Feasibility study of fracture interpretation using multicomponent seismic data: SEAM Phase II Barrett model

Abstract

Several P-wave azimuthal anisotropy studies have been conducted for the SEG Advanced Modeling Program Phase II Barrett model data. However, these analyses provide fracture property estimation that is inconsistent with the actual model properties. Therefore, we perform a feasibility study to understand the influence of the overburden and reservoir properties, and processing and inversion steps, which together determine the success of the fracture interpretation from seismic data. The 1D model properties (orthorhombic for overburden and reservoir) are first extracted from the actual Barrett model properties at two locations. Anisotropic prestack reflectivity modeling exposes the true orthorhombic response of the 1D medium in the form of common-offset and common-azimuth gathers. The true anisotropic response is obscured in the Barrett data (generated by finite-element modeling) due to the mild lateral velocity variations and orthorhombic anisotropy in the overburden. We then expose the reservoir anisotropic response using an isotropic overburden in the reflectivity modeling. This indicates that the P-wave velocity variation with azimuth (VVAZ) responses generated by the reservoir itself are weak, which leads to an unstable VVAZ inversion to estimate the interval normal moveout (NMO) velocity anisotropy. The reservoir thickness (125 m or 65 ms of two-way traveltime) or NMO velocity anisotropy (6%–7%) needs to be at least doubled to obtain a stable VVAZ inversion. Anisotropic geometric-spreading correction improves the amplitude variation with azimuth inversion results when reflectivity modeling models the orthorhombic overburden. The converted wave (C-wave) has a stronger VVAZ response compared to that of the P-wave. We suggest that the C-wave data could be useful to constrain fracture interpretation in the Barrett model. We conclude that the results of previous studies are due to the combination of the residual influence of overburden after processing and imaging and the weak anisotropy responses from the reservoir.