The Study On Gas Identification Based On Seismic Dispersion Attribute Inversion For Tight Sandstone Reservoirs

Seismic identification of high gas-saturation reservoirs is a primary goal in the exploration and development of tight sandstone gas reservoirs.The complex poroelastic behaviors associated with the fluid flow make the gas identification of tight sandstone reservoirs based on conventional elastic properties challenging due to the characteristics of tight sandstone,such as low porosity,low permeability,and high heterogeneity.When seismic waves propagate in tight sandstone gas reservoirs,waveinduced fluid flow in the pore space results in significant dispersion and attenuation of seismic waves.Therefore,it is necessary to study frequency-dependent seismic attributes for gas identification in tight sandstone reservoirs.In this paper,based on the analysis of the physical characteristics of tight sandstone gas reservoirs in the study area,rock physical modeling and analysis are conducted,and the study and application of seismic dispersion attribute inversion methods are developed to improve the accuracy of gas identification in tight sandstone reservoirs.Firstly,according to the analysis of geological and seismic data in the study area,the rock physical modeling of tight sandstone gas reservoirs for dispersion and attenuation is developed based on the patchy saturation theory with respect to the lithology,pore,and fluid properties of the tight sandstone gas reservoirs.The mechanism of seismic wave dispersion and attenuation related to the heterogeneous distribution of pore fluid is studied.The study provides an effective rock physical model for the subsequent study of the quantitative relationship between gas saturation and the response of seismic wave dispersion and attenuation.Also,it provides a theoretical basis for constructing seismic dispersion attributes with higher gas-bearing sensitivity.Secondly,the bulk modulus dispersion attribute inversion method is developed and applied.Based on the established rock physical model of the tight sandstone gas reservoirs,the variation characteristics of the bulk modulus with frequency are analyzed.The bulk modulus dispersion attribute inversion method is proposed on this basis.A theoretical model of interbedded sandstone-mudstone is established based on the structural characteristics of the target layer in the study area,and the bulk modulus dispersion attribute inversion method is tested and analyzed.The results show that the bulk modulus dispersion attribute is more sensitive to the variation of gas saturation than the conventional P-and S-wave velocity ratio.The field data application results show that the bulk modulus dispersion attributes are more consistent with the production status of the development wells than the conventional P-wave velocity dispersion attributes.Furthermore,the gas-bearing evaluation factor is constructed by integrating the bulk modulus dispersion attribute and elastic property,which can more accurately identify tight sandstone gas reservoirs and the spatial spreading of the sedimentary phase.Meanwhile,the nonlinear relationship between the gas-bearing evaluation factor and the gas saturation is fitted based on the logging data and applied to quantitatively predict the spatial distribution of gas saturation.The calculation results show that the quantitative prediction of gas saturation has a high degree of agreement with the actual production capacity of gas wells.Finally,the fluid dispersion attribute inversion method is developed and applied.Based on the analysis of the dispersion and attenuation characteristics of the fluid bulk modulus,the solid-fluid decoupled AVO approximation is extended to the frequencydependent form,and the fluid dispersion attribute inversion method is developed.In addition,a practical model that simultaneously considers reservoir log-scale heterogeneity and the characteristics of dispersion and attenuation is established by combining rock physical modeling and seismic forward simulation.The relationship between gas saturation and seismic dispersion response is studied and analyzed,and the advantages of fluid dispersion attributes in gas identification of tight sandstone reservoirs are verified.The gas identification based on fluid dispersion attributes for tight sandstone reservoirs is developed by applying field data.The calculation results show that the fluid dispersion attribute can provide more reliable gas prediction results than the traditional P-wave velocity dispersion and is more consistent with the geological characteristics of the study area.At last,the seismic dispersion attribute inversion based on different time-frequency analysis methods is compared.The calculation results show that the fluid dispersion attributes with higher gas-bearing sensitivity can improve the accuracy of gas identification more effectively than the way to improve the accuracy of time-frequency analysis methods.Therefore,the proposed fluid dispersion attribute in this paper can be used as a high-precision gas identification factor for tight sandstone reservoirs.