Reliable evaluation of rock mechanical properties is important to predict response to fracture treatment in organic-shale formations. However, characterization of rock mechanical properties in organic-shale formations is challenging, as the result of heterogeneity, anisotropy, and complex lithology. Although organic-shale formations are anisotropic and highly laminated, their mechanical properties are often estimated using conventional techniques, with the assumption of isotropy, either in the laboratory or from interpretation of borehole acoustic measuritalicents. Isotropic assumptions in conventional laboratory measuritalicent are not appropriate for organic-shale formations, and, therefore, they can result in uncertainties in stress prediction.
The objective of this paper is to estimate elastic properties, earth stress coefficient, and minimum horizontal stress of highly-laminated organic-shale formations using laboratory experiments. We use hydrostatic tests, conventional triaxial tests, and uniaxial strain tests to measure rock compressibility, anisotropic elastic properties, and earth stress coefficient. We introduce the measured elastic properties and earth stress coefficient, Ko, in the stiffness matrix of the TIV (transversely isotropic media with a vertical axis of symmetry) model and compare the values of the minimum horizontal stress for anisotropic elastic properties and for the earth stress coefficient, Ko.
We assessed mechanical anisotropy in the Haynesville shale-gas formation through experimental techniques. This formation is both highly laminated and naturally anisotropic on both, micro and macro scales. The estimated compressional modulus of organic-shale samples using the TIV stress profile model are more than 30% less than those estimated from conventional techniques. The Young's modulus and Poisson's ratio in the direction parallel to the bedding plane were higher than the values obtained in the normal direction to the bedding plane. The earth stress coefficient, Ko, which is a measure of rock stiffness can be used to determine mechanical behavior in organic-shale formations.
