Azimuthal elastic impedance inversion for fluid term and fracture weakness

Huaizhen Chen, Kristopher A. H. Innanen, Yuxin Ji, Xiucheng Wei

Fracture weaknesses and fluid factor are important parameters to identify the location of underground fractures and the type of fluids. An indirect method to predict fractures and discriminate the fluid is first using azimuthal seismic data to estimate the normal and tangential fracture weaknesses and then calculating the fracture fluid factor. The indirect method may create some uncertainties, and the estimated fracture weaknesses are affected by both the fluid and fractures. We demonstrate a direct method to estimate Lam constants and fracture weaknesses of the dry fractured rock, and fluid term from partially incident angle-stack seismic data, based on azimuthal elastic impedance (EI) parameterization and inversion. Combining stiffness parameter perturbations and scattering function, we first derive a linearized PP- wave reflection coefficient for the case of an interface separating two horizontal transverse isotropic (HTI) media, which can isolate the effects of fractures and fluids. Using the derived reflection coefficient, we propose the expression of azimuthal EI. The estimation of fluid term and fracture weaknesses is implemented as a two-step inversion, which includes inversion of partially-incident-stack seismic data for EI at different azimuths, and the estimation of fluid term and fracture weaknesses from the inverted results of azimuthal EI using a Bayesian Markov-chain Monte Carlo (MCMC) method. Tests on synthetic and real data can confirm the stability of the proposed inversion method, and the inversion method appears to be useful for fracture detection and fluid discrimination.