CO2 Monitoring by Using VSP-FWI - Timelapse Elastic Parameters Estimation

Geological carbon sequestration involves large-scale injection of carbon dioxide into underground geologic formations. Changes in reservoir properties resulting from the CO2 injection and migration can be monitored using time-lapse seismic data. Conventional analysis of time-lapse surface and/or borehole seismic data only gives qualitative information about the changes of the acoustic impedance contrasts in the reservoir. In order to differentiate the pore-pressure from CO2 saturation effects, it is necessary to evaluate the changes in the elastic properties of the reservoir. Borehole seismic data contain strong converted shear waves at the level of the reservoir that will allow determining S-velocity changes. Elastic full waveform inversion (FWI) is the appropriate method to estimate elastic parameters. We demonstrate the feasibility of a timelapse multi-Offset VSP inversion for CO2 sequestration monitoring, by inverting synthetic seismic data based on a virtual CO2 injection site study. The timelapse FWI recovers P-wave and S-wave velocity for the baseline model, i.e., the reference model; and, the perturbations of the velocity models associated to 3 year of CO2 injection are also well recovered for a distance of a few hundreds of meters from the well.