The orientation, spacing, and shape of drilling- induced disking, petal, and petal-centerline fractures in core commonly are remarkably uniform. These fractures result from concentrations of in-situ stress by the well-bore bottom-hole cavity, and in oriented core their strikes commonly have been used as indicators of the horizontal principal stress directions; however, an understanding of how these varied fractures are produced has been limited by the lack of detailed knowledge of the distribution of stresses near the bottom hole. In this paper, we present our result of studying these stress concentrations using full three-dimensional finite-element modeling for a variety of applied far-field in-situ stress conditions and as a function of core stub length. In nearly all cases, purely tensional concentrated stresses are generated within the core by the compressive in-situ stresses. The directions and magnitudes of these tensions vary with the applied stress, indicating the morphology of many of the observed drilling-induced core fractures. Cupped-shape disking fractures result from a state of uniform horizontal (biaxial) stress; these fractures also initiate within the rock at the root of the core stub. As the horizontal stresses become more anisotropic, the point of fracture initiation shifts to the surface of the core, and saddle-shaped core disks are possible. Such fractures strike in the direction of the most compressive in-situ horizontal principal stress. Increasing the magnitude of the overburden stress eventually results in petal and 

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

¹Manuscript received April 1, 1996; revised manuscript received December 2, 1996; final acceptance June 19, 1997.

²Institute of Geophysics, Meteorology, and Space Physics, Department of Physics, University of Alberta, Edmonton, Alberta, Canada; e-mail: [email protected], [email protected]

This work is supported in part by NSERC Lithoprobe Supporting Science Grant, NSERC, and the Alberta Oil Sand Technology and Research Authority. D. Schmitt acknowledges the assistance of the A. V. Humboldt foundation during revisions of this paper. Constructive reviews and comments of B. R. Kulander, J. C. Lorenz, and S. L. Dean were greatly appreciated. This paper is Lithoprobe submission number 882.