Instrumented nanoindentation and 3D mechanistic modeling of a shale at multiple scales

Nanoindentation tests, spanning various length scales ranging from 200 nm to 5 μm deep, were performed on a sample of organic-rich Woodford shale in both the bedding plane normal and bedding plane parallel directions. Focused ion beam milling, scanning electron microscopy, and energy dispersive X-ray spectroscopy were utilized to characterize the shale at the scale of the nanoindentation testing as being comprised predominantly of clay and other silicate minerals suspended in a mixed organic/clay matrix. The nanoindentation tests reveal the mechanical properties of the relatively homogeneous constituent materials as well as those of the highly heterogeneous composite material. Loads on the order of a few millinewtons produced shallower indents and demonstrated the elastic–plastic deformation response of the constituent materials, whereas higher loads of as much as a few hundred millinewtons produced deeper indents revealing the response of the composite matrix. In both cases, significant creep was observed. We use nonlinear finite element modeling utilizing an isotropic critical state theory with creep to capture the indentation response by calibrating plastic material parameters to the laboratory measurements. The simulations provide a means of extracting plastic material parameters from the nanoindentation measurements and reveal the capabilities as well as limitations of an isotropic model in capturing the response of an inherently anisotropic material.