Simultaneous Prediction for Fracture Density and a New Fluid Influence Factor in Oil-Bearing Reservoirs

Reliable characterization of subsurface fracture information within the oil-bearing reservoirs plays an essential role in reservoir description, particularly in assessing the fracture density and fluid storage capacity. Fracture density indicates the spatial distribution and extent of fracture development, which is traditionally estimated indirectly via tangential fracture weakness predicted from the OVT (offset vector tile) seismic data or azimuthal elliptical fitting techniques. The fracture fluid factor is also a critical parameter for identifying and characterizing fluids within fractured reservoirs. Conventional methods for estimating the fluid factor typically rely on establishing a linear correlation between the fracture fluid factor and anisotropy parameters, leading to challenging fluid identification and low accuracy. To address this limitation, we propose a novel approach that directly and simultaneously inverts the fracture density and a newly defined fluid influence factor (FIF) within a horizontally transverse isotropic (HTI) medium. The specific definition of frequency-dependent FIF is presented, and the reliable mechanism of FIF for identifying fracture-filling fluid is demonstrated based on an anisotropic petrophysical model. Furthermore, we derive a new PP-wave reflection coefficient for HTI media to facilitate azimuthal seismic inversion. A two-step inversion strategy is also introduced to invert weakly anisotropic parameters to enhance inversion accuracy. Combining the superiority of direct simultaneous inversion of fracture density and FIF, these results applied to the carbonate oil-bearing reservoir demonstrate that the proposed method can not only estimate fracture density directly but also predict FIF to identify fractured fluid, providing a valuable reference for the evaluation of fractured oil reservoirs.