Abstract

This work presents a pilot study of the application of a seismic inversion technique for fracture density and fracture orientation in an area where available well data revealed the presence of open fractures. The inversion method is based on a singular value decomposition of azimuthal AVO data. This decomposition allows us to calculate seismic attributes which are linked to the fracture density of the fracture network through anisotropic rock physics modeling. The outcome of this pilot study is promising since the obtained results are consistent with existing image log data; however, further testing and research on a larger area are needed to assess if the fracture characterization results are consistent with geological interpretations and other analysises of fracture networks in the reservoir.

Introduction

The characterization of fracture networks in naturally fractured reservoirs can potentially play an important role in the optimization and enhancement of hydrocarbon production. Accurate maps of fracture density can aid in identifying areas of high occurrence of fractures and sweet spots. However, the determination of fracture density is a difficult and challenging problem that often involves the integration of different types of data at different scales. At the log scale, image logs and dipole sonic logs are employed to characterize fractures locally, while at the seismic scale azimuthal AVO and shear wave splitting techniques are used in seismic fracture characterization studies.

In this work we study the application of an inversion method to obtain the distribution of fracture density and fracture orientation in a pilot study covering a small area of a tight gas sand reservoir known to be fractured at least at one well location. The inversion method is based on a singular value decomposition (SVD) of the representative azimuthal AVO responses for the target reservoir. By means of singular value decomposition it is possible to calculate seismic attributes that provide a direct mapping with fracture density. This mapping is then used in the inversion of fracture density. The singular value decomposition method was originally implemented by Causse and coauthors for conventional AVO inversion (Causse et al., 2007). Varela and coauthors extended this methodology to invert for fracture density using azimuthal AVO and applied it both to synthetic data (Varela et al., 2007) and to laboratory experiments in a geological scale model (Varela et al., 2009). It is worthwhile to mention that one of the main advantages of this inversion method is that it is based on exact solutions of the AVOAz response calculated according to a full anisotropic modelling of the fractured reservoir and does not rely on the standard AVOAz approximations (Ruger et al., 1997). The seismic modelling in this work is based on the anisotropic rock physics model of Chapman (Chapman, 2003).

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