Influence of fluid displacement patterns on seismic velocity during supercritical CO2 injection: Simulation study for evaluation of the relationship between seismic velocity and CO2 saturation

• Lattice Boltzmann simulation revealed supercritical CO2 behavior in rock pores.
• Relationships between CO2 saturation and P-wave velocity were constructed.
• CO2 displacement was changed by injection pressure or capillary number.
• VP response to CO2 saturation was influenced by displacement mechanism.
• Capillary number must be considered to estimate CO2 saturation from seismic velocity.

Monitoring of injected subsurface carbon dioxide (CO2) is essential for safe CO2 capture and storage. Quantitative monitoring requires knowledge of the relationship between CO2 saturation and P-wave velocity (VP). VP response to CO2 saturation is not unique and depends on the CO2 distribution within rock pores. This study evaluated the influence of CO2 distributions on VP–CO2 saturation relationships. We conducted two computational studies with different injection pressures, using a two-phase lattice Boltzmann method for CO2 injection simulation and wave propagation simulation with a finite difference approach for evaluation of VP change. The change of capillary number associated with various injection pressures affected the CO2 displacement patterns. Viscous fingering was typical at high capillary numbers, whereas both viscous and capillary fingering were observed at low capillary numbers. We identified a difference in VP–CO2 saturation relationships in these two cases; i.e., lower VP was observed in the high capillary number case than at low capillary number at the same saturation. The difference in VP response to CO2 saturation is caused by CO2 distribution features. We evaluated VP with consideration of the pore-scale CO2 distribution. This study demonstrates that capillary number at each reservoir location (e.g., distance from injection well) should be considered to accurately estimate CO2 saturation from seismic velocity.