Characterization of Fractures and Faults from High-Resolution Image Logs to Optimize the Geological Model of a Fractured Carbonate Reservoir, Offshore Abu Dhabi
Authors: Chakravorty Sandeep , Lesueur Jean-Louis , Poirier-Coutansais Xavier , Gory Jean-Yves , Srivastava Ashok , Reynolds Scott 
Published: December 2007

The oil-bearing Upper Jurassic Arab reservoirs of an offshore Abu Dhabi fractured carbonate field (Abu Al Bukhoosh) have been producing for more than thirty years. All the available informations indicate that the producing layers, subdivided into Upper and Lower Arab, are fractured to varying extents. As a result, a better understanding of the fracture networks and their relationship with major and sub-seismic faults in this field is now critical to optimize infill drilling and produce the remaining reserves.

The present paper focuses on the characterization of different types of fractures and faults identified on high-resolution image logs recorded in this field. The objective of this study is to provide a more detailed fracture network to explain the present-day reservoir behavior. A uniform classification scheme of fracture types was devised and specific properties, namely, density, orientation, apertures, and porosity of each fracture type, wherever applicable, were generated. These fracture properties were used as input to upgrade the existing reservoir model to study their influence on reservoir production. The relationship of the faults from the images with the seismic faults and the fractures were also studied.

Interpretation of high-resolution image data from 24 wells indicates the presence of an organized fracture network within the Arab reservoirs. The impact of such fracture systems on mature carbonate reservoirs can range from very restricted to conduit for fluid flow. The exact implications depend on the type of fracture filling, geometry, connectivity and density.

Total-ABK, PO Box 4058, Abu Dhabi, UAE Schlumberger Middle East S.A., PO Box 21, Abu Dhabi, UAE The fractures observed on the images have been classified into continuous and discontinuous open, continuous and discontinuous closed types. The Upper Arab reservoirs contain significantly more open than closed fractures, while Lower Arab reservoirs appear to have equal proportions of closed and open fractures. Open fractures show a dominant NE-SW strike direction for most of the wells both in the Upper and Lower Arab reservoirs. The closed fractures, on the other hand, are dispersed in their orientations. However, most of the studied wells in the Upper Arab show a major NW-SE trend, perpendicular to the direction of the open fractures. Only two areas within the field show closed fractures oriented in the same direction as the open fractures. This fact tends to confirm the interpretation of complex structural trends in these zones indicated from seismic and production data.

The faults identified on the images were subdivided into major and minor ones, depending on their scale and relationship with seismic faults present in the field. A total of 8 major and 51 minor faults were observed, with a dominant NW-SE strike direction, consistent with the trend of the major faults interpreted from the seismic data. The majority of these faults are closed in nature.