The Fate of R in Light of Field Shale Laboratory Tests

Summary Successful utilization of petroleum resources requires proper monitoring of the subsurface, where time-lapse seismic is a prominent source of knowledge. The R-factor model is commonly used to model the strain sensitivity of 4D seismic data. Field cores are systematically tested with different stress-change variants. We show that the strain sensitivity is significantly influenced by the stress path and the angle of wave propagation (offset). It is well known that shales are inherently anisotropic in terms of both static and dynamic stiffnesses. We find evidence that shales also obey anisotropic strain sensitivities in their velocities and show that the assumption of isotropic strain sensitivity implies a significant misfit to laboratory data. On the other hand, a satisfying fit to offset-variant time-shifts from laboratory data is achieved when transversely isotropic strain sensitivity is implemented. Such tests permit a calibrated higher order description of the strain sensitivity, and may provide an accurate basis with which to understand how suitable models and correlations may be implemented in 4D field data analysis.