Experimental Study of Shear Failure in Anisotropic Rocks

Donath, Fred A.

doi:10.1130/0016-7606(1961)72[985:esosfi]2.0.co;2

Cited by 334 publications

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“…These interpretations were consistent well with the sketches of failure morphology in Table 2. Similar phenomenon is also observed in tests carried out with Martinsburg slate by Donath [11]. However, there is a little difference between the two kinds of rocks in triaxial compression condition.…”

Section: Discussionsupporting

confidence: 63%

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Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

et al. 2016

Energies

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Abstract:With the exploitation of shale gas booming all over the world, more and more studies are focused on the core technology, hydraulic fracturing, to improve commercial exploitation. Shale gas resources in China are enormous. In this research, a series of tests were carried out with samples of black organic-rich shale from the Lower Silurian Longmaxi formation, south China. Samples were drilled from different directions and were subjected to uniaxial and triaxial condition with various confining pressures, aiming at studying its rock mechanics properties, so as to provide basis for research and breakthrough of hydraulic fracturing technology. According to the results of the study, the development and distribution of shale's bedding planes significantly impact its mechanical properties. Shale samples show obvious brittle characteristics under low confining pressure, and its mechanical behavior begins to transform from brittle to plastic characteristics with increasing confining pressure. Shale samples with different inclinations (β) have different sensitivities to the confining pressure. As a result, samples with 45 • inclinations (β) are least sensitive. The strength of bedding planes is significantly lower than that of shale matrix, and tensile failure and shear failure generally tend to occur along the bedding planes. When hydraulic fracturing was conducted in shale formation with depth less than 2.25 km, corresponding to original in-situ of 60 MPa, cracks will preferably occur at first along the inclination (β) angle of 45 • from the maximum principal stress, and the failure mode is most likely to be shear failure without volumetric strain. And, different modes of failure will occur at different locations in the reservoir, depending on the orientation of bedding inclined from the principle stress, which can probably explain the phenomenon why there are fractures along and cross the bedding planes during hydraulic fracturing treatment. When hydraulic fracturing was conducted in shale formation with depth greater than 2.25 km, hydraulic fractures may not crack along the bedding surfaces to some extent.

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Section: Discussionsupporting

confidence: 63%

“…Furthermore, the coefficient of anisotropy of strength of Longmaxi shale under confining pressure of 40 MPa, f is 0.45. The coefficient of anisotropy of strength of Martinsburg slate under confining pressure of 35 MPa is 0.72 [11]. So, the anisotropy of the Longmaxi shale is not as strong as that of the Martinsburg slate, obviously.…”

Section: Discussionmentioning

confidence: 85%

Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

et al. 2016

Energies

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“…4). Because the failure envelope for preexisting faults is linear and essentially passes through the origin, it implies that preexisting faults have no cohesion and that only sliding on preexisting faults should occur, provided the faults are favorably oriented (~15°-60° to σ,; Donath, 1961;Twiss and Moores, 1992). Extension fractures can still form, however, if the preexisting fractures are unfavorably oriented or if there is some cohesion across the fault due to surface roughness or cementation by hydrothermal minerals.…”

Section: Mechanics Of Hydrofracturing In Ocean Crustmentioning

confidence: 99%

Structural Evolution of Upper Layer 2, Hole 896A

Harper¹,

Tartarotti²

1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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An analysis of the state of stress in the upper ocean crust shows that high hydrostatic pore-fluid pressures will result from the thick overlying water column, providing that the upper crust is sufficiently permeable. High pore-fluid pore pressures will result in very low effective stresses in the upper ocean crust. Such conditions favor deformation by formation of extension fractures and mixed mode (combined extension and shear) fractures rather than by frictional sliding on faults. High pore-fluid pressures are considered very likely to have produced many of the structures in Hole 896A, including jigsaw-puzzle breccias and associated vein networks, cement-supported breccias, and fibrous veins. These structures record an overall volume increase, and their geometry and offset markers indicate formation as extensional fractures or, in some cases, mixed mode fracturing. The geometry and textures of jigsaw-puzzle breccias and associated vein networks suggest formation by longitudinal (axial) splitting, a type of brittle rock failure that occurs in rock deformation experiments done under stress conditions similar to those in the upper ocean crust (σ 2 ' and σ 3 ' < 0). Breccias can form by disaggregation during volume increase associated with extension or mixed mode fractures. The textures of many breccias formed in this way are very similar to those of sedimentary breccias and thus the sedimentary vs. tectonic origin of many breccias in the drill core is uncertain, especially because the margins of the breccias are not usually preserved in core. The fibrous veins generally postdate jigsaw-puzzle breccias and vein networks. They probably formed by repeated cycles of hydrofracturing and sealing of fractures (crack-seal mechanism). The extension directions indicated by both fibrous and nonfibrous veins are highly variable in orientation, even within a single piece, and vein networks suggest simultaneous extension in several directions. In addition, small faults having fibrous slickensides occurring below 334 m below seafloor indicate normal, reverse, and oblique slip. The style of brittle deformation observed in Hole 896A should produce microseismicity characterized by a nonuniform stress field and non-double-couple earthquakes. Such microseismicity characterized geothermal activity in the post-rifting stage at Krafla, Iceland.

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“…Conjugate shears can also result from the influence of a strong anisotropy in the failing medium (Jaroszewski 1974). The influence of a planar anisotropy (foliation) on the shear surfaces formed in static strength tests have been reported by, among others, Jaeger (1960), Donath (1961), Paterson (1958Paterson ( , 1978. These authors demonstrated that, if the mechanical anisotropy in the sample is strong, one of the two potential sets of shear surface develops along the weakest plane; meanwhile, the complementary shear plane either does not develop at all, or only develops poorly at an increased but still acute angle to its conjugate partner.…”

Section: Discussionmentioning

confidence: 99%

Ultrasonic constraint of the microfracture anisotropy of flysch rocks from the Podhale Synclinorium (Poland)

Kłopotowska

2018

Int J Earth Sci (Geol Rundsch)

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This paper attempts to show the relationship between joints observed in flysch formations in the field and microfracture fabrics invisible to the naked eye in hand specimens. Ultrasonic measurements demonstrate that the intensity and orientations of domains "memorised" by rock specimens are associated with the historical stresses within the rock mass rather than the rock lamination. The spatial orientations of these microfractures have been measured, and their dynamic-elastic properties have been found to correlate with the orientation of macroscopic joint sets measured in the field. The elastic properties measured vary because of sedimentary diagenetic processes that occured during the tectonic deformations of these flysch rocks in the Podhale Synclinorium of Poland. The structural discontinuities detected by ultrasonic measurements can be perceived as an incipient phase of the macroscopic joints already visible in the field and are attributed to the in situ residual tectonic stresses. Such historical stresses impart a hidden mechanical anisotropy to the entire flysch sequence. The microfractures will develop into macroscopic joints during future relaxation of the exposed rock mass. Understanding the nature and orientation of the invisible microfracture anisotropy that will become macroscopic in the future is vital for the safe and efficient engineering of any rock mass.

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Experimental Study of Shear Failure in Anisotropic Rocks

Cited by 334 publications

References 0 publications

Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

Structural Evolution of Upper Layer 2, Hole 896A

Ultrasonic constraint of the microfracture anisotropy of flysch rocks from the Podhale Synclinorium (Poland)

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