Borehole breakout in rock is induced by stress concentration on the surface of a borehole subjected to far-field stresses. For investigating the formation of borehole breakout in the laboratory, polyaxial compression tests on a block sample and simpler, "hollow-cylinder" compression tests are usually conducted. However, there is a possibility that we can conduct an even simpler borehole breakout experiment-as simple as a uniaxial compression test. This method compresses a small, cylindrical rock core with a narrowed section in the middle, which gives the test specimen an appearance of an hourglass or a dog bone. In this paper, we examine how the geometry of these "shaped-core" samples and their material properties affect the stress state in the sample via numerical modeling, and present some examples from laboratory experiments. Advantages and disadvantages of the method are also discussed.
Borehole breakout in rock is induced by stress concentration on the surface of a borehole subjected to far-field stress. If this far-field stress is anisotropic, the highest stress concentration occurs on the surface in the minimum principal stress direction within the plane perpendicular to the borehole axis. To examine the effect of anisotropic stress on the formation of borehole breakout in the laboratory, a cube-shaped rock sample containing an analogue borehole can be subjected to polyaxial stresses . An alternative, simpler test that can be conducted more routinely is the "hollow-cylinder compression test," which is a variant of the axially symmetrical triaxial compression test, using a rock core containing a circular hole at its axis  (Note that the stress for this test is axi-symmetric around the borehole.) There is a possibility that we can conduct an even simpler experiment to examine borehole breakout-as simple as conducting a uniaxial compression test. This method employs a small, cylindrical rock core with a narrowed section in the middle, which gives the test specimen an appearance of an hourglass or a dog bone (Figure 1). In this paper, we call these test specimens "shaped-core" samples. Shaped-core samples are used routinely for testing the tensile strength of metal and polymer samples, to induce localized tensile failure along a plane known a priori. For rock testing, shaped core samples have been used to study extensile failure involving tensile stress , and its transition to shear failure . When a shaped core is subjected to simple axial compression, both the reduced core cross section and the curved geometry of the core wall result in a near-core-wall stress distribution similar to an anisotropically loaded borehole. Therefore, a shapedcore uniaxial compression test can be used to examine borehole breakout in rocks.
Fig.1. A shaped-core test sample (synthetic sandstone [glasscemented sand]) (available in full paper)
This method, however, needs to be used with caution, because the stress distribution within a shaped core depends on both core geometry and material properties. For this reason, in this paper, we will first examine the stress distribution within a shaped core using a linear-elastic finite element code.