Microporomechanical modeling of shale

Abstract

Shale, a common type of sedimentary rock of significance to petroleum and reservoir engineering, has recently emerged as a crucial component in the design of sustainable carbon and nuclear waste storage solutions and as a prolific natural gas source. Despite its importance, the highly heterogeneous and anisotropic nature of shale has challenged the theoretical modeling and prediction of its mechanical properties. This thesis presents a comprehensive microporomechanics framework for developing predictive models for shale poroelasticity and strength. Modeling is accomplished through a multi-scale approach, in which the experimental evidence gathered from novel nanoindentation techniques and conventional macroscopic tests informs the development of a suit of micromechanics tools for linking composition and microstructure to material performance. Based on a closed loop approach of calibration and validation of elastic and strength properties at different length scales, it was possible to deconstruct shale to the scale of an elementary material unit with mechanical behaviors governed by invariant properties, and to upscale these behaviors from the nanoscale to the macroscale of engineering applications. The elementary building block for elasticity is an anisotropic solid characterizing the in situ stiffness of highly consolidated clay.

(cont.) This intrinsic behavior represents the composite response of clay platelets, interlayer galleries, and interparticle contacts, yielding an invariant stiffness with respect to clay mineralogy. The anisotropic nanogranular nature of the porous clay in shale as inferred from nanoindentation is confirmed through micromechanics modeling. The intrinsic anisotropy of the clay fabric is suggested as the dominant factor driving the multi-scale anisotropic poroelasticity of unfractured shale compared to the contributions of geometrical sources related to shapes and orientations of particles. For strength properties, the micromechanics approach revealed that the frictional behavior of the elementary unit of compacted clay is scale independent, whereas a scale effect modifies its cohesive behavior. Having established a fundamental material unit and the adequate micromechanics representation for the microstructure, the macroscopic diversity of shale predominantly depends on two volumetric properties derived from mineralogy and porosity: the clay packing density and the silt inclusion volume fraction. The proposed two-parameter microporoelastic and strength models represent appealing alternatives for use in geomechanics and geophysics applications.