Seismic Forward Modeling of Fractures and Fractured Media Inversion

Xiaoqin Cui

ABSTRACT

The goal of this thesis is to develop a methodology for enhancing fracture detection and correctly delineating reservoirs with fractures. The thesis deeply explores the mechanical formation of fractures and fractured media, presents an enhanced fracture detection technique that uses a new finite-difference scheme to accurately model fractures and analyze the fracture response in seismic traveltime and amplitude, and develops a method for accurate reservoir delineation by deriving new AVO fracture equations to correctly estimate the properties of the fractured medium, the host medium and fractured medium with impedance contrast.

With the long wavelength assumption, a linear slip interface is equivalent to a fracture interface that satisfies the nonwelded contact boundary conditions. Therefore, the fractured medium can be regarded as a combination of a fracture, or a set of fractures, and a host medium: a horizontally fractured medium is effectively composed of a horizontal fracture embedded into a homogeneous isotropic host medium; and a vertically fractured medium is effectively formed by inserting a vertical fracture into a homogeneous isotropic host medium; an orthogonally fractured medium is effectively assembled from a vertical fracture and a homogeneous VTI host medium, or a horizontal fracture and a homogeneous HTI host medium, or two orthorhombic fractures and a homogeneous isotropic host medium.

New finite-difference schemes for horizontal, vertical and orthorhombic fractures are implemented to generate seismograms that precisely illustrate the fracture representations in seismic data. The results indicate that the fractures are detectable, even though the fractured medium does not have impedance contrasts, and that the fractured medium can be characterized as a transversely isotropic medium. Through an analysis of how fractures are represented in seismic data can help in fracture detection in geoscience.

New exact equations for the reflection and transmission coefficients of a fractured medium with impedance contrast are derived that take into account the azimuthal parameter and the nonwelded contact boundary conditions. New approximate AVO equations that include fracture parameters are derived. Therefore, the fracture, the host medium and the fractured medium with impedance contrast properties can be estimated from seismic data to correctly delineate the reservoir characterization.

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