Behavior of Shales from Experiment to Modeling

Shale is the most ubiquitous material on the earth's surface and plays important roles in underground excavations and in geo-energy and geo-environmental applications. At the same time, shale is a very complicated material because of its mineralogical composition, heterogeneity, extremely low porosity and permeability, loading-rate dependency, anisotropy, and interactions with the pore fluid. Tight shale oil gas is becoming a valuable source of energy that produces less climate-changing CO₂ compared to other fossil fuels. Due to their very low permeability, economic shale gas/oil production is only possible by enhancing shale permeability through hydraulic fracturing techniques. Also, due to their low permeability, shales are a common cap rock layers for carbon dioxide (CO₂) sequestration in geological reservoirs such as deep saline aquifers. The caprock layers are expected to physically trap the CO₂ plume in the reservoir for an extended period. The fracturing of caprock shales would allow the sequestered CO₂ to escape to drinking water aquifers or the atmosphere, thereby reducing the effectiveness of geological sequestration. Shales are also the main cap rock in trapping of hydrocarbons in reservoirs. In oil and gas extraction in shale reservoirs, hydraulic fracturing is the most common form of reservoir stimulation by increasing the pore pressure until it exceeds the combined minimum effective principal stress and the rock’s tensile strength. Similarly, hydraulic or shear fracturing of shales can be one cause of leakage in CO₂ storage caprocks. This paper presents experimental data on different aspects of shale behavior including its compaction characteristics, poroelasticity, shear stress-strain relation, brittle-to-ductile transition, time-dependent stress-strain response, shear failure and strain softening, fracturing and damage, and eventual strain softening, and anisotropy. Using the experimental results, models to account for the multi-faceted shales behavior are presented.