Abstract

Naturally Fractured Reservoir (NFR) characterization represents an increased focus for oil and gas companies as it becomes more and more admitted that they represent a substantial part of their portfolio. However the complexity of the understanding of fractured reservoirs, in terms of fracturing mechanism, fracture density, orientation, and the complexity of their management issues (i.e. infill drilling, water production, steam injection, to list few of these issues) pushed several service and integrated companies to tackle the fractured reservoir characterization challenge. Moreover the use of integrated approaches with the help of 3D seismic and new technologies are started to show successful results. This paper will present two technologies where 3D seismic attributes along with geologic and engineering data are being used to characterize fractured reservoirs. The first technology will show how the use of post-stack seismic in an integrated approach, involving high resolution seismic inversion, spectral imaging and static geological modeling, provides an accurate fracture reservoir model that can be applied in the reservoir simulation and development stage. The second technology will highlight the use of pre-stack seismic to actually image the fracture distribution. Application of these technologies is presented on two different fields.

Introduction

Naturally Fractured Reservoirs (NFR) are becoming a major issue throughout the world both for old and new fields. The old fields are reaching maturity and something must be done to extend their life. The most common solution is to use secondary or tertiary recovery processes which are creating unpleasant surprises as they exaggerate the effects of fractures. Additionally, many newly discovered oil and gas fields happen to be fractured and their development constitute a real challenge in these difficult times where the E&P industry must deliver high returns to its shareholders. Fortunately, most of these companies have acquired 3D seismic data which provides a wealth of information that can assist in meeting the challenges.

The use of 3D seismic data in fractured reservoir modeling remains a major priority for most oil and gas companies operating throughout the world. The major problem facing these companies is to find the right technology or tool to fully utilize their data especially the recently acquired 3D surveys.

When facing the problem of modeling a fractured reservoir, oil and gas companies have three available approaches. The first one is the geomechanical approach where an attempt is made to reconstruct the tectonic history of the fractured reservoir. Unfortunately, all the existing tools in this approach employ overly simplistic models where the complex geology of the reservoir is ignored and homogenous and isotropic rock properties are assumed in the calculations. Furthermore, the end result of this approach is a strain map, which is typically very similar to a simple curvature map easily derived from the current structural surfaces. Another major deficiency in this approach is the assumption that present day open fractures are only related to tectonic events as if diagenesis and mineralization did not exist and had no effect on fractures after they were formed. In addition to the inability of this approach to account for the complex and heterogeneous geology of all fractured reservoirs, there is no room to incorporate any 3D seismic attribute in the geomechanical modeling process.

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