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On the anisotropy, creep, and damage of shale at nano- to micro-scales

This research combines state-of-the-art nanocharacterization experiments on a sample of organic-rich Woodford shale with the development of experimentally motivated constitutive theory. The experimental investigation has provided a description of the shale as a highly heterogeneous material comprised of near micrometer sized particles of clay, organics, and various other mineral particles embedded in a mixed organic/clay matrix. Specifically, we distinguish between a nano-scale picture of shale as being that of individual homogeneous particles, and a micro-scale picture of shale as being that of a heterogeneous composite material. Nanoindentation experiments have been carried out at both the particle (homogeneous) and composite (heterogeneous) scales in both bedding-plane parallel and bedding-plane perpendicular directions, providing in situ measurements of constituent material properties and also capturing the anisotropic stress-strain response of the composite material. The 3D geometry of pre- and post-indented regions has been imaged to nanometer resolution with focused ion beam - scanning electron microscopy (FIB-SEM). The 3D imaging provides novel insight into the mechanisms of deformation and fracture occurring at these very small scales. For example, significant micro-fractures preferentially aligned with the bedding plane direction have been observed near indented regions. Non-linear finite elements are used to develop a 3D mechanistic model for interpreting the results of nanoindentation tests and FIB-SEM imaging. Measurements of anisotropy, creep, and micro-fracture are incorporated into a theoretical framework of finite elasto-plasticity motivated by the experimental results. Simulation of nanoindentation experiments allows for calibration of material model parameters and for validation against the measured load-displacement responses.

Author(s):

Kane Bennett    
Stanford University
United States

Ronaldo Borja    
Stanford University
United States