Veröffentlichungen und Präsentationen

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 takes into account 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 (see Appelo & Postma (2004) for details). 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. The model requires the solution of an initial chemical speciation problem and then the integration of a set of partial differential equations (the mass balance equations of water and of the major primary species dissolved in water) together with a set of non-linear algebraic equations (to obtain the concentration of all the chemical species involved). The entire numerical solution has been obtained in Comsol Multiphysics, with the following interfaces: “Transport of diluted species” for primary species mass balance, “Darcy’ law” for the water mass balance and “Distributed ODEs” for the dissolution kinetics and speciation problem, as proposed by Lopez Vizcaino et al. (2021). The numerical model was validated according to the numerical benchmark proposed in Lu et al. (2022), developed to reproduce a geochemical scenario where mineral dissolution causes permanent changes in the transport properties of a porous medium, and then applied to reproduce simple scenarios involving the interaction between CO2 and calcite-rich caprock (see, e.g., Gramegna et al, 2023)