Exploring Trends in Microcrack Properties of Shales Using Stress Dependencies of Ultrasonic Velocities

Abstract

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