Frequency Dependent Elastic and Anelastic Properties of Clastic Rocks

This study focuses on measurements of clastic rocks ranging from 3 Hz up to 500 kHz in the laboratory and their application to well log analysis and a time-lapse study in the North Sea. Measurements of elastic properties over a large frequency spectrum can provide a better understanding of the dispersion and attenuation mechanisms in rocks and help to predict and model these effects. Sandstones show a significant dispersion if inhomogeneities in saturation or open boundaries are present. These open boundaries cause a pore pressure equilibration and large changes in the compressibility of rocks. A large difference between the seismic and ultrasonic frequencies can be observed for the P-wave velocities if the rock is partially saturated with brine. The seismic frequency measurements follow the Gassmann prediction, while the ultrasonic measurement results track an arithmetic velocity average. In the course of this work, I found a surprising result for shale. In the past, differences in velocities between VSPs and well logs, or well logs and ultrasonic laboratory measurements, have been reported, but no measurements have been done to cover the whole range from the seismic frequency to the ultrasonic frequency range. The results show that a shale with high porosity (26%), especially with the normal components of the stiffness tensor, show significant dispersion effects, while the shear stiffness shows no dispersion. Measurements of the normal and shear stiffness for a low porosity (8%) shale sample show no significant dispersion. All these laboratory results have little value if not applied and confirmed in the “field”. The time lapse seismic experiment at Sleipner is a good example to observe