Effects of VTI Anisotropy on Shale Reservoir Characterization

Abstract

AbstractPresently shale reservoirs are the one of the hottest plays in the oil industry. Our understanding of these reservoirs rapidly progressed from one of a continuous type, to that of a spatially varying type. Two key elastic parameters, Young’s modulus and Poisson’s ratio are critically relied upon, to seismically high-grade these spatially varying reservoirs in terms of their reservoir and completion qualities. Isotropic elastic properties are assumed in the delineation of ‘frac’able’ zones and sweet spots. However, these shale formations are highly anisotropic even in the absence of any in-situ fractures and there are multiple Young’s moduli and Poisson’s ratios in an anisotropic medium.In this paper we discuss the effects of vertical transverse isotropy (VTI) in characterizing shale reservoirs in terms of Young’s moduli and Poisson’s ratios. We begin by rewriting Young’s modulus and Poisson’s ratio formulae in terms of Thomsen’s anisotropy parameters (Thomsen,1986). Both approximate and exact expressions are given. Approximate relations in terms of anisotropy parameters are easier to interpret than those in terms of the stiffness coefficients. We then discuss the results in terms of relevant ranges of values of these anisotropy parameters. Also, an important contribution to fracture initiation and containment comes from the uniaxial stress ratio (Higgins et al., 2008;Sayers, 2010, Iverson, 1995). It is the ratio of the horizontal to the vertical stresses in the absence of any transverse strain. The effects of anisotropy on the uniaxial stress ratio are discussed in this context. Finally we discuss how P-wave surface seismic data can be integrated with borehole and other measurements to estimate relevant elastic attributes in these highly anisotropic shale formations.