A coupled anisotropic fluid indicator for seismic characterization of tight gas-bearing fractured reservoirs

Abstract

Discrimination of fluid properties in tight fractured reservoirs plays a significant role in seismic reservoir characterization for a fracture-induced horizontal transversely isotropic (HTI) medium. From a perspective of low-frequency anisotropic Gassmann's theory, we propose a new anisotropic fluid indicator coupling an isotropic fluid/porosity term and a dry term of normal fracture weakness by the use of linear slip theory and weak-anisotropy (WA) analytic assumption. The accuracy of WA approximation with and without the coupled anisotropic fluid indicator are demonstrated in application to the tight gas-bearing fractured reservoirs with relatively low values of fracture properties. Integrating the perturbations in WA elastic stiffness components of fluid-saturated fractured porous media and the scattering function, we then derive a linearized and weakly anisotropic PP-wave reflection coefficient in terms of isotropic fluid/porosity term, shear modulus, density, coupled anisotropic fluid indicator, and fracture weaknesses. Based on the sensitivity analysis of coupled anisotropic fluid indicator and two fracture weaknesses for the difference of PP-wave reflection amplitudes in two orthogonal azimuths, we find that the coupled anisotropic fluid indicator is less sensitive to the azimuthal amplitude differences than the fracture weaknesses. Combining the Bayesian framework with two regularization constraint terms of Cauchy sparse and initial model, we finally propose an iterative singular-value- decomposition (SVD)-based inversion method of seismic amplitude difference between two orthogonal azimuths to estimate the coupled anisotropic fluid indicator and two fracture weaknesses. Tests with noisy synthetic seismic gathers demonstrate that the estimated model parameters can be reasonably inverted from two azimuthal seismic data with three angles of incidence containing a moderate level of noises. Field data acquired over Sichuan basin in China is used to further confirm that the proposed inversion approach can get geologically reasonable estimates of coupled anisotropic fluid indicator and fracture weaknesses. Consequently, the coupled anisotropic fluid indicator is more sensitive to the gas-bearing fractured reservoirs than the conventional fracture weaknesses, which may provide a more effective manner for the fluid identification of tight gas-bearing fractured reservoirs.