Energy Reports (Nov 2022)
Anisotropy characterization of the elasticity and energy flow of Longmaxi shale under uniaxial compression
Abstract
The inherent anisotropy of shale makes it exhibit different mechanical properties when measured in different directions. Generally, the measurements of the anisotropic mechanical properties of shale are generally conducted on cylindrical samples, and the computation of elastic constants is usually an empirical approximation. To accurately estimate the elastic parameters of shale rocks, an iterative method for approaching the crack initiation stress threshold was proposed in this work. Several uniaxial compression experiments were performed on cuboid Longmaxi shale samples with different bedding plane orientations. With a newly presented method, the apparent elastic modulus and Poisson’s ratios and five elastic constants were simultaneously determined. By examining the volumetric strain reversal point, the crack damage stress was obtained for each sample. The results demonstrate that the elastic regime, which was the range in which the elastic parameters were determined, significantly varies with bedding plane orientation. A transverse anisotropy ratio of 1.28 between in-plane modulus E1and out-of-plane modulus E3was determined for Longmaxi shale. Additionally, with the obtained elastic parameters, the energy releasing and dissipating behaviors of the samples during uniaxial loading were quantitatively characterized. The energy dissipation was found to rapidly increase once the crack damage stress was reached. However, before rock failure occurred, the total dissipated energy in each sample was very limited, indicating that microcrack propagation and rock damage were quite limited before rock rupture and that the shale samples were essentially destroyed by rapid energy release.
