Experimental Study of Anisotropic Strength Properties of Shale

Abstract

ABSTRACT Led by new developments of horizontal drilling and hydraulic fracturing techniques, organic rich shales have been contributing significantly to a gas production in the US. In drilling processes, shales have been known to be the source of wellbore instability problems. 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 of vital importance to investigate the anisotropic strength of shale both theoretically and experimentally. The goal of this study is to evaluate anisotropic mechanical properties of shale by tri-axial tests and predict shale anisotropic 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 for different orientations. Simple Plane of Weakness and Modified Cam Clay failure criteria were applied to describe shear failure mechanism. Well logging data were used to connect experimental data and real field data. Compressional wave velocity was predicted with different inclination angle by stiffness parameters. The predicted compressional wave velocity for a 45-degree inclination angle showed a perfect fit with the field logging data. Mud window for a special shale formation was predicted by experimental data. This study provides an understanding for shale anisotropic strength of different bedding orientations, as well as an instruction of safety mud window calculation for directional drilling through shale formations. INTRODUCTION Traditionally, shales have been known to be the source of wellbore instability problems during drilling. The borehole may experience hole-enlargement, hole-reduction, lost circulation, poor hole cleaning, and well-control problems. The cause of shale instability is two-fold: physical (stress change vs. shale strength environment) and chemical (shale/fluid interaction— capillary pressure, osmotic pressure, borehole-fluid invasion into shale). Both physical and chemical process will induce changes in shale geo-mechanical properties, such as compressional strength. Solving wellbore instability problems relies on accurate evaluation of geo-mechanical properties of a shale formation.