The effect of kerogen on the elastic anisotropy of organic-rich shales

Abstract

The elastic properties of reservoir rocks are important for geomechanics applications; the most important of which are: analysis of stress changes due to production, analysis of rock deformation and failure, wellbore trajectory optimization, and the design of hydraulic fractures. Organic-rich shales are often observed to be strongly anisotropic due to the partial alignment of anisotropic clay minerals and the bedding-parallel lamination of organic material within the shale. Neglecting shale anisotropy may lead to incorrect estimates of the in situ stress or stress changes resulting from production. As a result, isotropic models may fail to describe geomechanical behavior correctly. The distribution of the organic phase plays an important role in determining the elastic properties of organic-rich shales, and this has a significant effect on production-induced stress changes. The presence of kerogen leads to a decrease in all of the elastic moduli, and has a significant effect on the geomechanical behavior of shales. The change in horizontal effective stress for a given change in pore pressure resulting from production is greater for kerogen-rich shales, and the neglect of anisotropy in predicting such stress changes may lead to significant errors.