4-D Rock Physics – a Heuristic Approach for Estimating Stress Induced Time Shifts and Changes in Elastic Properties

Rock physics models for fluid and stress dependency in reservoir rocks are essential for quantification and interpretation of 4-D seismic signatures during reservoir depletion and injection. However, our ability to predict the sensitivity to pressure from first principles is poor. The current state of the art requires that we calibrate the pressure dependence of velocity with core measurements. A major challenge is the fact that consolidated rocks often break up during coring, and hence the stress sensitivity is likely to be over-predicted in the laboratory relative to the in situ conditions (e.g., Furre et al., 2009). For unconsolidated sands, acquisition of core samples is not very feasible due to the friable nature of the sediments. In this study, we demonstrate how the “Hybrid” model by Avseth and Skjei (2011) can be used to quantify stress sensitivity in cemented sandstones from well log data alone. We show how to create dynamic rock physics templates (RPTs) of acoustic impedance versus Vp/Vs, and also how to predict time shifts and time shift derivatives. These are all useful rock physics tools that can be used in 4-D seismic interpretation.