Discussion On Physical Model Of Tight Sandstone Rock And Its Application In Inversion

Tight sandstone gas reservoirs have the characteristics of small porosity,low permeability,complex pore structure and strong heterogeneity.Traditional post-stack seismic exploration has limited effect in this type of reservoir identification and is no longer applicable.The pre-stack elastic inversion technique based on fine rock physical analysis can be used to study the heterogeneity of reservoirs,and to identify high quality tight sandstone gas reservoirs.In this paper,the Voigt-Reuss-Hill and Hashin-Shtrikman boundary models,the Gassmann equation,the Kuster-Tok?se model,the differential equivalent medium theory are introduced.And the applicability,advantages and disadvantages of the Wood formula and the Brie formula,which are mostly used for fluid elastic modulus calculation,are presented.On this basis above,combined with the characteristics of tight sandstone reservoirs,a new three-dimensional DEM-Brie rock physical model considering multiple minerals,multiple pore structures and fluid mixing methods was established.The laboratory core data is used to optimize the e-index in the Brie formula for calculating the fluid elastic modulus.Based on the three-dimensional DEM-Brie rock physical model,the wave velocity prediction of tight sandstone well is predicted.The relative errors of the predicted P-wave and S-wave velocity and the measured P-wave and S-wave velocity are less than 5%.The prediction accuracy is higher than the conventional rock physical model,which confirms that the DEM-Brie physical model is applicability and superiority in tight sandstone gas reservoirs.Combine the comprehensive logging interpretation,well production and shear wave velocity prediction results to do rock physical analysis.The elastic parameter intersection analysis shows that the P-wave velocity and S-wave velocity ratio and ?? have a good indication to the tight sandstone gas reservoir.The longitudinal wave impedance of the post-stack data inversion cannot identify the tight sandstone gas reservoir.So the pre-stack pure longitudinal wave inversion,pre-stack simultaneous inversion and pre-stack hybrid iterative inversion that guided by fine rock physical analysis,based on the modified treatment of pre-stack CRP gathers,are introduced to identify the fine reservoirs.And analyze the advantages and disadvantages of different inversion methods.Relatively speaking,pre-stack hybrid iterative inversion based on pre-stack pure longitudinal wave inversion and pre-stack simultaneous inversion can obtain higher precision and more stable P-wave and S-wave information,which remove the AVO effect and improve the resolution and accuracy of inversion results.In addition,the inversion method overcomes the shortcoming of pre-stack simultaneous inversion relying heavily on the initial Vp/Vs ratio.Based on the shear wave velocity predicted by the preferred rock physical model and guided by fine rock physical analysis,the optimal pre-stack elastic inversion technique is used as a mean to predict the tight sandstone reservoirs in the study area,the coincidence rate between the predicted results and the actual production of the well reached 85% or more.The comprehensive application of rock physical and pre-stack inversion is effective in the prediction of gas-bearing favorable areas of tight sandstone,which can provide a basis for subsequent exploration of such reservoirs.