Multi-band Seismic Rock Physics Modeling Of Carbonate Reservoirs

The main reservoir type of the Ordovician Yingshan Formation in the Gucheng area,Tarim basin is tight carbonate rocks.The buried depth of the reservoir is above 6000 meters.The reservoir has the characteristics of low porosity,low permeability and strong heterogeneity,and the reservoir distribution is extremely complicated.Seismic rock physics modeling of this kind of reservoir requires a comprehensive consideration of the pore structure and pore-fluid characteristics,which may affect the dispersion and attenuation of seismic waves.Firstly,assuming a linear-elastic solid containing pores of various shapes filled with non-viscous compressible fluid as the representative elementary volume,made the undrained approximation,considering the externally applied stress induced fluid pressures in cavities is coupled with the overall elastic response,the cracks-pores effective medium(CPEM)theory model is studied to analyze the effects of different conditions on the elastic properties and dispersion of rocks,including single pores,single cracks,coexistence of pores and cracks,matrix modulus variation and fluid saturation variation,etc.The numerical simulation results of the theoretical model show that the elastic property can be significantly impacted and the dispersion may be the largest when the difference in stiffness between the matrix and the pores is larger,or the existence of a small amount of cracks or a small amount of gas.For example,when the matrix is calcite with pore filling water and the aspect ratio equals 0.005,the bulk dispersion reaches 54.8% when the crack fraction is 0.18;when the aspect ratio equals 0.003,the bulk dispersion reaches 81.8% when the water saturation is 89.4%.Then,based on the CPEM theory model,a multi-band quantitative rock physics interpretation plate is constructed.The plate is applied to the analysis of the logging data of the carbonate reservoir,lie in the Ordovician Yingshan Formation in the Gucheng area,Tarim basin.The results illustrated that,considering the influence of dispersion,this rock physics interpretation plate regarding crack porosity and fluid changes can validly evaluate the crack porosity and fluid saturation of reservoir rocks,avoiding the misjudgment caused by using only the low frequency theory.Based on the CPEM theory model,using the elastic parameter and physical parameter of rock samples from Gucheng area,including the measured velocity,porosity and density,the inversion of crack porosity and dispersion is carried out.The results show that based on this theory,the crack porosity and dispersion can be quantitatively predicted.In addition,based on the information of P-wave and S-wave velocity,density,porosity and mineral composition from logging data,using the CPEM theory model,the pore type inversion and the prediction of P-wave and S-wave velocity are conducted.The results show that with the fully utilization of the existing logging data,based on this theory,the pore structure of the reservoir rock can be well described,and the S-wave velocity prediction results also show a high consistency with the actual logging results.