Study Of Multi-wave AVO Pre-stack Inversion And Application

Conventional seismic exploration,using only PP-wave seismic data,has been difficult to meet the needs of oil and gas exploration and progress,especially for unconventional reservoirs.As further in seismic prospecting,multi-wave and multi-component seismic exploration have great application potential in accurate imaging,fracture detection,reservoir lithology identification and fluid detection.As one of the most important techniques to obtain elastic parameters and identify lithology and fluid,AVO/AVA inversion based on pre-stack seismic data has been frequently used in the hydrocarbon industry.This paper mainly studies multi-wave AVO pre-stack inversion and presents a novel method and workflow to predict pore pressure directly based on inverted elastic parameters,which achieves obvious application effects in shale oil reservoirs.The main research results of this paper can be summarized as follows:(1)A sandstone rock physical model based on the equivalent medium theory is established.Then we analyze the AVO response of multi-waves(PP,SV-SV,P-SV,SV-P,SH-SH waves)with different saturations of oil and gas reservoirs and different incident angles of seismic waves.Besides,we acquire and process PP-wave,SV-SV wave and SH-SH wave artificial sandstone seismic physical model data,which containing different fluids,and research the wave field characteristics and AVO response characteristics.The AVO response characteristics and identifiable ability of PP wave,SV-SV wave and SH-SH wave with different fluid saturation,clay content and porosity are found.And it is found that PP wave has a strong distinguishable ability for gas saturation and good identification for clay content,especially for large incident angle(greater than 40°).And SH-SH wave has a good distinguishable ability for clay content and porosity.(2)In order to improve the accuracy of the pre-stack AVO inversion based on the approximations of Zoeppritz equations,a direct pre-stack AVO density,P-and S-wave velocities inversion method based on the precise Zoeppritz formula,which is suitable for strong reflection interface and large incident angle,is studied.Furthermore,an iteratively regularizing Levenberg-Marquardt(IRLM)algorithm is introduced to overcome the highly nonlinear and ill-posed problems and improve the robustness of the inversion method.Both model and field data cases verify the probability and usefulness of the presented method.(3)Since the S-wave velocity and density results of the pre-stack AVO inversion just using PP-wave seismic data are unstable and low precision,a joint PP-PS pre-stack AVO nonlinear inversion method based on the exact Zoeppritz equations is presented in this paper.IRLM scheme can optimize objective function while enhancing inversion stability.Model data demonstrates that this method can acquire more stable and precise P-and S-wave velocities and density results.(4)To obtain higher accuracy of Young’s modulus and Poisson’s ratio directly and simultaneously,a new form of the exact Zoeppritz equations using these elastic parameters is derived and rewritten and a robust AVO inversion strategy is presented,which avoid the cumulative error caused by indirect calculations and the low accuracy caused by approximate expressions.Synthetic test and field data cases demonstrate the feasibility and validity of the introduced method,which is of great significance to the"sweet spots"prediction in unconventional reservoirs.(5)The relationship between the ratio of P-and S-wave velocities(V_P/V_S),Poisson’s ratio and pore pressure with different fluid contents are different based on the laboratory data analysis of different rock samples.With the increase of pore pressure,Poisson’s ratio and V_P/V_S increase and the decrease of S-wave velocity is higher than P-wave velocity when water is saturated,which are opposite when gas is saturated.It is found that the cross-plot of Poisson’s ratio and V_P/V_S can identify pore pressure and pore fluid,which reduces the ambiguity and multiplicity of pore pressure prediction and reservoir description.(6)To improve the accuracy of pore pressure estimation results,a method and workflow to predict pore pressure based on elastic parameters inversion for shale oil reservoirs from Qianjiang depression in the middle of Jianghan salt-lake basin,in China,is proposed here.Firstly,high-precision density,P-and S-wave velocities are inverted by pre-stack AVO inversion;then a suitable elastic parameters-to-pore pressure is established to estimate pore pressure.The application of real well and seismic data examples illustrates the reliability and accuracy of the elastic parameters method,clarifies the spatial distribution characteristics of pore pressure,identifies the distribution of oil and gas qualitatively and guides the drilling design and seismic sweet spot prediction.