A study of the anisotropic static and dynamic elastic properties of transversely isotropic rocks

Abstract

The elastic properties of rock are major factors affecting hydraulic fracturing. Static elastic properties can be estimated using geomechanical laboratory tests, whereas dynamic properties can be estimated from elastic-wave velocity and rock density. We prepared two synthetic shales containing different clay minerals and one natural shale and focused on the elastic properties for the full tensor of elasticity and their anisotropy. The static and dynamic properties of these dry samples were obtained based on triaxial tests during loading and unloading. The results suggest that the synthetic and natural shale indicate high similarity in the static and dynamic properties. The dynamic Young’s modulus and Poisson’s ratio increase with increasing axial stress during loading and unloading. For the static properties, the static Poisson’s ratio increases with axial stress during loading and unloading. However, differences exist between the static and dynamic Young’s moduli during loading, with the static Young’s modulus decreases with the increasing axial stress at a high stress level. In addition, the static Young’s modulus is consistently lower than the dynamic Young’s modulus during loading and unloading, but the static Poisson’s ratio is larger or smaller than the dynamic Poisson’s ratio. During loading and unloading, there could be approximately a 30% difference when estimating static elastic properties from the static-dynamic relations, depending on which static moduli are used. Furthermore, the static and dynamic properties of the samples are strongly anisotropic, and the anisotropy of elastic properties is sensitive to the axial stress and the clay minerals.