The Study Of 4D Multicomponent Seismic Interpretation Methods For CO₂-EOR And Sequestration

4D seismic monitoring technology is the most effective method to monitor the oil displacement efficiency and dynamic changes of reservoir after CO₂ is injected into reservoirs,to track the migration path of CO₂ and verify the safety of CO₂ geological storage.The use of4D multi-component seismic method to monitor the safety of CO₂ geological storage is an important extension of the application scope of multi-component seismic technology.It is mainly by comparing differences of PS1 and PS2 amplitude and travel time between two vintages of converted-wave datasets,then we could determine the change of stress state and fracture in reservoir before and after CO₂ injection,and the combination of PP waves to determine the internal pore pressure and CO₂ saturation in the reservoir quantitatively.The research focus of this paper is the interpretation method of 4D converted-wave seismic data.To carry out and do a good research in 4D converted wave seismic data monitoring CO₂ flooding and geological storage,it is necessary to correctly understand the change of reservoir parameters after CO₂ injection into the reservoir,and simulate the change of elastic parameters and converted-wave seismic characteristics.Then,it is necessary to repeatedly perform consistent processing on the two vintages of converted-wave datasets,and finally obtain reasonable difference of converted-wave amplitude.For the carbonate reservoir of Weyburn Oilfield in Canada studied in this paper,two viewpoints and methods of porous media and fracture media need to be considered in analyzing 4D converted-wave datasets.Based on the theory of the porous media model,and based on the rock physics test data,our paper carried out P-wave and S-wave velocity prediction considering the changes of pore pressure,fluid saturation and porosity before and after CO₂ injection,and then we make converted-wave synthetic seismograms to calibrate 3D converted-wave monitor data,and to verify the accuracy of the shear wave velocity prediction after CO₂ injection;on this basis,we construct converted-wave AVO forward and inverse model in porous medium,and study the method of inverting the change of reservoir pore pressure and CO₂ saturation from 4D PP-PS wave information.Similarly,for the fracture medium model,we carry out the same fast and slow shear wave velocity prediction method,and establish the fracture medium AVAZ forward and inversion method and the reservoir parameter prediction and 4D converted-wave anisotropy theory interpretation method.For the actual 4D converted-wave seismic datasets,the 4D PP wave seismic data processing and interpretation methods are used to construct the 4D converted wave migrated data consistency processing technology,including geometry correction,phase correction,time shift correction and energy correction.After obtaining reasonable amplitude difference information of 4D converted-wave,the amplitude difference of the 4D converted-wave is explained from the perspective of the pore medium and the fracture medium.Especially under the theory of the porous medium,we could obtain plane change map of reservoir pore pressure and CO₂ saturation after preliminary inversion through the 4D PP-PS wave data.The primary problem faced by the quantitative interpretation of 4D converted-wave data is the lack of secondary logging data matched with secondary monitoring converted-wave seismic data,and the lack of shear wave logging information from well logs.Accurate shear wave velocity is the prerequisite for the interpretation of 4D converted-wave data.We established the relationship between the pore pressure and bulk modulus and shear modulus of the dry rock with Digby equation.By using the constraints of P-wave velocity,and on the basis of the rock physics test,we could obtain S-wave velocity considering the change of pore pressure.For fracture medium model,based on Thomsen's weak anisotropy theory,the fast and slow shear wave velocity curves with pore pressure changes are predicted.For the prediction of secondary time-lapse well logging information,this paper considers the change of reservoir porosity through water flooding,fracturing and other development processes,and we normalize the dissolution of the rock framework by CO₂ injection into the reservoir,the blockage of the fine particles in the reservoir pores,and the opening and closing of fractures caused by changes in the reservoir pore pressure to the change in effective porosity in reservoir.By constructing rock matrix models with different porosity to simulate changes in effective porosity of the reservoir,and then carried out fluid substation and elastic parameter prediction under varying porosity.Based on the prediction of time-lapse well logs,by constructing 4D converted-wave forward model,we can understand the changes of the converted-wave seismic response during CO₂ injection.In the porous media model,we use the Zoeppritz equation to study the rule of the converted-wave reflection coefficient with the change of pore pressure,CO₂ saturation and porosity.From the perspective of inversion,a method of joint 4D PP-PS data to separate pore pressure and CO₂ saturation is given by us.In the fractured media model,the reflection coefficients of PS1 and PS2 waves are calculated using the Ruger azimuth AVAZ formula,and we comprehensively analyze the effects of reservoir anisotropy parameters,elastic parameters,and PS1 and PS2 wave reflection coefficients influenced by pore pressure,CO₂ saturation,and porosity.In order to simulate the characteristics of the converted-wave under the change of the vertical characteristics of the reservoir in the study area,we made a 4D converted-wave synthetic seismograms from the well model,and analyzed the difference between the 4D converted-wave amplitude considering the change of pore pressure,CO₂ saturation,and porosity.Aiming at the difficulty of identifying thin reservoirs in research areas,our paper proposes a thin layer interpretation method for 4D converted-wave through the wedge model.The research results based on 4D converted-wave wedge model show that the tuning thickness of4D converted-wave is significantly affected by the pore pressure.The increase of the pore pressure will cause decrease of the converted-wave tuning thickness.In the interpretation of 4D converted-wave migration data in Weyburn field,we have adopted multiple phase corrections and time shift corrections,and used the dual constraints of amplitude difference and NRMS value to suppress noise of Baseline and Monitor 3D converted-wave data.And then we concentrated the difference of the 4D converted-wave seismic information near the reservoir,and obtained Baseline and Monitor data after 4D converted-wave consistency processing,as well as a reasonable distribution map of the 4D converted wave differences in the reservoir.The research results of 4D converted-wave show that the difference in two-way travel time of 4D converted-wave in Weyburn area cannot be used for the interpretation of 4D converted-wave seismic data.The difference in the amplitude of 4D converted-wave can be approximately regarded as the difference in pore pressure of the reservoir.Joint 4D PP and PS amplitude difference can roughly determine the changes in pore pressure and CO₂ saturation on the distribution of reservoir plane.From the perspective of the fractured medium,the difference in the amplitude of the 4D converted-wave can be approximated as the result of dual changes in pore pressure and reservoir fractures.We need to use the converted-wave pre-stack data to separate the change of the fracture,and determine the change of pore pressure in the reservoir.