Research On Seismic Anisotropic Response Characteristics And Inversion Methods For Unconventional Reservoirs

This dissertation mainly studies the forward modeling,inversion and attribute methods suitable for unconventional reservoirs.According to different structural characteristics of different reservoirs,the reasonable forward modeling methods are chosen for wave field response analysis and corresponding inversion research.For layered models,such as structural anisotropic reservoirs,propagator matrix?PM?,a full wave field simulation method is selected;for lithologic anisotropic condition,such as shale rocks,the Rüger approximation or the exact equation by Graebner is selected;for viscoelastic HTI media,such as fluid-saturated fractured or carbonate reservoirs,we use the forward modeling based on combining the Chapman theory and generalized propagator matrix?for viscoelastic anisotropic media?.In order to improve the inversion accuracy of layered model,a joint PP and PS inversion based on propagator matrix?PM?is proposed.The model tests show that this method is not only suitable for strong contrast,but also can deal with transmission losses,multiples,converted wave modes and so on.For thin interbedded model,PM-based inversion can effectively eliminate the interference of complex responses of thin interbedded to underlying strata and obtain better inversion results.The actual data application shows that the method is superior to the AVO inversion based on Zoeppritz equation.In addition,the joint PP-PS inversion based on PM does not require the PS-to-PP time alignment,and has better noise resistance and lower initial model dependence.Since the results of the joint PP-PS anisotropic five parameter direct inversion based on Rüger approximation are not ideal,a two-stage inversion strategy based on Rüger approximation and simulated annealing is presented here.Three elastic parameters are estimated by multi-wave joint isotropic AVO inversion in the first stage.The results are used to set the search window for the anisotropic AVO inversion in the second stage.It is found that the search window by using the first stage results can reduce the search range and improve the inversion efficiency.Because anisotropic parameters are more sensitive to large-angle data,in order to reduce inversion inaccuracy caused by low accuracy of Rüger approximation in large-angle range,we propose a multi-wave joint inversion strategy based on the Graebner exact equation.To improve the inversion accuracy of anisotropic parameters,the target parameters are set as the elastic stiffness parameters C_ij which have higher sensitivity.The model test shows that the inversion accuracy of anisotropic parameters has been improved.Inverse spectral decomposition?ISD?method can provide time-frequency spectrums with high resolution and high accuracy.In order to improve the accuracy of attenuation attribute analysis results,two methods for extracting attenuation parameters based on ISD method are proposed in this paper.One is the high-resolution fluid detection method by combining the improved ISD and FAVO inversion.FAVO inversion based on ISD can obtain high resolution P-wave dispersion attributes which can indicate gas reservoir.Combining with post-stack frequency anomalies and pre-stack P-wave dispersion attributes,gas-bearing reservoirs can be comprehensively analyzed.The other is a quality factor Q estimation method by logarithmic spectral ratio?LSR?combined with ISD and shaping regularization.The test results validate that the time-frequency spectrum with high resolution and accuracy by ISD can provide a good basis to Q estimation based on LSR.In addition,the inversion scheme with shaping regularization can improve the instability of the traditional LSR method and obtain the estimation results with better stability and continuity.By analyzing the full wave field responses of HTI fractured model,it is verified that the shear wave anisotropy information contains important fluid information.Three shear wave anisotropy attributes are studied to discriminate the saturated fluids in fractured reservoirs,including the time delay gradient differences of shear wave splitting,amplitude differences,and amplitude difference of frequency division cubes.Compared with the time delay gradient,the amplitude difference attribute has better practicality,and is a potential method for oil and water discrimination of fractured reservoirs.The model test shows that the amplitude difference between fast and slow shear waves varies with frequency.Frequency-dependent amplitude difference can be used to distinguish different fluid types.The difference between oil and water/gas and water is more prominent in mid-frequency band,and the difference of oil and gas is prominent in low-frequency component.