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A constitutive framework for shale based on anisotropic critical state plasticity

Shale is a fine-grained sedimentary rock with high degree of heterogeneity at multiple scales. It is well known that due to the layered structure of shale its mechanical properties in the direction parallel to the bedding planes is significantly different from its properties in the perpendicular direction. Although isotropic models simplify the modeling process, they fail to accurately describe the mechanical behavior of these rocks; therefore, anisotropic material models can provide a significant improvement in modeling of these materials. In the current study, we present an anisotropic material model based on the well-known Modified Cam-Clay (MCC) yield surface to incorporate the effects of transverse isotropy in both elastic and plastic behavior of the material, while taking advantage of applicability of MCC in modeling plasticity in geo-materials. The proposed material model is verified via comparison with available experimental data, and can be readily applied to existing finite element codes to predict the behavior of shale in different applications. On the other hand, since shale contains a high degree of heterogeneity and porosity at the scale of nanometers, studying its microstructure is essential to enlighten the key mechanisms responsible for the observed macroscopic behavior of shale, including its anisotropy. Quantification of nanometric composition and structure of shale, and incorporating it into a mechanical model requires high resolution imaging techniques, e.g. Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM), and Transmission X-ray Microscopy (TXM). In the present work, we also present and discuss the application of these techniques in illuminating the heterogeneity and anisotropy stemming from shale’s complex microstructure, for the purpose of establishing a micromechanical anisotropic framework for shale.

Author(s):

Shabnam Jandaghi Semnani    
Stanford University
United States

Joshua A. White    
Lawrence Livermore National Laboratory
United States

Ronaldo I. Borja    
Stanford University
United States