Bharath Shekar

Estimation of interval shear‐wave attenuation from mode‐converted data

Interval attenuation measurements provide valuable information for reservoir characterization and lithology discrimination. Here, we present a methodology for estimating the interval S-wave attenuation coefficient from mode-converted (PS) data. By identifying PP and PS events with shared ray segments and applying the PP+PS=SS method, we first perform kinematic construction of pure shear (SS) events in the target layer and overburden. Then, the modified spectral-ratio method is used to compute the effective shear-wave attenuation coefficient for the target reflection. Finally, application of the dynamic version of velocity-independent layer stripping to the constructed SS reflections yields the interval S-wave attenuation coefficient in the target layer. The algorithm does not require knowledge of velocity and attenuation in the overburden, as long as it is composed of laterally homogeneous layers with a horizontal symmetry plane. The attenuation coefficient estimated for a range of source-receiver offsets can be inverted for the interval attenuation-anisotropy parameters. The method is tested on multicomponent synthetic data from layered VTI (transversely isotropic with a vertical symmetry axis) media generated with the anisotropic reflectivity method.

Anisotropic Attenuation Analysis of a Cross-hole Data Set

Measurements of attenuation anisotropy can provide valuable information for reservoir characterization and monitoring. Here, we analyze cross-hole data generated by perforation shots fired in a horizontal borehole to induce hydraulic fracturing in a tight gas reservoir. The spectral-ratio method is applied to pairs of traces to set up a system of equations for directionally-dependent effective attenuation. The anisotropic attenuation coefficient is expanded in a quadratic function of the polar and azimuthal angles of the source-receiver line. The coefficients of this polynomial are found separately for each stage of perforation shots. Although the inversion provides clear evidence of attenuation anisotropy, the narrow range of propagation directions impairs the accuracy of anisotropy analysis. The observed variations of the attenuation coefficient between different perforation stages may be related to changes in the medium due to hydraulic fracturing and stimulation.