Abstract
Shale formations show obvious anisotropic characteristics in their mechanical properties due to pronounced bedding planes and natural fractures. This anisotropic behavior generally creates complex fracturing networks and is crucial to gas shale stimulation. Although much research has been done to study the anisotropic compression behaviors of shale with static and quasi-static strain rates, there are limited investigations addressing the anisotropic tensile behaviors of shale at quasi-static strain rate. In this work, the anisotropic tensile behaviors of Longmaxi shales were studied systematically at different strain rates from 10−5 to 10−2 s−1 by performing Brazilian splitting tests. Testing results reveal the tensile strength anisotropy, rate dependency, and the stimulated fracture pattern morphology. The results show that the orientation between the applied force and bedding direction has an obvious effect on the tensile strength and fracture pattern. The rate dependency of shale under different loading rates is different for shale samples with various orientations. It was suggested that a complex tensile fracture pattern can be easily formed when using a high loading rate. The result sheds light on how to stimulate a complex fracturing network during field hydraulic fracturing treatment.
Highlights
Characterization of anisotropy is critical in shale due to the grain alignments and the developed natural fractures
The tensile behavior was studied at four orientations on shale core samples drilled parallel to the bedding plane, and each cylindrical sample was polished having a diameter of 50 mm, and a thickness of 25 mm for the Brazilian test
Where P is the applied force, θ is the intersection angle between axial force and bedding plane, D is the diameter of the sample, t is sample thickness, E is the elastic modulus in the transverse isotropic plane, E’ and G’ are the elasticity modulus and shear modulus for the plane perpendicular to the transverse isotropic plane, v’ is the Poisson’s ratio in the transverse isotropic plane
Summary
Characterization of anisotropy is critical in shale due to the grain alignments and the developed natural fractures Platy minerals such as clays have a tendency to be aligned in a parallel orientation, and form geologic discontinuities. Hou et al [7] studied the anisotropic characteristics of the tensile strength and failure mode of black shale from the Longmaxi Formation using Brazilian tests. Shale sample behaviors under dynamic tension may be different from those under static loading conditions, for peak strength, and for the fracture propagation path, and the formation of complex fracturing network. A series of indirect dynamic tension tests for shale samples with inclinations of 0◦ , 30◦ , 60◦ , and 90◦ at different strain rates of 10−5 –10−2 s−1 was performed to investigate the rate dependency, the fracture network propagation, and the interactions between bedding planes and fractures. The effects of strain rate on the strength and failure characteristics for anisotropic Longmaxi shale have been documented first
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