--> Abstract: Re-Assessment of Criteria for Discrimination of Parallel Inclined Drilling-Induced Tensile Fractures and Natural Open Fractures in Electrical Borehole Images, by Pedram Zarian, Raleigh D. Blumstein, Robert H. Maddock, and Mike Kozimko; #90039 (2005)

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Re-Assessment of Criteria for Discrimination of Parallel Inclined Drilling-Induced Tensile Fractures and Natural Open Fractures in Electrical Borehole Images

Pedram Zarian1, Raleigh D. Blumstein1, Robert H. Maddock2, and Mike Kozimko3
1 Baker Atlas Geoscience, Houston, TX
2 Baker Atlas Geoscience, Aberdeen, Aberdeenshire AB21 7EA, United Kingdom
3 Yates Petroleum Corporation, Denver, CO

Drilling induced tensile fractures identified from borehole images are commonly used to constrain the orientations of the present day stress field. However, if the induced fractures are parallel to natural open fractures and show significantly inclined fracture traces on the borehole images, the interpreter faces difficulty in discriminating between natural and induced fractures, particularly in the absence of core data or other sources of information. The importance of this discrimination is in correct determination of principal stress orientation and correct assessment of natural fracture density (i.e. number, length or area per unit) for fractured reservoir modeling. This study from a wellbore in the Washakie Basin of Wyoming re-evaluates the criteria used by others in previous studies to discriminate between parallel inclined drilling induced tensile fractures and natural open fractures on electrical borehole images. The significantly inclined trace of the drilling induced tensile fractures in the studied near-vertical wellbore implies that the principal stresses are not coaxial with the borehole. The study shows that en-echelon drilling induced fractures are easily interpreted but stress-enhanced natural fractures look very similar to induced fractures. The fracture to fracture relationships as well as the effect of a change in lithology on fracture geometry is documented. It is observed in this wellbore that the more brittle lithologies contain steeper and longer fracture trace lengths than the more ductile lithologies. This study helped in the identification of the natural open fractures as potential contributors to the permeability of this deep basin-centered gas accumulation reservoir.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005