Elastic, strength, and fracture properties of Marcellus shale

TL;DR

This study comprehensively investigates the elastic, strength, and fracture properties of Marcellus shale through experimental methods, revealing anisotropic behaviors, size effects, and the applicability of transversely isotropic models for its mechanical characterization.

Contribution

It provides new experimental data on Marcellus shale's mechanical properties, including anisotropy and size effects, and validates the use of transversely isotropic models and Bazant's Size Effect Law.

Findings

Elastic behavior is transversely isotropic.

Strength varies with bedding orientation and loading direction.

Fracture properties are size-dependent and anisotropic.

Abstract

Shale, a fine-grained sedimentary rock, is the key source rock for many of the world's most important oil and natural gas deposits. A deep understanding of the mechanical properties of shale is of vital importance in various geotechnical applications, including oil and gas exploitation. In this work, deformability, strength, and fracturing properties of Marcellus shale were investigated through an experimental study. Firstly, uniaxial compression, direct tension, and Brazilian tests were performed on the Marcellus shale specimens in various bedding plane orientations with respect to loading directions to measure the static mechanical properties and their anisotropy. Furthermore, the deformability of Marcellus shale was also studied through seismic velocity measurements for comparison with the static measurements. The experimental results revealed that the transversely isotropic model is applicable for describing the elastic behaviors of Marcellus shale in pure tension and compression. The elastic properties measured from these two experiments, however, were not exactly the same. Strength results showed that differences exist between splitting (Brazilian) and direct tensile strengths, both of which varied with bedding plane orientations and loading directions and were associated with different failure modes. Finally, a series of three-point-bending tests were conducted on specimens of increasing size in three different principal notch orientations to investigate the fracture properties of the material. It was found that there exists a significant size effect on the fracture properties calculated from the measured peak loads and by using the Linear Elastic Fracture Mechanics (LEFM) theory. The fracture properties can be uniquely identified, however, by using Bazant's Size Effect Law and they were found to be anisotropic.