Rock physics modelling of 4D time‐shifts and time‐shift derivatives using well log data – a North Sea demonstration

Abstract

ABSTRACTRock physics models for fluid and stress dependency in reservoir rocks are essential for quantification and interpretation of 4D seismic signatures during reservoir depletion and injection. For siliciclastic sandstone reservoirs, the Gassmann theory successfully predicts changes in seismic properties associated with fluid changes. However, our ability to predict the sensitivity to pressure from first principles is poor, especially for cemented sandstones. In this study, we demonstrate how we can use a patchy cement rock physics model to quantify the combined effect of stress and fluid changes in terms of seismic time‐shifts and time‐shift derivatives during depletion or injection. The time‐shifts are estimated directly from well log data without core calibration of stress sensitivity. By assuming non‐uniform grain contacts where some grain contacts are cemented and others are loose, we can combine the contact theory for cemented sandstones with the contact theory for loose sands in order to predict stress sensitivity in a patchy cemented sandstone reservoir. Time‐shift derivatives are also useful estimates, as this parameter reveals which part of the reservoir is most stress sensitive and contributes most to the cumulative time‐shift.We test out our new approach on well log data from Troll East, North Sea and compare the predicted time‐shifts with observed 4D seismic time‐shifts. We find that there are good agreements between predicted time‐shifts and observed time‐shifts. Furthermore, we confirm that there are local geological trends controlling the fluid and stress sensitivity of the reservoir sands on Troll East. In particular, we observe a lateral stiffening of the reservoir from west to east, probably associated with the tectonic and burial history of the area. The combined effect of a thinning gas cap and stiffening reservoir sands amplifies the eastward decrease in time‐shifts associated with reservoir depletion. We manage to disentangle these two effects using rock physics analysis. It is essential to identify and map the static rock stiffness spatial trends before interpreting time‐shifts and time‐shift derivatives in terms of dynamic (i.e., 4D) pressure and fluid changes.