Influence of lithological contrast on elastic anisotropy of shales under true-triaxial stress and thermal conditions

The intrinsic anisotropy of shales, arising from their complex microstructure, including mineral alignment and bedding orientation, significantly influences seismic wave propagation, impacting subsurface imaging and reservoir characterization. Despite extensive research on shale anisotropy, studies employing cubic specimens under true-triaxial conditions, which provide a more realistic simulation of subsurface stress states, remain limited. This study investigates the elastic anisotropic behaviour of two distinct shale lithotypes: grey shale (GSH) and silty shale (SSH) from the Permian Barakar Formation of Lower Gondwana in the Jharia Basin, India, under varying isostatic pressures (8 MPa, 12 MPa, 25 MPa, 35 MPa, and 51 MPa) and temperatures (20 °C, 50 °C, 100 °C, 150 °C, and 200 °C) using a polyaxial loading apparatus. Compressional (Vp) and shear wave (Vs) velocities were measured in multiple orientations—parallel, perpendicular, and at 45° to the bedding plane—allowing for a comprehensive evaluation of elastic properties. The results reveal that both Vp and Vs exhibit a noticeable increase with rising isostatic pressure, indicating enhanced stiffness due to the closure of microcracks and pores. Moreover, GSH consistently demonstrated higher wave velocities compared to SSH, attributed to its higher quartz content and mineral alignment, which contributed to reduced shear wave splitting. The analysis of dynamic bulk modulus (K) indicated a consistent increase with pressure for both shale types, with GSH exhibiting a more linear response compared to the non-linear behaviour observed in SSH, influenced by its clay-rich composition. Temperature-induced changes in elastic properties were minimal, with only slight decrease in wave velocities observed at higher temperatures under constant pressure, inferring to the stability of the shale structure. This stability suggests that thermal expansion and mineralogical transformations do not significantly impact the elastic behaviour of these shales within the temperature range of the test. Furthermore, Thomsen anisotropy parameters (ε and γ) exhibited distinct trends under varying pressure and temperature conditions where overall a reduction in anisotropy was observed with increased pressure, reflecting a transition to more isotropic behaviour as pre-existing microcracks closed. The study provides a baseline understanding of variations in elastic properties of two different shale lithotypes to support subsurface resource exploitation strategies, underscoring the significance of employing cubic specimens under true-triaxial conditions to accurately simulate subsurface stress states.