Modeling sensitivity of 3D, 9-C wide azimuth data to changes in fluid content and crack density in cracked reservoirs

Abstract

The volume density of cracks and the fluids contained in them are salient aspects of characterization of cracked reservoirs. Thus, it is of practical importance to investigate whether variations in these reservoir properties are detectable in seismic observations. Eighth-order staggered-grid, 3D finite-difference simulations generate nine-component amplitude variations with offset and azimuth (AVOAZ) for reflections from the top of a vertically cracked zone embedded in an isotropic host. The T-matrix method is used to calculate elastic stiffness tensors. Responses for various crack densities and fluid contents show sensitivity to the spatial orientation of, and variation in, anisotropy. In isotropic media, when source and receiver components have the same orientation (such as XX and YY), reflection amplitude contours align approximately perpendicular to the particle motion. Mixed components (such as XY and YX) have amplitude patterns thatare symmetrical pairs of the same, or opposite, polarity on either side of the diagonal of the 9-C response matrix. In anisotropic media, AVOAZ data show the same basic patterns and symmetries as for isotropic media but with a superimposed tendency for alignment parallel to the strike of the vertical cracks. The data contain combined effects related to the source, receiver, and crack orientations. The sensitivity of data to changes in fluid content is quantified by comparing the differences between responses to various fluid conditions, to the maximum amplitude of oil-filled crack responses. For a crack density of 0.1, amplitude differences are [Formula: see text] for oil-dry and [Formula: see text] for oil-brine. The corresponding values for S-wave reflections are [Formula: see text] for oil-dry and [Formula: see text] for oil-brine. Amplitude changes caused by changing the oil-filled crack density from 0.1 to 0.2 are [Formula: see text] for P-wave reflections and [Formula: see text] for S-wave reflections. These differences are visible in AVOAZ data and are potentially diagnostic for reservoir characterization.