Seismic Characterization of Naturally Fractured Reservoirs with Monoclinic Symmetry Induced by Horizontal and Tilted Fractures from Amplitude Variation with Offset and Azimuth

Geological and geophysical data acquired from naturally fractured hydrocarbon reservoirs usually exhibit the presence of multiple sets of fractures. Two different nonorthorhombic sets of fractures embedded in isotropic background rocks can reveal the anisotropic characteristics of monoclinic symmetry with a horizontal symmetry plane. Here, we focus on the modeling and inversion of the effective elastic properties in fractured reservoirs with monoclinic symmetry induced by a set of horizontal fractures and a set of tilted fractures with rotationally invariant properties embedded in purely isotropic host rock. To facilitate the seismic characterization of fractured reservoirs with monoclinic symmetry, we introduce two sets of fracture weakness parameters to replace the Thomsen-type anisotropic parameters and describe these two nonorthogonal sets of rotationally invariant fractures. With the incident and reflected P wave in such a monoclinic anisotropic medium, we perform the seismic characterization of two nonorthogonal sets of rotationally invariant fractures based on the reflected amplitudes variation with offset and azimuth (AVOAz). Using the linear-slip theory, we first derive the effective elastic stiffness tensor as a function of background isotropic elastic moduli, normal and shear weakness parameters of horizontal and tilted fractures, and the dip angle. We then derive the first-order perturbations in elastic stiffness tensor and formulate the linearized PP-wave reflection coefficient based on the seismic scattering theory. Next, we propose a robust AVOAz inversion approach to characterize these two nonorthogonal fractures with rotationally invariant fractures. Finally, synthetic and real data examples are used to demonstrate that the background elastic moduli and fracture properties can be reasonably estimated using the azimuthal seismic reflected amplitudes. The new formulation and inversion approach may provide us a useful tool to characterize the naturally fractured reservoirs with monoclinic symmetry induced by horizontal and tilted fractures in a more reasonable manner than previous method.

Automated summary

This study focuses on the modeling and inversion of effective elastic properties in fractured reservoirs with monoclinic symmetry. By introducing two sets of fracture weakness parameters to describe nonorthogonal sets of rotationally invariant fractures, seismic characterization is performed using the reflected amplitudes variation with offset and azimuth. A robust AVOAz inversion approach is proposed to characterize these fractures.