Effect of fluid saturation on frequency-dependent P-wave velocity and anisotropy of fractured porous rock

Fluid saturation will change the P-wave velocity of fractured reservoirs, thereby affecting the dispersion characteristics and velocity anisotropy, increasing the complexity of seismic response, multiple solution of reservoir prediction and difficulty of fluid identification. In this paper, according to the characteristics of fractured reservoirs saturated with multi-phase fluids, a partially saturated fractured porous equivalent medium model is established based on Norris and KG models. The exact relationship of P-wave velocity and fluid saturation is further derived. Numerical simulation results show that when gas and water coexist, with the increase of water saturation, P-wave phase velocities gradually increase, with decreasing degree of anisotropy at high frequencies, while P-wave phase velocity gives opposite trend with unchanged degree of anisotropy at low frequencies. The velocity dispersion and its degree of anisotropy gradually increase. Combining the existing poroelasticity theoretical models, we fit the P-wave velocity of a laboratory artificial fracture-porous sandstone sample. The calculated curvea match the measured scatter values reasonably, indicating that the combined model set is effectiveness under the given parameters. These results can lay a solid theoretical foundation for further analysis of seismic response characteristics of multiphase fluid-saturated fractured reservoirs, and provide a reliable theoretical basis for improving the certainty of reservoir prediction and the accuracy of fluid identification.