Modeling and analysis of seismic wave dispersion based on the rock physics model

Abstract

Seismic wave attenuation and dispersion are largely related to the fluid content in reservoirs, which has been verified by rock physics models, numerical modeling and some field case studies. The main mechanism of seismic wave dispersion is wave-induced fluid flow due to the heterogeneity at different scales, including ‘macroscopic’, ‘microscopic’ and ‘mesoscopic’. Recently, studies on dispersion have focused on mesoscopic rock. Previous studies have shown that there are many factors influencing seismic wave dispersion and attenuation, and amongst all the parameters, gas saturation is a very important one. Based on the White model, in this paper we firstly model the seismic wave dispersion and attenuation characteristics with varying gas saturation and porosity, and then analyze the velocity and normal-incidence (NI) reflection coefficient using three sandstone models. The results show that the frequency-dependent reflection coefficient (FDRC) is influenced by gas saturation, which corresponds to the frequency-dependent velocity characteristic. Finally, we derive approximate FDRC expressions using the NI reflection coefficient method, which can be used to estimate the dispersion characteristic. Analysis of the cross-plots of NI and dispersion indicates that dispersion and attenuation are more sensitive to gas saturation and porosity. Therefore, dispersion and NI cross-plots provide a potential way to predict the gas saturation. The modeling results of different consolidated sandstone reservoirs also indicate that there is a critical gas saturation point at which the dispersion magnitude reaches maximum value. Based on the critical point, the cross-plot can be divided into two parts and we can predict the gas saturation in either of them.