Trends in micro-crack properties and reconstruction of TI elasticity tensors of shales

Abstract

Stress dependency of the TI elastic tensor of shales is important for seismic interpretation, overpressure prediction, 4D monitoring, etc. Using Sayers-Kachanov formalism, we develop a new model for transversely isotropic (TI) media which predicts stress sensitivity behaviour of all five elastic coefficients. The model is used to parameterize elastic properties of about 20 shales obtained from our laboratory measurements and also from a literature survey. The four fitting parameters (namely, tangential and normal compliances of a single crack, characteristic pressure, and crack orientation anisotropy parameter) show correlations with the depth from which the shale was extracted.With increasing depth, the tangential compliance broadly decreases exponentially and the ratio of normal to tangential compliance generally increases linearly. The crack orientation anisotropy parameter exponentially increases with the depth for most of the shales indicating that cracks may be more aligned in the bedding plane. The characteristic pressure shows no simple correlation with depth. The suggested model allows prediction of stress dependency of all five elastic coefficients if only three of them are known. This can be useful, for instance, for the reconstruction of all five elastic coefficients of shale from log data.