Abstract
Organic-rich shales are often found to be strongly anisotropic. Their dynamic and static elastic properties depend on their intrinsic anisotropy and the anisotropic in-situ stress field. We report pseudo-triaxial tests on Eagle Ford shales with axial load normal and parallel to beddings, respectively. From the experimental data, regardless of being from dynamic or static measurements, the elastic parameters present strong angular dependences: a much higher Young's modulus and a higher Poisson's ratio in the bedding-parallel direction. The deviatoric load orientation with respect to beddings leads to different nonlinearity and hysteresis in the stress-strain curves. From the microstructural point of view, the deviatoric load induces elastic compaction as well as some non-elastic processes such as frictional sliding and crushing of asperities at crack surfaces or grain boundaries. Hence, the statically derived parameters are sensitive to the anisotropic stress state and load-unload history. However, those microstructural alternations bring very small effects on the dynamic parameters. The dynamic Young's moduli are systematically higher than the static Young's moduli, whereas the dynamic Poisson's ratios are lower in the loading process and higher in the unloading process than the static Poisson's ratios. When the load is initially reversed, the static parameters approach the corresponding dynamic parameters, reflecting the rock bulk properties without any frictional sliding effects.
Introduction
Shales comprise more than 70% of the drilled formations in most sedimentary basins and form the seal or source rocks of many hydrocarbon reservoirs (Vernik and Nur, 1992). As the unconventional oil and gas boom, the organic-rich shales have drawn global attention in the past fifteen years. These shales serve as both source rocks and reservoirs in resource shale plays. Because of the extremely low porosity and permeability, extracting economic hydrocarbon flows from such reservoirs requires the applications of horizontal drilling and hydraulic fracture stimulation techniques (Rickman et al., 2008). To this end, their geo-mechanical properties, such as Young's modulus and Poisson's ratio, require a better understanding in consideration of the importance in predicting the in-situ stress profile, evaluating brittleness, and optimizing horizontal well and hydraulic fracture designs (Higgins et al., 2008; Rickman et al., 2008).
