Abstract:

Aiming at the feasibility of time-lapse seismic monitoring in the CO₂ 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 CO₂ injection. After that, a two-layer medium model was established based on the reservoir logging data of well Huang-234. Finally, the AVO gradient intercept varied with the formation pressure and CO₂ saturation after the CO₂ injection was calculated. It was found that with the increase of injection pressure, the gradient decreased and the intercept increased. As CO₂ saturation increased, the gradient and intercept decreased. The reflection coefficients before and after CO₂ 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.

Key words: CO₂ flooding and storage, two-layer model, time-lapse AVO, pressure change, shear wave velocity