WELL-BORE BOTTOM STRESS-CONCENTRATION AND INDUCED CORE FRACTURES

The orientation, spacing, and shape of drilling-induced disking, petal , and petal-centerline fractures in core commonly are remarkably unifo rm. These fractures result from concentrations of insitu stress by the well-bare bottom-hole cavity, and in oriented core their strikes comm only have been used as indicators of the horizontal principal stress d irections; however, an understanding of how these varied fractures are produced has been limited by the lack of detailed knowledge of the di stribution of stresses near the bottom hole. In this paper, we present our result of studying these stress concentrations using full three-d imensional finite-element modeling for a variety of applied far-field in-situ stress conditions and as a function of core stub length. In ne arly all cases, purely tensional concentrated stresses are generated w ithin 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 fracture s. Cupped-shape disking fractures result from a state of uniform horiz ontal (biaxial) stress; these fractures also initiate within the rock at the root of the core stub. As the horizontal stresses become more a nisotropic, the point of fracture initiation shifts to the surface of the core, and saddle-shaped core disks are possible. Such fractures st rike in the direction of the most compressive in-situ horizontal princ ipal stress. Increasing the magnitude of the overburden stress eventua lly results in petal and petal-centerline fractures. Centerline fractu ring can be produced by high overburden stress with a short core stub. The length of the core stub has substantial influence on the magnitud es of the concentrated stresses. The greatest tensile stress initially increases rapidly with core stub length, but reaches a plateau for le ngths greater than approximately 40% of core diameter, placing maximum bounds on the spacing between successive fractures along a core. Alth ough more work is required to accurately predict the shape of drilling -induced fractures, the present results indicate that the morphology o f the fracture alone contains substantial information on the in-situ s tress state existing in the rock mass prior to the drilling of the wel l bore.