Current analytical methods to assess rock strength for borehole stability, stimulated volume calculations and fracture and faulting almost exclusively involve the use of the Mohr-Coulomb failure criteria, which does not account for the effect of the intermediate principal stress. Conventional triaxial testing does not permit rock testing under anisotropic horizontal stresses. However, in-situ horizontal stresses are rarely isotropic, and a few studies on granitic and carbonate rocks have shown that the intermediate principal stress potentially has a very significant effect on rock strength and depends on rock type. In the case of shales and mudstones however, very little data exists on this topic due to experimental challenges involved in sample preparation and testing rock specimens under a true-triaxial state of stress. In this work, we present a series of true-triaxial tests on several shale specimens, and explore the effect of the intermediate principal stress on the failure properties of shale. Results indicate that the intermediate principal stress has a measurable contribution to rock strength, and impacts rock brittleness during the fracturing process. Failure laws of the Mogi-Coulomb form are used to predict rock strength under true-triaxial conditions, and methods to obtain true-triaxial strength parameters from conventional triaxial test data are discussed. Rock brittleness is studied as a function of minimum and intermediate principal stresses, and results indicate that increasing the intermediate principal stress may have an embrittling effect on rock.

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