Nonlinear Prestack Inversion Method for Fluid Factor Using the Reflectivity Method and PID-Based Search Algorithm

Fluid factors play an important role in reservoir fluid identification. Current fluid factor inversion methods rely on the exact Zoeppritz equations and their approximations, which neglect seismic wave propagation effects. This necessitates thorough multiple attenuation and amplitude compensation for inversion data. However, complex reservoirs with thin or interbedded strata often do not meet these requirements. The vectorized reflectivity method, based on a 1-D analytical solution of the full elastic wave equation, can accurately describe the true propagation response of seismic waves under the assumption of horizontally layered media. To address the aforementioned challenges, a new nonlinear inversion method for the Russell fluid factor is proposed by combining the reflectivity method with the fluid factor inversion approach. First, we rewrite the traditional reflection coefficient equations of the reflectivity method based on rock physics theory, obtaining new expressions that include the Russell fluid factor. To avoid instability in solving the first-order partial derivatives caused by changes in parameterization, we combine the new equations with a PID-based search algorithm (PSA) featuring global optimization properties, effectively ensuring the stability, accuracy, and convergence speed of the inversion algorithm. Additionally, the edge-preserving smoothing (EPS) operator is introduced into the PSA to enhance the reservoir boundary characterization ability of the fluid factor inversion results. Both synthetic and field data tests show that the proposed method significantly improves the estimation accuracy of the fluid factor in reservoirs with thin or interbedded strata, which is of great significance for advancing seismic fluid discrimination methods.