Anisotropy rather than transverse isotropy in Longmaxi shale and the potential role of tectonic stress

In the last few decades, the recovery of and exploration for shale gas has increased. This has led to increased interest in the mechanical characterization of shale formations as relates to enhancing the conductivity of such formations. Characterization and modeling of these materials typically assumes transverse isotropy associated with the direction of bedding, despite some studies showing anisotropy in these materials. In this work, we evaluate the validity of transverse isotropy for the Longmaxi Shale Formation, which has been targeted for shale gas exploration in China. This evaluation is done through analysis of the macro- and microstructure distribution, confined triaxial tests, and compression and shear wave velocity measurements. These tests were performed parallel and perpendicular to the bedding and in the directions interpreted as the tectonic principal stresses. The effect of tectonic stress on shale anisotropy is also discussed. We found that the assumption of transverse isotropy for the mechanical behavior of the Longmaxi Shale might not be adequate under low confinement stresses. The trends in the measured behavior suggest that transverse isotropy might provide adequate estimations as confinement increases. The data also suggest that tectonic stresses affected the Longmaxi Shale rock mass characteristics at the study site in terms of macro- and microstructure, as well as mechanical properties. The results also indicate that isotropic rocks may show anisotropy in subsurface engineering. Directing more attention to the possible mechanical differences in the field may provide implications for engineering safety.