Estimation of fracture weaknesses and integrated attenuation factors from azimuthal seismic data

Huaizhen Chen, Kristopher Innanen

ABSTRACT

Seismic wave propagation in fractured reservoirs exhibits anisotropy and attenuation that are related to fracture properties (e.g. fracture density) and fluid parameters (e.g., fluid moduli and viscosity). Based on the linear slip theory, we derive stiffness parameters for fractured and attenuative rocks, and present the integrated attenuation factors involving both host rock intrinsic attenuation and fracture-induced attenuation. Using the simplified stiffness parameters, we derive a linearized reflection coefficient in terms of fracture weaknesses and integrated attenuation factors. A two-step inversion approach is proposed, which involves an iterative damped least-squares algorithm to predict P- and S-wave moduli using angle gathers at the azimuthal angle approximately equal to fracture orientation azimuth, and a Bayesian inversion method to estimate fracture weaknesses and integrated attenuation factors from seismic amplitude differences among the data at different azimuthal angles. Tests on synthetic data confirm the proposed approach makes a stable inversion for fracture weaknesses and integrated attenuation factors in the presence of moderate data noise. The proposed approach is further confirmed on a fractured carbonate real data set, within which we observe that reasonable parameters (P- and S-wave moduli, fracture weaknesses and integrated attenuation factors) are determined. We conclude that the proposed inversion approach can provide reliable parameters for prediction of natural fractures and discrimi-nation of fluid type.

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