Effective pressure prediction from 4D seismic AVO data during CO2-EOR and storage

The use of time-lapse seismic difference analysis methods, time-lapse AVO forward and inversion technology to study and distinguish the changes in saturation and pressure caused by CO2 injection into reservoirs is one of the difficult issues in the current technology research for monitoring CO2 geological storage. Based on the time-lapse seismic AVO inversion method proposed by Landrø, this paper distinguishes the fluid saturation and pressure effect after CO2 injection. By establishing a petrophysical model with simultaneous changes in CO2 saturation and pressure, and performing an AVO forward and inversion simulation of two-layer media, we compare the accuracy of Landrø’s simplified formula with Smith and Gidlow's AVO simplified equation that before Landrø simplified in inverting the saturation differences (ΔS) and pressure differences (ΔP), and conclude that Smith and Gidlow's simplified formula has better accuracy. And then we get the relationship between the saturation differences (ΔS), pressure differences (ΔP) and the time-lapse AVO intercept differences (ΔR0), gradient differences (ΔG). The relationship is further applied to the actual time-lapse seismic AVO inversion to quantitatively analyze the saturation and pressure changes after CO2 injection. The effective pressure difference value obtained by inversion has a good match with the measured wellhead pressure difference value, which can well explain the pressure sweep range and the pressure difference change caused by CO2 injection in the reservoir, which provides a basis for monitoring the underground occurrence state and storage safety of CO2.