4.2.3 Well integrity modelling

Well integrity modelling focuses on the geochemical impacts of CO2 on the well completion itself. Well completions are made of a sheath of cement surrounding a casing (i.e., pipe) made of steel. The cement sheath seals the annulus between the casing and the borehole walls and prevents the migration of fluid between the formation rocks and the casing. Cements are also used to plug the casing in case of well abandonment. Over its life, the cement is exposed to fluids of varying composition (e.g. saline brine, CO2-rich phase fluids, two-component fluids). Chemical interactions will occur between the brine, the gas mixture and the cements and borehole steels. A typical reaction sequence is the carbonation of the constituent cement minerals by CO2. This reaction may impact the cement mineralogy and porosity and thus its transport and mechanical properties. For example, reaction may cause an increase of permeability, diffusivity, increased fissures and annular space between the casing and the cement sheath, thus creating paths for leakage to surface (Gaus et al., 2008).

For CO2 storage purposes, modelling of well cement degradation due to the presence of CO2 needs to be performed at low, acidic pH values. Jacquemet, 2006, Carey and Lichtner, 2007 and Carey et al., 2007 have modelled cement degradation using one dimensional models without accounting for the geometry of the well itself. Two studies, conducted by Pfingsten, 2002 and Burnol et al., 2006, although conducted for nuclear waste purposes, are recommended by Gaus et al., 2008 since carbonation of cement is modelled. Pfingsten, 2002 emphasises the need for additional data linking permeability and diffusivity to porosity, since carbonation is likely to significantly decrease porosity (Thiery, 2005; Jacquemet, 2006). As concluded by Burnol et al., 2006, future modelling should also account for possible carbonation in two-phase conditions, e.g. by intrusion of gas within the cement. Regnault et al., 2005 and Jacquemet, 2006 demonstrated experimentally that significant carbonation can also occur in complete absence of brine. In addition, reactions with other gaseous components, such as SO2 and SO3, and cement should also be considered.