One of the criteria that have to be fulfilled when assessing the impact of CO2 storage is the evolution of the storage site towards a situation of long-term stability. Mineral trapping is the incorporation of CO2 into minerals due to chemical precipitation. It is a time dependent process whose contribution to CO2 immobilisation increases slowly with time. To assess long-term mineral trapping a time scale of up to 10,000 years is needed.
Once the CO2 is in the subsurface, primary host rock minerals will start to dissolve due to low pH, releasing divalent cations which react with the dissolved bicarbonate species forming Ca, Mg and Fe carbonates. Aluminosilicate minerals such as clay minerals, micas, chlorites and feldspars that can function as cation donors, dissolve very slow at reservoir temperatures. Dissolution and precipitation kinetics of minerals is affected by the composition of the rock and the formation fluid, the CO2 fugacity as well as by temperature and pressure.
There arises the question: How should long-term mineral trapping capacity be assessed? Considering the complexity and interdependency of chemical and physical processes as well as the time factor, numerical modelling turns out to be the best tool to use, backed up where possible by laboratory experiments and findings from natural CO2 deposits/reservoirs.
With a focus on saline aquifers our investigation area is the Ketzin pilot site using geochemical modelling as a primary tool. Fluid and mineral composition are provided by site-specific core and fluid sample analysis. One of the main goals is to assess the influence of heterogeneous parameters like porosity, water saturation and dissolved CO2 concentrations on mineral trapping. The detailed level of knowledge and data needed for the modelling process is joined by uncertainties, especially with regard to kinetic of long-term reactions. Dealing with the different kind of uncertainties to make well-founded predictions of mineral trapping is thus a further objective.