Anisotropic elastic, strength, and fracture properties of Marcellus shale

Abstract Shales are considered to be both source and cap rocks, and play an important role in various geotechnical applications including oil and gas exploration and production. A deep understanding of the mechanical properties of shale is essential. In this work, deformability, strength, and fracturability of Marcellus shale were investigated through experimental studies, and a database of its mechanical properties was generated. Uniaxial compression, direct tension, and Brazilian tests were performed on Marcellus shale specimens in various bedding plane orientations with respect to loading directions to study the static mechanical properties of the material and their anisotropy. The experimental results revealed that the transversely isotropic model is applicable for describing the elastic behavior of Marcellus shale in pure tension and compression. However, the elastic properties measured from these two experiments were not exactly the same. In addition, differences exist between Brazilian and direct tensile strengths, both of which varied with bedding plane orientations and loading directions, and were associated with different failure modes. The deformability of Marcellus shale was also studied through seismic velocity measurements, as a means for comparison with the static measurements. 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 using Linear Elastic Fracture Mechanics (LEFM). The fracture properties of the material calculated by using Bažant's Size Effect Law (SEL) were independent of the testing method and were found to be anisotropic.