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: yongyi@phys.ualberta.ca,
doug@phys.ualberta.ca
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.
