Earch On Fracture Prediction Of Tight Sandstone Gas Reservoir Based On Anisotropic Seismic Inversion

Tight gas sandstone is an essential field of unconventional oil and gas exploration.The Upper Paleozoic tight gas sandstone reservoir in Ordos Basin has abundant natural gas resources.Because the development of fractures in tight sandstone can improve the permeability of tight reservoirs,provide a channel for natural gas migration,and facilitate the formation of fracture networks in the process of hydraulic fracturing.Therefore,fracture prediction is an important research content for the quality evaluation of tight gas sandstone reservoirs.In this paper,based on the simulation and analysis of seismic rock physical response,the anisotropic seismic inversion method is used to predict the horizontal and vertical fractures of tight gas sandstone reservoirs,which provides vital parameter information for reservoir description.This paper describes the elastic properties of the development of fractures in tight gas sandstone reservoirs based on the anisotropic equivalent medium theory.It analyzes the quantitative relationship between the seismic anisotropy response and fracture properties of tight gas sandstone reservoirs based on seismic AVO and AVAZ theories.It provides a theoretical basis for applying seismic data to characterize the spatial distribution of fracture strike and development degree.Based on the approximate formula of the PP-wave reflection coefficient put forward by Ruger in weakly anisotropic VTI media,seismic inversion of VTI anisotropic parameters is performed with the constraint of P-and S-wave velocity and density for elastic inversion.Then the fracture properties are characterized by rock physics theory.The field data application results of the tight gas sandstone reservoirs in Ordos Basin demonstrate the predicted horizontal fracture density is significantly correlated with logging permeability,indicating fracture development can improve the permeability of the tight gas sandstone reservoirs.In addition,fractures are also a critical reservoir space for natural gas,which is conducive to gas enrichment.This paper proposes an improved method based on azimuthal amplitude difference to calculate anisotropy parameters using wide azimuthal seismic data and further predict vertical fracture density and other attributes utilizing rock physical relations.In the inversion process,applying the Fourier series method to predict fracture orientation first and using the expected fracture orientation as the reference orientation to constructing the azimuth trace to eliminate the isotropic background and the corresponding anisotropic parameter inversion method to improve the sensitivity of fracture identification and the accuracy of anisotropic parameter inversion.The field data application indicates that the predicted fracture strike matches the tectonic stress field analysis results.At the same time,the predicted vertical fracture density has an obvious correlation with the gas production of tight gas sandstone reservoirs.Therefore,vertical fractures improve the permeability of tight gas sandstone reservoirs,and the distribution and development provide a basis for high-quality reservoir identification.Further analysis reveals that both fracture and natural gas enrichment degrees affect the productivity of tight gas sandstone reservoirs.Fracture density e can represent the degree of fracture development,and compression ratio 1/K is an effective indicator for gas content identification.Therefore,regard e×1/K as the comprehensive index of reservoir quality evaluation.This paper considers two main factors controlling reservoir gas production capacity to predict the spatial distribution of high-quality reservoirs.In addition,comprehensive prediction and analysis of the fracture system based on the inversion prediction of horizontal and vertical fractures.In summary,based on seismic rock physical simulation and analysis,this paper applies an anisotropic seismic inversion method to predict the fractures of tight gas sandstone reservoirs.On this basis,combined with the prediction results of reservoir gas-bearing capacity,a comprehensive reservoir evaluation factor is constructed,which can effectively predict the spatial distribution of high-quality tight gas sandstone reservoirs and provide an effective method for tight gas sandstone reservoir evaluation.