Carbon dioxide injection into natural reservoirs has been used for enhanced oil recovery as well as, more recently, for geologic sequestration. In both technological applications, of major interest is remote sensing of the progress of the injected carbon dioxide through the subsurface as well as its effects on the physical characteristics of the rock, including the elastic, storage, and transport properties. Whereas it may be safe to assume that the presence of carbon dioxide does not alter the mineral matrix in most clastic reservoirs, the situation is very different in carbonates where such alteration may occur in real time, in a matter of hours, days and, certainly, months.
Physical laboratory experiments have confirmed that the carbon dioxide interacts with formation water and, eventually, with the mineral matrix (Vanorio, T. et all, 2008). These chemical processes alter the pore space geometry in carbonates and can even create such flow conduits as relatively large wormholes. This pore-space and matrix alterations can definitely affect the elastic properties of carbonate rock, which are crucial in interpreting the 4D time-lapse seismic data for pore-fluid content in space and time. Because of the changes occurring in the mineral matrix during carbon dioxide injection, traditional fluid-substitution techniques often do not work in such reactive rock and, hence, can be misleading during seismic data interpretation.