Xilin Qin

The accurate characterization of fracture density and gas saturation is crucial for successful shale gas reservoir developments. An integrated approach was proposed that combines rock physics modeling with anisotropic frequency-dependent inversion to characterize shale reservoirs in the Zhaotong Demonstration Area, Sichuan Basin. First, based on drilling, logging, and geologic data, a shale model was constructed using self-consistent approximation, differential equivalent medium, and Chapman fracture models. The model was used to study the effects of fracture density and gas saturation on the dispersion and attenuation of P-wave velocity, S-wave velocity, and the anisotropic parameters ε and δ. Based on this analysis, a rock physics interpretation template was constructed to provide a theoretical foundation for the anisotropic frequency-dependent inversion. Parameter sensitivity analysis indicated that P-wave and S-wave velocity dispersions are sensitive to fracture density, whereas the dispersions of anisotropy parameters ε and δ are influenced by gas saturation. Time–frequency analysis of the near-, mid-, and far-angle gathers using inverse spectral decomposition based on the generalized S-transform was performed to obtain data volumes at different frequencies. By establishing an objective function based on L1 norm regularization using the Rüger approximation formula, the spectral projection gradient for L1 minimization was used to solve this objective function, thereby obtaining the dispersion properties. Frequency-dependent amplitude variation with offset inversion was subsequently applied using the derived Rüger approximation equation to extract dispersion attributes. In addition, the fracture density and gas saturation were predicted using a rock physics template and validated against well-log data to prove the effectiveness of the proposed method.

The frequency-dependent amplitude-versus-offset (FAVO) method has become a practical method for fluid detection in sand reservoirs. At present, most FAVO inversions are based on the assumption that reservoirs are isotropy, but the application effect is not satisfactory for fractured reservoirs. Hence, we analyse the frequency variation characteristics of anisotropy parameters in tight sandstone reservoirs based on a new petrophysical model, and propose a stepwise anisotropic FAVO inversion method to extract frequency-dependent attributes from prestack seismic field data. First, we combine the improved Brie's law with the fine-fracture model to analyse frequency-dependent characteristics of velocities and Thomsen anisotropy parameters at different gas saturations and fracture densities. Then, we derive an anisotropic FAVO inversion algorithm based on Rüger's approximation formula and propose a stepwise anisotropic FAVO inversion method to obtain the dispersions of anisotropy parameters. Finally, we propose a method that combines the inversion spectral decomposition with the stepwise anisotropy FAVO inversion and apply it to tight sand reservoirs in the Xinchang area. We use P-wave velocity dispersion and anisotropy parameter ε dispersion to optimise favourable areas. Numerical analysis results show that velocity dispersion of the P-wave is sensitive to fracture density, which can be used for fracture prediction in fractured reservoirs. In contrast, anisotropic parameter dispersion is sensitive to gas saturation and can be used for fluid detection. The seismic data inversion results show that velocity dispersion of the P-wave and anisotropic parameter dispersion are sensitive to fractured reservoirs in the second member of Xujiahe Group, which is consistent with logging interpretation results.

Most frequency-dependent AVO inversions are currently based on an approximate equation derived using an isotropic medium. However, actual reservoirs usually show anisotropy, such as shale reservoirs, tight sandstone reservoirs and fractured reservoirs. We propose a joint frequency-dependent AVO (JFAVO) inversion in an anisotropic medium based on a periodic layered double-porosity medium. This JFAVO will allow us to quantitatively study the influence of fluids on the dispersion of PP- and PS-wave velocities and anisotropic parameters. First, we used a double-porosity medium to analyse the frequency-dependent characteristics of velocities and anisotropy parameters. We found that the anisotropic parameters show obvious dispersions, similar to those of velocities. Then, we derived the JFAVO inversion based on Rüger's equation to extract the dispersion of velocities and anisotropic parameters. Finally, we analysed the stability and applicability of the inversion algorithm, and used three sets of models to analyse the sensitivity of dispersion properties to fluids. The numerical analysis results show that PP-wave velocity dispersion and anisotropic parameter δ dispersion are sensitive to fluids, whereas, the velocity dispersion of the PS-wave is not. When saturation exceeds 80%, the velocity dispersion and anisotropic parameter dispersion properties are not sensitive to fluids.

At present, most studies have considered partial saturation and aligned fractures separately. However, these two phenomena exist simultaneously in most fractured reservoirs, such as carbonate reservoirs and tight sandstone reservoirs. In this paper, we develop a periodically layered model with alternating fractured layer saturated with gas and water-bearing porous layer to simultaneously model partial saturation and aligned fractures, which is an equivalent viscoelastic vertical transversely isotropic (VTI) medium. We assume that the two types of attenuation are independent of each other. Firstly, we analyze the phase velocities and attenuation coefficients of quasi-P (P) and quasi-S (S) waves with frequency and incident angle, when the thickness fraction of fractured layer is 0.5. It shows that the attenuation coefficients of P-wave are much greater than the S-wave, when the incident angle is smaller than 30 degree. Then we analyze the frequency-dependent characteristics of the P-wave with different thickness of the fractured layer related to gas saturation. The attenuation of P-wave increases significantly with gas saturation in the frequency band from 10 to 100 Hz. Finally, we re-derived the frequency-dependent amplitude versus offset (AVO) equation of anisotropic media to analyze the dispersion characteristics of the P-wave reflection coefficients. The reflection magnitude decreases with frequency and the reflection coefficients show significant anisotropy at high gas saturation. The dispersion and attenuation of P-wave largely depends on the gas saturation.