S-wave Prediction And Time Lapse AVO Model On Porous And Fractured Porous Media For CO₂ Sequestration

Rock physics is the link between seismic attributes and reservoir properties.Rock physics can also provide methods and basis for interpreting time-lapse seismic data.In CO2 sequestration,time-lapse seismic technology is key to choose CO2 sequestration site,monitor status and distribution of CO2 stored underground,and verify if the injection volume equals to sequestration volume.During the process of CO2 injection,pore pressure as well as fluid composition and saturation in the reservoir will may be changed.It is the key to predict saturation and pressure of CO2 in the reservoir during injection and post-closure using time lapse seismic information.The focus of this paper is to use the rock physics method to predict the shear wave velocity at different stages of CO2 injection,i.e.,the change of pore pressure and fluid saturation and the prediction of shear wave velocity with different porosity.Then time lapse AVO forward model is constructed and provides foundation for seismic interpretation and seismic invesion.In the view of resevoir fluid change,a mix-fluid model is established to calculate mix fluid perproties of oil,water and CO2 by using wood's equaiton.Shear wave velocity is an indispensable parameter in the AVO forward modeling model without the shear wave velocity area.Shear wave velocity is sensitive to pressure and insensitive to fluids,and P wave is sensitive to both pressure and fluid during CO2 injection Therefore,predicting how shear wave velocity changes with pressure can be used to interpret 4D seismic monitoring data at different stages of CO2 injection.Digby model can be used to calculate pressure-dependent bulk modulus and shear modulus of dry rock.In the paper,we propose an improved method for calculating the unknown parameter-coordination number of the Digby model,which makes our prediction match measured shear-wave velocities quite well.Then the calculation method of modulus and shear modulus of dry rock mass with pressure and porosity is established by using Digby model,and then the Biot-Gassmann equation(1951)is used to predict the P and shear wave velocity after fluid substitution.A shear-wave splits into fast and slow model when cracks exist in reservoir.Fast and slow shear wave velocity prediction in the different stages of CO2 in the borehole is needed to interpret the change of the reservoir fractures or cracks at different stages of CO2 injection by using the time lapse multi-component seismic data.And evaluate the safety of CO2 sequestration.Therefore,based on the above-mentioned porous model with pressure change,the fast and slow shear wave velocity and the calculation method of anisotropy parameters of fractured porous media with pressure change are established by using Thomsen weak anisotropic medium theory.After getting P-wave velocities,shear wave velocities and density in different pressures,we an use time lapse AVO forward model to interpret time lapse siesmic data.For the porous media,Zoeppritz equation is used to calculate reflection coefficients.Firstly,time lapse synthetic seismograms and AVO gradient & intercept crossplos of two-layer models were used to identify the fluids using different pressures and P-wave and shear wave velocities at different saturation.Considering the two-layer model is difficult to reflect the multiple reservoirs of reservoirs and thin interbed information,we made time lapse synthetic seismograms based on the actual well logging data.For the fractured porous media,this paper uses the anisotropic azimuthal AVO formula derived from Rüger to compute the reflection coefficients of the fast and slow shear wave,i.e.PS1 wave and PS2 wave,and make the time lapse Converted-wave synthetic seismograms.After establishing shear wave predicting model and AVO forward model,these mothods are separately applied to carbonate reservoir in Weyburn oil field,Canada and sand shale interbed reservoir in Shengli oil field,China,which two typical different pressure conditions and reservoirs with different characteristics,to predict the shear wave velocity and make time lapse AVO forward modeling.The properties and pressure variation of these two prejects are classic and different.For carbonate reservoir of Weyburn field,P-wave and shaer wave velocities with different pressure and fluid saturation are predicted firstly.Then due to the fracture of reservoir developed,this paper also carries out the prediction of fast and slow shear wave velocity under different pressure conditions.And then use the Zoeppritz equation and Rüger's formula to establish the time lapse AVO forward model respectively.For the sand interbedding reservoir of Shengli Oilfield,the fluid substitution sand reservoir is extracted to carry our fluid substation and velocity prediction,and then the AVO forward model is established.The actual data show that the method of this paper shows that the pressure and porosity changes in the shear wave velocity prediction for the Weyburn oilfield carbonate rocks and Shengli oil field interbed sand reservoir with different pressure characteristics is effective and applicable.The method studied in this paper is also applicable to the prediction of the P-wave velocity of reservoirs with different porosity and pressure change in development later stage of oilfields and during CO2 injection.Of course,the P-wave velocity is both sensitive to pressure and fluid saturation.Based on this,the constructed time lapse AVO model,synthetic seismogram and the Converted-wave synthetic seismogram in the fractured media have improved our understanding of AVO and seismic attributes in time lapse seismic monitoring information in CO2 sequestration and provide the basis for the designing of time lapse seismic acquisition,analysis of time lapse seismic processing and interpretation.