Effect of Fluid Saturation on Frequency-Dependent AVAZ Seismic Response Characteristics of Fractured Reservoirs

In fractured reservoirs, the seismic response characteristics are more complex than in conventional reservoirs because of their inherent properties of anisotropy and dispersion-attenuation, increasing the multiplicity of the prediction solutions and making fluid identification more difficult. Since fluid saturation is a crucial parameter that can directly affect these inherent properties, its effect on F-AVAZ (frequency-dependent amplitude versus angle-azimuth) seismic response characteristics of fractured reservoirs is studied in this article. Using Norris and KG models, an effective partially saturated fractured porous medium is established. Based on the anisotropic reflectivity algorithm, the deterministic relationship between F-AVAZ and fluid saturation is obtained. The numerical simulation results of the three-layer model show that when oil–water coexists in fractured reservoirs, F-AVAZ gradually changes with growing water saturation. While the fluid changes from fully water saturated to gas bearing, F-AVAZ suddenly jumps because of the rapidly decreasing effective P-wave modulus. This abrupt increase of P-wave amplitude is the classic 'bright spot' phenomenon, which verifies the validity of the anisotropic reflectivity algorithm based on the Norris-KG model for fractured reservoirs. This study lays a solid theoretical foundation for the analysis of seismic response characteristics of multi-phase fluid saturated fractured reservoirs and provides a reliable theoretical basis for improving reservoir prediction and fluid identification accuracy.