Abstract

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 petal-centerline fractures. Centerline fracturing can be produced by high overburden stress with a short core stub. The length of the core stub has substantial influence on the magnitudes of the concentrated stresses. The greatest tensile stress initially increases rapidly with core stub length, but reaches a plateau for lengths greater than approximately 40% of core diameter, placing maximum bounds on the spacing between successive fractures along a core. Although more work i required to accurately predict the shape of drilling-induced fractures, the present results indicate that the morphology of the fracture alone contains substantial information on the in-situ stress state existing in the rock mass prior to the drilling of the well bore.

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