of the mechanisms involved are also supported by other experiments performed under SEM observations. [ABSTRACT FROM AUTHOR]

The moment tensor is a useful representation of seismic sources. It can be used for efficient solution of forward and inverse problems of seismic wave excitation and propagation. The full moment tensor solutions based on seismic waveform inversion e.g. induced by hydraulic fracturing and borehole breakout processes can be used to understand the volumetric failure mechanisms associated with microseismic events. The characteristics of the moment tensor can be linked to the stress state in the reservoir rocks. Yet, the physical interpretation of the seismic moment tensor remains somewhat ambiguous. The moment tensor as a seismic source has been interpreted by introducing a stress or strain "glut" of some inelastic nature which cannot be described by the Hooke's law. Here, we suggest an alternative interpretation of this inelastic strain in the framework of time-independent non-associated plasticity theory. Thus, the previously introduced an inelastic strain "glut" acquires a meaning of plastic strain tensor. We show that this interpretation is practical since: i) it is consistent with continuum mechanics approach, such as applied in numerical geomechanical modeling of fault and fracture growth; 2) captures results of laboratory deformation and acoustic emission experiments; 3) explains the observed non-double-couple component of moment tensor solutions (dilatant plastic failure); 4) gives correct first-order estimates of scalar moment of moment tensor. As a numerical setup, we consider a classical problem of wellbore failure when stress is applied externally. Our analytical model provides first-order insights into the magnitude of scalar moment and scaling relations. Numerical finite element model in 2D plane strain combines both the yield function and flow potential describing the shear and tensile failure regimes. We observe and classify several modes of deformation, obtained in numerical experiments such as localized (shear bands) and distributed failure around wellbore. We systematically explore the effects of boundary conditions, heterogeneity of material parameters and elastic anisotropy. The source type and distribution of seismic events, obtained in our numerical model, agrees with observations of borehole breakouts, tunnel collapse and early growth of fluid-filled cracks such as hydraulic fractures and magmatic dikes. [ABSTRACT FROM AUTHOR]