Processing and Interpretation of Time-Lapse Seismic Data at Gao 89 CO2-EOR Area

CO2-EOR greatly recovered oil production in Gao 89-1 block in Sinopec Shengli Oil Company. However, CO2-EOR efficiency was high in some wells but some production wells had little effect. In order to study CO2 sweep efficiency and CO2 distribution, 3D seismic monitoring acquisition was conducted in December, 2010. It is the unique time-lapse 3D seismic datasets to monitor CO2 distribution in China. But the geometry of baseline and monitor is different. Moreover, the depth of reservoir is about 3200 m and effective thickness of reservoir is only 10 m. In addition, the injection volume of CO2 was small and around 50,000 t. If time-lapse 3D seismic technique could discriminate CO2 distribution has been a controversial question. In this paper, we studied the problems during CO2 flooding and estimated the net CO2 storage volume. Then we proposed a time-lapse seismic processing method that improved the conventional geometry degradation and made a better seismic image from vantages of seismic monitoring. In order to calibrate monitoring seismic datasets by using the well log data measured before CO2 injection, P- and S- wave velocity logs versus pressure change method is proposed and verified by making synthetic seismograms. Our study proved that the time-lapse seismic difference mainly caused by the change of pore pressure during CO2 injection in this area. Finally, time-lapse seismic difference calibration method is proposed that assuming the caprock seismic attributes were consistent before and after CO2 injection. The time-lapse difference matched the oil production and CO2 injection well.