Usando o modelo de Thomsen para análise da composição e estrutura interna de folhelhos

Abstract

In terms of seismic wave propagation shale may be treated as a VTI medium. This is a homogeneous transversely isotopic medium whose axis of symmetry is vertical. In other words, it is assumed that shale horizontal bedding is a result of hard and soft platelets interlamination. Hard platelets may be composed of hard minerals like quartz or some clay minerals, whereas soft ones are filled of organic matter and its fluids. For a VTI medium there are three mutually independent phase velocities (VP, VSH and VSV) and five stiffness components (c11, c12, c33, c44 and c13). Thomsen suggested replacing these stiffness components with two vertical velocities (VP0 and VS0) and three dimensionless anisotropy parameters (γ, e and δ). The γ parameter depends on Poisson´s ratio of the solid and on the ratio of the soft platelets porosity over their aspect ratio, but e and δ parameters are, additionally, functions of fluid/solid bulk modulus ratio and soft platelets porosity. Thus, knowing the anisotropy Thomsen´s parameters through wave velocities measurements in shale samples and adopting a value for soft platelets aspect ratio, fluid/solid bulk modulus ratio and soft platelets porosity can be determined. Finally, these variables may be analyzed in terms of mineral/organic composition, fluid saturation and spatial internal structure of shales.