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The AAPG/Datapages Combined Publications Database

South Texas Geological Society Special Publications


Austin Chalk Exploration Symposium: Geology, Geophysics and Formation Evaluation, 1991
Pages 51-63

Fracture Description in the Austin Chalk Using Cast Images

Joseph F. Goetz, Douglas D. Seiler, Charles S. Edmiston


Reservoir description in the Austin Chalk by means of well logs reduces mainly to a detailed representation of the layering and natural fractures. Productivity is primarily related to sub-vertical fractures related to hinge lines. In both vertical and horizontal boreholes, open fractures appear as sharp anomalies on images produced by the Circumferential Acoustic Scanning Tool (CAST)

CAST images in vertical holes illustrate fractures in the Austin Chalk as large amplitude sine waves. The amplitude is a measure of the fracture dip angle. The phase of the sine wave indicates the fracture strike which is used to plan the directions of horizontal drainage holes. In horizontal wells, fractures appear as sine waves of varying amplitude reflecting the incidence angle of the hole vis-a-vis the fractures. Fracture dip and orientation can be computed either automatically or interactively on workstations.

Although Austin Chalk fractures tend to maintain a general directional trend, locally the strike can vary by up to 90°. The drainage efficiency of horizontal well depends critically on its orientation relative to the fractures. CAST images also monitor the exact elevation of a horizontal well with respect to the bedding.

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