The effect of anisotropy on the Young's moduli and Poisson's ratios of shales

Young's modulus and Poisson's ratio are required for geomechanics applications such as hydraulic fracture design, analysis of wellbore stability and rock failure, determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress. Shales are usually anisotropic and models that neglect shale anisotropy may fail to describe geomechanical behaviour correctly. Anisotropy in shales results from a partial alignment of anisotropic clay particles, kerogen inclusions, microcracks, low‐aspect ratio pores and layering. For shales, the Young's modulus measured parallel to bedding E 1 is usually greater than the Young's modulus measured perpendicular to bedding E 3 . However, the Poisson's ratio ν 31 corresponding to stress applied perpendicular to bedding and strain measured parallel to bedding can be greater than, equal to, or less than the Poisson's ratio ν 12 for stress applied parallel to bedding and strain measured parallel to bedding. For transverse isotropy, the elastic anisotropy resulting from a partial alignment of clay particles can be written in terms of the coefficients W 200 and W 400 , which describe the impact of clay particle orientation on the anisotropy of shales. Disorder in the orientation of clay particles acts to reduce W 400 faster than W 200 , since W 400 is a higher order moment of the clay particle orientation distribution function than W 200 . This is confirmed by analysis of measured anisotropy parameters for shales. A partial alignment of clay particles is consistent with the measured Young's moduli for shales and with values of Poisson's ratio ν 31 > ν 12 but not with values ν 31 < ν 12 . These values can be explained if there exist kerogen inclusions, microcracks, or low‐aspect ratio pores aligned parallel to the bedding plane.