Geological carbon storage: a FEM reactive transport model to assess caprock degradation

Increased concentrations of greenhouse gases in the atmosphere are known to be the main cause of global warming. Among the various decarbonisation strategies proposed, geological storage of CO2 is one of the most interesting options for reducing net carbon emissions to the atmosphere. The safe long-term storage of CO2 in spatially confined underground volumes requires the combination of a reservoir and an undamaged structural trap with a suitable low-permeability caprock, such as is potentially provided by deep saline aquifers, depleted oil and gas fields and unminable coal seams. In order to prevent CO2 leakage to the atmosphere over long geological time scales, the potential caprock alterations due to contact with CO2 need to be considered. In particular, CO2 dissolution and diffusion in the pore fluid leads to acidification of the in-situ brine, causing chemical reactions with some caprock minerals and potentially affecting mechanical and transport properties. This paper presents a reactive transport model to assess the effects of pore water acidification on caprock materials. The model includes the water mass balance equation for the saturated porous medium and the mass balance equation for all primary chemical species dissolved in water, following the theoretical approach presented in Steefel & Lasaga (1994). The proposed modelling approach considers both the aqueous (homogeneous) reactions of CO2 dissolved in water (assumed to be in equilibrium) and the dissolution kinetics of calcite in the acidic environment induced by CO2 injection. Calcite dissolution is finally coupled to porosity changes via the reactive surface area of the mineral and the reaction rate. The chemo-hydraulic coupling is addressed by considering porosity changes in the storage term of the balance equations and by introducing an appropriate link between hydraulic conductivity and current porosity.