Study On Methodology Of Pre-stack Seismic Inversion For Fractured Reservoirs Based On Rock Physics

Underground fractures play an important role in the storage and movement of hydrocarbon fluid.For cave-fractured carbonate reservoirs,the accurate prediction of fracture location and the identification of the type of fillings in fractures can help to improve the accuracy of the calculation of oil and gas reserves,and for unconventional reservoirs,especially shale gas reservoirs,the reliable prediction of fracture characterization is helpful for selecting the location which is suitable for fracturing,and this will make shale layers to be communicated by fracturing to form a network structure,and it will increase the production of shale gas reservoirs.This paper begins with the study on the basic theory of seismic anisotropy,and it shows us some types of strong seismic anisotropy,introduces anisotropy parameters(Thomsen parameters)which may be used to measure the strength of seismic anisotropy,and presents the definition of Thomsen parameters in the media which have a horizontal symmetry axis and a vertical symmetry axis,respectively.It also studies the relationship among these three anisotropic parameters using the experimental data.This may lay the foundation for follow-up research work.After the analysis of rock physics theory,we propose one anisotropic rock physics effective model for fractured layers.Combining with the log-interpretation results(Acoustic data,Density,Shale comtent,Porosity,Saturation,et al.),we may use this effective model to estimate P-and S-wave velocities and anisotropic parameters of fractured layers.Based on this effective rock physics model,we also do some research on the relationship among fracture parameters(fracture density,fracture aspect ratio and the type of fracture fillings),anisotropic parameters and fracture weaknesses parameters.According to the relationship between reflection and transmission for plane elastic waves in anisotropic media,we present the solution method which can be used to calculate the exact value of the reflection coefficient,and select the suitable approximate formula which may be used for seismic inversion in anisotropic media.By constructing different fractured reservoir numerical model,we study the effects of fracture parameters on the characteristic of the reflection coefficient.We also explore the method which can be used to extract AVAZ feature of fractured reservoirs,and verify this method in real work area.The prediction of the type of fracture fillings is an important research content for fracture characteristics description.Studies have shown that fracture weaknesses parameters are closely related to fracture density and the type of fracture filling fluid.By deriving an approximate formula which can indicate the relationship between fracture rock physics parameters and reflection coefficient,this paper explores the direct inversion method which uses seismic data to estimate the elastic parameters and fracture rock physics parameters of fractured layers.Synthetic seismic data using logging data and real seismic data from fractured reservoirs are used to verify this method.The results show that the fracture rock physics parameters can be estimated stably by using the proposed direct inversion method,and the method has high noise immunity.Using this inversion method,we can has a good estimation of the elastic parameters and fracture rock physics parameters of fractured reservoirs in the target work area.