A Thermo-mechanical Framework for Shale Based on Anisotropic Critical State Plasticity

Abstract:

Due to its layered structure at multiple scales, mechanical properties of shale in the direction parallel to the bedding planes are significantly different from its properties in the perpendicular direction. Isotropic material models simplify the process of modeling these materials; however, they often fail to accurately describe mechanical response when the layered structure plays an important role. Further, shales often exhibit temperature-dependent properties when significantly heated or cooled. In this work, we study the thermo-mechanical behavior of oil shale, a kerogen-rich source rock. In particular, we examine in-situ conversion processes in which the shale is heated to high temperatures (>300C) to convert the solid kerogen phase to useful oil and gas products. Multi-temperature triaxial tests on core samples reveal that changes in microstructure at high temperatures dramatically alter mechanical properties. Accordingly, the current study presents a thermo-mechanical framework for modeling oil shale and related materials. The underlying model is based on the well-known Modified Cam-Clay (MCC) yield surface, here modified to incorporate the effects of transverse isotropy. We also include temperature dependence in the elastic and plastic response. The proposed model is verified by comparison with available experimental data, and used to study the effect of anisotropy on deformation behavior during mechanical and thermal loading. We also study the formation of deformation bands as a result material instability, and the effect the bedding structure has on the shear localization process.