Formation Anisotropy Parameters Using Borehole Sonic Data

Abstract

ABSTRACT A new algorithm transforms the compressional, fast-shear, slow-shear, and Stoneley slownesses measured with respect to the borehole axes to anisotropic moduli referred to the earth anisotropy axes. The algorithm yields up to four anisotropic moduli of an orthorhombic or transversely isotropic (TI) formation using sonic data with known deviation from the vertical and true stratigraphic dip from an imaging tool in a single well. The algorithm is compensated for any tool effects on the borehole Stoneley and flexural modes. The output from this algorithm yields verifiable far-field compressional and shear moduli outside any near-wellbore altered annulus caused by stress concentrations or plastic yielding of the rock. Transversely isotropic anisotropy, also referred to as polar anisotropy, can be quantified by estimating the three Thomsen parameters e, γ, and δ. Borehole sonic data yields the γ parameter in terms of the vertical and horizontal shear moduli. The vertical shear moduli are routinely obtained from cross-dipole sonic logging. Equivalent shear rigidity in the borehole cross-sectional plane can be estimated from the Stoneley data in an arbitrarily anisotropic formation and in the presence of near-wellbore alteration that affects the Stoneley data at all frequencies. Generally speaking, a positive γ is an indicator of shale anisotropy, whereas a negative γ denotes a permeable formation. These anisotropic moduli can also be combined with the vertical seismic profile (VSP)-derived moduli to obtain seismic velocities as a function of propagation direction. Results from the field data analysis will be presented.