The detectability of free-phase migrating CO₂: a rock physics and seismic modelling feasibility study

Subsurface monitoring is essential for the successful implementation and public acceptance of CO2 storage. Injected CO2 will need to be monitored to verify the successful containment within its intended formation, and to ensure no loss of containment within the storage complex. The ability for seismic techniques to monitor structurally trapped CO2 has been successfully demonstrated due to the changes in the acoustic properties of the reservoir produced by the displacement of brine by less dense and more compressible CO2. However, the ability for seismic methods to detect free-phase migrating CO2 is still moderately understood. In order to assess the feasibility for seismic monitoring of a migrating front, we estimate the time-lapse signal over a theoretical, clean, homogeneous sandstone reservoir through the application of a three-stage model-driven workflow consisting of fluid-flow, rock physics and seismic forward modelling. To capture the range of responses which could be encountered, two end-member fluid distribution models were used: uniform saturation and the modified patchy saturation model. Analysis of the time- lapse survey highlights the importance of determining and understanding the fluid distribution model impacting the range of velocities prior to generating and interpreting the seismic response. This change in velocity is shown to be directly related to the volume of CO2 occupying the pore-space of a migrating plume front. This highlights the fact that the detectability of a migrating front is a site specific issue which not only depends on the geophysical parameters of the seismic survey but also on the geological variations and spatial distribution in the reservoir.