Due to their abundance, sedimentary rocks are playing a critical role in re-defining the world's energy landscape, leading to shifts in global geopolitics. The recovery of oil and gas from organic-rich shales, geological sequestration of CO2 in sandstone host rocks and storage of nuclear waste in impermeable rocks are among the most challenging problems worldwide. Technical challenges and environmental concerns slow the growth of organic-rich shale exploration and exploitation worldwide. Similarly, the technical challenges and lack of financial incentives continue to slow the growth of geological storage of CO¬2 worldwide. The engineering and scientific challenges emerge due to the extremely heterogeneous and anisotropic nature of these naturally occurring geo-composites at multiple length scales. Specifically, the effect of rate and speed of loading on fracture of organic rich shale, the effect of fabric and mineralogy on the mechanical behavior of organic-rich shales becomes of critical importance for petroleum engineers. Similarly, the effect of geochemical alterations in geomechanical behavior becomes of critical importance for storage of CO2 in deep saline formations.
Thus, this thesis develops a comprehensive nano-investigation approach to assess the microstructure as well as the stiffness, strength and fracture properties of these naturally occurring geo-materials. Specifically, the effects of anisotropy, organic content and mineralogy on mechanical behavior of organic-rich shale are investigated. Similarly, the effect of geochemical alterations on host rock mechanical response are investigated. This is achieved by a comprehensive experimental micromechanics approach. This study utilizes advanced experimental and analytical techniques, i.e. analytical nanoindentation, scratch testing, environmental scanning electron microscopy, to provide the basis for assessment of microstructure and material invariant properties. Nanoindentation experiments and analysis tools are designed to probe and infer the elastic and strength properties of the geo-composites. Furthermore, scratch testing experiments are designed to probe and infer the fracture toughness of the geo-composites.
The results of this investigation show that organic-rich shale exhibit exceptional toughness due to toughening mechanisms operating at the nanometer scale in the kerogen and at the clay-kerogen interface. Furthermore, fracture behavior is anisotropic as a combined result of a layered microstructure and the intrinsic fracture behavior of clay. Additionally, the effect of geochemical alterations on host rock, Mt. Simon, is investigated using nanoindentation testing. The result of this investigation indicates a dissolution of feldspar during incubation regimes in slightly acidic brine and in CO2-saturated brine. At the macroscopic length-scale, a weakening of the rock is predicted based on the formulated multiscale nanomechanics framework. These findings are important and will inform advanced physics-based constitutive materials law for geomechanics simulation in energy-related application such as hydraulic fracturing in unconventional reservoirs or geological CO2 sequestration in deep saline formations
