Aiming at the feasibility of time-lapse seismic monitoring in the CO

2

flooding-EOR process of tight reservoirs in the Ordos basin

and based on the tight oil-bearing sandstone reservoirs in the Sulige field

the volume modulus and shear modulus of dry rock were calculated by Digby model which considered the pressure changes. Then

Digby model and Gassmann equation were used to predict the primary and shear wave velocity

and to calculate their value varying with the pressure after CO

2

injection. After that

a two-layer medium model was e

stablished based on the reservoir logging data of well Huang-234. Finally

the AVO gradient intercept varied with the formation pressure and CO

2

saturation after the CO

2

injection was calculated. It was found that with the increase of injection pressure

the gradient decreased and the intercept increased. As CO

2

saturation increased

the gradient and intercept decreased. The reflection coefficients before and after CO

2

injection were significantly different. As for the tight sand oil reservoir

its AVO features were accorded with the third class of gas-bearing sandstone

which could be effectively detected by the secondary seismic monitoring.