Elastic anisotropy and deformation characteristics of Pennsylvania anthracite

Abstract

The mechanical behavior and elastic anisotropy of coal under stress are critical to understanding its structural integrity and performance in subsurface environments. Despite its significance, limited research has systematically analysed the elastic anisotropic responses of coals under such conditions. This study investigates the elastic anisotropy of three anthracite-rank coals, Primrose, Lattimer, and Mt. Carmel, subjected to conventional triaxial loading. P-wave (V_P) and S-wave (V_S) velocities, along with Thomsen parameters (ε and γ), were evaluated to elucidate the effects of increasing vertical stress on the structural integrity and anisotropy of each coal type. The results reveal that the Primrose coal exhibits the highest structural integrity, maintaining elevated V_P and V_S values and stable Thomsen parameters under stress due to its dense microstructure, higher inertinite content, and low porosity, which resist stress-induced microcracking. In contrast, the Lattimer coal demonstrates a significant reduction in VP and ε beyond 45 MPa, indicating greater susceptibility to microstructural damage and a trend towards isotropy as stress increases. The Mt. Carmel coal shows intermediate behavior, with moderate decreases in VP and ε but relatively stable γ values, reflecting a balanced resistance to structural degradation. S-wave anisotropy, as evidenced by shear wave splitting, remains most prominent in the Primrose coal, suggesting its superior ability to retain directional properties and resist stress-induced deformation. Principal component analysis highlights the role of rank, inertinite-to-vitrinite ratio, and aromaticity in influencing the mechanical responses of the coals, with Primrose coal consistently segregating as the most robust and anisotropically stable sample. These findings underscore the critical influence of compositional and microstructural differences on coal's anisotropic behavior under conventional-triaxial loading. They provide valuable insights for applications in subsurface energy extraction and storage, where understanding the mechanical and anisotropic properties of coal is essential for optimizing performance and mitigating risks.