Abstract
A carbonate sample extracted from the depth of about 10 kft was subjected to uniaxial loading while the confining stress remained constant. Post-experiment inspection of the sample showed an inclined crack at an angle less than 20° to the horizontal. This subhorizontal crack orientation was contrary to the expected 45° inclination, the plane of the maximum shear stress. Coincidentally, as shown by CT-scan prior to loading, there was a boundary between two layers of different density inside the sample located almost exactly where the crack appeared. This density difference has arguably translated into the contrast in the elastic properties at the boundary. The hypothesis is that because of this elastic heterogeneity, an incipient crack developed at the boundary due to the unavoidable tensile stressing of the sample as it was brought to the benchtop from its original state of high confining stress at depth. Controlled uniaxial compression made the sample slip along this crack, which then developed into a prominent feature. This assumption was corroborated by a numerical experiment showing a strong von Mises stress concentration at the elastic contrast boundary during hydrostatic tensile loading. Another sample, from the same formation, but without strong density heterogeneity, exhibited a classic 45° crack after uniaxial loading. These results provide a novel and important insight into the mechanics, breakage, and strength of natural rock.
Highlights
A carbonate sample extracted from the depth of about 10 kft was subjected to uniaxial loading while the confining stress remained constant
For an isotropic and homogeneous rock, classic strength of materials theory gives the following expression for the shear stress τ along the plane inclined at angle θ to the horizontal under uniaxial loading where the principal vertical stress σ3 exceeds the two equal horizontal stresses σ1 = σ2: τ = σ3 − σ1 sin 2θ, 2 (1)
An incipient crack appeared at the boundary inside the sample and developed into a major fracture during the uniaxial loading
Summary
The hypothesis is that an incipient subtle crack developed at the boundary as the sample was brought from its natural depth of about 10 kft to the ambient conditions at the benchtop. This is where the initial crack arguably developed as the sample was lifted from its original depth. The hypothesis put forward here is further supported by the testing and respective pre- and post CT-scan images of the third carbonate sample As in the first two samples, this intensity heterogeneity translated into density and elastic moduli heterogeneity, resulting in von Mises stress concentrations during tensile unloading of the core as it was evacuated from its original depth
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