4.2.1 Long-term integrity modelling

Modelling of long-term integrity aims to assess the ultimate fate of the injected CO2 and its impacts on physical properties. Four processes are distinguished: structural trapping, residual CO2 trapping, dissolution trapping and mineral trapping, described in the previous chapter.

Structural trapping represents the supercritical CO2 that is trapped within the pore space as a buoyant immiscible fluid phase, according to the heterogeneity of the storage zone lithology. Residual CO2 trapping represents the supercritical CO2 that is permanently trapped within small pores and cannot be remobilised. Dissolution trapping represents the CO2 dissolved in the liquid phase (oil or fluid). The final mechanism, mineral trapping, represents the CO2 that is incorporated into new secondary minerals due to chemical precipitation (Gaus et al., 2008).

Long-term integrity modelling aims to predict the ultimate fate of the injected CO2, accounting for the geometry of the reservoir in a simplified way. Studies can thus be based on one-dimensional (Knauss et al., 2005; Xu et al., 2005), two-dimensional (Audigane et al., 2007; Johnson et al., 2001; White et al., 2005) or three-dimensional (Nghiem et al., 2004; Le Gallo et al., 2006) transport. As long as geometries remain simple, it is possible to identify dominant geochemical interactions from the calculated species concentrations and the amounts of minerals dissolving and precipitating. This is also true for two-dimensional models involving a slightly more complex geology (Johnson et al., 2001; Johnson et al., 2004; Audigane et al., 2007). Gaus et al., 2008 noted, that when the complexity of the model grid and the number of layers increase, identification of dominant geochemical reactions becomes increasingly difficult.