Seismic signatures of time-dependent reservoir processes, necessary for the interpretation of 4-D seismic data, are still insufficiently described. This experiment was designed to monitor fluid-front movements and saturation changes and to identify the related seismic signatures.
Ultrasonic P- and S-wave transmission and reflection measurements were used to monitor the waterflooding of a porous sandstone. The sandstone was flooded in steps by filling a tank in which the room-dry cubic (50-cm side) block of rock was placed.
Waterflooding caused the velocity, amplitude, and frequency of the transmitted waves to diminish significantly; however, the changes were reversible by drying. The maximum reduction of the velocities was 7% and 12% for P- and S-waves, respectively. The velocity and amplitude behavior can be explained by the Biot-Gassmann's theory, local fluid flow, and grain-surface effects. The correct interpretation of seismic signatures of fluid processes in reservoirs thus involves a knowledge of rock physical relations and attenuation mechanisms.
Even at small saturations, reflections from the block bottom were strongly attenuated, but those from the upgoing water front could be monitored. The latter reflections were best observed in differential seismic traces, confirming that seismic monitoring can observe moving fronts directly.