Abstract
Rocks are generally anisotropic in nature—only the degree and type varies. The chapter explains the reservoir anisotropy concept and discusses various approaches for its parametric characterization. Special cases of weak anisotropy, strong anisotropy, and finely layered media are considered for anisotropy determination through Thomsen's definition. It also reviews the fluid effects on anisotropy parameters. The chapter discusses change in anisotropy parameters due to stress-induced anisotropy in triaxial and uniaxial stress cases. Chapter also discusses Shale anisotropy and explains the vertical transverse isotropy (VTI) model in clays, anisotropy in dry clay, and effects of fluid on shale anisotropy. The chapter details the anisotropy considerations in the borehole environment. It discusses the concepts and techniques of azimuthal and vertical anisotropy determination through various acoustic techniques of cross-dipole analysis, Stoneley waves, and dispersion crossovers. It also explains the characterization of intrinsic and stress-induced anisotropy and reviews the special case of fractured media. The chapter discusses the practical case of sonic anisotropy in an inclined borehole and provides necessary velocity corrections for sonic measurements in the deviated borehole.
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