The effect of water saturation on P-wave dispersion in a fractured porous medium with two immiscible fluids

Abstract

Most fractured reservoirs, such as carbonate rocks, are both partially saturated and contain aligned fractures. In this paper, we analyze the effects of water saturation and permeability on the dispersion of P waves from model data and actual seismic data from area S of the Tarim Basin, China. Firstly, we construct a fractured porous model by adding a set of horizontal fractures evenly distributed in a periodic layered porous model alternately saturated with natural gas and underground water. Then, we analyze the frequency-dependent velocity and attenuation of the P waves for five wells in the target layer. The numerical results show that there are two significant attenuations in different frequency bands. In the seismic frequency band from 10 Hz to 100 Hz the attenuation increase with water saturation when the P wave is at normal incidence. Finally, we investigate the frequency-dependent amplitude versus offset of P waves from model data and actual seismic data. The results show that the P-wave dispersion intensity of five wells is positively correlated with water saturation in the seismic frequency band except for well S5 which has a higher permeability than the others. Therefore, water saturation can be estimated from the seismic dispersion properties in study areas, when the permeability of the reservoir has been determined.