Study Of Velocity Model-building Method For The Fractured Vuggy Reservoir

Fractured-vuggy carbonate reservoirs have become the main force in the Tarim Oilfield.The main features of the fractured-vuggy carbonate reservoir are characterized by small geological targets,large burial depth and high heterogeneity which expressed as the rapidly varieties of the physical properties of the reservoir in the transverse direction and the large difference of the reservoir depth in the vertical direction.These characteristics not only cause the complex distribution of oil and gas in the fractured-type carbonate reservoirs,but also greatly increase the difficulty of the accurate seismic imaging for the space location.Therefore,the study and the establishment of a velocity modeling method which adapt to the cave-type carbonate reservoir is the most urgent solution to the core issues.In this paper,the influence of velocity field error on pre-stack depth migration imaging is studied.Based on tomography and full waveform inversion theory,this paper studies the shallow velocity modeling method of diving wave tomography inversion based on offset floating surface,single/multi-azimuthal tomography inversion velocity modeling method for mid-deep layers,the time domain multi-scale full wave inversion velocity modeling method,and the well velocity constrained velocity modeling method,which provides a more accurate velocity field for pre-stack depth migration and improves the imaging accuracy of the spatial location of the fractured carbonate reservoir,which lays the foundation for reservoir drilling and exploration and development for oil and gas.The diving wave tomography inversion method based on offset floating surface can acquire the fine modeling of shallow velocity.Shallow velocity modeling of fractured-vuggy carbonate reservoirs has been a difficult problem in seismic data processing.The shallow velocity is "blank" on the velocity spectrum.The pre-stack depth migration velocity modeling method uses the first reflection layer velocity filling the velocity of shallow layers which lead to a large velocity error,and the speed of first reflection layer cannot completely replace the shallow velocity.The primary wave tomography inversion is based on the true surface tomography method,which is mainly used to calculate the static correction.Although the method can invert the velocity,it is equivalent velocity,which cannot be directly used as shallow velocity for pre-stack depth migration.Based on the characteristics of offset floating surface and diving wave propagation,this paper uses the theory of primary wave tomography to study the shallow velocity modeling method of diving wave inversion,which greatly improves the accuracy of the shallow velocity field for carbonate reservoir.Single/multi-azimuthal robust reflection wave tomography can achieve mid-deep velocity modeling.It is very difficult to build the mid-deep velocity of the fractured-type carbonate reservoir,as a result of the influence of overburden igneous rocks and the difficulty to obtain the accurate high-frequency information.The single-azimuthal method of modeling the residual curvature of the uniaxial reflected wave is based on the azimuth information of the reflected wave,and only the lower frequency background velocity can be obtained.Multi-azimuthal reflection wave residual curvature tomography inversion modeling method can obtain the higher frequency component of velocity.However the multi-azimuth gather has low signal-to-noise ratio,which seriously affects the accuracy of residual curvature.In this paper,the method of single-azimuthal and multi-azimuthal reflection inversion is established.The method of deep velocity modeling in steady-state reflected wave tomography inversion is established.This method improves the efficiency of inversion and high-frequency information accuracy of med-deep velocity field.Time domain hierarchical multi-scale full waveform inversion can obtain high-frequency component of velocity field.Due to the limitation of the theory of seismic primary and reflection wave tomography inversion,we can only obtain the accurate medium and low frequency background velocity while the high frequency velocity component is inaccuracy.The full waveform inversion method is theoretically perfect and can accurately reflect the high-frequency components of the velocity field.However,it is difficult to converge to the correct result by FWI inversion due to the limitation of theoretical assumptions such as data quality,initial velocity model and forward wave.In this paper,we use the seismic data to meet the basic assumptions,and then design the fidelity preprocessing flow.The high-precision initial velocity model established by diving wave and reflection wave tomography is used to obtain the high frequency components of velocity field by full waveform inversion,which not only greatly reduces the number of iterations and avoids the local optimal solution,improves the accuracy of high frequency information,achieves the accurate imaging of the fractured reservoir,but also shows the application potential of the full waveform inversion for land seismic data.In situ seismic confinement field construction method using the real-time data obtained from drilling,such as acoustic logging or other logging data,using the theory of reflected wave tomography to further constrain and optimize the local subsurface model(accuracy of anisotropic velocity model)to further improve the pre-stack migration accuracy of ground seismic data around existing wells,then the velocity model is consistent with the new logging data.In the complex geological environment or in the area with high uncertainty before drilling,the seismic pre-stack migration imaging of the constrained and optimized velocity model is provided to update the position of the front dangerous zone and the target layer of the drill bit,which facilitates the drilling trajectory to the exact geological target,to avoid the drilling accident,improve the accuracy of the spatial location of the fractured-vuggy carbonate reservoir.In this paper,the application of pre-stack depth migration velocity field construction method is carried out for the fractured carbonate reservoir in Tarim Basin.The modeling method can not only improve the accuracy of the fractured-vuggy carbonate reservoir velocity model and the accuracy of reservoir space location,the reservoir one-time drilling rate,but also be good for the subsequent seismic interpretation and inversion.The results of the actual drilling show that this method is correct and the velocity modeling method is effective,which can provide a new idea for the study of seismic velocity field modeling with similar geological conditions.