Combined Experimental and Well Log Study of Anisotropic Strength of Shale

Abstract

Abstract Organic-rich shales have been contributing significantly to gas production in the US, led by new developments in horizontal drilling and hydraulic fracturing techniques. Shales have been known to be the source of wellbore instability problems in drilling processes. In the past practice, shales were treated as isotropic for mud window calculation and trajectory design. However, organic-rich shales are anisotropic due to their laminated structure and chemical properties. Thus, it is vital to theoretically and experimentally investigate the anisotropic strength of shale. This study aims to evaluate the anisotropic mechanical properties of shale by tri-axial tests and predict anisotropic shale properties by well-logging data interpretation. Shale mechanical properties (Young's Modulus, Shear Modulus, and Poisson's Ratio) of different bedding plane orientations (0/ 45 / 90 degrees) were studied. Both compressive strength and tensile strength were investigated in different directions. A simple Plane of Weakness and Modified Cam Clay failure criteria were applied to describe the shear failure mechanism. Well logging data were used to connect experimental data and actual field data. Compressional wave velocity was predicted with different inclination angles by stiffness parameters. The predicted compressional wave velocity for a 45-degree inclination angle perfectly fits the field logging data. Experimental data indicated the mud window for a particular shale formation. This study provides an understanding of shale anisotropic strength of different bedding orientations and instructions on safety mud window calculation for directional drilling through shale formations.