3.1.5 Dissolution

Some of the CO2 will dissolve into the formation water as it migrates through the formation. Simulation results show that up to 30% of the injected CO2 may dissolve in formation water over tens of years (Doughty et al., 2001). Large scale models suggest that the injected CO2 will eventually dissolve in formation water over centuries (McPherson and Cole, 2000; Ennis-King et al., 2003). It will take much longer for CO2 to be completely dissolved, if there is no flow of formation water because of reduced contact with unsaturated formation water. Water saturated with CO2 is slightly denser than fresh formation water, depending on salinity. Reservoir studies and simulations have shown that the denser CO2-saturated brine will eventually sink, which may trigger free convection and thus enhance the CO2 dissolution process (Lindeberg and Bergmo, 2003).

Solubility of CO2 in brine decreases with increasing pressure, decreasing temperature and increasing salinity. Calculations indicate that, depending on the salinity and depth, 20-60 kg CO2 can dissolve in 1 m3 of formation fluid (Holt et al., 1995; Koide et al., 1995).