Abstract

CO₂ is injected into deep geological formations for enhanced oil recovery and/or for sequestration. For ideal results, CO₂ is designated to be kept in the formation in supercritical state, pressure and temperature above the critical point (1071 psia and 88°F). Due to geological heterogeneity and changes in injection operation, the pressure or temperature might fall below the critical point in some parts of the formation. This will cause CO₂ transition from supercritical to subcritical. Seismic methods have been considered promising for monitoring the distribution of injected CO₂ under supercritical state. Can this method be used to monitor CO₂ state change? This paper presents experimental results for an Indiana limestone core sample saturated with CO₂ at 1100 psia (above critical pressure) and subject to axial and radial confining pressures of 3200 psia. We observed systematic changes in the compressional and shear wave velocities as the sample was heated and cooled across CO₂ critical temperature. These results suggest that seismic methods have the potential to monitor remotely temperature-induced CO₂ phase transition in subsurface geological formations.