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Abstract: 3D Characterization of Fracture Networks in Chalk Using Electrical Borehole Images, Danish Central Graben, North Sea

Chris Dart, Soren Priisholm

Electrical borehole image logs have been used to gain a continuous and detailed record of the 3D fracture network in the chalk fields of the Danish Central Graben. Data were collected from deviated and long horizontal wells. Results of the study were used to predict drainage patterns and have been incorporated into reservoir models.

Open fractures in chalk appear as conductive anomalies on electrical borehole images. Dip, strike and relative aperture were determined for each fracture plane. The circumferential continuity, and therefore orientation confidence, was also recorded and two, three and four pad classes were defined. Stereographic analysis allowed the identification and characterization of fracture sets and illustrated superposition of 'two pad' and 'three/four pad' clusters. Data from wells in a particular field were pooled to define fracture network domains. Variations of fracture density along each borehole trace were used to identify fracture swarms and calculate effective permeability.

Production data verified that areas of high fracture intensity enhanced permeability and created high production potential. Furthermore, well interference and well pressure test data demonstrated the lateral extent of fracture swarms, their influence on reservoir drainage, and also confirmed their orientations.

In addition to open fractures, bedding planes and faults were identified and measured. Potentially sealing, cemented fault planes appear as resistive anomalies. Borehole image data were combined with biostratigraphic zonations to construct geological reservoir maps and cross sections which illustrate the structural regime, and structural geometry below the current resolution of 3D seismic data.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France