Frequency and Pressure Effects on Fluid Substitution in High Porosity Reservoir Sandstones

Captain sandstone shows very high levels of ultrasonic attenuation in the laboratory suggesting significant velocity dispersion between seismic/sonic and ultrasonic frequency ranges. Some agreement was seen between squirt flow model predictions and the P-wave observations which suggests that P-wave velocity fluid substitution effects are consistent in the laboratory and in situ. However, uncertainly in model parameterisation (e.g., the true values of the unrelaxed dry frame moduli) makes it difficult to predict actual velocities and attenuations at seismic and sonic frequencies, needed for detailed interpretation of field data. More effort is needed to validate appropriate squirt flow and other elastic wave attenuation models over the whole frequency spectrum of interest.